net.sf.ehcache.constructs.concurrent

Class Mutex

public class Mutex extends Object implements Sync

Version: $Id: Mutex.java 59 2006-04-30 03:41:39Z gregluck $

Author: Doug Lea A simple non-reentrant mutual exclusion lock. The lock is free upon construction. Each acquire gets the lock, and each release frees it. Releasing a lock that is already free has no effect.

This implementation makes no attempt to provide any fairness or ordering guarantees. If you need them, consider using one of the Semaphore implementations as a locking mechanism.

Sample usage

Mutex can be useful in constructions that cannot be expressed using java synchronized blocks because the acquire/release pairs do not occur in the same method or code block. For example, you can use them for hand-over-hand locking across the nodes of a linked list. This allows extremely fine-grained locking, and so increases potential concurrency, at the cost of additional complexity and overhead that would normally make this worthwhile only in cases of extreme contention.

 class Node {
   Object item;
   Node next;
   Mutex lock = new Mutex(); // each node keeps its own lock
 

Node(Object x, Node n) { item = x; next = n; } }

class List { protected Node head; // pointer to first node of list

// Use plain java synchronization to protect head field. // (We could instead use a Mutex here too but there is no // reason to do so.) protected synchronized Node getHead() { return head; }

boolean search(Object x) throws InterruptedException { Node p = getHead(); if (p == null) return false;

// (This could be made more compact, but for clarity of illustration, // all of the cases that can arise are handled separately.)

p.lock.acquire(); // Prime loop by acquiring first lock. // (If the acquire fails due to // interrupt, the method will throw // InterruptedException now, // so there is no need for any // further cleanup.) for (;;) { if (x.equals(p.item)) { p.lock.release(); // release current before return return true; } else { Node nextp = p.next; if (nextp == null) { p.lock.release(); // release final lock that was held return false; } else { try { nextp.lock.acquire(); // get next lock before releasing current } catch (InterruptedException ex) { p.lock.release(); // also release current if acquire fails throw ex; } p.lock.release(); // release old lock now that new one held p = nextp; } } } }

synchronized void add(Object x) { // simple prepend // The use of `synchronized' here protects only head field. // The method does not need to wait out other traversers // who have already made it past head.

head = new Node(x, head); }

// ... other similar traversal and update methods ... }

[ Introduction to this package. ]

Field Summary
protected booleaninUse
The lock status *
Method Summary
voidacquire()
Wait (possibly forever) until successful passage.
booleanattempt(long msecs)
voidrelease()
Potentially enable others to pass.

Field Detail

inUse

protected boolean inUse
The lock status *

Method Detail

acquire

public void acquire()
Wait (possibly forever) until successful passage. Fail only upon interuption. Interruptions always result in `clean' failures. On failure, you can be sure that it has not been acquired, and that no corresponding release should be performed. Conversely, a normal return guarantees that the acquire was successful.

See Also: acquire

attempt

public boolean attempt(long msecs)

Parameters: msecs the number of milleseconds to wait. An argument less than or equal to zero means not to wait at all. However, this may still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention among threads.

Returns: true if acquired

See Also: Sync

release

public void release()
Potentially enable others to pass.

Because release does not raise exceptions, it can be used in `finally' clauses without requiring extra embedded try/catch blocks. But keep in mind that as with any java method, implementations may still throw unchecked exceptions such as Error or NullPointerException when faced with uncontinuable errors. However, these should normally only be caught by higher-level error handlers.

See Also: release