Page History

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• starting dependent services; where a parent component is dependent on the child components starting; where the start process may be asynchronous in different threads. e.g. there may be a recovery process on startup which you need to wait for.
• implementing master-slave type protocols where you need to monitor the state of the master and slave to make decisions on what to do; together with dealing with transitions from Started to Recovering to Running then maybe to FailingOver etc.
• implementing message orchestration. You may want to implement some simple orchestrations, waiting for either a response to arrive or a timeout to fire etc

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Code overview

State class

We have a State<T> interface which is identical to the AtomicReference<T> interface included in java.util.concurrent in Java 5 apart from that it allows you to register listeners to the state. This makes it easy to respond to the changes in state.

One added detail is the that State<T> allows you to specify the Notifier implementation used to perform the notifications (which always occur outside of the semaphore to allow things to be re-entrant).

The default notifier used is the SynchronousNotifier but its easy to switch to an AsynchronousNotifier by just passing it into the constructor of the State<T> object.

Activity class

An activity is just a POJO which implements the Activity interface; it has a simple lifecycle, you can start them and stop them and ask their status.

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