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

The short answer is: you're not supposed to change source code during annotation processing.

I have had a situation recently where that answer was not satisfactory (see this question). My solutions was to programmatically add the code I needed using the internal javac api. See my answer to my own question for details.

I took the inspiration to this from Project Lombok, starting out with their source code and throwing away everything I didn't need. I don't think you'll find a much better starting point.

BTW, Javassist probably won't help, because you are dealing with a source tree, not with byte code. If you want to use a byte code manipulation library you can do that either statically after compiling or dynamically when loading the classes, but not during annotation processing, because that's a pre-compile step.

Answer 2

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 forisolating 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);
  }
}

Answer 3

Annotation processing is not meant to alter existing classes - it is just for generating additional code/resources (on a class-by-class basis, otherwise you'll run into trouble when re-compiling the modified sources only).

Some time ago I tried Spoon for a similar problem: I liked the idea of a program processor very much (and the IDE integration even more), but it was not really stable at the time...

Depending on your use case, an AOP tool (eg: AspectJ) could suite you better than Spoon, and - of course - you could always use a source code generator or implement a full-blown DSL (take a look at the fantastic Xtext).

Depending on the size, turnover rate and "intellectual inertia" of your team mates - you could be better off bearing the pains of plain java instead of those of introducing a new tool/technology, forming co-workers and integrating the new tool in your CI system. Weight costs/benefits carefully.