Abstract supertype of resources which may be obtained at the beginning of a try statement and then released when the statement completes. Unlike a Destroyable resource, a single instance of Obtainable may be reused between multiple try statements or multiple executions of the same try statement.

Lock lock = ... ;
try (lock) {
    ...
}

• obtain() is called before the body of the try statement is executed, and
• release() is called when execution of the body of the try statement ends, even if an exception propagates out of the body of the try.

A class which satisfies Obtainable may impose constraints on the ordering and nesting of invocations of obtain() and release(). For example, it may be not be possible to obtain a resource that has already been obtained. Those methods should produce an AssertionError when any such constraint is violated.