使用注释处理器向类型添加属性

Question

I am trying to write an annotation processor that is supposed to run during compilation. It will first check, if a specific field (int id) will exists in a given class (after compilation) and if not, is supposed to add this field (if possible with an initial value of -1. I think I have the TypeElement which is to be edited. Can I for example insert a string in a line within the class file?

processingEnd.getFilter().getResource(...) seems to give me the file, but I am not sure how to do this and specify the parameters correctly. I can get a read and write stream, but I am not entirely sure if i can simply 'insert' what I want at some line within the class.

@Override
public boolean process(Set<? extends TypeElement> set,
                       RoundEnvironment round) {
    if (!set.contains(DBStorable.class)) {
        return false;
    }
    Set<? extends Element> elements =
            round.getElementsAnnotatedWith(DBStorable.class);
    for (Element element : elements) {
        if (element.getKind() == ElementKind.CLASS) {
            // edit processed class to include id field.
            TypeElement processedType = (TypeElement) element;
            boolean found = false;
            for (Element subElement : processedType.getEnclosedElements()) {
                if (subElement.getKind() == ElementKind.FIELD
                        && subElement.getSimpleName().equals("id")) {
                    found = true;
                    break;
                }
            }
            if (!found) {
                // This is where I don't get on...
                JavaFileObject javaFile = processingEnv.getFiler().getResource(
                        Location, // <- not sure how to get this.
                        processingEnv.getElementUtils().getPackageOf(processedType),
                        processedType.getSimpleName()); // <- is this the right value?

                // ... tbc
            }
        }
    }
    // TODO Auto-generated method stub
    return true;
}

Answer 1

You are not supposed to change any classes during processing. Check the documentation of the Filer class:

In general, processors must not knowingly attempt to overwrite existing files that were not generated by some processor.

You can however create subclasses that override the existing methods or add new properties:

Note that some of the effect of overwriting a file can be achieved by using a decorator-style pattern. Instead of modifying a class directly, the class is designed so that either its superclass is generated by annotation processing or subclasses of the class are generated by annotation processing. If the subclasses are generated, the parent class may be designed to use factories instead of public constructors so that only subclass instances would be presented to clients of the parent class.

If that is not an option for you, keep looking for other tools or workarounds. For example, there may be be some solutions for you in this question: Plugging in to Java compilers