Meta-Agent

Who instruments the instrumenter? This project is a Java agent that instruments Java agents, or specifically, it instruments the ClassFileTransformers of other agents to observe how they transform the bytecode of classes.

This is especially useful to check what libraries like Mockito do to your classes at runtime.

To run it, build the project with mvn package -DskipTests and run your Java program with the agent:

java -javaagent:target/meta-agent.jar -jar your-program.jar

# or run a Mockito based sample test
mvn package -DskipTests
mvn test -DargLine="-javaagent:target/meta-agent.jar"

The executed MockitoTest looks as follows:

@ExtendWith(MockitoExtension.class)
public class MockitoTest {

  @Mock
  List<String> mockedList;

  @Test
  public void whenNotUseMockAnnotation_thenCorrect() throws InterruptedException {
    mockedList.add("one");
    Mockito.verify(mockedList).add("one");
    assertEquals(0, mockedList.size());

    Mockito.when(mockedList.size()).thenReturn(100);
    assertEquals(100, mockedList.size());

    Thread.sleep(10000000L);
  }
}

Opening localhost will show you a list of available commands, most importantly

/help to show the help, available comands and decompilation and output options
/instrumentators to list all instrumentators (ClassFileTransformers) that have been used
/full-diff/instrumentator?pattern=.* to show the full diff for all instrumentators
/classes to list all classes that have been transformed
/full-diff/class?pattern=.* to show the full diff for all classes and all instrumentators
/all/decompile?pattern= to decompile the classes matching the pattern

In our example, we can see via /instrumentators that Mockito uses the org.mockito.internal.creation.bytebuddy.InlineByteBuddyMockMaker to transform classes. Using /full-diff/instrumentator/.*, we can see the diff of all transformations that this instrumentator has done:

Yet we also see via /classes that Mockito only transforms the java.util.List interface and all its parents:

How this works

The agent wraps all ClassFileTransformers with a custom transformer that records the diff of the bytecode. It then uses vineflower to decompile the bytecode and diff to compute the diff between the original and the transformed bytecode.

The front-end is implemented using the built-in HttpServer as a simple web server started by the agent.

This is essentially a more capable version of the classviewer-agent.-

Contributions

If you have sample programs where this tool helped to see something interesting, please share. Contributions, issues and PRs are welcome.

parttimenerd/meta-agent

Meta-Agent

How this works

Contributions

License