Jeta allows you to create custom annotation processor and you should take advantage of for sure.

In this post we’ll create a processor for our Hello, World sample.

First of all, we must create a separate module since a processor and all the corresponding code is needed at compile time only. Also, this module must be compiled into byte-code before being passed to javac as the annotation processor.

There are no special requirements for this module. It’s a plain java project. Assume we named it HelloWorldSampleApt and certainly used gradle to build:

apply plugin: 'java'

repositories {
    jcenter()
}

dependencies {
    compile 'org.brooth.jeta:jeta-apt:2.3'
    // @SayHello annotation dependency
}

Note Apt module depends on annotations it processes. In our case, we need to create a separate module for @SayHello annotation and add this module as the dependency for the apt module.

Next, we’ll create the processor. Be aware that this processor must implement org.brooth.jeta.apt.Processor or simply extend org.brooth.jeta.apt.processors.AbstractProcessor.

Pay attention that Jeta uses JavaPoet framework to generate java source code. Please, go to square/javapoet site on GitHub, before you write your first processor.

public class SayHelloProcessor extends AbstractProcessor {

    public SayHelloProcessor() {
        super(SayHello.class);
    }

    @Override
    public boolean process(Builder builder, RoundContext roundContext) {
        MetacodeContext context = roundContext.metacodeContext();
        ClassName masterClassName = ClassName.get(context.masterElement());
        builder.addSuperinterface(ParameterizedTypeName.get(
                ClassName.get(HelloWorldMetacode.class), masterClassName));

        MethodSpec.Builder methodBuilder = MethodSpec.methodBuilder("setHelloWorld")
                .addAnnotation(Override.class)
                .addModifiers(Modifier.PUBLIC)
                .returns(void.class)
                .addParameter(masterClassName, "master");

        for (Element element : roundContext.elements()) {
            String fieldName = element.getSimpleName().toString();
            methodBuilder.addStatement("master.$L = \"Hello, World!\"", fieldName);
        }

        builder.addMethod(methodBuilder.build());
        return false;
    }
}

Now, we need to add this module into apt classpath. Let’s amend dependencies of HelloWorldSample.

dependencies {
    apt project(':HelloWorldSampleApt')
    apt 'org.brooth.jeta:jeta-apt:+'
    compile 'org.brooth.jeta:jeta:+'
}

Also, say to Jeta to use our processor. To do that we add the option into jeta.processing:

processors.add = com.example.apt.SayHelloProcessor

You can find a similar example on GitHub.