I am writing a message oriented middleware, in which subscribers and publishers communicate via a queue in the middle class MessageBroker. publishers should drop messages into a queue of a topic if it is not full and subscribers should get the message of a subscribed topic if the queue is not empty. The problem now arises when I try to include wait() NotifyAll() in the receiveMessage() method of the subscriber. With the Publisher it works without any problems, but with the Subscriber I have the problem that they are not retrieved from the wait state, so that they do nothing.