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Javassist Maven Plugin

A simple Maven plugin that can apply Javassist tranformation on classes after compilation.

How to use it

Include the plugin on your pom.xml descriptor:

    <plugin>
        <groupId>de.icongmbh.oss.maven.plugins</groupId>
        <artifactId>javassist-maven-plugin</artifactId>
        <version>2.0.2</version>
        <configuration>
            <includeTestClasses>false</includeTestClasses>
            <transformerClasses>
                <transformerClass>
                    <className>com.domain.ToStringTransformer</className>
                    <properties>
                        <property>
                            <name>append.value</name>
                            <value> and ToStringTransformer</value>
                        </property>
                    </properties>
                </transformerClass>
            </transformerClasses>
        </configuration>
        <executions>
            <execution>
                <phase>process-classes</phase>
                <goals>
                    <goal>javassist</goal>
                </goals>
            </execution>
        </executions>
    </plugin>

You must implement Class transformers, here’s one simple example (at least, not a “logger” one).

/**
 * Silly transformer, used to hack the toString method.
 */
public class ToStringTransformer extends ClassTransformer {

  private static final String APPEND_VALUE_PROPERTY_KEY = "append.value";

  private String appendValue;

  /**
   * We'll only transform subtypes of MyInterface.
   */
  @Override public boolean shouldTransform(final CtClass candidateClass)
          throws JavassistBuildException {
    CtClass myInterface = ClassPool.getDefault().get(MyInterface.class.getName());
    try {
      return !candidateClass.equals(myInterface) && candidateClass.subtypeOf(myInterface);
    } catch (NotFoundException e) {
      throw new JavassistBuildException(e);
    }
  }

  /**
   * Hack the toString() method.
   */
  @Override public void applyTransformations(CtClass classToTransform)
          throws JavassistBuildException {
    try {
      // Actually you must test if it exists, but it's just an example...
      CtMethod toStringMethod = classToTransform.getDeclaredMethod("toString");
      classToTransform.removeMethod(toStringMethod);

      CtMethod hackedToStringMethod = CtNewMethod
              .make("public String toString() { return \"toString() hacked by Javassist" + (
                            this.appendValue != null ? this.appendValue : "") + "\"; }",
                    classToTransform);
      classToTransform.addMethod(hackedToStringMethod);
    } catch (CannotCompileException | NotFoundException e) {
      throw new JavassistBuildException(e);
    }
  }

  @Override public void configure(final Properties properties) {
    if (null == properties) {
      return;
    }
    this.appendValue = properties.getProperty(APPEND_VALUE_PROPERTY_KEY);
  }
}

Known limitations

• Don’t instrument classes inside .jar files, only classes on your project;
• Lack of unit tests and sample app;
• Further implementations of de.icongmbh.oss.maven.plugin.javassist.ClassTransformer can enable easier interactions with the Javassist API (provide some utilities).

How to contribute?

If you think this project is useful for you, then there’s a huge chance it’s useful to others, so please feel free
to fork, fix it, improve it and test it (the “Known limitations” above is a great way to start).

Documentation

• maven site
• wiki