Background

Chapter 17 - The BooleanLock Utility Java Thread Programming Paul Hyde

size=1>  size=2>Copyright � 1999 Sams Publishing

size=4>Background

size=2>Using the wait/notify mechanism effectively and correctly requires discipline. It is easy to erroneously code in such a way that a size=2>wait() is invoked just size=2>after a size=2>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.

size=2>In this chapter, I present the face="Courier New" size=2>BooleanLock face=Arial size=2> class. It simply encapsulates a color=#010100 face="Courier New" size=2>boolean color=#010100 face=Arial size=2> variable and controls access to it through size=2>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 color=#010100 face=Arial size=2>Waiting for the Full Timeout technique from color=#008000 face=Arial size=2>Chapter 14, “Waiting for the Full Timeout.”

face="Courier New" size=2>BooleanLock face=Arial size=2> can be used to conveniently signal between two threads. The first thread can block waiting until the value is set to size=2>true. When the second thread is ready, it can set the value to color=#010100 face="Courier New" size=2>true color=#010100 face=Arial size=2> and the first thread will be released from its blocked state.

face="Courier New" size=2>BooleanLock face=Arial size=2> can also be used to work around the problem of long size=2>synchronized blocks. Normally, when a thread is blocked waiting to acquire exclusive access to a lock (through face="Courier New" size=2>synchronized face=Arial size=2>), it will size=2>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 face="Courier New" size=2>synchronized face=Arial size=2> block. The face="Courier New" size=2>BooleanLock face=Arial size=2> utility can be used to provide an interruptible, timeout-capable, technique for providing exclusive access to a block of code.

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