Java’s standard java.net.URL class and standard handlers for various URL prefixes
unfortunately are not quite adequate enough for all access to low-level resources. For
example, there is no standardized URL implementation that may be used to access a
resource that needs to be obtained from the classpath, or relative to a
ServletContext. While it is possible to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as http:), this is generally
quite complicated, and the URL interface still lacks some desirable functionality,
such as a method to check for the existence of the resource being pointed to.
Spring’s Resource interface is meant to be a more capable interface for abstracting
access to low-level resources.
public interface Resource extends InputStreamSource {
boolean exists();
boolean isOpen();
URL getURL() throws IOException;
File getFile() throws IOException;
Resource createRelative(String relativePath) throws IOException;
String getFilename();
String getDescription();
}public interface InputStreamSource {
InputStream getInputStream() throws IOException;
}Some of the most important methods from the Resource interface are:
-
getInputStream(): locates and opens the resource, returning anInputStreamfor reading from the resource. It is expected that each invocation returns a freshInputStream. It is the responsibility of the caller to close the stream. -
exists(): returns abooleanindicating whether this resource actually exists in physical form. -
isOpen(): returns abooleanindicating whether this resource represents a handle with an open stream. Iftrue, theInputStreamcannot be read multiple times, and must be read once only and then closed to avoid resource leaks. Will befalsefor all usual resource implementations, with the exception ofInputStreamResource. -
getDescription(): returns a description for this resource, to be used for error output when working with the resource. This is often the fully qualified file name or the actual URL of the resource.
Other methods allow you to obtain an actual URL or File object representing the
resource (if the underlying implementation is compatible, and supports that
functionality).
The Resource abstraction is used extensively in Spring itself, as an argument type in
many method signatures when a resource is needed. Other methods in some Spring APIs
(such as the constructors to various ApplicationContext implementations), take a
String which in unadorned or simple form is used to create a Resource appropriate to
that context implementation, or via special prefixes on the String path, allow the
caller to specify that a specific Resource implementation must be created and used.
While the Resource interface is used a lot with Spring and by Spring, it’s actually
very useful to use as a general utility class by itself in your own code, for access to
resources, even when your code doesn’t know or care about any other parts of Spring.
While this couples your code to Spring, it really only couples it to this small set of
utility classes, which are serving as a more capable replacement for URL, and can be
considered equivalent to any other library you would use for this purpose.
It is important to note that the Resource abstraction does not replace functionality:
it wraps it where possible. For example, a UrlResource wraps a URL, and uses the
wrapped URL to do its work.
There are a number of Resource implementations that come supplied straight out of the
box in Spring:
The UrlResource wraps a java.net.URL, and may be used to access any object that is
normally accessible via a URL, such as files, an HTTP target, an FTP target, etc. All
URLs have a standardized String representation, such that appropriate standardized
prefixes are used to indicate one URL type from another. This includes file: for
accessing filesystem paths, http: for accessing resources via the HTTP protocol,
ftp: for accessing resources via FTP, etc.
A UrlResource is created by Java code explicitly using the UrlResource constructor,
but will often be created implicitly when you call an API method which takes a String
argument which is meant to represent a path. For the latter case, a JavaBeans
PropertyEditor will ultimately decide which type of Resource to create. If the path
string contains a few well-known (to it, that is) prefixes such as classpath:, it will
create an appropriate specialized Resource for that prefix. However, if it doesn’t
recognize the prefix, it will assume the this is just a standard URL string, and will
create a UrlResource.
This class represents a resource which should be obtained from the classpath. This uses either the thread context class loader, a given class loader, or a given class for loading resources.
This Resource implementation supports resolution as java.io.File if the class path
resource resides in the file system, but not for classpath resources which reside in a
jar and have not been expanded (by the servlet engine, or whatever the environment is)
to the filesystem. To address this the various Resource implementations always support
resolution as a java.net.URL.
A ClassPathResource is created by Java code explicitly using the ClassPathResource
constructor, but will often be created implicitly when you call an API method which
takes a String argument which is meant to represent a path. For the latter case, a
JavaBeans PropertyEditor will recognize the special prefix classpath: on the string
path, and create a ClassPathResource in that case.
This is a Resource implementation for java.io.File handles. It obviously supports
resolution as a File, and as a URL.
This is a Resource implementation for ServletContext resources, interpreting
relative paths within the relevant web application’s root directory.
This always supports stream access and URL access, but only allows java.io.File access
when the web application archive is expanded and the resource is physically on the
filesystem. Whether or not it’s expanded and on the filesystem like this, or accessed
directly from the JAR or somewhere else like a DB (it’s conceivable) is actually
dependent on the Servlet container.
A Resource implementation for a given InputStream. This should only be used if no
specific Resource implementation is applicable. In particular, prefer
ByteArrayResource or any of the file-based Resource implementations where possible.
In contrast to other Resource implementations, this is a descriptor for an already
opened resource - therefore returning true from isOpen(). Do not use it if you need
to keep the resource descriptor somewhere, or if you need to read a stream multiple
times.
The ResourceLoader interface is meant to be implemented by objects that can return
(i.e. load) Resource instances.
public interface ResourceLoader {
Resource getResource(String location);
}All application contexts implement the ResourceLoader interface, and therefore all
application contexts may be used to obtain Resource instances.
When you call getResource() on a specific application context, and the location path
specified doesn’t have a specific prefix, you will get back a Resource type that is
appropriate to that particular application context. For example, assume the following
snippet of code was executed against a ClassPathXmlApplicationContext instance:
Resource template = ctx.getResource("some/resource/path/myTemplate.txt");What would be returned would be a ClassPathResource; if the same method was executed
against a FileSystemXmlApplicationContext instance, you’d get back a
FileSystemResource. For a WebApplicationContext, you’d get back a
ServletContextResource, and so on.
As such, you can load resources in a fashion appropriate to the particular application context.
On the other hand, you may also force ClassPathResource to be used, regardless of the
application context type, by specifying the special classpath: prefix:
Resource template = ctx.getResource("classpath:some/resource/path/myTemplate.txt");Similarly, one can force a UrlResource to be used by specifying any of the standard
java.net.URL prefixes:
Resource template = ctx.getResource("file:///some/resource/path/myTemplate.txt");Resource template = ctx.getResource("http://myhost.com/resource/path/myTemplate.txt");The following table summarizes the strategy for converting Strings to Resources:
| Prefix | Example | Explanation |
|---|---|---|
classpath: |
|
Loaded from the classpath. |
file: |
|
Loaded as a |
http: |
Loaded as a |
|
(none) |
|
Depends on the underlying |
The ResourceLoaderAware interface is a special marker interface, identifying objects
that expect to be provided with a ResourceLoader reference.
public interface ResourceLoaderAware {
void setResourceLoader(ResourceLoader resourceLoader);
}When a class implements ResourceLoaderAware and is deployed into an application
context (as a Spring-managed bean), it is recognized as ResourceLoaderAware by the
application context. The application context will then invoke the
setResourceLoader(ResourceLoader), supplying itself as the argument (remember, all
application contexts in Spring implement the ResourceLoader interface).
Of course, since an ApplicationContext is a ResourceLoader, the bean could also
implement the ApplicationContextAware interface and use the supplied application
context directly to load resources, but in general, it’s better to use the specialized
ResourceLoader interface if that’s all that’s needed. The code would just be coupled
to the resource loading interface, which can be considered a utility interface, and not
the whole Spring ApplicationContext interface.
As of Spring 2.5, you can rely upon autowiring of the ResourceLoader as an alternative
to implementing the ResourceLoaderAware interface. The "traditional" constructor and
byType autowiring modes (as described in [beans-factory-autowire]) are now capable
of providing a dependency of type ResourceLoader for either a constructor argument or
setter method parameter respectively. For more flexibility (including the ability to
autowire fields and multiple parameter methods), consider using the new annotation-based
autowiring features. In that case, the ResourceLoader will be autowired into a field,
constructor argument, or method parameter that is expecting the ResourceLoader type as
long as the field, constructor, or method in question carries the @Autowired
annotation. For more information, see [beans-autowired-annotation].
If the bean itself is going to determine and supply the resource path through some sort
of dynamic process, it probably makes sense for the bean to use the ResourceLoader
interface to load resources. Consider as an example the loading of a template of some
sort, where the specific resource that is needed depends on the role of the user. If the
resources are static, it makes sense to eliminate the use of the ResourceLoader
interface completely, and just have the bean expose the Resource properties it needs,
and expect that they will be injected into it.
What makes it trivial to then inject these properties, is that all application contexts
register and use a special JavaBeans PropertyEditor which can convert String paths
to Resource objects. So if myBean has a template property of type Resource, it can
be configured with a simple string for that resource, as follows:
<bean id="myBean" class="...">
<property name="template" value="some/resource/path/myTemplate.txt"/>
</bean>Note that the resource path has no prefix, so because the application context itself is
going to be used as the ResourceLoader, the resource itself will be loaded via a
ClassPathResource, FileSystemResource, or ServletContextResource (as appropriate)
depending on the exact type of the context.
If there is a need to force a specific Resource type to be used, then a prefix may be
used. The following two examples show how to force a ClassPathResource and a
UrlResource (the latter being used to access a filesystem file).
<property name="template" value="classpath:some/resource/path/myTemplate.txt"><property name="template" value="file:///some/resource/path/myTemplate.txt"/>An application context constructor (for a specific application context type) generally takes a string or array of strings as the location path(s) of the resource(s) such as XML files that make up the definition of the context.
When such a location path doesn’t have a prefix, the specific Resource type built from
that path and used to load the bean definitions, depends on and is appropriate to the
specific application context. For example, if you create a
ClassPathXmlApplicationContext as follows:
ApplicationContext ctx = new ClassPathXmlApplicationContext("conf/appContext.xml");The bean definitions will be loaded from the classpath, as a ClassPathResource will be
used. But if you create a FileSystemXmlApplicationContext as follows:
ApplicationContext ctx =
new FileSystemXmlApplicationContext("conf/appContext.xml");The bean definition will be loaded from a filesystem location, in this case relative to the current working directory.
Note that the use of the special classpath prefix or a standard URL prefix on the
location path will override the default type of Resource created to load the
definition. So this FileSystemXmlApplicationContext…
ApplicationContext ctx =
new FileSystemXmlApplicationContext("classpath:conf/appContext.xml");-
will actually load its bean definitions from the classpath. However, it is still a
FileSystemXmlApplicationContext. If it is subsequently used as aResourceLoader, any unprefixed paths will still be treated as filesystem paths.
The ClassPathXmlApplicationContext exposes a number of constructors to enable
convenient instantiation. The basic idea is that one supplies merely a string array
containing just the filenames of the XML files themselves (without the leading path
information), and one also supplies a Class; the ClassPathXmlApplicationContext
will derive the path information from the supplied class.
An example will hopefully make this clear. Consider a directory layout that looks like this:
com/
foo/
services.xml
daos.xml
MessengerService.class
A ClassPathXmlApplicationContext instance composed of the beans defined in the
'services.xml' and 'daos.xml' could be instantiated like so…
ApplicationContext ctx = new ClassPathXmlApplicationContext(
new String[] {"services.xml", "daos.xml"}, MessengerService.class);Please do consult the ClassPathXmlApplicationContext javadocs for details
on the various constructors.
The resource paths in application context constructor values may be a simple path (as
shown above) which has a one-to-one mapping to a target Resource, or alternately may
contain the special "classpath*:" prefix and/or internal Ant-style regular expressions
(matched using Spring’s PathMatcher utility). Both of the latter are effectively
wildcards
One use for this mechanism is when doing component-style application assembly. All
components can 'publish' context definition fragments to a well-known location path, and
when the final application context is created using the same path prefixed via
classpath*:, all component fragments will be picked up automatically.
Note that this wildcarding is specific to use of resource paths in application context
constructors (or when using the PathMatcher utility class hierarchy directly), and is
resolved at construction time. It has nothing to do with the Resource type itself.
It’s not possible to use the classpath*: prefix to construct an actual Resource, as
a resource points to just one resource at a time.
When the path location contains an Ant-style pattern, for example:
/WEB-INF/*-context.xml com/mycompany/**/applicationContext.xml file:C:/some/path/*-context.xml classpath:com/mycompany/**/applicationContext.xml
The resolver follows a more complex but defined procedure to try to resolve the
wildcard. It produces a Resource for the path up to the last non-wildcard segment and
obtains a URL from it. If this URL is not a jar: URL or container-specific variant
(e.g. zip: in WebLogic, wsjar in WebSphere, etc.), then a java.io.File is
obtained from it and used to resolve the wildcard by traversing the filesystem. In the
case of a jar URL, the resolver either gets a java.net.JarURLConnection from it or
manually parses the jar URL and then traverses the contents of the jar file to resolve
the wildcards.
If the specified path is already a file URL (either explicitly, or implicitly because
the base ResourceLoader is a filesystem one), then wildcarding is guaranteed to work in
a completely portable fashion.
If the specified path is a classpath location, then the resolver must obtain the last
non-wildcard path segment URL via a Classloader.getResource() call. Since this is just
a node of the path (not the file at the end) it is actually undefined (in the
ClassLoader javadocs) exactly what sort of a URL is returned in this case. In
practice, it is always a java.io.File representing the directory, where the classpath
resource resolves to a filesystem location, or a jar URL of some sort, where the
classpath resource resolves to a jar location. Still, there is a portability concern on
this operation.
If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to
get a java.net.JarURLConnection from it, or manually parse the jar URL, to be able to
walk the contents of the jar, and resolve the wildcard. This will work in most
environments, but will fail in others, and it is strongly recommended that the wildcard
resolution of resources coming from jars be thoroughly tested in your specific
environment before you rely on it.
When constructing an XML-based application context, a location string may use the
special classpath*: prefix:
ApplicationContext ctx =
new ClassPathXmlApplicationContext("classpath*:conf/appContext.xml");This special prefix specifies that all classpath resources that match the given name
must be obtained (internally, this essentially happens via a
ClassLoader.getResources(…) call), and then merged to form the final application
context definition.
|
Note
|
The wildcard classpath relies on the |
The classpath*: prefix can also be combined with a PathMatcher pattern in the
rest of the location path, for example classpath*:META-INF/*-beans.xml. In this
case, the resolution strategy is fairly simple: a ClassLoader.getResources() call is
used on the last non-wildcard path segment to get all the matching resources in the
class loader hierarchy, and then off each resource the same PathMatcher resolution
strategy described above is used for the wildcard subpath.
Please note that classpath*: when combined with Ant-style patterns will only work
reliably with at least one root directory before the pattern starts, unless the actual
target files reside in the file system. This means that a pattern like
classpath*:*.xml might not retrieve files from the root of jar files but rather only
from the root of expanded directories.
Spring’s ability to retrieve classpath entries originates from the JDK’s
ClassLoader.getResources() method which only returns file system locations for a
passed-in empty string (indicating potential roots to search). Spring evaluates
URLClassLoader runtime configuration and the "java.class.path" manifest in jar files
as well but this is not guaranteed to lead to portable behavior.
|
Note
|
The scanning of classpath packages requires the presence of corresponding directory entries in the classpath. When you build JARs with Ant, make sure that you do not activate the files-only switch of the JAR task. Also, classpath directories may not get exposed based on security policies in some environments, e.g. standalone apps on JDK 1.7.0_45 and higher (which requires 'Trusted-Library' setup in your manifests; see http://stackoverflow.com/questions/19394570/java-jre-7u45-breaks-classloader-getresources). On JDK 9’s module path (Jigsaw), Spring’s classpath scanning generally works as expected. Putting resources into a dedicated directory is highly recommendable here as well, avoiding the aforementioned portability problems with searching the jar file root level. |
Ant-style patterns with classpath: resources are not guaranteed to find matching
resources if the root package to search is available in multiple class path locations.
This is because a resource such as
com/mycompany/package1/service-context.xml
may be in only one location, but when a path such as
classpath:com/mycompany/**/service-context.xml
is used to try to resolve it, the resolver will work off the (first) URL returned by
getResource("com/mycompany");. If this base package node exists in multiple
classloader locations, the actual end resource may not be underneath. Therefore,
preferably, use " `classpath*:`" with the same Ant-style pattern in such a case, which
will search all class path locations that contain the root package.
A FileSystemResource that is not attached to a FileSystemApplicationContext (that
is, a FileSystemApplicationContext is not the actual ResourceLoader) will treat
absolute vs. relative paths as you would expect. Relative paths are relative to the
current working directory, while absolute paths are relative to the root of the
filesystem.
For backwards compatibility (historical) reasons however, this changes when the
FileSystemApplicationContext is the ResourceLoader. The
FileSystemApplicationContext simply forces all attached FileSystemResource instances
to treat all location paths as relative, whether they start with a leading slash or not.
In practice, this means the following are equivalent:
ApplicationContext ctx =
new FileSystemXmlApplicationContext("conf/context.xml");ApplicationContext ctx =
new FileSystemXmlApplicationContext("/conf/context.xml");As are the following: (Even though it would make sense for them to be different, as one case is relative and the other absolute.)
FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("some/resource/path/myTemplate.txt");FileSystemXmlApplicationContext ctx = ...;
ctx.getResource("/some/resource/path/myTemplate.txt");In practice, if true absolute filesystem paths are needed, it is better to forgo the use
of absolute paths with FileSystemResource / FileSystemXmlApplicationContext, and
just force the use of a UrlResource, by using the file: URL prefix.
// actual context type doesn't matter, the Resource will always be UrlResource
ctx.getResource("file:///some/resource/path/myTemplate.txt");// force this FileSystemXmlApplicationContext to load its definition via a UrlResource
ApplicationContext ctx =
new FileSystemXmlApplicationContext("file:///conf/context.xml");