Annotation Interface ThreadSafeTypeParameter

@Documented
@Target(TYPE_PARAMETER)
@Retention(RUNTIME)
public @interface ThreadSafeTypeParameter

When a ThreadSafe class has type parameters, annotating a parameter with ThreadSafeTypeParameter enforces that declarations of this class must, for that type parameter, use a type that is itself thread-safe.

Additionally, only type parameters that are annotated with ThreadSafeTypeParameter can be used as field types that are not @GuardedBy.

In more detail, consider this (valid) class:

@ThreadSafe class MyThreadSafeClass<A, B, @ThreadSafeTypeParameter C> {

  @GuardedBy("this") B b;

  final C c;

  MyThreadSafeClass(B b, C c) {
    this.b = b;
    this.c = c;
  }
}

Each of these three type parameters is valid for a different reason: type parameter A is ok because it is simply not used as the type of a field; type parameter B is ok because it is used as the type of a field that is declared to be @GuardedBy; finally, type parameter C is ok because it is annotated with ThreadSafeTypeParameter. Furthermore, the declaration MyThreadSafeClass<Object, Object, String> is valid, since the type parameter C (i.e., String) is thread-safe, whereas a declaration MyThreadSafeClass<Object, Object, Object> would result in a compiler error.

Note: the ThreadSafeTypeParameter annotation has a secondary use case. If you annotate a type parameter of a method, then callers to that method are only allowed to pass in a type that is deemed thread-safe. For example, given the method declaration static <@ThreadSafeTypeParameter T> void foo(T foo) {}, a call to foo must pass a parameter that is deemed thread-safe.