Java Cookbook

Thread Communication: wait( ) and notifyAll( )

Problem

The synchronized keyword lets you lock out multiple threads, but doesn’t give you much communication between them.

Solution

Use wait( ) and notifyAll( ). Very carefully.

Discussion

Three methods appear in java.lang.Object that allow you to use any object as a synchronization target: wait( ), notify( ), and notifyAll( ).

wait( )

Causes the current thread to block in the given object until awakened by a notify( ) or notifyAll( ).

notify( )

Causes a randomly selected thread waiting on this object to be awakened. It must then try to regain the monitor lock. If the “wrong” thread is awakened, your program can deadlock.

notifyAll( )

Causes all threads waiting on the object to be awakened; each will then try to regain the monitor lock. Hopefully one will succeed.

The mechanism is a bit odd: there is no way to awaken only the thread that owns the lock. However, that’s how it works, and it’s the reason almost all programs use notifyAll( ) instead of notify( ). Also note that both wait( ) and the notification methods can be used only if you are already synchronized on the object; that is, you must be in a synchronized method within, or a code block synchronized on, the object that you wish your current thread to wait( ) or notify( ) upon.

For a simple introduction to wait( ) and notify( ), I’ll use a simple Producer-Consumer model. This pattern can be used to simulate a variety of real-world situations in which one object is creating ...