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. ]
protected boolean inUse
The lock status *
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
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
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