CSC/ECE 506 Spring 2012/9a ms

Synchronization in Java

The support for multi-threading at language level is the strength of Java programming language. Hence most of Java programming language is centered around coordinating the sharing of data among the multiple threads.

Memory Model for Data

The JVM organizes the data of a running Java application into several runtime data areas: one or more Java stacks, a heap, and a method area.

Each thread has it own Java stack. The stack contains data that cannot be accessed by other threads (including the local variables, parameters, and return values of each method the thread has invoked). The data on the stack is limited to primitive types and object references. The JVM has only one heap which is shared by all threads. The heap contains objects. The Method Area is another place where data can reside. It contains all the class (or static) variables used by the program. The method area is similar to the stack in that it contains only primitive types and object references. Unlike the stack, however, the class variables in the method area are shared by all threads.

Sharing and Locks

The sharing of data in a multiprocessor differ from that of the uniprocessor. In a uni-processor system, multiple threads do not execute concurrently but they time share the processor for execution. Whereas on multiprocessor, multiple threads execute concurrently on different processors. Thus they have a tight contention for locks and strong sharing rules on multi processor system.

As mentioned above, the heap and the method area contain all the data that is shared by multiple threads. To coordinate shared data access among multiple threads, the Java virtual machine associates a lock with each object and class. A lock is like a privilege that only one thread can "possess" at any one time. If a thread wants to lock a particular object or class, it asks the JVM. At some point after the thread asks the JVM for a lock -- maybe very soon, maybe later, possibly never -- the JVM gives the lock to the thread. When the thread no longer needs the lock, it returns it to the JVM. If another thread has requested the same lock, the JVM passes the lock to that thread. Class locks are actually implemented as object locks. When the JVM 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. Threads need not obtain a lock to access instance or class variables. If a thread does obtain a lock, however, no other thread can access the locked data until the thread that owns the lock releases it.

Monitors

The JVM uses locks in conjunction with monitors. A monitor is basically a guardian in that it watches over a sequence of code, making sure only one thread at a time executes the code. Each monitor is associated with an object reference. When a thread arrives at the first instruction in a block of code that is under the watchful eye of a monitor, 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.

Synchronization

A single thread is allowed to lock the same object multiple times. For each object, the JVM 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 incremented to one. Each time the thread acquires a lock on the same object, a count is incremented. 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.

For a java developer, the keyword synchronized is provided to enforce critical execution on a statement or a method. On entering a synchronized block, a lock is acquired. The block is not executed till a lock is acquired. The opcodes monitorenter and monitorexit are used while entering and exiting the synchronized block. When the JVM encounters monitorenter, it acquires the lock for the object referred. If the thread already owns the lock for the object, the lock count is incremented. Similarly, when monitorexit is executed by the JVM, the count is decremented. The monitor lock is released when the count reaches zero.

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Sun's Java virtual machine specification states that synchronization is based on monitors. This point is reinforced at the Java VM level by the presence of monitorenter and monitorexit instructions.

First suggested by E. W. Dijkstra in 1971, conceptualized by P. Brinch Hansen in 1972-1973, and refined by C. A. R. Hoare in 1974, a monitor is a concurrency construct that encapsulates data and functionality for allocating and releasing shared resources (such as network connections, memory buffers, printers, and so on). To accomplish resource allocation or release, a thread calls a monitor entry (a special function or procedure that serves as an entry point into a monitor). If there is no other thread executing code within the monitor, the calling thread is allowed to enter the monitor and execute the monitor entry's code. But if a thread is already inside of the monitor, the monitor makes the calling thread wait outside of the monitor until the other thread leaves the monitor. The monitor then allows the waiting thread to enter. Because synchronization is guaranteed, problems such as data being lost or scrambled are avoided. To learn more about monitors, study Hoare's landmark paper, <ref http://john.cs.olemiss.edu/~dwilkins/Seminar/S05/Monitors.pdf> ""Monitors: An Operating System Structuring Concept," </ref> first published by the Communications of the Association for Computing Machinery Inc. in 1974.

The Java virtual machine specification goes on to state that monitor behavior can be explained in terms of locks. Think of a lock as a token that a thread must acquire before a monitor allows that thread to execute inside of a monitor entry. That token is automatically released when the thread exits the monitor, to give another thread an opportunity to get the token and enter the monitor.

Java associates locks with objects: each object is assigned its own lock, and each lock is assigned to one object. A thread acquires an object's lock prior to entering the lock-controlled monitor entry, which Java represents at the source code level as either a <ref http://docs.oracle.com/javase/tutorial/essential/concurrency/syncmeth.html> synchronized method </ref> or a <ref http://www.javamex.com/tutorials/synchronization_concurrency_synchronized1.shtml> synchronized statement </ref>.

References

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