mirror of git://gcc.gnu.org/git/gcc.git
1128 lines
42 KiB
Java
1128 lines
42 KiB
Java
/*
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* Written by Doug Lea, Bill Scherer, and Michael Scott with
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* assistance from members of JCP JSR-166 Expert Group and released to
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* the public domain, as explained at
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* http://creativecommons.org/licenses/publicdomain
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*/
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package java.util.concurrent;
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import java.util.concurrent.locks.*;
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import java.util.concurrent.atomic.*;
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import java.util.*;
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/**
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* A {@linkplain BlockingQueue blocking queue} in which each insert
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* operation must wait for a corresponding remove operation by another
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* thread, and vice versa. A synchronous queue does not have any
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* internal capacity, not even a capacity of one. You cannot
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* <tt>peek</tt> at a synchronous queue because an element is only
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* present when you try to remove it; you cannot insert an element
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* (using any method) unless another thread is trying to remove it;
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* you cannot iterate as there is nothing to iterate. The
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* <em>head</em> of the queue is the element that the first queued
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* inserting thread is trying to add to the queue; if there is no such
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* queued thread then no element is available for removal and
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* <tt>poll()</tt> will return <tt>null</tt>. For purposes of other
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* <tt>Collection</tt> methods (for example <tt>contains</tt>), a
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* <tt>SynchronousQueue</tt> acts as an empty collection. This queue
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* does not permit <tt>null</tt> elements.
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*
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* <p>Synchronous queues are similar to rendezvous channels used in
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* CSP and Ada. They are well suited for handoff designs, in which an
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* object running in one thread must sync up with an object running
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* in another thread in order to hand it some information, event, or
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* task.
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*
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* <p> This class supports an optional fairness policy for ordering
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* waiting producer and consumer threads. By default, this ordering
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* is not guaranteed. However, a queue constructed with fairness set
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* to <tt>true</tt> grants threads access in FIFO order.
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*
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* <p>This class and its iterator implement all of the
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* <em>optional</em> methods of the {@link Collection} and {@link
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* Iterator} interfaces.
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*
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* <p>This class is a member of the
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* <a href="{@docRoot}/../technotes/guides/collections/index.html">
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* Java Collections Framework</a>.
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*
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* @since 1.5
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* @author Doug Lea and Bill Scherer and Michael Scott
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* @param <E> the type of elements held in this collection
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*/
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public class SynchronousQueue<E> extends AbstractQueue<E>
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implements BlockingQueue<E>, java.io.Serializable {
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private static final long serialVersionUID = -3223113410248163686L;
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/*
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* This class implements extensions of the dual stack and dual
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* queue algorithms described in "Nonblocking Concurrent Objects
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* with Condition Synchronization", by W. N. Scherer III and
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* M. L. Scott. 18th Annual Conf. on Distributed Computing,
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* Oct. 2004 (see also
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* http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
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* The (Lifo) stack is used for non-fair mode, and the (Fifo)
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* queue for fair mode. The performance of the two is generally
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* similar. Fifo usually supports higher throughput under
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* contention but Lifo maintains higher thread locality in common
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* applications.
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*
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* A dual queue (and similarly stack) is one that at any given
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* time either holds "data" -- items provided by put operations,
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* or "requests" -- slots representing take operations, or is
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* empty. A call to "fulfill" (i.e., a call requesting an item
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* from a queue holding data or vice versa) dequeues a
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* complementary node. The most interesting feature of these
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* queues is that any operation can figure out which mode the
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* queue is in, and act accordingly without needing locks.
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*
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* Both the queue and stack extend abstract class Transferer
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* defining the single method transfer that does a put or a
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* take. These are unified into a single method because in dual
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* data structures, the put and take operations are symmetrical,
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* so nearly all code can be combined. The resulting transfer
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* methods are on the long side, but are easier to follow than
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* they would be if broken up into nearly-duplicated parts.
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*
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* The queue and stack data structures share many conceptual
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* similarities but very few concrete details. For simplicity,
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* they are kept distinct so that they can later evolve
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* separately.
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*
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* The algorithms here differ from the versions in the above paper
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* in extending them for use in synchronous queues, as well as
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* dealing with cancellation. The main differences include:
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*
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* 1. The original algorithms used bit-marked pointers, but
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* the ones here use mode bits in nodes, leading to a number
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* of further adaptations.
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* 2. SynchronousQueues must block threads waiting to become
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* fulfilled.
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* 3. Support for cancellation via timeout and interrupts,
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* including cleaning out cancelled nodes/threads
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* from lists to avoid garbage retention and memory depletion.
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*
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* Blocking is mainly accomplished using LockSupport park/unpark,
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* except that nodes that appear to be the next ones to become
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* fulfilled first spin a bit (on multiprocessors only). On very
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* busy synchronous queues, spinning can dramatically improve
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* throughput. And on less busy ones, the amount of spinning is
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* small enough not to be noticeable.
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*
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* Cleaning is done in different ways in queues vs stacks. For
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* queues, we can almost always remove a node immediately in O(1)
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* time (modulo retries for consistency checks) when it is
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* cancelled. But if it may be pinned as the current tail, it must
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* wait until some subsequent cancellation. For stacks, we need a
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* potentially O(n) traversal to be sure that we can remove the
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* node, but this can run concurrently with other threads
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* accessing the stack.
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*
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* While garbage collection takes care of most node reclamation
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* issues that otherwise complicate nonblocking algorithms, care
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* is taken to "forget" references to data, other nodes, and
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* threads that might be held on to long-term by blocked
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* threads. In cases where setting to null would otherwise
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* conflict with main algorithms, this is done by changing a
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* node's link to now point to the node itself. This doesn't arise
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* much for Stack nodes (because blocked threads do not hang on to
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* old head pointers), but references in Queue nodes must be
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* aggressively forgotten to avoid reachability of everything any
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* node has ever referred to since arrival.
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*/
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/**
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* Shared internal API for dual stacks and queues.
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*/
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static abstract class Transferer {
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/**
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* Performs a put or take.
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*
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* @param e if non-null, the item to be handed to a consumer;
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* if null, requests that transfer return an item
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* offered by producer.
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* @param timed if this operation should timeout
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* @param nanos the timeout, in nanoseconds
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* @return if non-null, the item provided or received; if null,
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* the operation failed due to timeout or interrupt --
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* the caller can distinguish which of these occurred
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* by checking Thread.interrupted.
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*/
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abstract Object transfer(Object e, boolean timed, long nanos);
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}
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/** The number of CPUs, for spin control */
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static final int NCPUS = Runtime.getRuntime().availableProcessors();
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/**
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* The number of times to spin before blocking in timed waits.
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* The value is empirically derived -- it works well across a
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* variety of processors and OSes. Empirically, the best value
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* seems not to vary with number of CPUs (beyond 2) so is just
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* a constant.
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*/
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static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
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/**
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* The number of times to spin before blocking in untimed waits.
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* This is greater than timed value because untimed waits spin
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* faster since they don't need to check times on each spin.
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*/
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static final int maxUntimedSpins = maxTimedSpins * 16;
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/**
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* The number of nanoseconds for which it is faster to spin
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* rather than to use timed park. A rough estimate suffices.
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*/
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static final long spinForTimeoutThreshold = 1000L;
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/** Dual stack */
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static final class TransferStack extends Transferer {
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/*
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* This extends Scherer-Scott dual stack algorithm, differing,
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* among other ways, by using "covering" nodes rather than
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* bit-marked pointers: Fulfilling operations push on marker
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* nodes (with FULFILLING bit set in mode) to reserve a spot
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* to match a waiting node.
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*/
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/* Modes for SNodes, ORed together in node fields */
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/** Node represents an unfulfilled consumer */
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static final int REQUEST = 0;
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/** Node represents an unfulfilled producer */
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static final int DATA = 1;
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/** Node is fulfilling another unfulfilled DATA or REQUEST */
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static final int FULFILLING = 2;
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/** Return true if m has fulfilling bit set */
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static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
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/** Node class for TransferStacks. */
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static final class SNode {
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volatile SNode next; // next node in stack
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volatile SNode match; // the node matched to this
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volatile Thread waiter; // to control park/unpark
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Object item; // data; or null for REQUESTs
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int mode;
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// Note: item and mode fields don't need to be volatile
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// since they are always written before, and read after,
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// other volatile/atomic operations.
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SNode(Object item) {
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this.item = item;
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}
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static final AtomicReferenceFieldUpdater<SNode, SNode>
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nextUpdater = AtomicReferenceFieldUpdater.newUpdater
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(SNode.class, SNode.class, "next");
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boolean casNext(SNode cmp, SNode val) {
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return (cmp == next &&
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nextUpdater.compareAndSet(this, cmp, val));
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}
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static final AtomicReferenceFieldUpdater<SNode, SNode>
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matchUpdater = AtomicReferenceFieldUpdater.newUpdater
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(SNode.class, SNode.class, "match");
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/**
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* Tries to match node s to this node, if so, waking up thread.
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* Fulfillers call tryMatch to identify their waiters.
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* Waiters block until they have been matched.
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*
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* @param s the node to match
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* @return true if successfully matched to s
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*/
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boolean tryMatch(SNode s) {
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if (match == null &&
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matchUpdater.compareAndSet(this, null, s)) {
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Thread w = waiter;
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if (w != null) { // waiters need at most one unpark
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waiter = null;
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LockSupport.unpark(w);
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}
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return true;
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}
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return match == s;
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}
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/**
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* Tries to cancel a wait by matching node to itself.
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*/
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void tryCancel() {
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matchUpdater.compareAndSet(this, null, this);
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}
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boolean isCancelled() {
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return match == this;
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}
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}
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/** The head (top) of the stack */
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volatile SNode head;
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static final AtomicReferenceFieldUpdater<TransferStack, SNode>
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headUpdater = AtomicReferenceFieldUpdater.newUpdater
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(TransferStack.class, SNode.class, "head");
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boolean casHead(SNode h, SNode nh) {
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return h == head && headUpdater.compareAndSet(this, h, nh);
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}
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/**
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* Creates or resets fields of a node. Called only from transfer
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* where the node to push on stack is lazily created and
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* reused when possible to help reduce intervals between reads
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* and CASes of head and to avoid surges of garbage when CASes
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* to push nodes fail due to contention.
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*/
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static SNode snode(SNode s, Object e, SNode next, int mode) {
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if (s == null) s = new SNode(e);
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s.mode = mode;
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s.next = next;
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return s;
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}
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/**
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* Puts or takes an item.
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*/
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Object transfer(Object e, boolean timed, long nanos) {
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/*
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* Basic algorithm is to loop trying one of three actions:
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*
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* 1. If apparently empty or already containing nodes of same
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* mode, try to push node on stack and wait for a match,
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* returning it, or null if cancelled.
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*
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* 2. If apparently containing node of complementary mode,
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* try to push a fulfilling node on to stack, match
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* with corresponding waiting node, pop both from
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* stack, and return matched item. The matching or
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* unlinking might not actually be necessary because of
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* other threads performing action 3:
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*
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* 3. If top of stack already holds another fulfilling node,
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* help it out by doing its match and/or pop
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* operations, and then continue. The code for helping
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* is essentially the same as for fulfilling, except
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* that it doesn't return the item.
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*/
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SNode s = null; // constructed/reused as needed
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int mode = (e == null)? REQUEST : DATA;
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for (;;) {
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SNode h = head;
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if (h == null || h.mode == mode) { // empty or same-mode
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if (timed && nanos <= 0) { // can't wait
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if (h != null && h.isCancelled())
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casHead(h, h.next); // pop cancelled node
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else
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return null;
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} else if (casHead(h, s = snode(s, e, h, mode))) {
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SNode m = awaitFulfill(s, timed, nanos);
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if (m == s) { // wait was cancelled
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clean(s);
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return null;
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}
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if ((h = head) != null && h.next == s)
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casHead(h, s.next); // help s's fulfiller
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return mode == REQUEST? m.item : s.item;
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}
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} else if (!isFulfilling(h.mode)) { // try to fulfill
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if (h.isCancelled()) // already cancelled
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casHead(h, h.next); // pop and retry
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else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
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for (;;) { // loop until matched or waiters disappear
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SNode m = s.next; // m is s's match
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if (m == null) { // all waiters are gone
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casHead(s, null); // pop fulfill node
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s = null; // use new node next time
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break; // restart main loop
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}
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SNode mn = m.next;
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if (m.tryMatch(s)) {
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casHead(s, mn); // pop both s and m
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return (mode == REQUEST)? m.item : s.item;
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} else // lost match
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s.casNext(m, mn); // help unlink
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}
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}
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} else { // help a fulfiller
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SNode m = h.next; // m is h's match
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if (m == null) // waiter is gone
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casHead(h, null); // pop fulfilling node
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else {
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SNode mn = m.next;
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if (m.tryMatch(h)) // help match
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casHead(h, mn); // pop both h and m
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else // lost match
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h.casNext(m, mn); // help unlink
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}
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}
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}
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}
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/**
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* Spins/blocks until node s is matched by a fulfill operation.
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*
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* @param s the waiting node
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* @param timed true if timed wait
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* @param nanos timeout value
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* @return matched node, or s if cancelled
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*/
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SNode awaitFulfill(SNode s, boolean timed, long nanos) {
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/*
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* When a node/thread is about to block, it sets its waiter
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* field and then rechecks state at least one more time
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* before actually parking, thus covering race vs
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* fulfiller noticing that waiter is non-null so should be
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* woken.
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*
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* When invoked by nodes that appear at the point of call
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* to be at the head of the stack, calls to park are
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* preceded by spins to avoid blocking when producers and
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* consumers are arriving very close in time. This can
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* happen enough to bother only on multiprocessors.
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*
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* The order of checks for returning out of main loop
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* reflects fact that interrupts have precedence over
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* normal returns, which have precedence over
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* timeouts. (So, on timeout, one last check for match is
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* done before giving up.) Except that calls from untimed
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* SynchronousQueue.{poll/offer} don't check interrupts
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* and don't wait at all, so are trapped in transfer
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* method rather than calling awaitFulfill.
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*/
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long lastTime = (timed)? System.nanoTime() : 0;
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Thread w = Thread.currentThread();
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SNode h = head;
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int spins = (shouldSpin(s)?
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(timed? maxTimedSpins : maxUntimedSpins) : 0);
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for (;;) {
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if (w.isInterrupted())
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s.tryCancel();
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SNode m = s.match;
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if (m != null)
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return m;
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if (timed) {
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long now = System.nanoTime();
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nanos -= now - lastTime;
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lastTime = now;
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if (nanos <= 0) {
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s.tryCancel();
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continue;
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}
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}
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if (spins > 0)
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spins = shouldSpin(s)? (spins-1) : 0;
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else if (s.waiter == null)
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s.waiter = w; // establish waiter so can park next iter
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else if (!timed)
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LockSupport.park(this);
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else if (nanos > spinForTimeoutThreshold)
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LockSupport.parkNanos(this, nanos);
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}
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}
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/**
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* Returns true if node s is at head or there is an active
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* fulfiller.
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*/
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boolean shouldSpin(SNode s) {
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SNode h = head;
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return (h == s || h == null || isFulfilling(h.mode));
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}
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/**
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* Unlinks s from the stack.
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*/
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void clean(SNode s) {
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s.item = null; // forget item
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s.waiter = null; // forget thread
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/*
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* At worst we may need to traverse entire stack to unlink
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* s. If there are multiple concurrent calls to clean, we
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* might not see s if another thread has already removed
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* it. But we can stop when we see any node known to
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* follow s. We use s.next unless it too is cancelled, in
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* which case we try the node one past. We don't check any
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* further because we don't want to doubly traverse just to
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* find sentinel.
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*/
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SNode past = s.next;
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if (past != null && past.isCancelled())
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past = past.next;
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// Absorb cancelled nodes at head
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SNode p;
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while ((p = head) != null && p != past && p.isCancelled())
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casHead(p, p.next);
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// Unsplice embedded nodes
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while (p != null && p != past) {
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SNode n = p.next;
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if (n != null && n.isCancelled())
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p.casNext(n, n.next);
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else
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p = n;
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}
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}
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}
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|
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/** Dual Queue */
|
|
static final class TransferQueue extends Transferer {
|
|
/*
|
|
* This extends Scherer-Scott dual queue algorithm, differing,
|
|
* among other ways, by using modes within nodes rather than
|
|
* marked pointers. The algorithm is a little simpler than
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|
* that for stacks because fulfillers do not need explicit
|
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* nodes, and matching is done by CAS'ing QNode.item field
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* from non-null to null (for put) or vice versa (for take).
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*/
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|
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/** Node class for TransferQueue. */
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static final class QNode {
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volatile QNode next; // next node in queue
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volatile Object item; // CAS'ed to or from null
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|
volatile Thread waiter; // to control park/unpark
|
|
final boolean isData;
|
|
|
|
QNode(Object item, boolean isData) {
|
|
this.item = item;
|
|
this.isData = isData;
|
|
}
|
|
|
|
static final AtomicReferenceFieldUpdater<QNode, QNode>
|
|
nextUpdater = AtomicReferenceFieldUpdater.newUpdater
|
|
(QNode.class, QNode.class, "next");
|
|
|
|
boolean casNext(QNode cmp, QNode val) {
|
|
return (next == cmp &&
|
|
nextUpdater.compareAndSet(this, cmp, val));
|
|
}
|
|
|
|
static final AtomicReferenceFieldUpdater<QNode, Object>
|
|
itemUpdater = AtomicReferenceFieldUpdater.newUpdater
|
|
(QNode.class, Object.class, "item");
|
|
|
|
boolean casItem(Object cmp, Object val) {
|
|
return (item == cmp &&
|
|
itemUpdater.compareAndSet(this, cmp, val));
|
|
}
|
|
|
|
/**
|
|
* Tries to cancel by CAS'ing ref to this as item.
|
|
*/
|
|
void tryCancel(Object cmp) {
|
|
itemUpdater.compareAndSet(this, cmp, this);
|
|
}
|
|
|
|
boolean isCancelled() {
|
|
return item == this;
|
|
}
|
|
|
|
/**
|
|
* Returns true if this node is known to be off the queue
|
|
* because its next pointer has been forgotten due to
|
|
* an advanceHead operation.
|
|
*/
|
|
boolean isOffList() {
|
|
return next == this;
|
|
}
|
|
}
|
|
|
|
/** Head of queue */
|
|
transient volatile QNode head;
|
|
/** Tail of queue */
|
|
transient volatile QNode tail;
|
|
/**
|
|
* Reference to a cancelled node that might not yet have been
|
|
* unlinked from queue because it was the last inserted node
|
|
* when it cancelled.
|
|
*/
|
|
transient volatile QNode cleanMe;
|
|
|
|
TransferQueue() {
|
|
QNode h = new QNode(null, false); // initialize to dummy node.
|
|
head = h;
|
|
tail = h;
|
|
}
|
|
|
|
static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
|
|
headUpdater = AtomicReferenceFieldUpdater.newUpdater
|
|
(TransferQueue.class, QNode.class, "head");
|
|
|
|
/**
|
|
* Tries to cas nh as new head; if successful, unlink
|
|
* old head's next node to avoid garbage retention.
|
|
*/
|
|
void advanceHead(QNode h, QNode nh) {
|
|
if (h == head && headUpdater.compareAndSet(this, h, nh))
|
|
h.next = h; // forget old next
|
|
}
|
|
|
|
static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
|
|
tailUpdater = AtomicReferenceFieldUpdater.newUpdater
|
|
(TransferQueue.class, QNode.class, "tail");
|
|
|
|
/**
|
|
* Tries to cas nt as new tail.
|
|
*/
|
|
void advanceTail(QNode t, QNode nt) {
|
|
if (tail == t)
|
|
tailUpdater.compareAndSet(this, t, nt);
|
|
}
|
|
|
|
static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
|
|
cleanMeUpdater = AtomicReferenceFieldUpdater.newUpdater
|
|
(TransferQueue.class, QNode.class, "cleanMe");
|
|
|
|
/**
|
|
* Tries to CAS cleanMe slot.
|
|
*/
|
|
boolean casCleanMe(QNode cmp, QNode val) {
|
|
return (cleanMe == cmp &&
|
|
cleanMeUpdater.compareAndSet(this, cmp, val));
|
|
}
|
|
|
|
/**
|
|
* Puts or takes an item.
|
|
*/
|
|
Object transfer(Object e, boolean timed, long nanos) {
|
|
/* Basic algorithm is to loop trying to take either of
|
|
* two actions:
|
|
*
|
|
* 1. If queue apparently empty or holding same-mode nodes,
|
|
* try to add node to queue of waiters, wait to be
|
|
* fulfilled (or cancelled) and return matching item.
|
|
*
|
|
* 2. If queue apparently contains waiting items, and this
|
|
* call is of complementary mode, try to fulfill by CAS'ing
|
|
* item field of waiting node and dequeuing it, and then
|
|
* returning matching item.
|
|
*
|
|
* In each case, along the way, check for and try to help
|
|
* advance head and tail on behalf of other stalled/slow
|
|
* threads.
|
|
*
|
|
* The loop starts off with a null check guarding against
|
|
* seeing uninitialized head or tail values. This never
|
|
* happens in current SynchronousQueue, but could if
|
|
* callers held non-volatile/final ref to the
|
|
* transferer. The check is here anyway because it places
|
|
* null checks at top of loop, which is usually faster
|
|
* than having them implicitly interspersed.
|
|
*/
|
|
|
|
QNode s = null; // constructed/reused as needed
|
|
boolean isData = (e != null);
|
|
|
|
for (;;) {
|
|
QNode t = tail;
|
|
QNode h = head;
|
|
if (t == null || h == null) // saw uninitialized value
|
|
continue; // spin
|
|
|
|
if (h == t || t.isData == isData) { // empty or same-mode
|
|
QNode tn = t.next;
|
|
if (t != tail) // inconsistent read
|
|
continue;
|
|
if (tn != null) { // lagging tail
|
|
advanceTail(t, tn);
|
|
continue;
|
|
}
|
|
if (timed && nanos <= 0) // can't wait
|
|
return null;
|
|
if (s == null)
|
|
s = new QNode(e, isData);
|
|
if (!t.casNext(null, s)) // failed to link in
|
|
continue;
|
|
|
|
advanceTail(t, s); // swing tail and wait
|
|
Object x = awaitFulfill(s, e, timed, nanos);
|
|
if (x == s) { // wait was cancelled
|
|
clean(t, s);
|
|
return null;
|
|
}
|
|
|
|
if (!s.isOffList()) { // not already unlinked
|
|
advanceHead(t, s); // unlink if head
|
|
if (x != null) // and forget fields
|
|
s.item = s;
|
|
s.waiter = null;
|
|
}
|
|
return (x != null)? x : e;
|
|
|
|
} else { // complementary-mode
|
|
QNode m = h.next; // node to fulfill
|
|
if (t != tail || m == null || h != head)
|
|
continue; // inconsistent read
|
|
|
|
Object x = m.item;
|
|
if (isData == (x != null) || // m already fulfilled
|
|
x == m || // m cancelled
|
|
!m.casItem(x, e)) { // lost CAS
|
|
advanceHead(h, m); // dequeue and retry
|
|
continue;
|
|
}
|
|
|
|
advanceHead(h, m); // successfully fulfilled
|
|
LockSupport.unpark(m.waiter);
|
|
return (x != null)? x : e;
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Spins/blocks until node s is fulfilled.
|
|
*
|
|
* @param s the waiting node
|
|
* @param e the comparison value for checking match
|
|
* @param timed true if timed wait
|
|
* @param nanos timeout value
|
|
* @return matched item, or s if cancelled
|
|
*/
|
|
Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
|
|
/* Same idea as TransferStack.awaitFulfill */
|
|
long lastTime = (timed)? System.nanoTime() : 0;
|
|
Thread w = Thread.currentThread();
|
|
int spins = ((head.next == s) ?
|
|
(timed? maxTimedSpins : maxUntimedSpins) : 0);
|
|
for (;;) {
|
|
if (w.isInterrupted())
|
|
s.tryCancel(e);
|
|
Object x = s.item;
|
|
if (x != e)
|
|
return x;
|
|
if (timed) {
|
|
long now = System.nanoTime();
|
|
nanos -= now - lastTime;
|
|
lastTime = now;
|
|
if (nanos <= 0) {
|
|
s.tryCancel(e);
|
|
continue;
|
|
}
|
|
}
|
|
if (spins > 0)
|
|
--spins;
|
|
else if (s.waiter == null)
|
|
s.waiter = w;
|
|
else if (!timed)
|
|
LockSupport.park(this);
|
|
else if (nanos > spinForTimeoutThreshold)
|
|
LockSupport.parkNanos(this, nanos);
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Gets rid of cancelled node s with original predecessor pred.
|
|
*/
|
|
void clean(QNode pred, QNode s) {
|
|
s.waiter = null; // forget thread
|
|
/*
|
|
* At any given time, exactly one node on list cannot be
|
|
* deleted -- the last inserted node. To accommodate this,
|
|
* if we cannot delete s, we save its predecessor as
|
|
* "cleanMe", deleting the previously saved version
|
|
* first. At least one of node s or the node previously
|
|
* saved can always be deleted, so this always terminates.
|
|
*/
|
|
while (pred.next == s) { // Return early if already unlinked
|
|
QNode h = head;
|
|
QNode hn = h.next; // Absorb cancelled first node as head
|
|
if (hn != null && hn.isCancelled()) {
|
|
advanceHead(h, hn);
|
|
continue;
|
|
}
|
|
QNode t = tail; // Ensure consistent read for tail
|
|
if (t == h)
|
|
return;
|
|
QNode tn = t.next;
|
|
if (t != tail)
|
|
continue;
|
|
if (tn != null) {
|
|
advanceTail(t, tn);
|
|
continue;
|
|
}
|
|
if (s != t) { // If not tail, try to unsplice
|
|
QNode sn = s.next;
|
|
if (sn == s || pred.casNext(s, sn))
|
|
return;
|
|
}
|
|
QNode dp = cleanMe;
|
|
if (dp != null) { // Try unlinking previous cancelled node
|
|
QNode d = dp.next;
|
|
QNode dn;
|
|
if (d == null || // d is gone or
|
|
d == dp || // d is off list or
|
|
!d.isCancelled() || // d not cancelled or
|
|
(d != t && // d not tail and
|
|
(dn = d.next) != null && // has successor
|
|
dn != d && // that is on list
|
|
dp.casNext(d, dn))) // d unspliced
|
|
casCleanMe(dp, null);
|
|
if (dp == pred)
|
|
return; // s is already saved node
|
|
} else if (casCleanMe(null, pred))
|
|
return; // Postpone cleaning s
|
|
}
|
|
}
|
|
}
|
|
|
|
/**
|
|
* The transferer. Set only in constructor, but cannot be declared
|
|
* as final without further complicating serialization. Since
|
|
* this is accessed only at most once per public method, there
|
|
* isn't a noticeable performance penalty for using volatile
|
|
* instead of final here.
|
|
*/
|
|
private transient volatile Transferer transferer;
|
|
|
|
/**
|
|
* Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
|
|
*/
|
|
public SynchronousQueue() {
|
|
this(false);
|
|
}
|
|
|
|
/**
|
|
* Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
|
|
*
|
|
* @param fair if true, waiting threads contend in FIFO order for
|
|
* access; otherwise the order is unspecified.
|
|
*/
|
|
public SynchronousQueue(boolean fair) {
|
|
transferer = (fair)? new TransferQueue() : new TransferStack();
|
|
}
|
|
|
|
/**
|
|
* Adds the specified element to this queue, waiting if necessary for
|
|
* another thread to receive it.
|
|
*
|
|
* @throws InterruptedException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
*/
|
|
public void put(E o) throws InterruptedException {
|
|
if (o == null) throw new NullPointerException();
|
|
if (transferer.transfer(o, false, 0) == null) {
|
|
Thread.interrupted();
|
|
throw new InterruptedException();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Inserts the specified element into this queue, waiting if necessary
|
|
* up to the specified wait time for another thread to receive it.
|
|
*
|
|
* @return <tt>true</tt> if successful, or <tt>false</tt> if the
|
|
* specified waiting time elapses before a consumer appears.
|
|
* @throws InterruptedException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
*/
|
|
public boolean offer(E o, long timeout, TimeUnit unit)
|
|
throws InterruptedException {
|
|
if (o == null) throw new NullPointerException();
|
|
if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
|
|
return true;
|
|
if (!Thread.interrupted())
|
|
return false;
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Inserts the specified element into this queue, if another thread is
|
|
* waiting to receive it.
|
|
*
|
|
* @param e the element to add
|
|
* @return <tt>true</tt> if the element was added to this queue, else
|
|
* <tt>false</tt>
|
|
* @throws NullPointerException if the specified element is null
|
|
*/
|
|
public boolean offer(E e) {
|
|
if (e == null) throw new NullPointerException();
|
|
return transferer.transfer(e, true, 0) != null;
|
|
}
|
|
|
|
/**
|
|
* Retrieves and removes the head of this queue, waiting if necessary
|
|
* for another thread to insert it.
|
|
*
|
|
* @return the head of this queue
|
|
* @throws InterruptedException {@inheritDoc}
|
|
*/
|
|
public E take() throws InterruptedException {
|
|
Object e = transferer.transfer(null, false, 0);
|
|
if (e != null)
|
|
return (E)e;
|
|
Thread.interrupted();
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Retrieves and removes the head of this queue, waiting
|
|
* if necessary up to the specified wait time, for another thread
|
|
* to insert it.
|
|
*
|
|
* @return the head of this queue, or <tt>null</tt> if the
|
|
* specified waiting time elapses before an element is present.
|
|
* @throws InterruptedException {@inheritDoc}
|
|
*/
|
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException {
|
|
Object e = transferer.transfer(null, true, unit.toNanos(timeout));
|
|
if (e != null || !Thread.interrupted())
|
|
return (E)e;
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Retrieves and removes the head of this queue, if another thread
|
|
* is currently making an element available.
|
|
*
|
|
* @return the head of this queue, or <tt>null</tt> if no
|
|
* element is available.
|
|
*/
|
|
public E poll() {
|
|
return (E)transferer.transfer(null, true, 0);
|
|
}
|
|
|
|
/**
|
|
* Always returns <tt>true</tt>.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @return <tt>true</tt>
|
|
*/
|
|
public boolean isEmpty() {
|
|
return true;
|
|
}
|
|
|
|
/**
|
|
* Always returns zero.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @return zero.
|
|
*/
|
|
public int size() {
|
|
return 0;
|
|
}
|
|
|
|
/**
|
|
* Always returns zero.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @return zero.
|
|
*/
|
|
public int remainingCapacity() {
|
|
return 0;
|
|
}
|
|
|
|
/**
|
|
* Does nothing.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*/
|
|
public void clear() {
|
|
}
|
|
|
|
/**
|
|
* Always returns <tt>false</tt>.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @param o the element
|
|
* @return <tt>false</tt>
|
|
*/
|
|
public boolean contains(Object o) {
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Always returns <tt>false</tt>.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @param o the element to remove
|
|
* @return <tt>false</tt>
|
|
*/
|
|
public boolean remove(Object o) {
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Returns <tt>false</tt> unless the given collection is empty.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @param c the collection
|
|
* @return <tt>false</tt> unless given collection is empty
|
|
*/
|
|
public boolean containsAll(Collection<?> c) {
|
|
return c.isEmpty();
|
|
}
|
|
|
|
/**
|
|
* Always returns <tt>false</tt>.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @param c the collection
|
|
* @return <tt>false</tt>
|
|
*/
|
|
public boolean removeAll(Collection<?> c) {
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Always returns <tt>false</tt>.
|
|
* A <tt>SynchronousQueue</tt> has no internal capacity.
|
|
*
|
|
* @param c the collection
|
|
* @return <tt>false</tt>
|
|
*/
|
|
public boolean retainAll(Collection<?> c) {
|
|
return false;
|
|
}
|
|
|
|
/**
|
|
* Always returns <tt>null</tt>.
|
|
* A <tt>SynchronousQueue</tt> does not return elements
|
|
* unless actively waited on.
|
|
*
|
|
* @return <tt>null</tt>
|
|
*/
|
|
public E peek() {
|
|
return null;
|
|
}
|
|
|
|
static class EmptyIterator<E> implements Iterator<E> {
|
|
public boolean hasNext() {
|
|
return false;
|
|
}
|
|
public E next() {
|
|
throw new NoSuchElementException();
|
|
}
|
|
public void remove() {
|
|
throw new IllegalStateException();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Returns an empty iterator in which <tt>hasNext</tt> always returns
|
|
* <tt>false</tt>.
|
|
*
|
|
* @return an empty iterator
|
|
*/
|
|
public Iterator<E> iterator() {
|
|
return new EmptyIterator<E>();
|
|
}
|
|
|
|
/**
|
|
* Returns a zero-length array.
|
|
* @return a zero-length array
|
|
*/
|
|
public Object[] toArray() {
|
|
return new Object[0];
|
|
}
|
|
|
|
/**
|
|
* Sets the zeroeth element of the specified array to <tt>null</tt>
|
|
* (if the array has non-zero length) and returns it.
|
|
*
|
|
* @param a the array
|
|
* @return the specified array
|
|
* @throws NullPointerException if the specified array is null
|
|
*/
|
|
public <T> T[] toArray(T[] a) {
|
|
if (a.length > 0)
|
|
a[0] = null;
|
|
return a;
|
|
}
|
|
|
|
/**
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
|
* @throws ClassCastException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
|
*/
|
|
public int drainTo(Collection<? super E> c) {
|
|
if (c == null)
|
|
throw new NullPointerException();
|
|
if (c == this)
|
|
throw new IllegalArgumentException();
|
|
int n = 0;
|
|
E e;
|
|
while ( (e = poll()) != null) {
|
|
c.add(e);
|
|
++n;
|
|
}
|
|
return n;
|
|
}
|
|
|
|
/**
|
|
* @throws UnsupportedOperationException {@inheritDoc}
|
|
* @throws ClassCastException {@inheritDoc}
|
|
* @throws NullPointerException {@inheritDoc}
|
|
* @throws IllegalArgumentException {@inheritDoc}
|
|
*/
|
|
public int drainTo(Collection<? super E> c, int maxElements) {
|
|
if (c == null)
|
|
throw new NullPointerException();
|
|
if (c == this)
|
|
throw new IllegalArgumentException();
|
|
int n = 0;
|
|
E e;
|
|
while (n < maxElements && (e = poll()) != null) {
|
|
c.add(e);
|
|
++n;
|
|
}
|
|
return n;
|
|
}
|
|
|
|
/*
|
|
* To cope with serialization strategy in the 1.5 version of
|
|
* SynchronousQueue, we declare some unused classes and fields
|
|
* that exist solely to enable serializability across versions.
|
|
* These fields are never used, so are initialized only if this
|
|
* object is ever serialized or deserialized.
|
|
*/
|
|
|
|
static class WaitQueue implements java.io.Serializable { }
|
|
static class LifoWaitQueue extends WaitQueue {
|
|
private static final long serialVersionUID = -3633113410248163686L;
|
|
}
|
|
static class FifoWaitQueue extends WaitQueue {
|
|
private static final long serialVersionUID = -3623113410248163686L;
|
|
}
|
|
private ReentrantLock qlock;
|
|
private WaitQueue waitingProducers;
|
|
private WaitQueue waitingConsumers;
|
|
|
|
/**
|
|
* Save the state to a stream (that is, serialize it).
|
|
*
|
|
* @param s the stream
|
|
*/
|
|
private void writeObject(java.io.ObjectOutputStream s)
|
|
throws java.io.IOException {
|
|
boolean fair = transferer instanceof TransferQueue;
|
|
if (fair) {
|
|
qlock = new ReentrantLock(true);
|
|
waitingProducers = new FifoWaitQueue();
|
|
waitingConsumers = new FifoWaitQueue();
|
|
}
|
|
else {
|
|
qlock = new ReentrantLock();
|
|
waitingProducers = new LifoWaitQueue();
|
|
waitingConsumers = new LifoWaitQueue();
|
|
}
|
|
s.defaultWriteObject();
|
|
}
|
|
|
|
private void readObject(final java.io.ObjectInputStream s)
|
|
throws java.io.IOException, ClassNotFoundException {
|
|
s.defaultReadObject();
|
|
if (waitingProducers instanceof FifoWaitQueue)
|
|
transferer = new TransferQueue();
|
|
else
|
|
transferer = new TransferStack();
|
|
}
|
|
|
|
}
|