JarCastingJavassist 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.ClassTransformercan 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).
