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Caldum

Caldum (Latin: "hot") is a microframework for writing function hooks targeting the Java Virtual Machine (JVM). It provides additional scaffolding on top of the venerable Byte Buddy, and provides several features that enable faster and easier development of Byte Buddy-based Java agents.

Caldum's main feature is its hot-reloading capabilities that enable hooks to be updated at runtime. It additionally provides annotation-based dependency injection similar to JAX-RS, enabling simpler development workflows that do not require application restarts. Lastly, Caldum provides an interface to extend function hooks themselves with dynamic instrumentation.

Caldum (and VulcanLoader [see below]) support Java 6 and above (most recently tested against Java 21) and have been tested on both the HotSpot and OpenJ9 JVMs.

Building

The following will build everything in the current environment:

./build.sh

This is more or less equvalent to:

cd caldum && ./gradlew; cd ..
cd vulcanloader && ./gradlew; cd ..
cd embeddedagentplugin && ./gradlew; cd ..

But if you want a saner/cleaner build, the following will build everything in a Docker container:

sudo ./build-docker.sh

Testing

The following will run all of the "static" tests:

sudo ./tests/test.sh

Howver, individual tests can be performed under various configurations:

sudo ./test.sh 'java89' -- 'premain' ---- eclipse-temurin:21-jdk ibm-semeru-runtimes:open-21-jdk
sudo ./test.sh 'java67' -- 'agentmain' ---- openjdk:6-jdk

There is a more "dynamic" test in the works in tests/hotreload, but it's a bit more involved to run at the moment and has not been dockerized yet:

cd tests/hotreload/testapp
JAVA_HOME=path/to/testingjdk ./test.sh 5

VulcanLoader

VulcanLoader (VL) is a thin-wrapper that builds Caldum as a Java agent and provides a minimal interface to set up, reconfigure, and unload Caldum-compatible Java agents. Agent state is maintained based on the JAR filename, but not the entire file path; attempting to load a new agent JAR with same filename as an existing agent JAR will result in the original being unloaded prior to the new one being loaded.

Building

VulcanLoader is intended to be rebuilt for each target host when targeting JRE versions below 9. When doing so, the respective platform's tools.jar (from its JDK, not JRE) must be copied to the vulcanloader/tools/ directory prior to building.

cd vulcanloader && ./gradlew

Interface

  • -javaagent:path/to/vl.jar=path/to/config
  • -javaagent:path/to/vl.jar=path/to/agent.jar
  • java -jar path/to/vl.jar <pid> -c path/to/config
  • java -jar path/to/vl.jar <pid> <agent.jar...> -- <agent opts>

Configuration Format

path/to/agent1.jar: <agent1 opts>
path/to/agent2.jar: <agent2 opts>
...

Unloading Agents

  • Unload one agent:
    • java -jar path/to/vl.jar <pid> -u path/to/agent.jar
    • java -jar path/to/vl.jar <pid> path/to/agent.jar -- unload
  • Unload all agents:
    • java -jar path/to/vl.jar <pid> -u

Structure

Caldum and VulcanLoader are designed to directly expose a single bundled version of Byte Buddy directly to compatible Java agents, such that they do not need to bundle the entire Byte buddy library and may build to relatively small JAR files. The ClassLoader used to load agent JARs will first check itself when attempting to load classes. This enables developers to embed generally whatever libraries they may need. However, care should be taken to ensure that dependencies intended for use within hook code are injected so that they are accessible from the ClassLoader of the instrumented class.

While it is not strictly necessary to include the normal JAR manifest attributes used for Java agents (all loaded JARs inherit Caldum/VL's agent capabilities), Caldum/VL will respect any Premain-Class and Agent-Class attributes and invoke the associated entry points prior to scanning for and applying annotated function hooks. This enables developers to provide additional static configuration to annotated classes and support legacy instrumentation code. Caldum/VL will additionally invoke any public static void unload() method within the entry class when the agent is unloaded (e.g. manually or when reloading).

Java Version Support

Caldum and VulcanLoader currently attempt to maintain support for the same versions of Java supported by Byte Buddy (currently Java 6 through 9+). Due to this, the codebases of Caldum and VulcanLoader themselves are built as Java 6 (using OpenJDK 11); however, Caldum-compatible agents may target newer versions of Java supported by the JVM being instrumented.

Annotations (trust.nccgroup.caldum.annotation.*)

Class Annotations

@Hook

This annotation identifies a Caldum hook class to be automatically applied on load.

Options:

  • The wrappers value may be used to supply a list of hook wrapping classes that will be applied in order to instrument the hook code.
  • The redefinition value may be used to configure an AgentBuilder.RedefinitionStrategy (e.g. DISABLED (new loads only), REDEFINITION, or RETRANSFORMATION).

Hook classes are structured as follows:

@Hook
class ExampleHook {

  static class SomeSettings {
    ... // type/method matchers, local dependency injection providers
  }

  ... // static fields (copied or dependency injected)

  @Advice.OnMethodEnter
  static void enter() {
    ...
  }

  @Advice.OnMethodExit
  static void exit() {
    ...
  }
}

@Dynamic

This annotation identifies that a Caldum hook class is to be preprocessed using the DynVarsAgent/DynamicFields classes. They rewrite classes to proxy their static and non-static fields (and associated field accesses) into fixed Map objects that it adds to the classes. This makes it possible to make a wider range of hot-reload modifications to already loaded hooks beyond what is traditionally supported by mainstream JVMs (e.g. not adding/removing/reordering fields/methods, and not editing method signatures). Generally speaking, this annotation should be used primarily during the development of Caldum-based agents/hooks as its modifications can make analyzing one's own hooks at runtime a bit annoying.

@Debug

This annotation is used to configure Byte Buddy to write events and exceptions of an @Hook-annotated class to the standard output of the process being hooked. This is implemented using a new AgentBuilder.Listener.StreamWriting(System.out).

@DI.Provider

This annotation identifies a global dependency injection provider.

Dependency Injection Annotations

Dependency injection in Caldum is designed to support local, in-file configuration, with opt-in support for global defaults without violating the principle of least surprise. By default, locally configured @DI.Provide-d values will be applied to any static fields within the @Hook class that have a matching name without validating type compatibility.

@DI.Provide

This annotation may be applied to a static field or static method within a global @DI.Provider class or a "settings" class nested within an @Hook class.

If applied to a field without an overriding name value, the name of the field will be used to match against injection targets.

If applied to a static method returning a Map<String,Object>, the method will be invoked to provide a dependency injection mapping of field names to values.

@DI.Inject

This annotation may be applied to a static field within a @Hook class to opt-in to dependency injection from a matching @DI.Provider class-supplied @DI.Provide-d field. However, locally-declared @DI.Provide-d fields will take precedence.

@DI.AgentClassLoader

This annotation may be applied to a static field within a @Hook class to inject the ClassLoader of the @Hook class (i.e. the ClassLoader of the Caldum-compatible agent JAR).

Matcher Annotations

Each of these annotations may be applied to a static Byte Buddy ElementMatcher field or a static method returning an ElementMatcher.

@Matcher.Ignore

The ElementMatcher will be used to configure net.bytebuddy.agent.builder.AgentBuilder::ignore. net.bytebuddy.agent.builder.AgentBuilder::ignore.

@Matcher.Type

The ElementMatcher will be used to configure the type matcher of net.bytebuddy.agent.builder.AgentBuilder::type.

@Matcher.Loader

The ElementMatcher will be used to configure the ClassLoader matcher of net.bytebuddy.agent.builder.AgentBuilder::type.

@Matcher.Module

The ElementMatcher will be used to configure the module matcher of net.bytebuddy.agent.builder.AgentBuilder::type.

@Matcher.Raw

The ElementMatcher will be used to configure net.bytebuddy.agent.builder.AgentBuilder::type(AgentBuilder.RawMatcher matcher).

@Matcher.Member

The ElementMatcher will be used to configure net.bytebuddy.asm.Advice::on(ElementMatcher<? super MethodDescription> matcher).

Hook Wrapper Annotations

Caldum supports the instrumentation of @Hook classes through the use of its wrappers value. Classes specified in this manner must contain static nested classes annotated with at least one of the following annotations.

@Wrapper.OnMethodEnter

This class will be applied as Byte Buddy Advice to the @Advice.OnMethodEnter-annotated method of the @Hook class. It should specify at least one @Advice.OnMethodEnter-/@Advice.OnMethodExit-annotated static method. Care should be taken when specifying @Advice dependency injected method arguments; these will be applied to the @Hook-annotated class itself and not the classes it instruments.

@Wrapper.OnMethodExit

This class will be applied as Byte Buddy Advice to the @Advice.OnMethodExit-annotated method of the @Hook class. It should specify at least one @Advice.OnMethodEnter-/@Advice.OnMethodExit-annotated static method. Care should be taken when specifying @Advice dependency injected method arguments; these will be applied to the @Hook-annotated class itself and not the classes it instruments.

Included Hook Wrappers (trust.nccgroup.caldum.wrappers.*)

  • NoRecursion: This wrapper ensures that a @Hook-annotated class configured with it will not invoke the injected hook code when invoked from the call-stack of a NoRecursion-wrapped @Hook-annotated class. This is useful for performing invocations within hook code that may result in infinite recursion otherwise.

License

Caldum (and VulcanLoader) are licensed under the Apache License, Version 2.0. Exceptions to this (i.e. for certain vendored source files) are explicitly noted in the relevant source files.