Using Java 6 processors in Eclipse

I couldn’t find any complete tutorial on this and wasted a couple of hours figuring this stuff out.

So here you are, hopefully it’ll save you some time.

JDK5 introduced the APT (Annotation Processing Tool).
It was part of the sdk but the classes were part of the unofficial com.sun.* namespace, and you had to use the “apt” tool to process the source code.

JDK6 cleaned up the api and integrated this stuff it into javac itself so you didn’t need to use the separate apt tool anymore.

Apparently built for processing source code with annotations before they are compiled into classes, it can also be used for all kinds of fun like code generation and code analyzers which are IDE independent; and you don’t even need to use annotations necessarily. I think the JPA2 meta-model used in its criteria api is implemented with this.

I made one very contrived example of java6 processor usage with Eclipse. All of this is possible to integrate into a maven build but I’m leaving that out and focusing on how I got this processing to work within Eclipse.

So we’re creating a processor which will generate a new class for each class in projects compiled using this processor. Additionally we’ll create a Warning for each class which starts with a T. Yes it’s silly.

Step 1: Create the processor project

SillyProcessor.java:

@SupportedAnnotationTypes(value= {"*"})
@SupportedSourceVersion(SourceVersion.RELEASE_6)
public class SillyProcessor extends AbstractProcessor { 

	private Filer filer;
	private Messager messager;

	@Override
	public void init(ProcessingEnvironment env) {
		filer = env.getFiler();
		messager = env.getMessager();
	}

	@Override
	public boolean process(Set<? extends TypeElement> elements, RoundEnvironment env) {

		for (Element element : env.getRootElements()) {

			if (element.getSimpleName().toString().startsWith("Silly")) {
				// We don't want generate new silly classes 
				// for auto-generated silly classes
				continue;
			}

			if (element.getSimpleName().toString().startsWith("T")) {
				messager.printMessage(Kind.WARNING, 
					"This class name starts with a T!", 
					element);	
			}

			String sillyClassName = "Silly" + element.getSimpleName();
			String sillyClassContent = 
					"package silly;\n" 
				+	"public class " + sillyClassName + " {\n"
				+	"	public String foobar;\n"
				+	"}";

			JavaFileObject file = null;

			try {
				file = filer.createSourceFile(
						"silly/" + sillyClassName, 
						element);
				file.openWriter()
					.append(sillyClassContent)
					.close();
			} catch (IOException e) {
				e.printStackTrace();
			}

		}

		return true;
	}
}

Without creating this META-INF entry I couldn’t get the processor to register in Eclipse.

META-INF/services/javax.annotation.processing.Processor:

com.kerebus.annotation.processor.SillyProcessor

Its only contents is the name of the Processor implementation. I guess you might be able to list several processors here, although I’m not sure.

That’s it. Now export it as a jar and use that jar in other projects where you wish to use the processor.

STEP 2: Create a project which uses your processor.

In the properties for your new project go to Java Compiler -> Annotation Processing
Check the “Enable Project Specific Settings” and make sure “Enable annotation processing” is checked. I also changed the generated source directory to a name which didn’t start with a dot so it wouldn’t be hidden in the package explorer (files or directories which start with a dot are by default filtered away in eclipse).

Next off go to Java Compiler -> Annotation Processing -> Factory Path
Here you should add the jar of your processor project. You cannot use project references.
Press the “Advanced” button and you’ll be presented with a dialog which contains the processor you defined in your META-INF/services/javax.annotation.processing.Processor file. Select it and press ok.

Step 3: Build!

We’re practically done. Here’s what it looks like for me in my project:

So we get a warning for the Thing class because its class name start with a “T” and for each class in the project we get corresponding “Silly” classes generated. These are compiled and usable just like any other normal class.

For more info check out the eclipse jdt/apt docs, this bit about creating a code analyzer or the offical docs