CSC/ECE 506 Spring 2012/9a ms

Introduction

Synchronization in Java

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>.

Problems with Monitors

Thin Lock Mechanism

Biased Lock Mechanism

Conclusion