| /* |
| * Written by Doug Lea with assistance from members of JCP JSR-166 |
| * Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/licenses/publicdomain |
| */ |
| |
| package java.util.concurrent.locks; |
| import java.util.*; |
| import java.util.concurrent.*; |
| import java.util.concurrent.atomic.*; |
| import sun.misc.Unsafe; |
| |
| /** |
| * 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 <tt>int</tt> 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 <tt>int</tt> |
| * value manipulated using methods {@link #getState}, {@link |
| * #setState} and {@link #compareAndSetState} is tracked with respect |
| * to synchronization. |
| * |
| * <p>Subclasses should be defined as non-public internal helper |
| * classes that are used to implement the synchronization properties |
| * of their enclosing class. Class |
| * <tt>AbstractQueuedSynchronizer</tt> does not implement any |
| * synchronization interface. Instead it defines methods such as |
| * {@link #acquireInterruptibly} that can be invoked as |
| * appropriate by concrete locks and related synchronizers to |
| * implement their public methods. |
| * |
| * <p>This class supports either or both a default <em>exclusive</em> |
| * mode and a <em>shared</em> 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 |
| * {@link ReadWriteLock}. Subclasses that support only exclusive or |
| * only shared modes need not define the methods supporting the unused mode. |
| * |
| * <p>This class defines a nested {@link ConditionObject} class that |
| * can be used as a {@link Condition} implementation by subclasses |
| * supporting exclusive mode for which method {@link |
| * #isHeldExclusively} reports whether synchronization is exclusively |
| * held with respect to the current thread, method {@link #release} |
| * invoked with the current {@link #getState} value fully releases |
| * this object, and {@link #acquire}, given this saved state value, |
| * eventually restores this object to its previous acquired state. No |
| * <tt>AbstractQueuedSynchronizer</tt> method otherwise creates such a |
| * condition, so if this constraint cannot be met, do not use it. The |
| * behavior of {@link ConditionObject} depends of course on the |
| * semantics of its synchronizer implementation. |
| * |
| * <p> 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 <tt>AbstractQueuedSynchronizer</tt> for their |
| * synchronization mechanics. |
| * |
| * <p> Serialization of this class stores only the underlying atomic |
| * integer maintaining state, so deserialized objects have empty |
| * thread queues. Typical subclasses requiring serializability will |
| * define a <tt>readObject</tt> method that restores this to a known |
| * initial state upon deserialization. |
| * |
| * <h3>Usage</h3> |
| * |
| * <p> 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 {@link #getState}, {@link |
| * #setState} and/or {@link #compareAndSetState}: |
| * |
| * <ul> |
| * <li> {@link #tryAcquire} |
| * <li> {@link #tryRelease} |
| * <li> {@link #tryAcquireShared} |
| * <li> {@link #tryReleaseShared} |
| * <li> {@link #isHeldExclusively} |
| *</ul> |
| * |
| * Each of these methods by default throws {@link |
| * UnsupportedOperationException}. Implementations of these methods |
| * must be internally thread-safe, and should in general be short and |
| * not block. Defining these methods is the <em>only</em> supported |
| * means of using this class. All other methods are declared |
| * <tt>final</tt> because they cannot be independently varied. |
| * |
| * <p> 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: |
| * |
| * <pre> |
| * Acquire: |
| * while (!tryAcquire(arg)) { |
| * <em>enqueue thread if it is not already queued</em>; |
| * <em>possibly block current thread</em>; |
| * } |
| * |
| * Release: |
| * if (tryRelease(arg)) |
| * <em>unblock the first queued thread</em>; |
| * </pre> |
| * |
| * (Shared mode is similar but may involve cascading signals.) |
| * |
| * <p> Because checks in acquire are invoked before enqueuing, a newly |
| * acquiring thread may <em>barge</em> ahead of others that are |
| * blocked and queued. However, you can, if desired, define |
| * <tt>tryAcquire</tt> and/or <tt>tryAcquireShared</tt> to disable |
| * barging by internally invoking one or more of the inspection |
| * methods. In particular, a strict FIFO lock can define |
| * <tt>tryAcquire</tt> to immediately return <tt>false</tt> if {@link |
| * #getFirstQueuedThread} does not return the current thread. A |
| * normally preferable non-strict fair version can immediately return |
| * <tt>false</tt> only if {@link #hasQueuedThreads} returns |
| * <tt>true</tt> and <tt>getFirstQueuedThread</tt> is not the current |
| * thread; or equivalently, that <tt>getFirstQueuedThread</tt> is both |
| * non-null and not the current thread. Further variations are |
| * possible. |
| * |
| * <p> Throughput and scalability are generally highest for the |
| * default barging (also known as <em>greedy</em>, |
| * <em>renouncement</em>, and <em>convoy-avoidance</em>) 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 <tt>tryAcquire</tt> 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 {@link #hasContended} |
| * and/or {@link #hasQueuedThreads} to only do so if the synchronizer |
| * is likely not to be contended. |
| * |
| * <p> This class provides an efficient and scalable basis for |
| * synchronization in part by specializing its range of use to |
| * synchronizers that can rely on <tt>int</tt> 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 |
| * {@link java.util.concurrent.atomic atomic} classes, your own custom |
| * {@link java.util.Queue} classes, and {@link LockSupport} blocking |
| * support. |
| * |
| * <h3>Usage Examples</h3> |
| * |
| * <p>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. It also supports conditions and exposes |
| * one of the instrumentation methods: |
| * |
| * <pre> |
| * class Mutex implements Lock, java.io.Serializable { |
| * |
| * // Our internal helper class |
| * private static class Sync extends AbstractQueuedSynchronizer { |
| * // Report whether in locked state |
| * protected boolean isHeldExclusively() { |
| * return getState() == 1; |
| * } |
| * |
| * // Acquire the lock if state is zero |
| * public boolean tryAcquire(int acquires) { |
| * assert acquires == 1; // Otherwise unused |
| * return compareAndSetState(0, 1); |
| * } |
| * |
| * // Release the lock by setting state to zero |
| * protected boolean tryRelease(int releases) { |
| * assert releases == 1; // Otherwise unused |
| * if (getState() == 0) throw new IllegalMonitorStateException(); |
| * 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(); |
| * |
| * 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)); |
| * } |
| * } |
| * </pre> |
| * |
| * <p> Here is a latch class that is like a {@link CountDownLatch} |
| * except that it only requires a single <tt>signal</tt> to |
| * fire. Because a latch is non-exclusive, it uses the <tt>shared</tt> |
| * acquire and release methods. |
| * |
| * <pre> |
| * 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); |
| * } |
| * } |
| * |
| * </pre> |
| * |
| * @since 1.5 |
| * @author Doug Lea |
| */ |
| public abstract class AbstractQueuedSynchronizer implements java.io.Serializable { |
| private static final long serialVersionUID = 7373984972572414691L; |
| |
| /** |
| * Creates a new <tt>AbstractQueuedSynchronizer</tt> instance |
| * with initial synchronization state of zero. |
| */ |
| protected AbstractQueuedSynchronizer() { } |
| |
| /** |
| * Wait queue node class. |
| * |
| * <p> The wait queue is a variant of a "CLH" (Craig, Landin, and |
| * Hagersten) lock queue. CLH locks are normally used for |
| * spinlocks. 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 a |
| * specific-notification-style monitor holding a single waiting |
| * thread. The status field does NOT control whether threads are |
| * granted locks etc though. A thread may try to acquire if it is |
| * first 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. |
| * |
| * <p>To enqueue into a CLH lock, you atomically splice it in as new |
| * tail. To dequeue, you just set the head field. |
| * <pre> |
| * +------+ prev +-----+ +-----+ |
| * head | | <---- | | <---- | | tail |
| * +------+ +-----+ +-----+ |
| * </pre> |
| * |
| * <p>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, dequeing |
| * 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. |
| * |
| * <p>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-cancelled |
| * predecessor. For explanation of similar mechanics in the case |
| * of spin locks, see the papers by Scott and Scherer at |
| * http://www.cs.rochester.edu/u/scott/synchronization/ |
| * |
| * <p>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 set |
| * the "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.) |
| * |
| * <p>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. |
| * |
| * <p>CLH queues need a dummy header node to get started. But |
| * we don't create them on construction, because it would be wasted |
| * effort if there is never contention. Instead, the node |
| * is constructed and head and tail pointers are set upon first |
| * contention. |
| * |
| * <p>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. |
| * |
| * <p>Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill |
| * Scherer and Michael Scott, along with members of JSR-166 |
| * expert group, for helpful ideas, discussions, and critiques |
| * on the design of this class. |
| */ |
| static final class Node { |
| /** waitStatus value to indicate thread has cancelled */ |
| static final int CANCELLED = 1; |
| /** waitStatus value to indicate thread needs unparking */ |
| static final int SIGNAL = -1; |
| /** waitStatus value to indicate thread is waiting on condition */ |
| static final int CONDITION = -2; |
| /** Marker to indicate a node is waiting in shared mode */ |
| static final Node SHARED = new Node(); |
| /** Marker to indicate a node is waiting in exclusive mode */ |
| static final Node EXCLUSIVE = null; |
| |
| /** |
| * Status field, taking on only the values: |
| * SIGNAL: The successor of this node is (or will soon be) |
| * blocked (via park), so the current node must |
| * unpark its successor when it releases or |
| * cancels. To avoid races, acquire methods must |
| * first indicate they need a signal, |
| * then retry the atomic acquire, and then, |
| * on failure, block. |
| * CANCELLED: Node is cancelled due to timeout or interrupt |
| * Nodes never leave this state. In particular, |
| * a thread with cancelled node never again blocks. |
| * CONDITION: Node is currently on a condition queue |
| * It will not be used as a sync queue node until |
| * transferred. (Use of this value here |
| * has nothing to do with the other uses |
| * of the field, but simplifies mechanics.) |
| * 0: None of the above |
| * |
| * The values are arranged numerically to simplify use. |
| * Non-negative values mean that a node doesn't need to |
| * signal. So, most code doesn't need to check for particular |
| * values, just for sign. |
| * |
| * The field is initialized to 0 for normal sync nodes, and |
| * CONDITION for condition nodes. It is modified only using |
| * CAS. |
| */ |
| volatile int waitStatus; |
| |
| /** |
| * Link to predecessor node that current node/thread relies on |
| * for checking waitStatus. Assigned during enqueing, and nulled |
| * out (for sake of GC) only upon dequeuing. Also, upon |
| * cancellation of a predecessor, we short-circuit while |
| * finding a non-cancelled one, which will always exist |
| * because the head node is never cancelled: A node becomes |
| * head only as a result of successful acquire. A |
| * cancelled thread never succeeds in acquiring, and a thread only |
| * cancels itself, not any other node. |
| */ |
| volatile Node prev; |
| |
| /** |
| * Link to the successor node that the current node/thread |
| * unparks upon release. Assigned once during enqueuing, and |
| * nulled out (for sake of GC) when no longer needed. Upon |
| * cancellation, we cannot adjust this field, but can notice |
| * status and bypass the node if cancelled. The enq operation |
| * does not assign next field of a predecessor until after |
| * attachment, so seeing a null next field does not |
| * necessarily mean that node is at end of queue. However, if |
| * a next field appears to be null, we can scan prev's from |
| * the tail to double-check. |
| */ |
| volatile Node next; |
| |
| /** |
| * The thread that enqueued this node. Initialized on |
| * construction and nulled out after use. |
| */ |
| volatile Thread thread; |
| |
| /** |
| * Link to next node waiting on condition, or the special |
| * value SHARED. Because condition queues are accessed only |
| * when holding in exclusive mode, we just need a simple |
| * linked queue to hold nodes while they are waiting on |
| * conditions. They are then transferred to the queue to |
| * re-acquire. And because conditions can only be exclusive, |
| * we save a field by using special value to indicate shared |
| * mode. |
| */ |
| Node nextWaiter; |
| |
| /** |
| * Returns true if node is waiting in shared mode |
| */ |
| final boolean isShared() { |
| return nextWaiter == SHARED; |
| } |
| |
| /** |
| * Returns previous node, or throws NullPointerException if |
| * null. Use when predecessor cannot be null. |
| * @return the predecessor of this node |
| */ |
| final Node predecessor() throws NullPointerException { |
| Node p = prev; |
| if (p == null) |
| throw new NullPointerException(); |
| else |
| return p; |
| } |
| |
| Node() { // Used to establish initial head or SHARED marker |
| } |
| |
| Node(Thread thread, Node mode) { // Used by addWaiter |
| this.nextWaiter = mode; |
| this.thread = thread; |
| } |
| |
| Node(Thread thread, int waitStatus) { // Used by Condition |
| this.waitStatus = waitStatus; |
| this.thread = thread; |
| } |
| } |
| |
| /** |
| * Head of the wait queue, lazily initialized. Except for |
| * initialization, it is modified only via method setHead. Note: |
| * If head exists, its waitStatus is guaranteed not to be |
| * CANCELLED. |
| */ |
| private transient volatile Node head; |
| |
| /** |
| * Tail of the wait queue, lazily initialized. Modified only via |
| * method enq to add new wait node. |
| */ |
| private transient volatile Node tail; |
| |
| /** |
| * The synchronization state. |
| */ |
| private volatile int state; |
| |
| /** |
| * Returns the current value of synchronization state. |
| * This operation has memory semantics of a <tt>volatile</tt> read. |
| * @return current state value |
| */ |
| protected final int getState() { |
| return state; |
| } |
| |
| /** |
| * Sets the value of synchronization state. |
| * This operation has memory semantics of a <tt>volatile</tt> write. |
| * @param newState the new state value |
| */ |
| protected final void setState(int newState) { |
| state = newState; |
| } |
| |
| /** |
| * Atomically sets synchronization state to the given updated |
| * value if the current state value equals the expected value. |
| * This operation has memory semantics of a <tt>volatile</tt> read |
| * and write. |
| * @param expect the expected value |
| * @param update the new value |
| * @return true if successful. False return indicates that |
| * the actual value was not equal to the expected value. |
| */ |
| protected final boolean compareAndSetState(int expect, int update) { |
| // See below for intrinsics setup to support this |
| return unsafe.compareAndSwapInt(this, stateOffset, expect, update); |
| } |
| |
| // Queuing utilities |
| |
| /** |
| * Insert node into queue, initializing if necessary. See picture above. |
| * @param node the node to insert |
| * @return node's predecessor |
| */ |
| private Node enq(final Node node) { |
| for (;;) { |
| Node t = tail; |
| if (t == null) { // Must initialize |
| Node h = new Node(); // Dummy header |
| h.next = node; |
| node.prev = h; |
| if (compareAndSetHead(h)) { |
| tail = node; |
| return h; |
| } |
| } |
| else { |
| node.prev = t; |
| if (compareAndSetTail(t, node)) { |
| t.next = node; |
| return t; |
| } |
| } |
| } |
| } |
| |
| /** |
| * Create and enq node for given thread and mode |
| * @param current the thread |
| * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared |
| * @return the new node |
| */ |
| private Node addWaiter(Node mode) { |
| Node node = new Node(Thread.currentThread(), mode); |
| // Try the fast path of enq; backup to full enq on failure |
| Node pred = tail; |
| if (pred != null) { |
| node.prev = pred; |
| if (compareAndSetTail(pred, node)) { |
| pred.next = node; |
| return node; |
| } |
| } |
| enq(node); |
| return node; |
| } |
| |
| /** |
| * Set head of queue to be node, thus dequeuing. Called only by |
| * acquire methods. Also nulls out unused fields for sake of GC |
| * and to suppress unnecessary signals and traversals. |
| * @param node the node |
| */ |
| private void setHead(Node node) { |
| head = node; |
| node.thread = null; |
| node.prev = null; |
| } |
| |
| /** |
| * Wake up node's successor, if one exists. |
| * @param node the node |
| */ |
| private void unparkSuccessor(Node node) { |
| /* |
| * Try to clear status in anticipation of signalling. It is |
| * OK if this fails or if status is changed by waiting thread. |
| */ |
| compareAndSetWaitStatus(node, Node.SIGNAL, 0); |
| |
| /* |
| * Thread to unpark is held in successor, which is normally |
| * just the next node. But if cancelled or apparently null, |
| * traverse backwards from tail to find the actual |
| * non-cancelled successor. |
| */ |
| Thread thread; |
| Node s = node.next; |
| if (s != null && s.waitStatus <= 0) |
| thread = s.thread; |
| else { |
| thread = null; |
| for (s = tail; s != null && s != node; s = s.prev) |
| if (s.waitStatus <= 0) |
| thread = s.thread; |
| } |
| LockSupport.unpark(thread); |
| } |
| |
| /** |
| * Set head of queue, and check if successor may be waiting |
| * in shared mode, if so propagating if propagate > 0. |
| * @param pred the node holding waitStatus for node |
| * @param node the node |
| * @param propagate the return value from a tryAcquireShared |
| */ |
| private void setHeadAndPropagate(Node node, int propagate) { |
| setHead(node); |
| if (propagate > 0 && node.waitStatus != 0) { |
| /* |
| * Don't bother fully figuring out successor. If it |
| * looks null, call unparkSuccessor anyway to be safe. |
| */ |
| Node s = node.next; |
| if (s == null || s.isShared()) |
| unparkSuccessor(node); |
| } |
| } |
| |
| // Utilities for various versions of acquire |
| |
| /** |
| * Cancel an ongoing attempt to acquire. |
| * @param node the node |
| */ |
| private void cancelAcquire(Node node) { |
| if (node != null) { // Ignore if node doesn't exist |
| node.thread = null; |
| // Can use unconditional write instead of CAS here |
| node.waitStatus = Node.CANCELLED; |
| unparkSuccessor(node); |
| } |
| } |
| |
| /** |
| * Checks and updates status for a node that failed to acquire. |
| * Returns true if thread should block. This is the main signal |
| * control in all acquire loops. Requires that pred == node.prev |
| * @param pred node's predecessor holding status |
| * @param node the node |
| * @return true if thread should block |
| */ |
| private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { |
| int s = pred.waitStatus; |
| if (s < 0) |
| /* |
| * This node has already set status asking a release |
| * to signal it, so it can safely park |
| */ |
| return true; |
| if (s > 0) |
| /* |
| * Predecessor was cancelled. Move up to its predecessor |
| * and indicate retry. |
| */ |
| node.prev = pred.prev; |
| else |
| /* |
| * Indicate that we need a signal, but don't park yet. Caller |
| * will need to retry to make sure it cannot acquire before |
| * parking. |
| */ |
| compareAndSetWaitStatus(pred, 0, Node.SIGNAL); |
| return false; |
| } |
| |
| /** |
| * Convenience method to interrupt current thread. |
| */ |
| private static void selfInterrupt() { |
| Thread.currentThread().interrupt(); |
| } |
| |
| /** |
| * Convenience method to park and then check if interrupted |
| * @return true if interrupted |
| */ |
| private static boolean parkAndCheckInterrupt() { |
| LockSupport.park(); |
| return Thread.interrupted(); |
| } |
| |
| /* |
| * Various flavors of acquire, varying in exclusive/shared and |
| * control modes. Each is mostly the same, but annoyingly |
| * different. Only a little bit of factoring is possible due to |
| * interactions of exception mechanics (including ensuring that we |
| * cancel if tryAcquire throws exception) and other control, at |
| * least not without hurting performance too much. |
| */ |
| |
| /** |
| * Acquire in exclusive uninterruptible mode for thread already in |
| * queue. Used by condition wait methods as well as acquire. |
| * @param node the node |
| * @param arg the acquire argument |
| * @return true if interrupted while waiting |
| */ |
| final boolean acquireQueued(final Node node, int arg) { |
| try { |
| boolean interrupted = false; |
| for (;;) { |
| final Node p = node.predecessor(); |
| if (p == head && tryAcquire(arg)) { |
| setHead(node); |
| p.next = null; // help GC |
| return interrupted; |
| } |
| if (shouldParkAfterFailedAcquire(p, node) && |
| parkAndCheckInterrupt()) |
| interrupted = true; |
| } |
| } catch (RuntimeException ex) { |
| cancelAcquire(node); |
| throw ex; |
| } |
| } |
| |
| /** |
| * Acquire in exclusive interruptible mode |
| * @param arg the acquire argument |
| */ |
| private void doAcquireInterruptibly(int arg) |
| throws InterruptedException { |
| final Node node = addWaiter(Node.EXCLUSIVE); |
| try { |
| for (;;) { |
| final Node p = node.predecessor(); |
| if (p == head && tryAcquire(arg)) { |
| setHead(node); |
| p.next = null; // help GC |
| return; |
| } |
| if (shouldParkAfterFailedAcquire(p, node) && |
| parkAndCheckInterrupt()) |
| break; |
| } |
| } catch (RuntimeException ex) { |
| cancelAcquire(node); |
| throw ex; |
| } |
| // Arrive here only if interrupted |
| cancelAcquire(node); |
| throw new InterruptedException(); |
| } |
| |
| /** |
| * Acquire in exclusive timed mode |
| * @param arg the acquire argument |
| * @param nanosTimeout max wait time |
| * @return true if acquired |
| */ |
| private boolean doAcquireNanos(int arg, long nanosTimeout) |
| throws InterruptedException { |
| long lastTime = System.nanoTime(); |
| final Node node = addWaiter(Node.EXCLUSIVE); |
| try { |
| for (;;) { |
| final Node p = node.predecessor(); |
| if (p == head && tryAcquire(arg)) { |
| setHead(node); |
| p.next = null; // help GC |
| return true; |
| } |
| if (nanosTimeout <= 0) { |
| cancelAcquire(node); |
| return false; |
| } |
| if (shouldParkAfterFailedAcquire(p, node)) { |
| LockSupport.parkNanos(nanosTimeout); |
| if (Thread.interrupted()) |
| break; |
| long now = System.nanoTime(); |
| nanosTimeout -= now - lastTime; |
| lastTime = now; |
| } |
| } |
| } catch (RuntimeException ex) { |
| cancelAcquire(node); |
| throw ex; |
| } |
| // Arrive here only if interrupted |
| cancelAcquire(node); |
| throw new InterruptedException(); |
| } |
| |
| /** |
| * Acquire in shared uninterruptible mode |
| * @param arg the acquire argument |
| */ |
| private void doAcquireShared(int arg) { |
| final Node node = addWaiter(Node.SHARED); |
| try { |
| boolean interrupted = false; |
| for (;;) { |
| final Node p = node.predecessor(); |
| if (p == head) { |
| int r = tryAcquireShared(arg); |
| if (r >= 0) { |
| setHeadAndPropagate(node, r); |
| p.next = null; // help GC |
| if (interrupted) |
| selfInterrupt(); |
| return; |
| } |
| } |
| if (shouldParkAfterFailedAcquire(p, node) && |
| parkAndCheckInterrupt()) |
| interrupted = true; |
| } |
| } catch (RuntimeException ex) { |
| cancelAcquire(node); |
| throw ex; |
| } |
| } |
| |
| /** |
| * Acquire in shared interruptible mode |
| * @param arg the acquire argument |
| */ |
| private void doAcquireSharedInterruptibly(int arg) |
| throws InterruptedException { |
| final Node node = addWaiter(Node.SHARED); |
| try { |
| for (;;) { |
| final Node p = node.predecessor(); |
| if (p == head) { |
| int r = tryAcquireShared(arg); |
| if (r >= 0) { |
| setHeadAndPropagate(node, r); |
| p.next = null; // help GC |
| return; |
| } |
| } |
| if (shouldParkAfterFailedAcquire(p, node) && |
| parkAndCheckInterrupt()) |
| break; |
| } |
| } catch (RuntimeException ex) { |
| cancelAcquire(node); |
| throw ex; |
| } |
| // Arrive here only if interrupted |
| cancelAcquire(node); |
| throw new InterruptedException(); |
| } |
| |
| /** |
| * Acquire in shared timed mode |
| * @param arg the acquire argument |
| * @param nanosTimeout max wait time |
| * @return true if acquired |
| */ |
| private boolean doAcquireSharedNanos(int arg, long nanosTimeout) |
| throws InterruptedException { |
| |
| long lastTime = System.nanoTime(); |
| final Node node = addWaiter(Node.SHARED); |
| try { |
| for (;;) { |
| final Node p = node.predecessor(); |
| if (p == head) { |
| int r = tryAcquireShared(arg); |
| if (r >= 0) { |
| setHeadAndPropagate(node, r); |
| p.next = null; // help GC |
| return true; |
| } |
| } |
| if (nanosTimeout <= 0) { |
| cancelAcquire(node); |
| return false; |
| } |
| if (shouldParkAfterFailedAcquire(p, node)) { |
| LockSupport.parkNanos(nanosTimeout); |
| if (Thread.interrupted()) |
| break; |
| long now = System.nanoTime(); |
| nanosTimeout -= now - lastTime; |
| lastTime = now; |
| } |
| } |
| } catch (RuntimeException ex) { |
| cancelAcquire(node); |
| throw ex; |
| } |
| // Arrive here only if interrupted |
| cancelAcquire(node); |
| throw new InterruptedException(); |
| } |
| |
| // Main exported methods |
| |
| /** |
| * Attempts to acquire in exclusive mode. This method should query |
| * if the state of the object permits it to be acquired in the |
| * exclusive mode, and if so to acquire it. |
| * |
| * <p>This method is always invoked by the thread performing |
| * acquire. If this method reports failure, the acquire method |
| * may queue the thread, if it is not already queued, until it is |
| * signalled by a release from some other thread. This can be used |
| * to implement method {@link Lock#tryLock()}. |
| * |
| * <p>The default |
| * implementation throws {@link UnsupportedOperationException} |
| * |
| * @param arg the acquire argument. This value |
| * is always the one passed to an acquire method, |
| * or is the value saved on entry to a condition wait. |
| * The value is otherwise uninterpreted and can represent anything |
| * you like. |
| * @return true if successful. Upon success, this object has been |
| * acquired. |
| * @throws IllegalMonitorStateException if acquiring would place |
| * this synchronizer in an illegal state. This exception must be |
| * thrown in a consistent fashion for synchronization to work |
| * correctly. |
| * @throws UnsupportedOperationException if exclusive mode is not supported |
| */ |
| protected boolean tryAcquire(int arg) { |
| throw new UnsupportedOperationException(); |
| } |
| |
| /** |
| * Attempts to set the state to reflect a release in exclusive |
| * mode. <p>This method is always invoked by the thread |
| * performing release. |
| * |
| * <p>The default implementation throws |
| * {@link UnsupportedOperationException} |
| * @param arg the release argument. This value |
| * is always the one passed to a release method, |
| * or the current state value upon entry to a condition wait. |
| * The value is otherwise uninterpreted and can represent anything |
| * you like. |
| * @return <tt>true</tt> if this object is now in a fully released state, |
| * so that any waiting threads may attempt to acquire; and <tt>false</tt> |
| * otherwise. |
| * @throws IllegalMonitorStateException if releasing would place |
| * this synchronizer in an illegal state. This exception must be |
| * thrown in a consistent fashion for synchronization to work |
| * correctly. |
| * @throws UnsupportedOperationException if exclusive mode is not supported |
| */ |
| protected boolean tryRelease(int arg) { |
| throw new UnsupportedOperationException(); |
| } |
| |
| /** |
| * Attempts to acquire in shared mode. This method should query if |
| * the state of the object permits it to be acquired in the shared |
| * mode, and if so to acquire it. |
| * |
| * <p>This method is always invoked by the thread performing |
| * acquire. If this method reports failure, the acquire method |
| * may queue the thread, if it is not already queued, until it is |
| * signalled by a release from some other thread. |
| * |
| * <p>The default implementation throws {@link |
| * UnsupportedOperationException} |
| * |
| * @param arg the acquire argument. This value |
| * is always the one passed to an acquire method, |
| * or is the value saved on entry to a condition wait. |
| * The value is otherwise uninterpreted and can represent anything |
| * you like. |
| * @return a negative value on failure, zero on exclusive success, |
| * and a positive value if non-exclusively successful, in which |
| * case a subsequent waiting thread must check |
| * availability. (Support for three different return values |
| * enables this method to be used in contexts where acquires only |
| * sometimes act exclusively.) Upon success, this object has been |
| * acquired. |
| * @throws IllegalMonitorStateException if acquiring would place |
| * this synchronizer in an illegal state. This exception must be |
| * thrown in a consistent fashion for synchronization to work |
| * correctly. |
| * @throws UnsupportedOperationException if shared mode is not supported |
| */ |
| protected int tryAcquireShared(int arg) { |
| throw new UnsupportedOperationException(); |
| } |
| |
| /** |
| * Attempts to set the state to reflect a release in shared mode. |
| * <p>This method is always invoked by the thread performing release. |
| * <p> The default implementation throws |
| * {@link UnsupportedOperationException} |
| * @param arg the release argument. This value |
| * is always the one passed to a release method, |
| * or the current state value upon entry to a condition wait. |
| * The value is otherwise uninterpreted and can represent anything |
| * you like. |
| * @return <tt>true</tt> if this object is now in a fully released state, |
| * so that any waiting threads may attempt to acquire; and <tt>false</tt> |
| * otherwise. |
| * @throws IllegalMonitorStateException if releasing would place |
| * this synchronizer in an illegal state. This exception must be |
| * thrown in a consistent fashion for synchronization to work |
| * correctly. |
| * @throws UnsupportedOperationException if shared mode is not supported |
| */ |
| protected boolean tryReleaseShared(int arg) { |
| throw new UnsupportedOperationException(); |
| } |
| |
| /** |
| * Returns true if synchronization is held exclusively with respect |
| * to the current (calling) thread. This method is invoked |
| * upon each call to a non-waiting {@link ConditionObject} method. |
| * (Waiting methods instead invoke {@link #release}.) |
| * <p>The default implementation throws {@link |
| * UnsupportedOperationException}. This method is invoked |
| * internally only within {@link ConditionObject} methods, so need |
| * not be defined if conditions are not used. |
| * |
| * @return true if synchronization is held exclusively; |
| * else false |
| * @throws UnsupportedOperationException if conditions are not supported |
| */ |
| protected boolean isHeldExclusively() { |
| throw new UnsupportedOperationException(); |
| } |
| |
| /** |
| * Acquires in exclusive mode, ignoring interrupts. Implemented |
| * by invoking at least once {@link #tryAcquire}, |
| * returning on success. Otherwise the thread is queued, possibly |
| * repeatedly blocking and unblocking, invoking {@link |
| * #tryAcquire} until success. This method can be used |
| * to implement method {@link Lock#lock} |
| * @param arg the acquire argument. |
| * This value is conveyed to {@link #tryAcquire} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| */ |
| public final void acquire(int arg) { |
| if (!tryAcquire(arg) && |
| acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) |
| selfInterrupt(); |
| } |
| |
| /** |
| * Acquires in exclusive mode, aborting if interrupted. |
| * Implemented by first checking interrupt status, then invoking |
| * at least once {@link #tryAcquire}, returning on |
| * success. Otherwise the thread is queued, possibly repeatedly |
| * blocking and unblocking, invoking {@link #tryAcquire} |
| * until success or the thread is interrupted. This method can be |
| * used to implement method {@link Lock#lockInterruptibly} |
| * @param arg the acquire argument. |
| * This value is conveyed to {@link #tryAcquire} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| * @throws InterruptedException if the current thread is interrupted |
| */ |
| public final void acquireInterruptibly(int arg) throws InterruptedException { |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| if (!tryAcquire(arg)) |
| doAcquireInterruptibly(arg); |
| } |
| |
| /** |
| * Attempts to acquire in exclusive mode, aborting if interrupted, |
| * and failing if the given timeout elapses. Implemented by first |
| * checking interrupt status, then invoking at least once {@link |
| * #tryAcquire}, returning on success. Otherwise, the thread is |
| * queued, possibly repeatedly blocking and unblocking, invoking |
| * {@link #tryAcquire} until success or the thread is interrupted |
| * or the timeout elapses. This method can be used to implement |
| * method {@link Lock#tryLock(long, TimeUnit)}. |
| * @param arg the acquire argument. |
| * This value is conveyed to {@link #tryAcquire} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| * @param nanosTimeout the maximum number of nanoseconds to wait |
| * @return true if acquired; false if timed out |
| * @throws InterruptedException if the current thread is interrupted |
| */ |
| public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| return tryAcquire(arg) || |
| doAcquireNanos(arg, nanosTimeout); |
| } |
| |
| /** |
| * Releases in exclusive mode. Implemented by unblocking one or |
| * more threads if {@link #tryRelease} returns true. |
| * This method can be used to implement method {@link Lock#unlock} |
| * @param arg the release argument. |
| * This value is conveyed to {@link #tryRelease} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| * @return the value returned from {@link #tryRelease} |
| */ |
| public final boolean release(int arg) { |
| if (tryRelease(arg)) { |
| Node h = head; |
| if (h != null && h.waitStatus != 0) |
| unparkSuccessor(h); |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Acquires in shared mode, ignoring interrupts. Implemented by |
| * first invoking at least once {@link #tryAcquireShared}, |
| * returning on success. Otherwise the thread is queued, possibly |
| * repeatedly blocking and unblocking, invoking {@link |
| * #tryAcquireShared} until success. |
| * @param arg the acquire argument. |
| * This value is conveyed to {@link #tryAcquireShared} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| */ |
| public final void acquireShared(int arg) { |
| if (tryAcquireShared(arg) < 0) |
| doAcquireShared(arg); |
| } |
| |
| /** |
| * Acquires in shared mode, aborting if interrupted. Implemented |
| * by first checking interrupt status, then invoking at least once |
| * {@link #tryAcquireShared}, returning on success. Otherwise the |
| * thread is queued, possibly repeatedly blocking and unblocking, |
| * invoking {@link #tryAcquireShared} until success or the thread |
| * is interrupted. |
| * @param arg the acquire argument. |
| * This value is conveyed to {@link #tryAcquireShared} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| * @throws InterruptedException if the current thread is interrupted |
| */ |
| public final void acquireSharedInterruptibly(int arg) throws InterruptedException { |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| if (tryAcquireShared(arg) < 0) |
| doAcquireSharedInterruptibly(arg); |
| } |
| |
| /** |
| * Attempts to acquire in shared mode, aborting if interrupted, and |
| * failing if the given timeout elapses. Implemented by first |
| * checking interrupt status, then invoking at least once {@link |
| * #tryAcquireShared}, returning on success. Otherwise, the |
| * thread is queued, possibly repeatedly blocking and unblocking, |
| * invoking {@link #tryAcquireShared} until success or the thread |
| * is interrupted or the timeout elapses. |
| * @param arg the acquire argument. |
| * This value is conveyed to {@link #tryAcquireShared} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| * @param nanosTimeout the maximum number of nanoseconds to wait |
| * @return true if acquired; false if timed out |
| * @throws InterruptedException if the current thread is interrupted |
| */ |
| public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| return tryAcquireShared(arg) >= 0 || |
| doAcquireSharedNanos(arg, nanosTimeout); |
| } |
| |
| /** |
| * Releases in shared mode. Implemented by unblocking one or more |
| * threads if {@link #tryReleaseShared} returns true. |
| * @param arg the release argument. |
| * This value is conveyed to {@link #tryReleaseShared} but is |
| * otherwise uninterpreted and can represent anything |
| * you like. |
| * @return the value returned from {@link #tryReleaseShared} |
| */ |
| public final boolean releaseShared(int arg) { |
| if (tryReleaseShared(arg)) { |
| Node h = head; |
| if (h != null && h.waitStatus != 0) |
| unparkSuccessor(h); |
| return true; |
| } |
| return false; |
| } |
| |
| // Queue inspection methods |
| |
| /** |
| * Queries whether any threads are waiting to acquire. Note that |
| * because cancellations due to interrupts and timeouts may occur |
| * at any time, a <tt>true</tt> return does not guarantee that any |
| * other thread will ever acquire. |
| * |
| * <p> In this implementation, this operation returns in |
| * constant time. |
| * |
| * @return true if there may be other threads waiting to acquire |
| * the lock. |
| */ |
| public final boolean hasQueuedThreads() { |
| return head != tail; |
| } |
| |
| /** |
| * Queries whether any threads have ever contended to acquire this |
| * synchronizer; that is if an acquire method has ever blocked. |
| * |
| * <p> In this implementation, this operation returns in |
| * constant time. |
| * |
| * @return true if there has ever been contention |
| */ |
| public final boolean hasContended() { |
| return head != null; |
| } |
| |
| /** |
| * Returns the first (longest-waiting) thread in the queue, or |
| * <tt>null</tt> if no threads are currently queued. |
| * |
| * <p> In this implementation, this operation normally returns in |
| * constant time, but may iterate upon contention if other threads are |
| * concurrently modifying the queue. |
| * |
| * @return the first (longest-waiting) thread in the queue, or |
| * <tt>null</tt> if no threads are currently queued. |
| */ |
| public final Thread getFirstQueuedThread() { |
| // handle only fast path, else relay |
| return (head == tail)? null : fullGetFirstQueuedThread(); |
| } |
| |
| /** |
| * Version of getFirstQueuedThread called when fastpath fails |
| */ |
| private Thread fullGetFirstQueuedThread() { |
| /* |
| * This loops only if the queue changes while we read sets of |
| * fields. |
| */ |
| for (;;) { |
| Node h = head; |
| if (h == null) // No queue |
| return null; |
| |
| /* |
| * The first node is normally h.next. Try to get its |
| * thread field, ensuring consistent reads: If thread |
| * field is nulled out or s.prev is no longer head, then |
| * some other thread(s) concurrently performed setHead in |
| * between some of our reads, so we must reread. |
| */ |
| Node s = h.next; |
| if (s != null) { |
| Thread st = s.thread; |
| Node sp = s.prev; |
| if (st != null && sp == head) |
| return st; |
| } |
| |
| /* |
| * Head's next field might not have been set yet, or may |
| * have been unset after setHead. So we must check to see |
| * if tail is actually first node, in almost the same way |
| * as above. |
| */ |
| Node t = tail; |
| if (t == h) // Empty queue |
| return null; |
| |
| if (t != null) { |
| Thread tt = t.thread; |
| Node tp = t.prev; |
| if (tt != null && tp == head) |
| return tt; |
| } |
| } |
| } |
| |
| /** |
| * Returns true if the given thread is currently queued. |
| * |
| * <p> This implementation traverses the queue to determine |
| * presence of the given thread. |
| * |
| * @param thread the thread |
| * @return true if the given thread in on the queue |
| * @throws NullPointerException if thread null |
| */ |
| public final boolean isQueued(Thread thread) { |
| if (thread == null) |
| throw new NullPointerException(); |
| for (Node p = tail; p != null; p = p.prev) |
| if (p.thread == thread) |
| return true; |
| return false; |
| } |
| |
| // Instrumentation and monitoring methods |
| |
| /** |
| * Returns an estimate of the number of threads waiting to |
| * acquire. The value is only an estimate because the number of |
| * threads may change dynamically while this method traverses |
| * internal data structures. This method is designed for use in |
| * monitoring system state, not for synchronization |
| * control. |
| * |
| * @return the estimated number of threads waiting for this lock |
| */ |
| public final int getQueueLength() { |
| int n = 0; |
| for (Node p = tail; p != null; p = p.prev) { |
| if (p.thread != null) |
| ++n; |
| } |
| return n; |
| } |
| |
| /** |
| * Returns a collection containing threads that may be waiting to |
| * acquire. Because the actual set of threads may change |
| * dynamically while constructing this result, the returned |
| * collection is only a best-effort estimate. The elements of the |
| * returned collection are in no particular order. This method is |
| * designed to facilitate construction of subclasses that provide |
| * more extensive monitoring facilities. |
| * @return the collection of threads |
| */ |
| public final Collection<Thread> getQueuedThreads() { |
| ArrayList<Thread> list = new ArrayList<Thread>(); |
| for (Node p = tail; p != null; p = p.prev) { |
| Thread t = p.thread; |
| if (t != null) |
| list.add(t); |
| } |
| return list; |
| } |
| |
| /** |
| * Returns a collection containing threads that may be waiting to |
| * acquire in exclusive mode. This has the same properties |
| * as {@link #getQueuedThreads} except that it only returns |
| * those threads waiting due to an exclusive acquire. |
| * @return the collection of threads |
| */ |
| public final Collection<Thread> getExclusiveQueuedThreads() { |
| ArrayList<Thread> list = new ArrayList<Thread>(); |
| for (Node p = tail; p != null; p = p.prev) { |
| if (!p.isShared()) { |
| Thread t = p.thread; |
| if (t != null) |
| list.add(t); |
| } |
| } |
| return list; |
| } |
| |
| /** |
| * Returns a collection containing threads that may be waiting to |
| * acquire in shared mode. This has the same properties |
| * as {@link #getQueuedThreads} except that it only returns |
| * those threads waiting due to a shared acquire. |
| * @return the collection of threads |
| */ |
| public final Collection<Thread> getSharedQueuedThreads() { |
| ArrayList<Thread> list = new ArrayList<Thread>(); |
| for (Node p = tail; p != null; p = p.prev) { |
| if (p.isShared()) { |
| Thread t = p.thread; |
| if (t != null) |
| list.add(t); |
| } |
| } |
| return list; |
| } |
| |
| /** |
| * Returns a string identifying this synchronizer, as well as its |
| * state. The state, in brackets, includes the String "State |
| * =" followed by the current value of {@link #getState}, and |
| * either "nonempty" or "empty" depending on |
| * whether the queue is empty. |
| * |
| * @return a string identifying this synchronizer, as well as its state. |
| */ |
| public String toString() { |
| int s = getState(); |
| String q = hasQueuedThreads()? "non" : ""; |
| return super.toString() + |
| "[State = " + s + ", " + q + "empty queue]"; |
| } |
| |
| |
| // Internal support methods for Conditions |
| |
| /** |
| * Returns true if a node, always one that was initially placed on |
| * a condition queue, is now waiting to reacquire on sync queue. |
| * @param node the node |
| * @return true if is reacquiring |
| */ |
| final boolean isOnSyncQueue(Node node) { |
| if (node.waitStatus == Node.CONDITION || node.prev == null) |
| return false; |
| if (node.next != null) // If has successor, it must be on queue |
| return true; |
| /* |
| * node.prev can be non-null, but not yet on queue because |
| * the CAS to place it on queue can fail. So we have to |
| * traverse from tail to make sure it actually made it. It |
| * will always be near the tail in calls to this method, and |
| * unless the CAS failed (which is unlikely), it will be |
| * there, so we hardly ever traverse much. |
| */ |
| return findNodeFromTail(node); |
| } |
| |
| /** |
| * Returns true if node is on sync queue by searching backwards from tail. |
| * Called only when needed by isOnSyncQueue. |
| * @return true if present |
| */ |
| private boolean findNodeFromTail(Node node) { |
| Node t = tail; |
| for (;;) { |
| if (t == node) |
| return true; |
| if (t == null) |
| return false; |
| t = t.prev; |
| } |
| } |
| |
| /** |
| * Transfers a node from a condition queue onto sync queue. |
| * Returns true if successful. |
| * @param node the node |
| * @return true if successfully transferred (else the node was |
| * cancelled before signal). |
| */ |
| final boolean transferForSignal(Node node) { |
| /* |
| * If cannot change waitStatus, the node has been cancelled. |
| */ |
| if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) |
| return false; |
| |
| /* |
| * Splice onto queue and try to set waitStatus of predecessor to |
| * indicate that thread is (probably) waiting. If cancelled or |
| * attempt to set waitStatus fails, wake up to resync (in which |
| * case the waitStatus can be transiently and harmlessly wrong). |
| */ |
| Node p = enq(node); |
| int c = p.waitStatus; |
| if (c > 0 || !compareAndSetWaitStatus(p, c, Node.SIGNAL)) |
| LockSupport.unpark(node.thread); |
| return true; |
| } |
| |
| /** |
| * Transfers node, if necessary, to sync queue after a cancelled |
| * wait. Returns true if thread was cancelled before being |
| * signalled. |
| * @param current the waiting thread |
| * @param node its node |
| * @return true if cancelled before the node was signalled. |
| */ |
| final boolean transferAfterCancelledWait(Node node) { |
| if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { |
| enq(node); |
| return true; |
| } |
| /* |
| * If we lost out to a signal(), then we can't proceed |
| * until it finishes its enq(). Cancelling during an |
| * incomplete transfer is both rare and transient, so just |
| * spin. |
| */ |
| while (!isOnSyncQueue(node)) |
| Thread.yield(); |
| return false; |
| } |
| |
| /** |
| * Invoke release with current state value; return saved state. |
| * Cancel node and throw exception on failure. |
| * @param node the condition node for this wait |
| * @return previous sync state |
| */ |
| final int fullyRelease(Node node) { |
| try { |
| int savedState = getState(); |
| if (release(savedState)) |
| return savedState; |
| } catch(RuntimeException ex) { |
| node.waitStatus = Node.CANCELLED; |
| throw ex; |
| } |
| // reach here if release fails |
| node.waitStatus = Node.CANCELLED; |
| throw new IllegalMonitorStateException(); |
| } |
| |
| // Instrumentation methods for conditions |
| |
| /** |
| * Queries whether the given ConditionObject |
| * uses this synchronizer as its lock. |
| * @param condition the condition |
| * @return <tt>true</tt> if owned |
| * @throws NullPointerException if condition null |
| */ |
| public final boolean owns(ConditionObject condition) { |
| if (condition == null) |
| throw new NullPointerException(); |
| return condition.isOwnedBy(this); |
| } |
| |
| /** |
| * Queries whether any threads are waiting on the given condition |
| * associated with this synchronizer. Note that because timeouts |
| * and interrupts may occur at any time, a <tt>true</tt> return |
| * does not guarantee that a future <tt>signal</tt> will awaken |
| * any threads. This method is designed primarily for use in |
| * monitoring of the system state. |
| * @param condition the condition |
| * @return <tt>true</tt> if there are any waiting threads. |
| * @throws IllegalMonitorStateException if exclusive synchronization |
| * is not held |
| * @throws IllegalArgumentException if the given condition is |
| * not associated with this synchronizer |
| * @throws NullPointerException if condition null |
| */ |
| public final boolean hasWaiters(ConditionObject condition) { |
| if (!owns(condition)) |
| throw new IllegalArgumentException("Not owner"); |
| return condition.hasWaiters(); |
| } |
| |
| /** |
| * Returns an estimate of the number of threads waiting on the |
| * given condition associated with this synchronizer. Note that |
| * because timeouts and interrupts may occur at any time, the |
| * estimate serves only as an upper bound on the actual number of |
| * waiters. This method is designed for use in monitoring of the |
| * system state, not for synchronization control. |
| * @param condition the condition |
| * @return the estimated number of waiting threads. |
| * @throws IllegalMonitorStateException if exclusive synchronization |
| * is not held |
| * @throws IllegalArgumentException if the given condition is |
| * not associated with this synchronizer |
| * @throws NullPointerException if condition null |
| */ |
| public final int getWaitQueueLength(ConditionObject condition) { |
| if (!owns(condition)) |
| throw new IllegalArgumentException("Not owner"); |
| return condition.getWaitQueueLength(); |
| } |
| |
| /** |
| * Returns a collection containing those threads that may be |
| * waiting on the given condition associated with this |
| * synchronizer. Because the actual set of threads may change |
| * dynamically while constructing this result, the returned |
| * collection is only a best-effort estimate. The elements of the |
| * returned collection are in no particular order. |
| * @param condition the condition |
| * @return the collection of threads |
| * @throws IllegalMonitorStateException if exclusive synchronization |
| * is not held |
| * @throws IllegalArgumentException if the given condition is |
| * not associated with this synchronizer |
| * @throws NullPointerException if condition null |
| */ |
| public final Collection<Thread> getWaitingThreads(ConditionObject condition) { |
| if (!owns(condition)) |
| throw new IllegalArgumentException("Not owner"); |
| return condition.getWaitingThreads(); |
| } |
| |
| /** |
| * Condition implementation for a {@link |
| * AbstractQueuedSynchronizer} serving as the basis of a {@link |
| * Lock} implementation. |
| * |
| * <p> Method documentation for this class describes mechanics, |
| * not behavioral specifications from the point of view of Lock |
| * and Condition users. Exported versions of this class will in |
| * general need to be accompanied by documentation describing |
| * condition semantics that rely on those of the associated |
| * <tt>AbstractQueuedSynchronizer</tt>. |
| * |
| * <p> This class is Serializable, but all fields are transient, |
| * so deserialized conditions have no waiters. |
| */ |
| public class ConditionObject implements Condition, java.io.Serializable { |
| private static final long serialVersionUID = 1173984872572414699L; |
| /** First node of condition queue. */ |
| private transient Node firstWaiter; |
| /** Last node of condition queue. */ |
| private transient Node lastWaiter; |
| |
| /** |
| * Creates a new <tt>ConditionObject</tt> instance. |
| */ |
| public ConditionObject() { } |
| |
| // Internal methods |
| |
| /** |
| * Add a new waiter to wait queue |
| * @return its new wait node |
| */ |
| private Node addConditionWaiter() { |
| Node node = new Node(Thread.currentThread(), Node.CONDITION); |
| Node t = lastWaiter; |
| if (t == null) |
| firstWaiter = node; |
| else |
| t.nextWaiter = node; |
| lastWaiter = node; |
| return node; |
| } |
| |
| /** |
| * Remove and transfer nodes until hit non-cancelled one or |
| * null. Split out from signal in part to encourage compilers |
| * to inline the case of no waiters. |
| * @param first (non-null) the first node on condition queue |
| */ |
| private void doSignal(Node first) { |
| do { |
| if ( (firstWaiter = first.nextWaiter) == null) |
| lastWaiter = null; |
| first.nextWaiter = null; |
| } while (!transferForSignal(first) && |
| (first = firstWaiter) != null); |
| } |
| |
| /** |
| * Remove and transfer all nodes. |
| * @param first (non-null) the first node on condition queue |
| */ |
| private void doSignalAll(Node first) { |
| lastWaiter = firstWaiter = null; |
| do { |
| Node next = first.nextWaiter; |
| first.nextWaiter = null; |
| transferForSignal(first); |
| first = next; |
| } while (first != null); |
| } |
| |
| // public methods |
| |
| /** |
| * Moves the longest-waiting thread, if one exists, from the |
| * wait queue for this condition to the wait queue for the |
| * owning lock. |
| * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| * returns false |
| */ |
| public final void signal() { |
| if (!isHeldExclusively()) |
| throw new IllegalMonitorStateException(); |
| Node first = firstWaiter; |
| if (first != null) |
| doSignal(first); |
| } |
| |
| /** |
| * Moves all threads from the wait queue for this condition to |
| * the wait queue for the owning lock. |
| * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| * returns false |
| */ |
| public final void signalAll() { |
| if (!isHeldExclusively()) |
| throw new IllegalMonitorStateException(); |
| Node first = firstWaiter; |
| if (first != null) |
| doSignalAll(first); |
| } |
| |
| /** |
| * Implements uninterruptible condition wait. |
| * <ol> |
| * <li> Save lock state returned by {@link #getState} |
| * <li> Invoke {@link #release} with |
| * saved state as argument, throwing |
| * IllegalMonitorStateException if it fails. |
| * <li> Block until signalled |
| * <li> Reacquire by invoking specialized version of |
| * {@link #acquire} with saved state as argument. |
| * </ol> |
| */ |
| public final void awaitUninterruptibly() { |
| Node node = addConditionWaiter(); |
| int savedState = fullyRelease(node); |
| boolean interrupted = false; |
| while (!isOnSyncQueue(node)) { |
| LockSupport.park(); |
| if (Thread.interrupted()) |
| interrupted = true; |
| } |
| if (acquireQueued(node, savedState) || interrupted) |
| selfInterrupt(); |
| } |
| |
| /* |
| * For interruptible waits, we need to track whether to throw |
| * InterruptedException, if interrupted while blocked on |
| * condition, versus reinterrupt current thread, if |
| * interrupted while blocked waiting to re-acquire. |
| */ |
| |
| /** Mode meaning to reinterrupt on exit from wait */ |
| private static final int REINTERRUPT = 1; |
| /** Mode meaning to throw InterruptedException on exit from wait */ |
| private static final int THROW_IE = -1; |
| |
| /** |
| * Check for interrupt, returning THROW_IE if interrupted |
| * before signalled, REINTERRUPT if after signalled, or |
| * 0 if not interrupted. |
| */ |
| private int checkInterruptWhileWaiting(Node node) { |
| return (Thread.interrupted()) ? |
| ((transferAfterCancelledWait(node))? THROW_IE : REINTERRUPT) : |
| 0; |
| } |
| |
| /** |
| * Throw InterruptedException, reinterrupt current thread, or |
| * do nothing, depending on mode. |
| */ |
| private void reportInterruptAfterWait(int interruptMode) |
| throws InterruptedException { |
| if (interruptMode == THROW_IE) |
| throw new InterruptedException(); |
| else if (interruptMode == REINTERRUPT) |
| Thread.currentThread().interrupt(); |
| } |
| |
| /** |
| * Implements interruptible condition wait. |
| * <ol> |
| * <li> If current thread is interrupted, throw InterruptedException |
| * <li> Save lock state returned by {@link #getState} |
| * <li> Invoke {@link #release} with |
| * saved state as argument, throwing |
| * IllegalMonitorStateException if it fails. |
| * <li> Block until signalled or interrupted |
| * <li> Reacquire by invoking specialized version of |
| * {@link #acquire} with saved state as argument. |
| * <li> If interrupted while blocked in step 4, throw exception |
| * </ol> |
| * |
| * @throws InterruptedException if the current thread is interrupted (and |
| * interruption of thread suspension is supported). |
| */ |
| public final void await() throws InterruptedException { |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| Node node = addConditionWaiter(); |
| int savedState = fullyRelease(node); |
| int interruptMode = 0; |
| while (!isOnSyncQueue(node)) { |
| LockSupport.park(); |
| if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| break; |
| } |
| if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| interruptMode = REINTERRUPT; |
| if (interruptMode != 0) |
| reportInterruptAfterWait(interruptMode); |
| } |
| |
| /** |
| * Implements timed condition wait. |
| * <ol> |
| * <li> If current thread is interrupted, throw InterruptedException |
| * <li> Save lock state returned by {@link #getState} |
| * <li> Invoke {@link #release} with |
| * saved state as argument, throwing |
| * IllegalMonitorStateException if it fails. |
| * <li> Block until signalled, interrupted, or timed out |
| * <li> Reacquire by invoking specialized version of |
| * {@link #acquire} with saved state as argument. |
| * <li> If interrupted while blocked in step 4, throw InterruptedException |
| * </ol> |
| * |
| * @param nanosTimeout the maximum time to wait, in nanoseconds |
| * @return A value less than or equal to zero if the wait has |
| * timed out; otherwise an estimate, that |
| * is strictly less than the <tt>nanosTimeout</tt> argument, |
| * of the time still remaining when this method returned. |
| * |
| * @throws InterruptedException if the current thread is interrupted (and |
| * interruption of thread suspension is supported). |
| */ |
| public final long awaitNanos(long nanosTimeout) throws InterruptedException { |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| Node node = addConditionWaiter(); |
| int savedState = fullyRelease(node); |
| long lastTime = System.nanoTime(); |
| int interruptMode = 0; |
| while (!isOnSyncQueue(node)) { |
| if (nanosTimeout <= 0L) { |
| transferAfterCancelledWait(node); |
| break; |
| } |
| LockSupport.parkNanos(nanosTimeout); |
| if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| break; |
| long now = System.nanoTime(); |
| nanosTimeout -= now - lastTime; |
| lastTime = now; |
| } |
| if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| interruptMode = REINTERRUPT; |
| if (interruptMode != 0) |
| reportInterruptAfterWait(interruptMode); |
| return nanosTimeout - (System.nanoTime() - lastTime); |
| } |
| |
| /** |
| * Implements absolute timed condition wait. |
| * <ol> |
| * <li> If current thread is interrupted, throw InterruptedException |
| * <li> Save lock state returned by {@link #getState} |
| * <li> Invoke {@link #release} with |
| * saved state as argument, throwing |
| * IllegalMonitorStateException if it fails. |
| * <li> Block until signalled, interrupted, or timed out |
| * <li> Reacquire by invoking specialized version of |
| * {@link #acquire} with saved state as argument. |
| * <li> If interrupted while blocked in step 4, throw InterruptedException |
| * <li> If timed out while blocked in step 4, return false, else true |
| * </ol> |
| * |
| * @param deadline the absolute time to wait until |
| * @return <tt>false</tt> if the deadline has |
| * elapsed upon return, else <tt>true</tt>. |
| * |
| * @throws InterruptedException if the current thread is interrupted (and |
| * interruption of thread suspension is supported). |
| */ |
| public final boolean awaitUntil(Date deadline) throws InterruptedException { |
| if (deadline == null) |
| throw new NullPointerException(); |
| long abstime = deadline.getTime(); |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| Node node = addConditionWaiter(); |
| int savedState = fullyRelease(node); |
| boolean timedout = false; |
| int interruptMode = 0; |
| while (!isOnSyncQueue(node)) { |
| if (System.currentTimeMillis() > abstime) { |
| timedout = transferAfterCancelledWait(node); |
| break; |
| } |
| LockSupport.parkUntil(abstime); |
| if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| break; |
| } |
| if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| interruptMode = REINTERRUPT; |
| if (interruptMode != 0) |
| reportInterruptAfterWait(interruptMode); |
| return !timedout; |
| } |
| |
| /** |
| * Implements timed condition wait. |
| * <ol> |
| * <li> If current thread is interrupted, throw InterruptedException |
| * <li> Save lock state returned by {@link #getState} |
| * <li> Invoke {@link #release} with |
| * saved state as argument, throwing |
| * IllegalMonitorStateException if it fails. |
| * <li> Block until signalled, interrupted, or timed out |
| * <li> Reacquire by invoking specialized version of |
| * {@link #acquire} with saved state as argument. |
| * <li> If interrupted while blocked in step 4, throw InterruptedException |
| * <li> If timed out while blocked in step 4, return false, else true |
| * </ol> |
| * |
| * @param time the maximum time to wait |
| * @param unit the time unit of the <tt>time</tt> argument. |
| * @return <tt>false</tt> if the waiting time detectably elapsed |
| * before return from the method, else <tt>true</tt>. |
| * @throws InterruptedException if the current thread is interrupted (and |
| * interruption of thread suspension is supported). |
| */ |
| public final boolean await(long time, TimeUnit unit) throws InterruptedException { |
| if (unit == null) |
| throw new NullPointerException(); |
| long nanosTimeout = unit.toNanos(time); |
| if (Thread.interrupted()) |
| throw new InterruptedException(); |
| Node node = addConditionWaiter(); |
| int savedState = fullyRelease(node); |
| long lastTime = System.nanoTime(); |
| boolean timedout = false; |
| int interruptMode = 0; |
| while (!isOnSyncQueue(node)) { |
| if (nanosTimeout <= 0L) { |
| timedout = transferAfterCancelledWait(node); |
| break; |
| } |
| LockSupport.parkNanos(nanosTimeout); |
| if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) |
| break; |
| long now = System.nanoTime(); |
| nanosTimeout -= now - lastTime; |
| lastTime = now; |
| } |
| if (acquireQueued(node, savedState) && interruptMode != THROW_IE) |
| interruptMode = REINTERRUPT; |
| if (interruptMode != 0) |
| reportInterruptAfterWait(interruptMode); |
| return !timedout; |
| } |
| |
| // support for instrumentation |
| |
| /** |
| * Returns true if this condition was created by the given |
| * synchronization object |
| * @return true if owned |
| */ |
| final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { |
| return sync == AbstractQueuedSynchronizer.this; |
| } |
| |
| /** |
| * Queries whether any threads are waiting on this condition. |
| * Implements {@link AbstractQueuedSynchronizer#hasWaiters} |
| * @return <tt>true</tt> if there are any waiting threads. |
| * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| * returns false |
| */ |
| protected final boolean hasWaiters() { |
| if (!isHeldExclusively()) |
| throw new IllegalMonitorStateException(); |
| for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
| if (w.waitStatus == Node.CONDITION) |
| return true; |
| } |
| return false; |
| } |
| |
| /** |
| * Returns an estimate of the number of threads waiting on |
| * this condition. |
| * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength} |
| * @return the estimated number of waiting threads. |
| * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| * returns false |
| */ |
| protected final int getWaitQueueLength() { |
| if (!isHeldExclusively()) |
| throw new IllegalMonitorStateException(); |
| int n = 0; |
| for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
| if (w.waitStatus == Node.CONDITION) |
| ++n; |
| } |
| return n; |
| } |
| |
| /** |
| * Returns a collection containing those threads that may be |
| * waiting on this Condition. |
| * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads} |
| * @return the collection of threads |
| * @throws IllegalMonitorStateException if {@link #isHeldExclusively} |
| * returns false |
| */ |
| protected final Collection<Thread> getWaitingThreads() { |
| if (!isHeldExclusively()) |
| throw new IllegalMonitorStateException(); |
| ArrayList<Thread> list = new ArrayList<Thread>(); |
| for (Node w = firstWaiter; w != null; w = w.nextWaiter) { |
| if (w.waitStatus == Node.CONDITION) { |
| Thread t = w.thread; |
| if (t != null) |
| list.add(t); |
| } |
| } |
| return list; |
| } |
| } |
| |
| /** |
| * Setup to support compareAndSet. We need to natively implement |
| * this here: For the sake of permitting future enhancements, we |
| * cannot explicitly subclass AtomicInteger, which would be |
| * efficient and useful otherwise. So, as the lesser of evils, we |
| * natively implement using hotspot intrinsics API. And while we |
| * are at it, we do the same for other CASable fields (which could |
| * otherwise be done with atomic field updaters). |
| */ |
| // BEGIN android-changed |
| private static final Unsafe unsafe = UnsafeAccess.THE_ONE; |
| // END android-changed |
| private static final long stateOffset; |
| private static final long headOffset; |
| private static final long tailOffset; |
| private static final long waitStatusOffset; |
| |
| static { |
| try { |
| stateOffset = unsafe.objectFieldOffset |
| (AbstractQueuedSynchronizer.class.getDeclaredField("state")); |
| headOffset = unsafe.objectFieldOffset |
| (AbstractQueuedSynchronizer.class.getDeclaredField("head")); |
| tailOffset = unsafe.objectFieldOffset |
| (AbstractQueuedSynchronizer.class.getDeclaredField("tail")); |
| waitStatusOffset = unsafe.objectFieldOffset |
| (Node.class.getDeclaredField("waitStatus")); |
| |
| } catch(Exception ex) { throw new Error(ex); } |
| } |
| |
| /** |
| * CAS head field. Used only by enq |
| */ |
| private final boolean compareAndSetHead(Node update) { |
| return unsafe.compareAndSwapObject(this, headOffset, null, update); |
| } |
| |
| /** |
| * CAS tail field. Used only by enq |
| */ |
| private final boolean compareAndSetTail(Node expect, Node update) { |
| return unsafe.compareAndSwapObject(this, tailOffset, expect, update); |
| } |
| |
| /** |
| * CAS waitStatus field of a node. |
| */ |
| private final static boolean compareAndSetWaitStatus(Node node, |
| int expect, |
| int update) { |
| return unsafe.compareAndSwapInt(node, waitStatusOffset, |
| expect, update); |
| } |
| |
| } |
