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Java Source Code / Java Documentation » Database DBMS » Quadcap Embeddable Database » javax.concurrent 
Source Cross Reference  Class Diagram Java Document (Java Doc) 


java.lang.Object
   javax.concurrent.Mutex

Mutex
public class Mutex implements Sync(Code)
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 ...
 }
 


See Also:   Semaphore
See Also:   

[ Introduction to this package. ]



Field Summary
protected  booleaninuse_
    


Method Summary
public  voidacquire()
    
public  booleanattempt(long msecs)
    
public synchronized  voidrelease()
    

Field Detail
inuse_
protected boolean inuse_(Code)
The lock status *





Method Detail
acquire
public void acquire() throws InterruptedException(Code)



attempt
public boolean attempt(long msecs) throws InterruptedException(Code)



release
public synchronized void release()(Code)



Methods inherited from java.lang.Object
native protected Object clone() throws CloneNotSupportedException(Code)(Java Doc)
public boolean equals(Object obj)(Code)(Java Doc)
protected void finalize() throws Throwable(Code)(Java Doc)
final native public Class getClass()(Code)(Java Doc)
native public int hashCode()(Code)(Java Doc)
final native public void notify()(Code)(Java Doc)
final native public void notifyAll()(Code)(Java Doc)
public String toString()(Code)(Java Doc)
final native public void wait(long timeout) throws InterruptedException(Code)(Java Doc)
final public void wait(long timeout, int nanos) throws InterruptedException(Code)(Java Doc)
final public void wait() throws InterruptedException(Code)(Java Doc)

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