public class Mutex extends Object implements Sync
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 ...
}
| Modifier and Type | Field and Description |
|---|---|
protected boolean |
inuse_
The lock status
|
ONE_CENTURY, ONE_DAY, ONE_HOUR, ONE_MINUTE, ONE_SECOND, ONE_WEEK, ONE_YEAR| Constructor and Description |
|---|
Mutex() |
public void acquire()
throws InterruptedException
Syncacquire in interface SyncInterruptedExceptionpublic void release()
SyncBecause 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.
public boolean attempt(long msecs)
throws InterruptedException
SyncThe 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.
attempt in interface Syncmsecs - 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.InterruptedExceptionCopyright © 2012 JBoss by Red Hat. All Rights Reserved.