Java 6 - Annotation processor and code addition

Question

I wrote a custom annotation containing metadata for a property and an AnnotationProcessor:

@SupportedAnnotationTypes({"<package>.Property"})
public class PropertyProcessor extends AbstractProcessor {

    @Override
    public boolean process(Set<? extends TypeElement> annotations,
            RoundEnvironment roundEnv) {
        // Get messager object
        Messager messager = processingEnv.getMessager();
        // Iterate through the annotations
        for(TypeElement typeElement : annotations) {
            // Iterate through the annotated elements
            for(Element element : roundEnv.getElementsAnnotatedWith(typeElement)) {
                // Get Property annotation
                Property property = element.getAnnotation(Property.class);

            }
        }
        return false;
    }

}

Here is the question, I have used Javassist before but it was depending on the class loader and I think it's not appropriate for OSGi applications. I want to change the generated bytecode when a class with Property annotation is compiled.

Answer 1

Have you tried Google Guice?

Google Guice lets you do a bit of Aspect Oriented Programming by intercepting methods. If that's all you need to do, you can implement a MethodInterceptor that'll let you override methods at runtime. It's really neat for isolating cross-cutting concerns.

For example, lets say you want to prevent a certain methods from being executed on weekends, you can annotate them as so:

@Property
public class SomeClass {
    public Receipt doSometing() {
        // Do something
    }
}

Define a MethodInterceptor:

public class PropertyInterceptor implements MethodInterceptor {
  public Object invoke(MethodInvocation invocation) throws Throwable {
    // For example prevent the classes annotated with @Property
    // from being called on weekends
    Calendar today = new GregorianCalendar();
    if (today.getDisplayName(DAY_OF_WEEK, LONG, ENGLISH).startsWith("S")) {
      throw new IllegalStateException(
          invocation.getMethod().getName() + " not allowed on weekends!");
    }
    return invocation.proceed();
  }
}

And then bind the interceptor to the annotation:

public class PropertyModule extends AbstractModule {
  protected void configure() {
        PropertyInterceptor propertyInterceptor = new PropertyInterceptor();        
        bindInterceptor(Matchers.annotatedWith(Property.class), 
        Matchers.any(), propertyInterceptor);
  }
}