| A simple 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 ...
}
This version adds some debugging capability: it will detect
an attempt by a thread that does not hold the mutex to release it.
This version is reentrant: the same thread may acquire a mutex multiple
times, in which case it must release the mutex the same number of times
as it was acquired before another thread can acquire the mutex. | 