Provides a framework for implementing blocking locks and related
synchronizers (semaphores, events, etc) that rely on first-in-first-out
(FIFO) wait queues. This class is designed to be a useful basis for most
kinds of synchronizers that rely on a single atomic int value to
represent state. Classes must implement the
TryLockObject interface
(or extend an implementation like
ExclusiveSharedSynchronizer to
define the methods that change this state, and which define what that state
means in terms of this object being acquired or released while containing an
object of this class to represent the thread waiter queue. Given these, the
other methods in this class carry out all queuing and blocking mechanics.
Lock classes can maintain other state fields, but only the atomically updated
int value manipulated in the
TryLockObject interface
implementations is tracked with respect to synchronization.
Classes should be defined as non-public internal helper classes that are used
to implement the synchronization properties of their enclosing class. Class
QueuedSynchronizer does not implement any synchronization interface.
Instead it defines methods such as
#tryAcquireNanos,
#tryAcquireSharedNanos that can be invoked as appropriate by concrete
locks and related synchronizers to implement their public methods.
This class supports either or both a default exclusive mode and a
shared mode or more modes as required depending on the optional
integer argument passed to the lock acquire/release methods. When acquired in
exclusive mode, attempted acquires by other threads cannot succeed. Shared
mode acquires by multiple threads may (but need not) succeed. This class does
not "understand" these differences except in the mechanical sense
that when a shared mode acquire succeeds, the next waiting thread (if one
exists) must also determine whether it can acquire as well. Threads waiting
in the different modes share the same FIFO queue. Usually, implementation
subclasses support only one of these modes, but both can come into play for
example in a ReadWriteLock. Subclasses that support only exclusive or only
shared modes need not define the methods supporting the unused mode.
This class provides inspection, instrumentation, and monitoring methods for
the internal queue, as well as similar methods for condition objects. These
can be exported as desired into classes using an QueuedSynchronizer
for their synchronization mechanics.
Usage
To use this class as the basis of a synchronizer, redefine the following
methods, as applicable, by inspecting and/or modifying the synchronization
state in the
TryLockObject interface implementation:
-
TryLockObject#tryAcquire
-
TryLockObject#tryRelease
-
TryLockObject#tryAcquireShared
-
TryLockObject#tryReleaseShared
Implementations of these methods must be internally thread-safe, and should
in general be short and not block. Defining these methods is the
only supported means of using this class. All methods in this class
are declared final because they cannot be independently varied.
Even though this class is based on an internal FIFO queue, it does not
automatically enforce FIFO acquisition policies. The core of exclusive
synchronization takes the form:
Acquire:
while (!lock.tryAcquire(arg, owner)) {
<em>enqueue thread if it is not already queued</em>;
<em>possibly block current thread</em>;
}
Release:
if (lock.tryRelease(arg, owner))
<em>unblock the first queued thread</em>;
(Shared mode is similar but may involve cascading signals.)
Because checks in acquire are invoked before enqueuing, a newly acquiring
thread may barge ahead of others that are blocked and queued.
However, you can, if desired, define tryAcquire and/or
tryAcquireShared to disable barging by internally invoking one or
more of the inspection methods. In particular, a strict FIFO lock can define
tryAcquire to immediately return false if
first queued thread is not the current thread. A normally
preferable non-strict fair version can immediately return false only
if
#hasQueuedThreads returns null and
getFirstQueuedThread is not the current thread; or equivalently,
that getFirstQueuedThread is both non-null and not the current
thread. Further variations are possible.
Throughput and scalability are generally highest for the default barging
(also known as greedy, renouncement, and
convoy-avoidance) strategy. While this is not guaranteed to be fair
or starvation-free, earlier queued threads are allowed to recontend before
later queued threads, and each recontention has an unbiased chance to succeed
against incoming threads. Also, while acquires do not "spin" in the
usual sense, they may perform multiple invocations of tryAcquire
interspersed with other computations before blocking. This gives most of the
benefits of spins when exclusive synchronization is only briefly held,
without most of the liabilities when it isn't. If so desired, you can augment
this by preceding calls to acquire methods with "fast-path" checks, possibly
prechecking
#hasContended and/or
#hasQueuedThreads to only do
so if the synchronizer is likely not to be contended.
This class provides an efficient and scalable basis for synchronization in
part by specializing its range of use to synchronizers that can rely on
int state, acquire, and release parameters, and an internal FIFO
wait queue. When this does not suffice, you can build synchronizers from a
lower level using java.util.concurrent.atomic atomic classes, your own custom
java.util.Queue classes, and
LockSupport blocking support.
Usage Examples
Here is a non-reentrant mutual exclusion lock class that uses the value zero
to represent the unlocked state, and one to represent the locked state. While
a non-reentrant lock does not strictly require recording of the current owner
thread, this class does so anyway to make usage easier to monitor. It also
supports conditions and exposes one of the instrumentation methods:
class Mutex implements Lock, java.io.Serializable {
// Our internal helper class
private static class Sync implements TryLockObject {
// Report whether in locked state
public boolean hasExclusiveLock() {
return getState() == 1;
}
// Acquire the lock if state is zero
public boolean tryAcquire(int acquires, Object owner) {
assert acquires == 1; // Otherwise unused
if (compareAndSetState(0, 1)) {
setExclusiveOwner(Thread.currentThread());
return true;
}
return false;
}
// Release the lock by setting state to zero
protected boolean tryRelease(int releases, Object owner) {
assert releases == 1; // Otherwise unused
if (getState() == 0) {
throw new IllegalMonitorStateException();
}
setExclusiveOwner(null);
setState(0);
return true;
}
// Provide a Condition
Condition newCondition() {
return new ConditionObject();
}
// Deserialize properly
private void readObject(ObjectInputStream s) throws IOException,
ClassNotFoundException {
s.defaultReadObject();
setState(0); // reset to unlocked state
}
}
// The sync object does all the hard work. We just forward to it.
private final Sync sync = new Sync();
private final QueuedSynchronizer waiterQueue = new QueuedSynchronizer();
public void lock() {
waiterQueue.acquire(1, Thread.currentThread(), this.sync);
}
public boolean tryLock() {
return sync.tryAcquire(1);
}
public void unlock() {
waiterQueue.release(1, Thread.currentThread(), this.sync);
}
public Condition newCondition() {
return sync.newCondition();
}
public boolean isLocked() {
return sync.hasExclusiveLock(Thread.currentThread());
}
public boolean hasQueuedThreads() {
return waiterQueue.hasQueuedThreads();
}
public void lockInterruptibly() throws InterruptedException {
attemptSharedLock(1, Thread.currentThread(), this.sync);
}
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return waiterQueue.tryAcquireNanos(1, Thread.currentThread(), this.sync,
unit.toNanos(timeout));
}
}
Here is a latch class that is like a CountDownLatch
except that
it only requires a single signal to fire. Because a latch is
non-exclusive, it uses the shared acquire and release methods.
class BooleanLatch implements TryLockObject {
private final QueuedSynchronizer sync = new QueuedSynchronizer();
public boolean isSignalled() {
return getState() != 0;
}
public int tryAcquireShared(int ignore, Object owner) {
return isSignalled() ? 1 : -1;
}
public boolean tryReleaseShared(int ignore, Object owner) {
setState(1);
return true;
}
public void signal() {
this.sync.releaseShared(1, null, this);
}
public void await() throws InterruptedException {
this.sync.acquireSharedInterruptibly(1, null, this);
}
}