Java annotation to set field to a static instance?

Question

I've been playing with annotations, and I'm wondering how to go about doing this. What I'd like to do is to be able to have a field declared in a class and annotated such that the field will be initialized with a static instance of the class.

Given an annotation like this:

@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME) //or would this be RetentionPolicy.CLASS?
public @interface SetThisField {
}

Something like this:

public class Foo {

    @SetThisField
    private Bar bar;
}

I've played around with using a parser and setting this at runtime, which works but isn't as elegant as I'd like.

I can't find any really good examples of RetentionPolicy.CLASS but the documentation seems to indicate that I could somehow make the declaration of "bar" get compiled into this:

private Bar bar = Bar.getInstance();

It wouldn't look that way in the source code of course, but it would in the byte code and it would behave like that at runtime.

So am I off base here? Is this possible? Or is the parser the way to go with it?

UPDATE: This is the guts of the parser I'm using

public static void parse(Object instance) throws Exception {

    Field[] fields = instance.getClass().getDeclaredFields();

    for (Field field : fields) {
        //"Property" annotated fields get set to an application.properties value
        //using the value of the annotation as the key into the properties
        if (field.isAnnotationPresent(Property.class)) {
            Property property = field.getAnnotation(Property.class);

            String value = property.value();

            if (!"".equals(value)) {
                setFieldValue(instance, field, properties.getProperty(value));
            }
        }

        //"Resource" annotated fields get static instances of the class allocated
        //based upon the type of the field.
        if (field.isAnnotationPresent(Resource.class)) {
            String name = field.getType().getName();
            setFieldValue(instance,  field, MyApplication.getResources().get(name));
        }
    }
}

private static void setFieldValue(Object instance, Field field, Object value) throws IllegalAccessException {
    boolean accessibleState = field.isAccessible();
    field.setAccessible(true);
    field.set(instance, value);
    field.setAccessible(accessibleState);
}

Answer 1

I would suggest doing the replacement at run time. This is much simpler to implement and test. Changing the byte code at build time is relatively error prone and tricky to get right. For example you would need to understand how byte code is structured and in this case how to add the code to all the constructors in the right place in the code.

If you make the retention RUNTIME, you can have a library which examines the annotation and sets the value after the object is created.