Summary
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| Copyright 1999 Sams Publishing |
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| Background |
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| Using the wait/notify mechanism effectively and correctly requires discipline. It is easy to erroneously code in such a way that a wait() is invoked just after a notify() has been issued, resulting in the notification being completely missed. This kind of mistake results in a very subtle race condition that only shows up occasionally and can be painstakingly difficult to track down. Wherever possible, the wait/notify mechanism should be encapsulated within a class to insulate external classes from the signaling complexity. |
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| In this chapter, I present the BooleanLock class. It simply encapsulates a boolean variable and controls access to it through synchronized methods . Multiple threads can safely interact with it. The wait/notify mechanism is hidden within the class and missed notifications are impossible . It also uses the Waiting for the Full Timeout technique from Chapter 14, Waiting for the Full Timeout. |
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| BooleanLock can be used to conveniently signal between two threads. The first thread can block waiting until the value is set to true . When the second thread is ready, it can set the value to true and the first thread will be released from its blocked state. |
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| BooleanLock can also be used to work around the problem of long synchronized blocks. Normally, when a thread is blocked waiting to acquire exclusive access to a lock (through synchronized ), it will not respond to interrupts. In addition, there is no mechanism for specifying a timeout value that indicates how long a thread should wait to try to get into a synchronized block. The BooleanLock utility can be used to provide an interruptible, timeout-capable, technique for providing exclusive access to a block of code. |
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