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Using wait(), notify() and notifyAll()
When to use wait/notify?
Common bugs with wait/notify
Java BlockingQueue (common Java 5 alternative to wait/notify)
Java synchronization and concurrency (overview of related articles on this site)
The Java language includes three important methods that effectively allow one thread to signal to another. Without this facility, various constructs used in concurrent programming would be difficult and inefficient to implement, at least prior to Java 5 (see below).
Put simply, this is how signalling between threads works using 'wait and notify':
- We can call the wait() method of any Java object, which suspends the current thread. The thread is said to be "waiting on" the given object.
- Another thread calls the notify() method of the same Java object. This "wakes up" one of the threads waiting on that object.
In some of the following discussion, we'll use the term wait/notify mechanism, and refer to the two methods wait() and notify(). However, we'll see that much of the discussion includes notifyAll() and we'll look at when to use the two variants below.
How to use wait/notifty?
The following issues then generally arise:
- When do you use wait/notify?: we'll look first at a simple thread pool, then later at thread coordination;
- How do you use wait/notify?;
- when to use notifyAll (which 'wakes up' multiple waiting threads as opposed to just one)?
- using timed waits for cases where we don't want the waiting thread to be allowed to wait forever.
See also: commun bugs with wait/notify.
The wait()/notify() construct in Java 5
In Java 5, it's less common to need to call wait() and notify() explicitly. This is because various library classes implement the types of object that typically use wait()/notify() mechanism. For more information, see:
- the section on how to implement a producer-consumer model in Java 5, using the new blocking queue implementation;
- the explicit Java 5 lock classes, which provide more control than the standard synchronized keyword, such as lock acquisition with a time limit; prior to Java 5, this functionality would require wait()/notify();
- how to control a resource pool with a Semaphore.
Further recommended reading
This site actually covers much of the information you are likely to need about wait()/notify() and related synchronization and concurrency issues in Java. However, it is also recommended that you check out the following:
- Goetz, B. et al, Java Concurrency in Practice
- A clearly written, authoritative guide to the modern concurrency features and libraries in Java. This book is likely to become something of a bible for anybody serious about issues of synchronization and concurrent programming in Java. In particular, this book gives good (and comprehensible!) coverage of the Java memory model, and much better coverage of the new Java 5 and 6 concurrency features than many other publications.
- Bloch, J. Effective Java (2nd Edition)
- A valuable resource for programmers of all levels. See in particular the chapter on Concurrency and Item #69: "Prefer concurrency utilities to wait and notify".
- Oaks, S Java Threads
- Another good resource on multithreading in Java. See Chapter 4 on Thread Notification, plus other sections such as Chapter 11 on Task Scheduling for coverage of certain Java 5 concurrency features.
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