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 to ensure or at least flag to other developers that all implementations of this method need to be thread-safe.

What is the best way to do this?

Is there a keyword or annotation to this effect?

I have already tried abstract synchronized but that combination of keywords is not allowed.

Answer 1

You can’t do it directly. One thing you can do is have the method be concrete, but invoke an abstract method:

public synchronized final void foo() {
    doFoo();
}
protected abstract void doFoo();

That way, doFoo() will always* be invoked under the synchronization established by foo().

* unless someone invokes it directly, so you should name and document it to make it clear that they shouldn’t.