java.util.concurrent.locks.AbstractQueuedSynchronizer
下面的英文内容摘录于jdk1.8的源码文档
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. Subclasses must define the protected methods that change this state, and which define what that state means in terms of this object being acquired or released. Given these, the other methods in this class carry out all queuing and blocking mechanics. Subclasses can maintain other state fields, but only the atomically updated int value manipulated using methods getState, setState and compareAndSetState is tracked with respect to synchronization.
Subclasses should be defined as non-public internal helper classes that are used to implement the synchronization properties of their enclosing class. Class AbstractQueuedSynchronizer does not implement an ysynchronization interface. Instead it defines methods such as acquireInterruptibly 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. 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 defines a nested ConditionObject class that can be used as a Condition implementation by subclasses supporting exclusive mode for which method isHeldExclusively reports whether synchronization is exclusively held with respect to the current thread, method release invoked with the current getState value fully releases this object, and acquire, given this saved state value,eventually restores this object to its previous acquired state. No AbstractQueuedSynchronizer method otherwise creates such a condition, so if this constraint cannot be met, do not use it. The behavior of ConditionObject depends of course on these mantics of its synchronizer implementation.
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 AbstractQueuedSynchronizer for their synchronization mechanics.
Serialization of this class stores only the underlying atomicinteger maintaining state, so deserialized objects have empty thread queues. Typical subclasses requiring serializability will define a readObject method that restores this to a known initial state upon deserialization.
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 using getState, setState and/or compareAndSetState:
• tryAcquire
• tryRelease
• tryAcquireShared
• tryReleaseShared
• isHeldExclusively
Each of these methods by default throws UnsupportedOperationException. 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 other methods are declared final because they cannot be independently varied.
You may also find the inherited methods from AbstractOwnableSynchronizer useful to keep track of the thread owning an exclusive synchronizer. You are encouraged to use them– this enables monitoring and diagnostic tools to assist users indetermining which threads hold locks.
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 (!tryAcquire(arg)) {
enqueue thread if it is not already queued;
possibly block current thread;
}
Release:
if (tryRelease(arg))
unblock the first queued thread;
(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, thereby providing a fair FIFO acquisition order.In particular, most fair synchronizers can define tryAcquire to return false if hasQueuedPredecessors (a method specifically designed to be used by fair synchronizers) returns true. Other 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 hasContendedand/or hasQueuedThreads to only do so if the synchronizeris 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 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 usesthe 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 extends AbstractQueuedSynchronizer {
// Reports whether in locked state
protected boolean isHeldExclusively() {
return getState() == 1;
}
// Acquires the lock if state is zero
public boolean tryAcquire(int acquires) {
assert acquires == 1; // Otherwise unused
if (compareAndSetState(0, 1)) {
setExclusiveOwnerThread(Thread.currentThread());
return true;
}
return false;
}
// Releases the lock by setting state to zero
protected boolean tryRelease(int releases) {
assert releases == 1; // Otherwise unused
if (getState() == 0) throw new IllegalMonitorStateException();
setExclusiveOwnerThread(null);
setState(0);
return true;
}
// Provides a Condition
Condition newCondition() { return new ConditionObject(); }
// Deserializes 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();
public void lock() { sync.acquire(1); }
public boolean tryLock() { return sync.tryAcquire(1); }
public void unlock() { sync.release(1); }
public Condition newCondition() { return sync.newCondition(); }
public boolean isLocked() { return sync.isHeldExclusively(); }
public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); }
public void lockInterruptibly() throws InterruptedException {
sync.acquireInterruptibly(1);
}
public boolean tryLock(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireNanos(1, 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 {
private static class Sync extends AbstractQueuedSynchronizer {
boolean isSignalled() { return getState() != 0; }
protected int tryAcquireShared(int ignore) {
return isSignalled() ? 1 : -1;
}
protected boolean tryReleaseShared(int ignore) {
setState(1);
return true;
}
}
private final Sync sync = new Sync();
public boolean isSignalled() { return sync.isSignalled(); }
public void signal() { sync.releaseShared(1); }
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
}}java.util.concurrent.locks.AbstractQueuedSynchronizer.Node
Wait queue node class.
The wait queue is a variant of a “CLH” (Craig, Landin, and Hagersten) lock queue. CLH locks are normally used forspinlocks. We instead use them for blocking synchronizers, but use the same basic tactic of holding some of the control information about a thread in the predecessor of its node. A “status” field in each node keeps track of whether a thread should block. A node is signalled when its predecessor releases. Each node of the queue otherwise serves as aspecific-notification-style monitor holding a single waiting thread. The status field does NOT control whether threads aregranted locks etc though. A thread may try to acquire if it isfirst in the queue. But being first does not guarantee success;it only gives the right to contend. So the currently released contender thread may need to rewait.
To enqueue into a CLH lock, you atomically splice it in as new tail. To dequeue, you just set the head field.
+——+ prev +—–+ +—–+
head | | <—- | | <—- | | tail
+——+ +—–+ +—–+
Insertion into a CLH queue requires only a single atomic operation on “tail”, so there is a simple atomic point of demarcation from unqueued to queued. Similarly, dequeuing involves only updating the “head”. However, it takes a bit more work for nodes to determine who their successors are,in part to deal with possible cancellation due to timeouts and interrupts.
The “prev” links (not used in original CLH locks), are mainly needed to handle cancellation. If a node is cancelled, its successor is (normally) relinked to a non-cancelledpredecessor. For explanation of similar mechanics in the caseof spin locks, see the papers by Scott and Scherer at http://www.cs.rochester.edu/u/scott/synchronization/
We also use “next” links to implement blocking mechanics.The thread id for each node is kept in its own node, so a predecessor signals the next node to wake up by traversing next link to determine which thread it is. Determination of successor must avoid races with newly queued nodes to setthe “next” fields of their predecessors. This is solved when necessary by checking backwards from the atomically updated “tail” when a node’s successor appears to be null.(Or, said differently, the next-links are an optimization so that we don’t usually need a backward scan.)
Cancellation introduces some conservatism to the basic algorithms. Since we must poll for cancellation of other nodes, we can miss noticing whether a cancelled node is ahead or behind us. This is dealt with by always unparking successors upon cancellation, allowing them to stabilize on a new predecessor, unless we can identify an uncancelled predecessor who will carry this responsibility.
CLH queues need a dummy header node to get started. Butwe don’t create them on construction, because it would be wasted effort if there is never contention. Instead, the nodeis constructed and head and tail pointers are set upon first contention.
Threads waiting on Conditions use the same nodes, but use an additional link. Conditions only need to link nodes in simple (non-concurrent) linked queues because they are only accessed when exclusively held. Upon await, a node is inserted into a condition queue. Upon signal, the node is transferred to the main queue. A special value of status field is used to mark which queue a node is on.
Thanks go to Dave Dice, Mark Moir, Victor Luchangco, BillScherer and Michael Scott, along with members of JSR-166expert group, for helpful ideas, discussions, and critiqueson the design of this class.
