org.metasyntactic.thread.concurrent
Class Mutex

java.lang.Object
  |
  +--org.metasyntactic.thread.concurrent.Mutex
All Implemented Interfaces:
Monitor

public class Mutex
extends java.lang.Object
implements Monitor

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

Field Summary
 
Fields inherited from interface org.metasyntactic.thread.concurrent.Monitor
ONE_CENTURY, ONE_DAY, ONE_HOUR, ONE_MINUTE, ONE_SECOND, ONE_WEEK, ONE_YEAR
 
Constructor Summary
Mutex()
           
 
Method Summary
 void acquire()
          Wait (possibly forever) until successful passage.
 boolean attempt(long msecs)
          Wait at most msecs to pass; report whether passed.
 void release()
          Potentially enable others to pass.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

Mutex

public Mutex()
Method Detail

acquire

public void acquire()
             throws java.lang.InterruptedException
Description copied from interface: Monitor
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.

Specified by:
acquire in interface Monitor
java.lang.InterruptedException

release

public void release()
Description copied from interface: Monitor
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.

Specified by:
release in interface Monitor

attempt

public boolean attempt(long msecs)
                throws java.lang.InterruptedException
Description copied from interface: Monitor
Wait at most msecs to pass; report whether passed.

The method has best-effort semantics: The msecs bound cannot be guaranteed to be a precise upper bound on wait time in Java. Implementations generally can only attempt to return as soon as possible after the specified bound. Also, timers in Java do not stop during garbage collection, so timeouts can occur just because a GC intervened. So, msecs arguments should be used in a coarse-grained manner. Further, implementations cannot always guarantee that this method will return at all without blocking indefinitely when used in unintended ways. For example, deadlocks may be encountered when called in an unintended context.

Specified by:
attempt in interface Monitor
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
java.lang.InterruptedException