Java 6 - Annotation processor and code addition
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>开发者_Go百科 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.
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);
}
}
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.
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. Weigh costs/benefits carefully.
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