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Chapter 20 of Inside the Java Virtual Machine
Thread Synchronization
by Bill Venners

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Object Locking

As mentioned in earlier chapters, some of the Java virtual machine's runtime data areas are shared by all threads, others are private to individual threads. Because the heap and method area are shared by all threads, Java programs need to coordinate multi-threaded access to two kinds of data: o instance variables, which are stored on the heap o class variables, which are stored in the method area Programs never need to coordinate access to local variables, which reside on Java stacks, because data on the Java stack is private to the thread to which the Java stack belongs.

In the Java virtual machine, every object and class is logically associated with a monitor. For objects, the associated monitor protects the object's instance variables. For classes, the monitor protects the class's class variables. If an object has no instance variables, or a class has no class variables, the associated monitor protects no data.

To implement the mutual exclusion capability of monitors, the Java virtual machine associates a lock (sometimes called a mutex) with each object and class. A lock is like a privilege that only one thread can "own" at any one time. Threads need not obtain a lock to access instance or class variables. If a thread does obtain a lock, however, no other thread can obtain a lock on the same data until the thread that owns the lock releases it. (To "lock an object" is to acquire the monitor associated with that object.)

Class locks are actually implemented as object locks. As mentioned in earlier chapters, when the Java virtual machine loads a class file, it creates an instance of class java.lang.Class. When you lock a class, you are actually locking that class's Class object.

A single thread is allowed to lock the same object multiple times. For each object, the Java virtual machine maintains a count of the number of times the object has been locked. An unlocked object has a count of zero. When a thread acquires the lock for the first time, the count is again incremented to one. Each time the thread acquires a lock on the same object, the count is again incremented. (Only the thread that already owns an object's lock is allowed to lock it again. As mentioned previously, no other thread can lock the object until the owning thread releases the lock.) Each time the thread releases the lock, the count is decremented. When the count reaches zero, the lock is released and made available to other threads.

A thread in the Java virtual machine requests a lock when it arrives at the beginning of a monitor region. In Java, there are two kinds of monitor regions: synchronized statements and synchronized methods. (These are described in detail later in this chapter.) Each monitor region in a Java program is associated with an object reference. When a thread arrives at the first instruction in a monitor region, the thread must obtain a lock on the referenced object. The thread is not allowed to execute the code until it obtains the lock. Once it has obtained the lock, the thread enters the block of protected code. When the thread leaves the block, no matter how it leaves the block, it releases the lock on the associated object.

Note that as a Java programmer, you never explicitly lock an object. Object locks are internal to the Java virtual machine. In your Java programs, you identify the monitor regions of your program by writing synchronized statements and methods. As the Java virtual machine runs your program, it automatically locks an object or class every time it encounters a monitor region.

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