How can I ensure that an overridden method is synchronized

Question

I have a class of common code that is thread safe.

One of the methods in that class is abstract and needs to be overridden for different implementations.

I need

Answer 1

This link to the JLS confirms that we can't mix abstract and synchronized.

Though much weaker than a keyword or standard annotation, but stronger than documentation: perhaps try a Marker interface?

... provides a means to associate metadata with a class where the language does not have explicit support for such metadata.

This is a stretch, but might help, in that the derived class makes a declaration (edit: new example tests the declaration):

interface EatMethodIsThreadSafe {}

abstract class Animal {
    public Animal() {
        if (! (this instanceof EatMethodIsThreadSafe)) { 
            throw new IllegalArgumentException("eat method must be thread safe");
        }
    }
    public abstract void eat();
}

public class Bear extends Animal implements EatMethodIsThreadSafe {
    public synchronized void eat() {}
    public static void main(String...args) { Bear b = new Bear(); } 
}