The original web framework included in the Spring Framework, Spring Web MVC, was purpose built for the Servlet API and Servlet containers. The reactive stack, web framework, Spring WebFlux, was added later in version 5.0. It is fully non-blocking, supports Reactive Streams back pressure, and runs on servers such as Netty, Undertow, and Servlet 3.1+ containers.
Both web frameworks mirror the names of their source modules
spring-webmvc and
spring-webflux
and co-exist side by side in the Spring Framework. Each module is optional.
Applications may use one or the other module, or in some cases both — e.g. Spring MVC controllers with the reactive WebClient
.
Why was Spring WebFlux created?
Part of the answer is the need for a non-blocking web stack to handle concurrency with a
small number of threads and scale with less hardware resources. Servlet 3.1 did provide
an API for non-blocking I/O. However, using it leads away from the rest of the Servlet API
where contracts are synchronous (Filter
, Servlet
) or blocking (getParameter
,
getPart
). This was the motivation for a new common API to serve as a foundation across
any non-blocking runtime. That is important because of servers such as Netty that are well
established in the async, non-blocking space.
The other part of the answer is functional programming. Much like the addition of annotations
in Java 5 created opportunities — e.g. annotated REST controllers or unit tests, the addition
of lambda expressions in Java 8 created opportunities for functional APIs in Java.
This is a boon for non-blocking applications and continuation style APIs — as popularized
by CompletableFuture
and ReactiveX, that allow declarative
composition of asynchronous logic. At the programming model level Java 8 enabled Spring
WebFlux to offer functional web endpoints alongside with annotated controllers.
We touched on non-blocking and functional but what does reactive mean?
The term "reactive" refers to programming models that are built around reacting to change — network component reacting to I/O events, UI controller reacting to mouse events, etc. In that sense non-blocking is reactive because instead of being blocked we are now in the mode of reacting to notifications as operations complete or data becomes available.
There is also another important mechanism that we on the Spring team associate with "reactive" and that is non-blocking back pressure. In synchronous, imperative code, blocking calls serve as a natural form of back pressure that forces the caller to wait. In non-blocking code it becomes important to control the rate of events so that a fast producer does not overwhelm its destination.
Reactive Streams is a small spec, also adopted in Java 9, that defines the interaction between asynchronous components with back pressure. For example a data repository — acting as Publisher, can produce data that an HTTP server — acting as Subscriber, can then write to the response. The main purpose of Reactive Streams is to allow the subscriber to control how fast or how slow the publisher will produce data.
Note
|
Common question: what if a publisher can’t slow down? |
Reactive Streams plays an important role for interoperability. It is of interest to libraries
and infrastructure components but less useful as an application API because it is too
low level. What applications need is a higher level and richer, functional API to
compose async logic — similar to the Java 8 Stream
API but not only for collections.
This is the role that reactive libraries play.
Reactor is the reactive library of choice for Spring WebFlux. It provides the Mono and Flux API types to work on data sequences of 0..1 and 0..N through a rich set of operators aligned with the ReactiveX vocabulary of operators. Reactor is a Reactive Streams library and therefore all of its operators support non-blocking back pressure. Reactor has a strong focus on server-side Java. It is developed in close collaboration with Spring.
WebFlux requires Reactor as a core dependency but it is interoperable with other reactive
libraries via Reactive Streams. As a general rule WebFlux APIs accept a plain Publisher
as input, adapt it to Reactor types internally, use those, and then return either
Flux
or Mono
as output. So you can pass any Publisher
as input and you can apply
operations on the output, but you’ll need to adapt the output for use with another reactive library.
Whenever feasible — e.g. annotated controllers, WebFlux adapts transparently to the use
of RxJava or other reactive library. See [webflux-reactive-libraries] for more details.
The spring-web
module contains the reactive foundation that underlies Spring WebFlux
including HTTP abstractions, Reactive Streams adapters for supported
servers, codecs, and a core WebHandler API comparable to
the Servlet API but with non-blocking contracts.
On that foundation Spring WebFlux provides a choice of two programming models:
-
Annotated Controllers — consistent with Spring MVC, and based on the same annotations from the
spring-web
module. Both Spring MVC and WebFlux controllers support reactive (Reactor, RxJava) return types and as a result it is not easy to tell them apart. One notable difference is that WebFlux also supports reactive@RequestBody
arguments. -
[webflux-fn] — lambda-based, lightweight, functional programming model. Think of this as a small library or a set of utilities that an application can use to route and handle requests. The big difference with annotated controllers is that the application is in charge of request handling from start to finish vs declaring intent through annotations and being called back.
Spring MVC or WebFlux?
A natural question to ask but one that sets up an unsound dichotomy. It’s actually both working together to expand the range of available options. The two are designed for continuity and consistency with each other, they are available side by side, and feedback from each side benefits both sides. The diagram below shows how the two relate, what they have in common, and what each supports uniquely:
Below are some specific points to consider:
-
If you have a Spring MVC application that works fine, there is no need to change. Imperative programming is the easiest way to write, understand, and debug code. You have maximum choice of libraries since historically most are blocking.
-
If you are already shopping for a non-blocking web stack, Spring WebFlux offers the same execution model benefits as others in this space and also provides a choice of servers — Netty, Tomcat, Jetty, Undertow, Servlet 3.1+ containers, a choice of programming models — annotated controllers and functional web endpoints, and a choice of reactive libraries — Reactor, RxJava, or other.
-
If you are interested in a lightweight, functional web framework for use with Java 8 lambdas or Kotlin then use the Spring WebFlux functional web endpoints. That can also be a good choice for smaller applications or microservices with less complex requirements that can benefit from greater transparency and control.
-
In a microservice architecture you can have a mix of applications with either Spring MVC or Spring WebFlux controllers, or with Spring WebFlux functional endpoints. Having support for the same annotation-based programming model in both frameworks makes it easier to re-use knowledge while also selecting the right tool for the right job.
-
A simple way to evaluate an application is to check its dependencies. If you have blocking persistence APIs (JPA, JDBC), or networking APIs to use, then Spring MVC is the best choice for common architectures at least. It is technically feasible with both Reactor and RxJava to perform blocking calls on a separate thread but you wouldn’t be making the most of a non-blocking web stack.
-
If you have a Spring MVC application with calls to remote services, try the reactive
WebClient
. You can return reactive types (Reactor, RxJava, or other) directly from Spring MVC controller methods. The greater the latency per call, or the interdependency among calls, the more dramatic the benefits. Spring MVC controllers can call other reactive components too. -
If you have a large team, keep in mind the steep learning curve in the shift to non-blocking, functional, and declarative programming. A practical way to start without a full switch is to use the reactive
WebClient
. Beyond that start small and measure the benefits. We expect that for a wide range of applications the shift is unnecessary. If you are unsure what benefits to look for, start by learning about how non-blocking I/O works (e.g. concurrency on single-threaded Node.js) and its effects.
Spring WebFlux is supported on Tomcat, Jetty, Servlet 3.1+ containers, as well as on non-Servlet runtimes such as Netty and Undertow. All servers are adapted to a low-level, common API so that higher level programming models can be supported across servers.
Spring WebFlux does not have built-in support to start or stop a server. However it is easy to assemble an application from Spring configuration, and WebFlux infrastructure, and run it with a few lines of code.
Spring Boot has a WebFlux starter that automates these steps. By default the starter uses Netty but it is easy to switch to Tomcat, Jetty, or Undertow simply by changing your Maven or Gradle dependencies. Spring Boot defaults to Netty because it is more widely used in the async, non-blocking space, and provides a client and a server share resources.
Tomcat and Jetty can be used with both Spring MVC and WebFlux. Keep in mind however that the way they’re used is very differently. Spring MVC relies on Servlet blocking I/O and allows applications to use the Servlet API directly if they need to. Spring WebFlux relies on Servlet 3.1 non-blocking I/O and uses the Servlet API behind a low-level adapter and not exposed for direct use.
For Undertow, Spring WebFlux uses Undertow APIs directly without the Servlet API.
Performance has many characteristics and meanings. Reactive and non-blocking generally
do not make applications run faster. They can, in some cases, for example if using the
WebClient
to execute remote calls in parallel. On the whole it requires more work to do
things the non-blocking way and that can increase slightly the required processing time.
The key expected benefit of reactive and non-blocking is the ability to scale with a small, fixed number of threads and less memory. That makes applications more resilient under load because they scale in a more predictable way. In order to observe those benefits however you need to have some latency including a mix of slow and unpredictable network I/O. That’s where the reactive stack begins to show its strengths and the differences can be dramatic.
Both Spring MVC and Spring WebFlux support annotated controllers, but there is a key difference in the concurrency model and default assumptions for blocking and threads.
In Spring MVC, and servlet applications in general, it is assumed that applications may block the current thread, e.g. for remote calls, and for this reason servlet containers use a large thread pool, to absorb potential blocking during request handling.
In Spring WebFlux, and non-blocking servers in general, it is assumed that applications will not block, and therefore non-blocking servers use a small, fixed-size thread pool (event loop workers) to handle requests.
Tip
|
To "scale" and "small number of threads" may sound contradictory but to never block the current thread, and rely on callbacks instead, means you don’t need extra threads as there are no blocking calls to absorb. |
Invoking a Blocking API
What if you do need to use a blocking library? Both Reactor and RxJava provide the
publishOn
operator to continue processing on a different thread. That means there is an
easy escape latch. Keep in mind however that blocking APIs are not a good fit for
this concurrency model.
Mutable State
In Reactor and RxJava, logic is declared through operators, and at runtime, a reactive pipeline is formed where data is processed sequentially, in distinct stages. A key benefit of that is that it frees applications from having to protect mutable state because application code within that pipeline is never invoked concurrently.
Threading Model
What threads should you expect to see on a server running with Spring WebFlux?
-
On a "vanilla" Spring WebFlux server (e.g. no data access, nor other optional dependencies), you can expect one thread for the server, and several others for request processing (typically as many as the number of CPU cores). Servlet containers, however, may start with more threads (e.g. 10 on Tomcat), in support of both servlet, blocking I/O and servlet 3.1, non-blocking I/O usage.
-
The reactive
WebClient
operates in event loop style. So you’ll see a small, fixed number of processing threads related to that, e.g. "reactor-http-nio-" with the Reactor Netty connector. However if Reactor Netty is used for both client and server, the two will share event loop resources by default. -
Reactor and RxJava provide thread pool abstractions, called Schedulers, to use with the
publishOn
operator that is used to switch processing to a different thread pool. The schedulers have names that suggest a specific concurrency strategy, e.g. "parallel" for CPU-bound work with a limited number of threads, or "elastic" for I/O-bound work with a large number of threads. If you see such threads it means some code is using a specific thread poolScheduler
strategy. -
Data access libraries and other 3rd party dependencies may also create and use threads of their own.
Configuring
The Spring Framework does not provide support for starting and stopping servers. To configure the threading model for a server, you’ll need to use server-specific config APIs, or if using Spring Boot, check the Spring Boot configuration options for each server. The WebClient can be configured directly. For all other libraries, refer to their respective documentation.
The spring-web
module provides low level infrastructure and HTTP abstractions — client
and server, to build reactive web applications. All public APIs are build around Reactive
Streams with Reactor as a backing implementation.
Server support is organized in two layers:
-
HttpHandler and server adapters — the most basic, common API for HTTP request handling with Reactive Streams back pressure.
-
WebHandler API — slightly higher level but still general purpose server web API with filter chain style processing.
Every HTTP server has some API for HTTP request handling. {api-spring-framework}/http/server/reactive/HttpHandler.html[HttpHandler] is a simple contract with one method to handle a request and response. It is intentionally minimal. Its main purpose is to provide a common, Reactive Streams based API for HTTP request handling over different servers.
The spring-web
module contains adapters for every supported server. The table below shows
the server APIs are used and where Reactive Streams support comes from:
Server name | Server API used | Reactive Streams support |
---|---|---|
Netty |
Netty API |
|
Undertow |
Undertow API |
spring-web: Undertow to Reactive Streams bridge |
Tomcat |
Servlet 3.1 non-blocking I/O; Tomcat API to read and write ByteBuffers vs byte[] |
spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge |
Jetty |
Servlet 3.1 non-blocking I/O; Jetty API to write ByteBuffers vs byte[] |
spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge |
Servlet 3.1 container |
Servlet 3.1 non-blocking I/O |
spring-web: Servlet 3.1 non-blocking I/O to Reactive Streams bridge |
Here are required dependencies, supported versions, and code snippets for each server:
Server name | Group id | Artifact name |
---|---|---|
Reactor Netty |
io.projectreactor.ipc |
reactor-netty |
Undertow |
io.undertow |
undertow-core |
Tomcat |
org.apache.tomcat.embed |
tomcat-embed-core |
Jetty |
org.eclipse.jetty |
jetty-server, jetty-servlet |
Reactor Netty:
HttpHandler handler = ...
ReactorHttpHandlerAdapter adapter = new ReactorHttpHandlerAdapter(handler);
HttpServer.create(host, port).newHandler(adapter).block();
Undertow:
HttpHandler handler = ...
UndertowHttpHandlerAdapter adapter = new UndertowHttpHandlerAdapter(handler);
Undertow server = Undertow.builder().addHttpListener(port, host).setHandler(adapter).build();
server.start();
Tomcat:
HttpHandler handler = ...
Servlet servlet = new TomcatHttpHandlerAdapter(handler);
Tomcat server = new Tomcat();
File base = new File(System.getProperty("java.io.tmpdir"));
Context rootContext = server.addContext("", base.getAbsolutePath());
Tomcat.addServlet(rootContext, "main", servlet);
rootContext.addServletMappingDecoded("/", "main");
server.setHost(host);
server.setPort(port);
server.start();
Jetty:
HttpHandler handler = ...
Servlet servlet = new JettyHttpHandlerAdapter(handler);
Server server = new Server();
ServletContextHandler contextHandler = new ServletContextHandler(server, "");
contextHandler.addServlet(new ServletHolder(servlet), "/");
contextHandler.start();
ServerConnector connector = new ServerConnector(server);
connector.setHost(host);
connector.setPort(port);
server.addConnector(connector);
server.start();
Note
|
To deploy as a WAR to a Servlet 3.1+ container, wrap |
The WebHandler API is a general purpose, server, web API for processing requests through a
chain of {api-spring-framework}/web/server/WebExceptionHandler.html[WebExceptionHandler’s],
{api-spring-framework}/web/server/WebFilter.html[WebFilter’s], and a target
{api-spring-framework}/web/server/WebHandler.html[WebHandler]. The chain can be assembled
with WebHttpHandlerBuilder
either by adding components to the builder or by having them
detected from a Spring ApplicationContext
. The builder returns an
HttpHandler that can then be used to run on any of the supported servers.
While HttpHandler
aims to be the most minimal contract across HTTP servers, the
WebHandler API provides essential features commonly used to build web applications.
For example, the ServerWebExchange
available to WebHandler API components provides
access not only to the request and response, but also to request and session attributes,
access to parsed form data, multipart data, and more.
The table below lists the components that WebHttpHandlerBuilder
detects:
Bean name | Bean type | Count | Description |
---|---|---|---|
<any> |
|
0..N |
Provide handling for exceptions from the chain of |
<any> |
|
0..N |
Apply interception style logic to before and after the rest of the filter chain and
the target |
"webHandler" |
|
1 |
The handler for the request. |
"webSessionManager" |
|
0..1 |
The manager for |
"serverCodecConfigurer" |
|
0..1 |
For access to |
"localeContextResolver" |
|
0..1 |
The resolver for |
ServerWebExchange
exposes the following method for access to form data:
Mono<MultiValueMap<String, String>> getFormData();
The DefaultServerWebExchange
uses the configured HttpMessageReader
to parse form data
("application/x-www-form-urlencoded") into a MultiValueMap
. By default
FormHttpMessageReader
is configured for use via the ServerCodecConfigurer
bean
(see Web Handler API).
ServerWebExchange
exposes the following method for access to multipart data:
Mono<MultiValueMap<String, Part>> getMultipartData();
The DefaultServerWebExchange
uses the configured
HttpMessageReader<MultiValueMap<String, Part>>
to parse "multipart/form-data" content
into a MultiValueMap
. At present
Synchronoss NIO Multipart is the only 3rd
party library supported, and the only library we know for non-blocking parsing of
multipart requests. It is enabled through the ServerCodecConfigurer
bean
(see Web Handler API).
To parse multipart data in streaming fashion, use the Flux<Part>
returned from an
HttpMessageReader<Part>
instead. For example in an annotated controller use of
@RequestPart
implies Map-like access to individual parts by name, and hence requires
parsing multipart data in full. By contrast @RequestBody
can be used to decode the
content to Flux<Part>
without collecting to a MultiValueMap
.
The spring-web
module defines the
{api-spring-framework}/http/codec/HttpMessageReader.html[HttpMessageReader] and
{api-spring-framework}/http/codec/HttpMessageWriter.html[HttpMessageWriter] contracts
for encoding and decoding the body of HTTP requests and responses via Rective Streams
Publisher
's. These contacts are used on the client side, e.g. in the WebClient
,
and on the server side, e.g. in annotated controllers and functional endpoints.
The spring-core
module defines the
{api-spring-framework}/core/codec/Encoder.html[Encoder] and
{api-spring-framework}/core/codec/Decoder.html[Decoder] contracts that are independent of
HTTP and rely on the {api-spring-framework}/core/io/buffer/DataBuffer.html[DataBuffer]
contract that abstracts different byte buffer representations such as the Netty ByteBuf
and java.nio.ByteBuffer
(see Data Buffers and Codecs).
An Encoder
can be wrapped with EncoderHttpMessageWriter
to be used as an
HttpMessageWriter
while a Decoder
can be wrapped with DecoderHttpMessageReader
to
be used as an HttpMessageReader
.
The spring-core
module contains basic Encoder
and Decoder
implementations for
byte[]
, ByteBuffer
, DataBuffer
, Resource
, and String
. The spring-web
module
adds Encoder
's and Decoder
's for Jackson JSON, Jackson Smile, and JAXB2.
The spring-web
module also contains some web-specific readers and writers for
server-sent events, form data, and multipart requests.
To configure or customize the readers and writers to use applications will typically use
ClientCodecConfigurer
or ServerCodecConfigurer
.
The decoder relies on Jackson’s non-blocking, byte array parser to parse a stream of byte
chunks into a TokenBuffer
stream, which can then be turned into Objects with Jackson’s
ObjectMapper
. JSON and Smile
(binary JSON) data formats are currently supported.
The encoder processes a Publisher<?>
as follows:
-
if the
Publisher
is aMono
(i.e. single value), the value is encoded when available. -
if media type is
application/stream+json
for JSON orapplication/stream+x-jackson-smile
for Smile, each value produced by thePublisher
is encoded individually (and followed by a new line in JSON). -
otherwise all items from the
Publisher
are gathered in withFlux#collectToList()
and the resulting collection is encoded as an array.
As a special case to the above rules the ServerSentEventHttpMessageWriter
feeds items
emitted from its input Publisher
individually into the Jackson2JsonEncoder
as a
Mono<?>
.
Note that both the Jackson JSON encoder and decoder explicitly back out of rendering
elements of type String
. Instead String
's are treated as low level content, (i.e.
serialized JSON) and are rendered as-is by the CharSequenceEncoder
. If you want a
Flux<String>
rendered as a JSON array, you’ll have to use Flux#collectToList()
and
provide a Mono<List<String>>
instead.
When a multi-value, reactive type such as Flux
is used for response rendering, it may
be collected to a List
and rendered as a whole (e.g. JSON array), or it may be treated
as an infinite stream with each item flushed immediately. The determination for which is
which is made based on content negotiation and the selected media type which may imply a
streaming format (e.g. "text/event-stream", "application/stream+json"), or not
(e.g. "application/json").
When streaming to the HTTP response, regardless of the media type (e.g. text/event-stream, application/stream+json), it is important to send data periodically, since the write would fail if the client has disconnected. The send could take the form of an empty (comment-only) SSE event, or any other data that the other side would have to interpret as a heartbeat and ignore.
In the WebHandler API, a WebFilter
can be used to apply interception-style
logic before and after the rest of the processing chain of filters and the target
WebHandler
. When using the WebFlux Config, registering a WebFilter
is as simple
as declaring it as a Spring bean, and optionally expressing precedence via @Order
on
the bean declaration or by implementing Ordered
.
The following describe the available WebFilter
implementations:
As a request goes through proxies such as load balancers the host, port, and scheme may change presenting a challenge for applications that need to create links to resources since the links should reflect the host, port, and scheme of the original request as seen from a client perspective.
RFC 7239 defines the "Forwarded" HTTP header for proxies to use to provide information about the original request. There are also other non-standard headers in use such as "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".
ForwardedHeaderFilter
detects, extracts, and uses information from the "Forwarded"
header, or from "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".
It wraps the request in order to overlay its host, port, and scheme and also "hides"
the forwarded headers for subsequent processing.
Note that there are security considerations when using forwarded headers as explained in Section 8 of RFC 7239. At the application level it is difficult to determine whether forwarded headers can be trusted or not. This is why the network upstream should be configured correctly to filter out untrusted forwarded headers from the outside.
Applications that don’t have a proxy and don’t need to use forwarded headers can
configure the ForwardedHeaderFilter
to remove and ignore such headers.
Spring WebFlux provides fine-grained support for CORS configuration through annotations on
controllers. However when used with Spring Security it is advisable to rely on the built-in
CorsFilter
that must be ordered ahead of Spring Security’s chain of filters.
See the section on [webflux-cors] and the [webflux-cors-webfilter] for more details.
In the WebHandler API, a WebExceptionHandler
can be used to to handle
exceptions from the chain of WebFilter
's and the target WebHandler
. When using the
WebFlux Config, registering a WebExceptionHandler
is as simple as declaring it as a
Spring bean, and optionally expressing precedence via @Order
on the bean declaration or
by implementing Ordered
.
Below are the available WebExceptionHandler
implementations:
Exception Handler | Description |
---|---|
|
Provides handling for exceptions of type {api-spring-framework}/web/server/ResponseStatusException.html[ResponseStatusException] by setting the response to the HTTP status code of the exception. |
|
Extension of This handler is declared in the WebFlux Config. |
Spring WebFlux, like Spring MVC, is designed around the front controller pattern where a
central WebHandler
, the DispatcherHandler
, provides a shared algorithm for request
processing while actual work is performed by configurable, delegate components.
This model is flexible and supports diverse workflows.
DispatcherHandler
discovers the delegate components it needs from Spring configuration.
It is also designed to be a Spring bean itself and implements ApplicationContextAware
for access to the context it runs in. If DispatcherHandler
is declared with the bean
name "webHandler" it is in turn discovered by
{api-spring-framework}/web/server/adapter/WebHttpHandlerBuilder.html[WebHttpHandlerBuilder]
which puts together a request processing chain as described in
WebHandler API.
Spring configuration in a WebFlux application typically contains:
-
DispatcherHandler
with the bean name "webHandler" -
WebFilter
andWebExceptionHandler
beans -
Others
The configuration is given to WebHttpHandlerBuilder
to build the processing chain:
ApplicationContext context = ...
HttpHandler handler = WebHttpHandlerBuilder.applicationContext(context);
The resulting HttpHandler
is ready for use with a
server adapter.
The DispatcherHandler
delegates to special beans to process requests and render the
appropriate responses. By "special beans" we mean Spring-managed, Object instances that
implement WebFlux framework contracts. Those usually come with built-in contracts but
you can customize their properties, extend then, or replaced.
The table below lists the special beans detected by the DispatcherHandler
. Note that
there are also some other beans detected at a lower level, see
Special bean types in the Web Handler API.
Bean type | Explanation |
---|---|
HandlerMapping |
Map a request to a handler. The mapping is based on some criteria the details of
which vary by The main |
HandlerAdapter |
Help the |
HandlerResultHandler |
Process the result from the handler invocation and finalize the response. See Result Handling. |
Applications can declare the infrastructure beans listed under Web Handler API and DispatcherHandler that are required to process requests. However in most cases the WebFlux Config is the best starting point. It declares the required beans and provides a higher level configuration callback API to customize it.
Note
|
Spring Boot relies on the WebFlux config to configure Spring WebFlux and also provides many extra convenient options. |
The DispatcherHandler
processes requests as follows:
-
Each
HandlerMapping
is asked to find a matching handler and the first match is used. -
If a handler is found, it is executed through an appropriate
HandlerAdapter
which exposes the return value from the execution asHandlerResult
. -
The
HandlerResult
is given to an appropriateHandlerResultHandler
to complete processing by writing to the response directly or using a view to render.
The return value from the invocation of a handler, through a HandlerAdapter
, is wrapped
as HandlerResult
, along with some additional context, and passed to the first
HandlerResultHandler
that claims support for it. The table below shows the available
HandlerResultHandler
implementations all of which are declared in the WebFlux Config:
Result Handler Type | Return Values | Default Order |
---|---|---|
|
|
0 |
|
|
0 |
|
Handle return values from |
100 |
|
Also see View Resolution. |
|
The HandlerResult
returned from a HandlerAdapter
may expose a function for error
handling based on some handler-specific mechanism. This error function is called if:
-
the handler (e.g.
@Controller
) invocation fails. -
handling of the handler return value through a
HandlerResultHandler
fails.
The error function can change the response, e.g. to an error status, as long as an error signal occurs before the reactive type returned from the handler produces any data items.
This is how @ExceptionHandler
methods in @Controller
classes are supported.
By contrast, support for the same in Spring MVC is built on a HandlerExceptionResolver
.
This generally shouldn’t matter, however, keep in mind that in WebFlux you cannot use a
@ControllerAdvice
to handle exceptions that occur before a handler is chosen.
See also Exceptions in the Annotated Controller section, or Exceptions in the WebHandler API section.
View resolution enables rendering to a browser with an HTML template and a model without
tying you to a specific view technology. In Spring WebFlux, view resolution is
supported through a dedicated HandlerResultHandler that uses
ViewResolver
's to map a String, representing a logical view name, to a View
instance. The View
is then used to render the response.
The HandlerResult
passed into ViewResolutionResultHandler
contains the return value
from the handler, and also the model that contains attributes added during request
handling. The return value is processed as one of the following:
-
String
,CharSequence
— a logical view name to be resolved to aView
through the list of configuredViewResolver
's. -
void
— select a default view name based on the request path minus the leading and trailing slash, and resolve it to aView
. The same also happens when a view name was not provided, e.g. model attribute was returned, or an async return value, e.g.Mono
completed empty. -
{api-spring-framework}/web/reactive/result/view/Rendering.html[Rendering] — API for view resolution scenarios; explore the options in your IDE with code completion.
-
Model
,Map
— extra model attributes to be added to the model for the request. -
Any other — any other return value (except for simple types, as determined by {api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty]) is treated as a model attribute to be added to the model. The attribute name is derived from the Class name, using {api-spring-framework}/core/Conventions.html[Conventions], unless a handler method
@ModelAttribute
annotation is present.
The model can contain asynchronous, reactive types (e.g. from Reactor, RxJava). Prior
to rendering, AbstractView
resolves such model attributes into concrete values
and updates the model. Single-value reactive types are resolved to a single
value, or no value (if empty) while multi-value reactive types, e.g. Flux<T>
are
collected and resolved to List<T>
.
To configure view resolution is as simple as adding a ViewResolutionResultHandler
bean
to your Spring configuration. WebFlux Config provides a
dedicated configuration API for view resolution.
See [webflux-view] for more on the view technologies integrated with Spring WebFlux.
The special redirect:
prefix in a view name allows you to perform a redirect. The
UrlBasedViewResolver
(and sub-classes) recognize this as an instruction that a
redirect is needed. The rest of the view name is the redirect URL.
The net effect is the same as if the controller had returned a RedirectView
or
Rendering.redirectTo("abc").build()
, but now the controller itself can simply
operate in terms of logical view names. A view name such as
redirect:/some/resource
is relative to the current application, while the view name
redirect:http://example.com/arbitrary/path
redirects to an absolute URL.
ViewResolutionResultHandler
supports content negotiation. It compares the request
media type(s) with the media type(s) supported by each selected View
. The first View
that supports the requested media type(s) is used.
In order to support media types such as JSON and XML, Spring WebFlux provides
HttpMessageWriterView
which is a special View
that renders through an
HttpMessageWriter. Typically you would configure these as default
views through the WebFlux Config. Default views are
always selected and used if they match the requested media type.
Spring WebFlux provides an annotation-based programming model where @Controller
and
@RestController
components use annotations to express request mappings, request input,
exception handling, and more. Annotated controllers have flexible method signatures and
do not have to extend base classes nor implement specific interfaces.
Here is a basic example:
@RestController
public class HelloController {
@GetMapping("/hello")
public String handle() {
return "Hello WebFlux";
}
}
In this example the methods returns a String to be written to the response body.
You can define controller beans using a standard Spring bean definition.
The @Controller
stereotype allows for auto-detection, aligned with Spring general support
for detecting @Component
classes in the classpath and auto-registering bean definitions
for them. It also acts as a stereotype for the annotated class, indicating its role as
a web component.
To enable auto-detection of such @Controller
beans, you can add component scanning to
your Java configuration:
@Configuration
@ComponentScan("org.example.web")
public class WebConfig {
// ...
}
@RestController
is a composed annotation that is
itself meta-annotated with @Controller
and @ResponseBody
indicating a controller whose
every method inherits the type-level @ResponseBody
annotation and therefore writes
directly to the response body vs view resolution and rendering with an HTML template.
The @RequestMapping
annotation is used to map requests to controllers methods. It has
various attributes to match by URL, HTTP method, request parameters, headers, and media
types. It can be used at the class-level to express shared mappings or at the method level
to narrow down to a specific endpoint mapping.
There are also HTTP method specific shortcut variants of @RequestMapping
:
-
@GetMapping
-
@PostMapping
-
@PutMapping
-
@DeleteMapping
-
@PatchMapping
The above are Custom Annotations that are provided out of the box
because arguably most controller methods should be mapped to a specific HTTP method vs
using @RequestMapping
which by default matches to all HTTP methods. At the same an
@RequestMapping
is still needed at the class level to express shared mappings.
Below is an example with type and method level mappings:
@RestController
@RequestMapping("/persons")
class PersonController {
@GetMapping("/{id}")
public Person getPerson(@PathVariable Long id) {
// ...
}
@PostMapping
@ResponseStatus(HttpStatus.CREATED)
public void add(@RequestBody Person person) {
// ...
}
}
You can map requests using glob patterns and wildcards:
-
?
matches one character -
*
matches zero or more characters within a path segment -
**
match zero or more path segments
You can also declare URI variables and access their values with @PathVariable
:
@GetMapping("/owners/{ownerId}/pets/{petId}")
public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
// ...
}
URI variables can be declared at the class and method level:
@Controller
@RequestMapping("/owners/{ownerId}")
public class OwnerController {
@GetMapping("/pets/{petId}")
public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
// ...
}
}
URI variables are automatically converted to the appropriate type or`TypeMismatchException`
is raised. Simple types — int
, long
, Date
, are supported by default and you can
register support for any other data type.
See Type Conversion and DataBinder.
URI variables can be named explicitly — e.g. @PathVariable("customId")
, but you can
leave that detail out if the names are the same and your code is compiled with debugging
information or with the -parameters
compiler flag on Java 8.
The syntax {*varName}
declares a URI variable that matches zero or more remaining
path segments. For example /resources/{*path}
matches all files /resources/
and the
"path"
variable captures the complete relative path.
The syntax {varName:regex}
declares a URI variable with a regular expressions with the
syntax {varName:regex}
— e.g. given URL "/spring-web-3.0.5 .jar"
, the below method
extracts the name, version, and file extension:
@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
public void handle(@PathVariable String version, @PathVariable String ext) {
// ...
}
URI path patterns can also have embedded ${…}
placeholders that are resolved on startup
via PropertyPlaceHolderConfigurer
against local, system, environment, and other property
sources. This can be used for example to parameterize a base URL based on some external
configuration.
Note
|
Spring WebFlux uses |
Spring WebFlux does not support suffix pattern matching — unlike Spring MVC, where a
mapping such as /person
also matches to /person.*
. For URL based content
negotiation, if needed, we recommend using a query parameter, which is simpler, more
explicit, and less vulnerable to URL path based exploits.
When multiple patterns match a URL, they must be compared to find the best match. This is done
with PathPattern.SPECIFICITY_COMPARATOR
which looks for patterns that more specific.
For every pattern, a score is computed based the number of URI variables and wildcards where a URI variable scores lower than a wildcard. A pattern with a lower total score wins. If two patterns have the same score, then the longer is chosen.
Catch-all patterns, e.g. **
, {*varName}
, are excluded from the scoring and are always
sorted last instead. If two patterns are both catch-all, the longer is chosen.
You can narrow the request mapping based on the Content-Type
of the request:
@PostMapping(path = "/pets", consumes = "application/json")
public void addPet(@RequestBody Pet pet) {
// ...
}
The consumes attribute also supports negation expressions — e.g. !text/plain
means any
content type other than "text/plain".
You can declare a shared consumes attribute at the class level. Unlike most other request mapping attributes however when used at the class level, a method-level consumes attribute overrides rather than extend the class level declaration.
Tip
|
|
You can narrow the request mapping based on the Accept
request header and the list of
content types that a controller method produces:
@GetMapping(path = "/pets/{petId}", produces = "application/json;charset=UTF-8")
@ResponseBody
public Pet getPet(@PathVariable String petId) {
// ...
}
The media type can specify a character set. Negated expressions are supported — e.g.
!text/plain
means any content type other than "text/plain".
You can declare a shared produces attribute at the class level. Unlike most other request mapping attributes however when used at the class level, a method-level produces attribute overrides rather than extend the class level declaration.
Tip
|
|
You can narrow request mappings based on query parameter conditions. You can test for the
presence of a query parameter ("myParam"
), for the absence ("!myParam"
), or for a
specific value ("myParam=myValue"
):
@GetMapping(path = "/pets/{petId}", params = "myParam=myValue")
public void findPet(@PathVariable String petId) {
// ...
}
You can also use the same with request header conditions:
@GetMapping(path = "/pets", headers = "myHeader=myValue")
public void findPet(@PathVariable String petId) {
// ...
}
@GetMapping
— and also @RequestMapping(method=HttpMethod.GET)
, support HTTP HEAD
transparently for request mapping purposes. Controller methods don’t need to change.
A response wrapper, applied in the HttpHandler
server adapter, ensures a "Content-Length"
header is set to the number of bytes written and without actually writing to the response.
By default HTTP OPTIONS is handled by setting the "Allow" response header to the list of HTTP
methods listed in all @RequestMapping
methods with matching URL patterns.
For a @RequestMapping
without HTTP method declarations, the "Allow" header is set to
"GET,HEAD,POST,PUT,PATCH,DELETE,OPTIONS"
. Controller methods should always declare the
supported HTTP methods for example by using the HTTP method specific variants — @GetMapping
, @PostMapping
, etc.
@RequestMapping
method can be explicitly mapped to HTTP HEAD and HTTP OPTIONS, but that
is not necessary in the common case.
Spring WebFlux supports the use of composed annotations
for request mapping. Those are annotations that are themselves meta-annotated with
@RequestMapping
and composed to redeclare a subset (or all) of the @RequestMapping
attributes with a narrower, more specific purpose.
@GetMapping
, @PostMapping
, @PutMapping
, @DeleteMapping
, and @PatchMapping
are
examples of composed annotations. They’re provided out of the box because arguably most
controller methods should be mapped to a specific HTTP method vs using @RequestMapping
which by default matches to all HTTP methods. If you need an example of composed
annotations, look at how those are declared.
Spring WebFlux also supports custom request mapping attributes with custom request matching
logic. This is a more advanced option that requires sub-classing
RequestMappingHandlerMapping
and overriding the getCustomMethodCondition
method where
you can check the custom attribute and return your own RequestCondition
.
@RequestMapping
handler methods have a flexible signature and can choose from a range of
supported controller method arguments and return values.
The table below shows supported controller method arguments.
Reactive types (Reactor, RxJava, or other) are supported on arguments that require blocking I/O, e.g. reading the request body, to be resolved. This is marked in the description column. Reactive types are not expected on arguments that don’t require blocking.
JDK 1.8’s java.util.Optional
is supported as a method argument in combination with
annotations that have a required
attribute — e.g. @RequestParam
, @RequestHeader
,
etc, and is equivalent to required=false
.
Controller method argument | Description |
---|---|
|
Access to the full |
|
Access to the HTTP request or response. |
|
Access to the session; this does not force the start of a new session unless attributes are added. Supports reactive types. |
|
Currently authenticated user; possibly a specific |
|
The HTTP method of the request. |
|
The current request locale, determined by the most specific |
|
The time zone associated with the current request, as determined by a |
|
For access to URI template variables. See URI Patterns. |
|
For access to name-value pairs in URI path segments. See Matrix variables. |
|
For access to Servlet request parameters. Parameter values are converted to the declared method argument type. See @RequestParam. Note that use of |
|
For access to request headers. Header values are converted to the declared method argument type. See @RequestHeader. |
|
For access to cookies. Cookies values are converted to the declared method argument type. See @CookieValue. |
|
For access to the HTTP request body. Body content is converted to the declared method
argument type using |
|
For access to request headers and body. The body is converted with |
|
For access to a part in a "multipart/form-data" request. Supports reactive types. See Multipart and Multipart data. |
|
For access to the model that is used in HTML controllers and exposed to templates as part of view rendering. |
|
For access to an existing attribute in the model (instantiated if not present) with data binding and validation applied. See @ModelAttribute as well as Model and DataBinder. Note that use of |
|
For access to errors from validation and data binding for a command object
(i.e. |
|
For marking form processing complete which triggers cleanup of session attributes
declared through a class-level |
|
For preparing a URL relative to the current request’s host, port, scheme, context path, and
the literal part of the servlet mapping also taking into account |
|
For access to any session attribute; in contrast to model attributes stored in the session
as a result of a class-level |
|
For access to request attributes. See @RequestAttribute for more details. |
Any other argument |
If a method argument is not matched to any of the above, by default it is resolved as
an |
The table below shows supported controller method return values. Note that reactive types from libraries such as Reactor, RxJava, or other are generally supported for all return values.
Controller method return value | Description |
---|---|
|
The return value is encoded through |
|
The return value specifies the full response including HTTP headers and body be encoded
through |
|
For returning a response with headers and no body. |
|
A view name to be resolved with |
|
A |
|
Attributes to be added to the implicit model with the view name implicitly determined based on the request path. |
|
An attribute to be added to the model with the view name implicitly determined based on the request path. Note that |
|
An API for model and view rendering scenarios. |
|
A method with a If none of the above is true, a |
|
Emit server-sent events; the |
Any other return value |
If a return value is not matched to any of the above, by default it is treated as a view
name, if it is |
Some annotated controller method arguments that represent String-based request input — e.g.
@RequestParam
, @RequestHeader
, @PathVariable
, @MatrixVariable
, and @CookieValue
,
may require type conversion if the argument is declared as something other than String
.
For such cases type conversion is automatically applied based on the configured converters.
By default simple types such as int
, long
, Date
, etc. are supported. Type conversion
can be customized through a WebDataBinder
, see [mvc-ann-initbinder], or by registering
Formatters
with the FormattingConversionService
, see
Spring Field Formatting.
RFC 3986 discusses name-value pairs in path segments. In Spring WebFlux we refer to those as "matrix variables" based on an "old post" by Tim Berners-Lee but they can be also be referred to as URI path parameters.
Matrix variables can appear in any path segment, each variable separated by semicolon and
multiple values separated by comma, e.g. "/cars;color=red,green;year=2012"
. Multiple
values can also be specified through repeated variable names, e.g.
"color=red;color=green;color=blue"
.
Unlike Spring MVC, in WebFlux the presence or absence of matrix variables in a URL does not affect request mappings. In other words you’re not required to use a URI variable to mask variable content. That said if you want to access matrix variables from a controller method you need to add a URI variable to the path segment where matrix variables are expected. Below is an example:
// GET /pets/42;q=11;r=22
@GetMapping("/pets/{petId}")
public void findPet(@PathVariable String petId, @MatrixVariable int q) {
// petId == 42
// q == 11
}
Given that all path segments may contain matrix variables, sometimes you may need to disambiguate which path variable the matrix variable is expected to be in. For example:
// GET /owners/42;q=11/pets/21;q=22
@GetMapping("/owners/{ownerId}/pets/{petId}")
public void findPet(
@MatrixVariable(name="q", pathVar="ownerId") int q1,
@MatrixVariable(name="q", pathVar="petId") int q2) {
// q1 == 11
// q2 == 22
}
A matrix variable may be defined as optional and a default value specified:
// GET /pets/42
@GetMapping("/pets/{petId}")
public void findPet(@MatrixVariable(required=false, defaultValue="1") int q) {
// q == 1
}
To get all matrix variables, use a MultiValueMap
:
// GET /owners/42;q=11;r=12/pets/21;q=22;s=23
@GetMapping("/owners/{ownerId}/pets/{petId}")
public void findPet(
@MatrixVariable MultiValueMap<String, String> matrixVars,
@MatrixVariable(pathVar="petId"") MultiValueMap<String, String> petMatrixVars) {
// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
// petMatrixVars: ["q" : 22, "s" : 23]
}
Use the @RequestParam
annotation to bind query parameters to a method argument in a
controller. The following code snippet shows the usage:
@Controller
@RequestMapping("/pets")
public class EditPetForm {
// ...
@GetMapping
public String setupForm(@RequestParam("petId") int petId, Model model) {
Pet pet = this.clinic.loadPet(petId);
model.addAttribute("pet", pet);
return "petForm";
}
// ...
}
Tip
|
Unlike the Servlet API "request paramater" concept that conflate query parameters, form
data, and multiparts into one, in WebFlux each is accessed individually through the
|
Method parameters using using the @RequestParam
annotation are required by default, but
you can specify that a method parameter is optional by setting @RequestParam
's
required
flag to false
or by declaring the argument with an java.util.Optional
wrapper.
Type conversion is applied automatically if the target method parameter type is not
String
. See [mvc-ann-typeconversion].
When an @RequestParam
annotation is declared as Map<String, String>
or
MultiValueMap<String, String>
argument, the map is populated with all query parameters.
Note that use of @RequestParam
is optional, e.g. to set its attributes.
By default any argument that is a simple value type, as determined by
{api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty],
and is not resolved by any other argument resolver, is treated as if it was annotated
with @RequestParam
.
Use the @RequestHeader
annotation to bind a request header to a method argument in a
controller.
Given request with headers:
Host localhost:8080 Accept text/html,application/xhtml+xml,application/xml;q=0.9 Accept-Language fr,en-gb;q=0.7,en;q=0.3 Accept-Encoding gzip,deflate Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive 300
The following gets the value of the Accept-Encoding
and Keep-Alive
headers:
@GetMapping("/demo")
public void handle(
@RequestHeader("Accept-Encoding") String encoding,
@RequestHeader("Keep-Alive") long keepAlive) {
//...
}
Type conversion is applied automatically if the target method parameter type is not
String
. See [mvc-ann-typeconversion].
When an @RequestHeader
annotation is used on a Map<String, String>
,
MultiValueMap<String, String>
, or HttpHeaders
argument, the map is populated
with all header values.
Tip
|
Built-in support is available for converting a comma-separated string into an
array/collection of strings or other types known to the type conversion system. For
example a method parameter annotated with |
Use the @CookieValue
annotation to bind the value of an HTTP cookie to a method argument
in a controller.
Given request with the following cookie:
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
The following code sample demonstrates how to get the cookie value:
@GetMapping("/demo")
public void handle(@CookieValue("JSESSIONID") String cookie) {
//...
}
Type conversion is applied automatically if the target method parameter type is not
String
. See [mvc-ann-typeconversion].
Use the @ModelAttribute
annotation on a method argument to access an attribute from the
model, or have it instantiated if not present. The model attribute is also overlaid with
values of query parameters and form fields whose names match to field names. This is
referred to as data binding and it saves you from having to deal with parsing and
converting individual query parameters and form fields. For example:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute Pet pet) { }
The Pet
instance above is resolved as follows:
-
From the model if already added via Model.
-
From the HTTP session via @SessionAttributes.
-
From the invocation of a default constructor.
-
From the invocation of a "primary constructor" with arguments matching to query parameters or form fields; argument names are determined via JavaBeans
@ConstructorProperties
or via runtime-retained parameter names in the bytecode.
After the model attribute instance is obtained, data binding is applied. The
WebExchangeDataBinder
class matches names of query parameters and form fields to field
names on the target Object. Matching fields are populated after type conversion is applied
where necessary. For more on data binding (and validation) see
Validation. For more on customizing data binding see
DataBinder.
Data binding may result in errors. By default a WebExchangeBindException
is raised but
to check for such errors in the controller method, add a BindingResult
argument
immediately next to the @ModelAttribute
as shown below:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) {
if (result.hasErrors()) {
return "petForm";
}
// ...
}
Validation can be applied automatically after data binding by adding the
javax.validation.Valid
annotation or Spring’s @Validated
annotation (also see
Bean validation and
Spring validation). For example:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) {
if (result.hasErrors()) {
return "petForm";
}
// ...
}
Spring WebFlux, unlike Spring MVC, supports reactive types in the model, e.g.
Mono<Account>
or io.reactivex.Single<Account>
. An @ModelAttribute
argument can be
declared with or without a reactive type wrapper, and it will be resolved accordingly,
to the actual value if necessary. Note however that in order to use a BindingResult
argument, you must declare the @ModelAttribute
argument before it without a reactive
type wrapper, as shown earlier. Alternatively, you can handle any errors through the
reactive type:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public Mono<String> processSubmit(@Valid @ModelAttribute("pet") Mono<Pet> petMono) {
return petMono
.flatMap(pet -> {
// ...
})
.onErrorResume(ex -> {
// ...
});
}
Note that use of @ModelAttribute
is optional, e.g. to set its attributes.
By default any argument that is not a simple value type, as determined by
{api-spring-framework}/beans/BeanUtils.html#isSimpleProperty-java.lang.Class-[BeanUtils#isSimpleProperty],
and is not resolved by any other argument resolver, is treated as if it was annotated
with @ModelAttribute
.
@SessionAttributes
is used to store model attributes in the WebSession
between
requests. It is a type-level annotation that declares session attributes used by a
specific controller. This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session for subsequent
requests to access.
For example:
@Controller
@SessionAttributes("pet")
public class EditPetForm {
// ...
}
On the first request when a model attribute with the name "pet" is added to the model,
it is automatically promoted to and saved in the WebSession
. It remains there until
another controller method uses a SessionStatus
method argument to clear the storage:
@Controller
@SessionAttributes("pet")
public class EditPetForm {
// ...
@PostMapping("/pets/{id}")
public String handle(Pet pet, BindingResult errors, SessionStatus status) {
if (errors.hasErrors) {
// ...
}
status.setComplete();
// ...
}
}
}
If you need access to pre-existing session attributes that are managed globally,
i.e. outside the controller (e.g. by a filter), and may or may not be present
use the @SessionAttribute
annotation on a method parameter:
@GetMapping("/")
public String handle(@SessionAttribute User user) {
// ...
}
For use cases that require adding or removing session attributes consider injecting
WebSession
into the controller method.
For temporary storage of model attributes in the session as part of a controller
workflow consider using SessionAttributes
as described in
@SessionAttributes.
Similar to @SessionAttribute
the @RequestAttribute
annotation can be used to
access pre-existing request attributes created earlier, e.g. by a WebFilter
:
@GetMapping("/")
public String handle(@RequestAttribute Client client) {
// ...
}
As explained in Multipart data, ServerWebExchange
provides access to multipart
content. The best way to handle a file upload form (e.g. from a browser) in a controller
is through data binding to a command object:
class MyForm {
private String name;
private MultipartFile file;
// ...
}
@Controller
public class FileUploadController {
@PostMapping("/form")
public String handleFormUpload(MyForm form, BindingResult errors) {
// ...
}
}
Multipart requests can also be submitted from non-browser clients in a RESTful service scenario. For example a file along with JSON:
POST /someUrl Content-Type: multipart/mixed --edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp Content-Disposition: form-data; name="meta-data" Content-Type: application/json; charset=UTF-8 Content-Transfer-Encoding: 8bit { "name": "value" } --edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp Content-Disposition: form-data; name="file-data"; filename="file.properties" Content-Type: text/xml Content-Transfer-Encoding: 8bit ... File Data ...
You can access individual parts with @RequestPart
:
@PostMapping("/")
public String handle(@RequestPart("meta-data") Part metadata,
@RequestPart("file-data") FilePart file) {
// ...
}
To deserialize the raw part content, for example to JSON (similar to @RequestBody
),
simply declare a concrete target Object, instead of Part
:
@PostMapping("/")
public String handle(@RequestPart("meta-data") MetaData metadata) {
// ...
}
@RequestPart
can be used in combination with javax.validation.Valid
, or Spring’s
@Validated
annotation, which causes Standard Bean Validation to be applied.
By default validation errors cause a WebExchangeBindException
which is turned
into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally
within the controller through an Errors
or BindingResult
argument:
@PostMapping("/")
public String handle(@Valid @RequestPart("meta-data") MetaData metadata,
BindingResult result) {
// ...
}
To access all multipart data in as a MultiValueMap
use @RequestBody
:
@PostMapping("/")
public String handle(@RequestBody Mono<MultiValueMap<String, Part>> parts) {
// ...
}
To access multipart data sequentially, in streaming fashion, use @RequestBody
with
Flux<Part>
instead. For example:
@PostMapping("/")
public String handle(@RequestBody Flux<Part> parts) {
// ...
}
Use the @RequestBody
annotation to have the request body read and deserialized into an
Object through an HttpMessageReader.
Below is an example with an @RequestBody
argument:
@PostMapping("/accounts")
public void handle(@RequestBody Account account) {
// ...
}
Unlike Spring MVC, in WebFlux the @RequestBody
method argument supports reactive types
and fully non-blocking reading and (client-to-server) streaming:
@PostMapping("/accounts")
public void handle(@RequestBody Mono<Account> account) {
// ...
}
You can use the HTTP message codecs option of the WebFlux Config to configure or customize message readers.
@RequestBody
can be used in combination with javax.validation.Valid
, or Spring’s
@Validated
annotation, which causes Standard Bean Validation to be applied.
By default validation errors cause a WebExchangeBindException
which is turned
into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally
within the controller through an Errors
or BindingResult
argument:
@PostMapping("/accounts")
public void handle(@Valid @RequestBody Account account, BindingResult result) {
// ...
}
HttpEntity
is more or less identical to using @RequestBody but based on a
container object that exposes request headers and body. Below is an example:
@PostMapping("/accounts")
public void handle(HttpEntity<Account> entity) {
// ...
}
Use the @ResponseBody
annotation on a method to have the return serialized to the
response body through an HttpMessageWriter. For example:
@GetMapping("/accounts/{id}")
@ResponseBody
public Account handle() {
// ...
}
@ResponseBody
is also supported at the class level in which case it is inherited by
all controller methods. This is the effect of @RestController
which is nothing more
than a meta-annotation marked with @Controller
and @ResponseBody
.
@ResponseBody
supports reactive types which means you can return Reactor or RxJava
types and have the asynchronous values they produce rendered to the response.
For additional details, see HTTP Streaming and
JSON rendering.
@ResponseBody
methods can be combined with JSON serialization views.
See Jackson JSON for details.
You can use the HTTP message codecs option of the WebFlux Config to configure or customize message writing.
ResponseEntity
is more or less identical to using @ResponseBody but based
on a container object that specifies request headers and body. Below is an example:
@PostMapping("/something")
public ResponseEntity<String> handle() {
// ...
URI location = ...
return new ResponseEntity.created(location).build();
}
Spring WebFlux provides built-in support for
Jackson’s Serialization Views
which allows rendering only a subset of all fields in an Object. To use it with
@ResponseBody
or ResponseEntity
controller methods, use Jackson’s
@JsonView
annotation to activate a serialization view class:
@RestController
public class UserController {
@GetMapping("/user")
@JsonView(User.WithoutPasswordView.class)
public User getUser() {
return new User("eric", "7!jd#h23");
}
}
public class User {
public interface WithoutPasswordView {};
public interface WithPasswordView extends WithoutPasswordView {};
private String username;
private String password;
public User() {
}
public User(String username, String password) {
this.username = username;
this.password = password;
}
@JsonView(WithoutPasswordView.class)
public String getUsername() {
return this.username;
}
@JsonView(WithPasswordView.class)
public String getPassword() {
return this.password;
}
}
Note
|
|
The @ModelAttribute
annotation can be used:
-
On a method argument in
@RequestMapping
methods to create or access an Object from the model, and to bind it to the request through aWebDataBinder
. -
As a method-level annotation in
@Controller
or@ControllerAdvice
classes helping to initialize the model prior to any@RequestMapping
method invocation. -
On a
@RequestMapping
method to mark its return value is a model attribute.
This section discusses @ModelAttribute
methods, or the 2nd from the list above.
A controller can have any number of @ModelAttribute
methods. All such methods are
invoked before @RequestMapping
methods in the same controller. A @ModelAttribute
method can also be shared across controllers via @ControllerAdvice
. See the section on
Controller Advice for more details.
@ModelAttribute
methods have flexible method signatures. They support many of the same
arguments as @RequestMapping
methods except for @ModelAttribute
itself nor anything
related to the request body.
An example @ModelAttribute
method:
@ModelAttribute
public void populateModel(@RequestParam String number, Model model) {
model.addAttribute(accountRepository.findAccount(number));
// add more ...
}
To add one attribute only:
@ModelAttribute
public Account addAccount(@RequestParam String number) {
return accountRepository.findAccount(number);
}
Note
|
When a name is not explicitly specified, a default name is chosen based on the Object
type as explained in the Javadoc for
{api-spring-framework}/core/Conventions.html[Conventions].
You can always assign an explicit name by using the overloaded |
Spring WebFlux, unlike Spring MVC, explicitly supports reactive types in the model,
e.g. Mono<Account>
or io.reactivex.Single<Account>
. Such asynchronous model
attributes may be transparently resolved (and the model updated) to their actual values
at the time of @RequestMapping
invocation, providing a @ModelAttribute
argument is
declared without a wrapper, for example:
@ModelAttribute
public void addAccount(@RequestParam String number) {
Mono<Account> accountMono = accountRepository.findAccount(number);
model.addAttribute("account", accountMono);
}
@PostMapping("/accounts")
public String handle(@ModelAttribute Account account, BindingResult errors) {
// ...
}
In addition any model attributes that have a reactive type wrapper are resolved to their actual values (and the model updated) just prior to view rendering.
@ModelAttribute
can also be used as a method-level annotation on @RequestMapping
methods in which case the return value of the @RequestMapping
method is interpreted as a
model attribute. This is typically not required, as it is the default behavior in HTML
controllers, unless the return value is a String
which would otherwise be interpreted
as a view name. @ModelAttribute
can also help to customize the model attribute name:
@GetMapping("/accounts/{id}")
@ModelAttribute("myAccount")
public Account handle() {
// ...
return account;
}
@Controller
or @ControllerAdvice
classes can have @InitBinder
methods in order to
initialize instances of WebDataBinder
, and those in turn are used to:
-
Bind request parameters (i.e. form data or query) to a model object.
-
Convert String-based request values such as request parameters, path variables, headers, cookies, and others, to the target type of controller method arguments.
-
Format model object values as String values when rendering HTML forms.
@InitBinder
methods can register controller-specific java.bean.PropertyEditor
, or
Spring Converter
and Formatter
components. In addition, the
WebFlux Java config can be used to register Converter
and
Formatter
types in a globally shared FormattingConversionService
.
@InitBinder
methods support many of the same arguments that a @RequestMapping
methods
do, except for @ModelAttribute
(command object) arguments. Typically they’re are declared
with a WebDataBinder
argument, for registrations, and a void
return value.
Below is an example:
@Controller
public class FormController {
@InitBinder
public void initBinder(WebDataBinder binder) {
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
dateFormat.setLenient(false);
binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
}
// ...
}
Alternatively when using a Formatter
-based setup through a shared
FormattingConversionService
, you could re-use the same approach and register
controller-specific Formatter
's:
@Controller
public class FormController {
@InitBinder
protected void initBinder(WebDataBinder binder) {
binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
}
// ...
}
@Controller
and @ControllerAdvice classes can have
@ExceptionHandler
methods to handle exceptions from controller methods. For example:
@Controller
public class SimpleController {
// ...
@ExceptionHandler
public ResponseEntity<String> handle(IOException ex) {
// ...
}
}
The annotation can list the exception types to match. Or simply declare the target
exception as a method argument as shown above. When multiple exception methods match,
a root exception match is generally preferred to a cause exception match. More formally
the ExceptionDepthComparator
is used to sort exceptions based on their depth from the
thrown exception type.
In a multi-@ControllerAdvice
arrangement, please declare your primary root exception
mappings on a @ControllerAdvice
prioritized with a corresponding order. While a root
exception match is preferred to a cause, this is mainly among the methods of a given
controller or @ControllerAdvice
. That means a cause match on a higher-priority
@ControllerAdvice
is preferred to any match (e.g. root) on a lower-priority
@ControllerAdvice
.
Support for @ExceptionHandler
methods in Spring WebFlux is provided by the
HandlerAdapter
for @RequestMapping
methods. See Exceptions
under the DispatcherHandler
section for more details.
An @ExceptionHandler
method in WebFlux supports the same method arguments and return
values as an @RequestMapping
method does with the exception of request body and
@ModelAttribute
related method arguments.
A common requirement for REST services is to include error details in the body of the
response. The Spring Framework does not automatically do this because the representation
of error details in the response body is application specific. However a
@RestController
may use @ExceptionHandler
methods with a ResponseEntity
return
value to set the status and the body of the response. Such methods may also be declared
in @ControllerAdvice
classes to apply them globally.
Note
|
Note that Spring WebFlux does not have an equivalent for the Spring MVC
|
Typically @ExceptionHandler
, @InitBinder
, and @ModelAttribute
methods apply within
the @Controller
class (or class hierarchy) they are declared in. If you want such
methods to apply more globally, across controllers, you can declare them in a class
marked with @ControllerAdvice
or @RestControllerAdvice
.
@ControllerAdvice
is marked with @Component
which means such classes can be registered
as Spring beans via component scanning.
@RestControllerAdvice
is also a meta-annotation marked with both @ControllerAdvice
and
@ResponseBody
which essentially means @ExceptionHandler
methods are rendered to the
response body via message conversion (vs view resolution/template rendering).
On startup, the infrastructure classes for @RequestMapping
and @ExceptionHandler
methods
detect Spring beans of type @ControllerAdvice
, and then apply their methods at runtime.
Global @ExceptionHandler
methods (from an @ControllerAdvice
) are applied after local
ones (from the @Controller
). By contrast global @ModelAttribute
and @InitBinder
methods are applied before local ones.
By default @ControllerAdvice
methods apply to every request, i.e. all controllers, but
you can narrow that down to a subset of controllers via attributes on the annotation:
// Target all Controllers annotated with @RestController
@ControllerAdvice(annotations = RestController.class)
public class ExampleAdvice1 {}
// Target all Controllers within specific packages
@ControllerAdvice("org.example.controllers")
public class ExampleAdvice2 {}
// Target all Controllers assignable to specific classes
@ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class})
public class ExampleAdvice3 {}
Keep in mind the above selectors are evaluated at runtime and may negatively impact performance if used extensively. See the {api-spring-framework}/web/bind/annotation/ControllerAdvice.html[@ControllerAdvice] Javadoc for more details.
This section describes various options available in the Spring Framework to prepare URIs.
The Spring Security project provides support for protecting web applications from malicious exploits. Check out the Spring Security reference documentation including:
-
{doc-root}/spring-security/site/docs/current/reference/html5/#jc-webflux[WebFlux Security]
-
{doc-root}/spring-security/site/docs/current/reference/html5/#test-webflux["WebFlux Testing Support"]
-
{doc-root}/spring-security/site/docs/current/reference/html5/#csrf[CSRF Protection]
-
{doc-root}/spring-security/site/docs/current/reference/html5/#headers[Security Response Headers]
The WebFlux Java config declares components required to process requests with annotated
controllers or functional endpoints, and it offers an API to customize the configuration.
That means you do not need to understand the underlying beans created by the Java config
but, if you want to, it’s very easy to see them in WebFluxConfigurationSupport
or read more
what they are in Special bean types.
For more advanced customizations, not available in the configuration API, it is also possible to gain full control over the configuration through the Advanced config mode.
Use the @EnableWebFlux
annotation in your Java config:
@Configuration
@EnableWebFlux
public class WebConfig {
}
The above registers a number of Spring WebFlux infrastructure beans also adapting to dependencies available on the classpath — for JSON, XML, etc.
In your Java config implement the WebFluxConfigurer
interface:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
// Implement configuration methods...
}
By default formatters for Number
and Date
types are installed, including support for
the @NumberFormat
and @DateTimeFormat
annotations. Full support for the Joda-Time
formatting library is also installed if Joda-Time is present on the classpath.
To register custom formatters and converters:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void addFormatters(FormatterRegistry registry) {
// ...
}
}
Note
|
See FormatterRegistrar SPI
and the |
By default if Bean Validation is present
on the classpath — e.g. Hibernate Validator, the LocalValidatorFactoryBean
is registered
as a global Validator for use with @Valid
and Validated
on
@Controller
method arguments.
In your Java config, you can customize the global Validator
instance:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public Validator getValidator(); {
// ...
}
}
Note that you can also register Validator
's locally:
@Controller
public class MyController {
@InitBinder
protected void initBinder(WebDataBinder binder) {
binder.addValidators(new FooValidator());
}
}
Tip
|
If you need to have a |
You can configure how Spring WebFlux determines the requested media types for
@Controller
's from the request. By default only the "Accept" header is checked but you
can also enable a query parameter based strategy.
To customize the requested content type resolution:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configureContentTypeResolver(RequestedContentTypeResolverBuilder builder) {
// ...
}
}
To customize how the request and response body are read and written:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configureHttpMessageCodecs(ServerCodecConfigurer configurer) {
// ...
}
}
ServerCodecConfigurer
provides a set of default readers and writers. You can use it to add
more readers and writers, customize the default ones, or replace the default ones completely.
For Jackson JSON and XML, consider using the {api-spring-framework}/http/converter/json/Jackson2ObjectMapperBuilder.html[Jackson2ObjectMapperBuilder] which customizes Jackson’s default properties with the following ones:
-
DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES
is disabled. -
MapperFeature.DEFAULT_VIEW_INCLUSION
is disabled.
It also automatically registers the following well-known modules if they are detected on the classpath:
-
jackson-datatype-jdk7: support for Java 7 types like
java.nio.file.Path
. -
jackson-datatype-joda: support for Joda-Time types.
-
jackson-datatype-jsr310: support for Java 8 Date & Time API types.
-
jackson-datatype-jdk8: support for other Java 8 types like
Optional
.
To configure view resolution:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
// ...
}
}
The ViewResolverRegistry
has shortcuts for view technologies that the Spring Framework
integrates with. Here is an example with FreeMarker which also requires configuring the
underlying FreeMarker view technology:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.freeMarker();
}
// Configure Freemarker...
@Bean
public FreeMarkerConfigurer freeMarkerConfigurer() {
FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
configurer.setTemplateLoaderPath("classpath:/templates");
return configurer;
}
}
You can also plug in any ViewResolver
implementation:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
ViewResolver resolver = ... ;
registry.viewResolver(resolver);
}
}
To support Content negotiation and rendering other formats
through view resolution, besides HTML, you can configure one or more default views based
on the HttpMessageWriterView
implementation which accepts any of the available
Message Codecs from spring-web
:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.freeMarker();
Jackson2JsonEncoder encoder = new Jackson2JsonEncoder();
registry.defaultViews(new HttpMessageWriterView(encoder));
}
// ...
}
See [webflux-view] for more on the view technologies integrated with Spring WebFlux.
This option provides a convenient way to serve static resources from a list of {api-spring-framework}/core/io/Resource.html[Resource]-based locations.
In the example below, given a request that starts with "/resources"
, the relative path is
used to find and serve static resources relative to "/static"
on the classpath. Resources
will be served with a 1-year future expiration to ensure maximum use of the browser cache
and a reduction in HTTP requests made by the browser. The Last-Modified
header is also
evaluated and if present a 304
status code is returned.
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
registry.addResourceHandler("/resources/**")
.addResourceLocations("/public", "classpath:/static/")
.setCachePeriod(31556926);
}
}
The resource handler also supports a chain of {api-spring-framework}/web/reactive/resource/ResourceResolver.html[ResourceResolver]'s and {api-spring-framework}/web/reactive/resource/ResourceTransformer.html[ResourceTransformer]'s. which can be used to create a toolchain for working with optimized resources.
The VersionResourceResolver
can be used for versioned resource URLs based on an MD5 hash
computed from the content, a fixed application version, or other. A
ContentVersionStrategy
(MD5 hash) is a good choice with some notable exceptions such as
JavaScript resources used with a module loader.
For example in your Java config;
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
registry.addResourceHandler("/resources/**")
.addResourceLocations("/public/")
.resourceChain(true)
.addResolver(new VersionResourceResolver().addContentVersionStrategy("/**"));
}
}
You can use ResourceUrlProvider
to rewrite URLs and apply the full chain of resolvers and
transformers — e.g. to insert versions. The WebFlux config provides a ResourceUrlProvider
so it can be injected into others.
Unlike Spring MVC at present in WebFlux there is no way to transparently rewrite static
resource URLs since there are no view technologies that can make use of a non-blocking chain
of resolvers and transformers (e.g. resources on Amazon S3). When serving only local
resources the workaround is to use ResourceUrlProvider
directly (e.g. through a custom
tag) and block for 0 seconds.
WebJars is also supported via WebJarsResourceResolver
and automatically registered when "org.webjars:webjars-locator"
is present on the
classpath. The resolver can re-write URLs to include the version of the jar and can also
match to incoming URLs without versions — e.g. "/jquery/jquery.min.js"
to
"/jquery/1.2.0/jquery.min.js"
.
Spring WebFlux uses parsed representation of path patterns — i.e. PathPattern
, and also
the incoming request path — i.e. RequestPath
, which eliminates the need to indicate
whether to decode the request path, or remove semicolon content, since PathPattern
can now access decoded path segment values and match safely.
Spring WebFlux also does not support suffix pattern matching so effectively there are only two
minor options to customize related to path matching — whether to match trailing slashes
(true
by default) and whether the match is case-sensitive (false
).
To customize those options:
@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {
@Override
public void configurePathMatch(PathMatchConfigurer configurer) {
// ...
}
}
@EnableWebFlux
imports DelegatingWebFluxConfiguration
that (1) provides default
Spring configuration for WebFlux applications and (2) detects and delegates to
WebFluxConfigurer
's to customize that configuration.
For advanced mode, remove @EnableWebFlux
and extend directly from
DelegatingWebFluxConfiguration
instead of implementing WebFluxConfigurer
:
@Configuration
public class WebConfig extends DelegatingWebFluxConfiguration {
// ...
}
You can keep existing methods in WebConfig
but you can now also override bean declarations
from the base class and you can still have any number of other WebMvcConfigurer
's on
the classpath.
Servlet 4 containers are required to support HTTP/2 and Spring Framework 5 is compatible with Servlet API 4. From a programming model perspective there is nothing specific that applications need to do. However there are considerations related to server configuration. For more details please check out the HTTP/2 wiki page.
Currently Spring WebFlux does not support HTTP/2 with Netty. There is also no support for pushing resources programmatically to the client.