| /* |
| * Written by Doug Lea, Bill Scherer, and Michael Scott 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; |
| |
| import java.util.concurrent.atomic.AtomicInteger; |
| import java.util.concurrent.atomic.AtomicReference; |
| import java.util.concurrent.locks.LockSupport; |
| |
| /** |
| * A synchronization point at which threads can pair and swap elements |
| * within pairs. Each thread presents some object on entry to the |
| * {@link #exchange exchange} method, matches with a partner thread, |
| * and receives its partner's object on return. An Exchanger may be |
| * viewed as a bidirectional form of a {@link SynchronousQueue}. |
| * Exchangers may be useful in applications such as genetic algorithms |
| * and pipeline designs. |
| * |
| * <p><b>Sample Usage:</b> |
| * Here are the highlights of a class that uses an {@code Exchanger} |
| * to swap buffers between threads so that the thread filling the |
| * buffer gets a freshly emptied one when it needs it, handing off the |
| * filled one to the thread emptying the buffer. |
| * <pre>{@code |
| * class FillAndEmpty { |
| * Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>(); |
| * DataBuffer initialEmptyBuffer = ... a made-up type |
| * DataBuffer initialFullBuffer = ... |
| * |
| * class FillingLoop implements Runnable { |
| * public void run() { |
| * DataBuffer currentBuffer = initialEmptyBuffer; |
| * try { |
| * while (currentBuffer != null) { |
| * addToBuffer(currentBuffer); |
| * if (currentBuffer.isFull()) |
| * currentBuffer = exchanger.exchange(currentBuffer); |
| * } |
| * } catch (InterruptedException ex) { ... handle ... } |
| * } |
| * } |
| * |
| * class EmptyingLoop implements Runnable { |
| * public void run() { |
| * DataBuffer currentBuffer = initialFullBuffer; |
| * try { |
| * while (currentBuffer != null) { |
| * takeFromBuffer(currentBuffer); |
| * if (currentBuffer.isEmpty()) |
| * currentBuffer = exchanger.exchange(currentBuffer); |
| * } |
| * } catch (InterruptedException ex) { ... handle ...} |
| * } |
| * } |
| * |
| * void start() { |
| * new Thread(new FillingLoop()).start(); |
| * new Thread(new EmptyingLoop()).start(); |
| * } |
| * } |
| * }</pre> |
| * |
| * <p>Memory consistency effects: For each pair of threads that |
| * successfully exchange objects via an {@code Exchanger}, actions |
| * prior to the {@code exchange()} in each thread |
| * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
| * those subsequent to a return from the corresponding {@code exchange()} |
| * in the other thread. |
| * |
| * @since 1.5 |
| * @author Doug Lea and Bill Scherer and Michael Scott |
| * @param <V> The type of objects that may be exchanged |
| */ |
| public class Exchanger<V> { |
| /* |
| * Algorithm Description: |
| * |
| * The basic idea is to maintain a "slot", which is a reference to |
| * a Node containing both an Item to offer and a "hole" waiting to |
| * get filled in. If an incoming "occupying" thread sees that the |
| * slot is null, it CAS'es (compareAndSets) a Node there and waits |
| * for another to invoke exchange. That second "fulfilling" thread |
| * sees that the slot is non-null, and so CASes it back to null, |
| * also exchanging items by CASing the hole, plus waking up the |
| * occupying thread if it is blocked. In each case CAS'es may |
| * fail because a slot at first appears non-null but is null upon |
| * CAS, or vice-versa. So threads may need to retry these |
| * actions. |
| * |
| * This simple approach works great when there are only a few |
| * threads using an Exchanger, but performance rapidly |
| * deteriorates due to CAS contention on the single slot when |
| * there are lots of threads using an exchanger. So instead we use |
| * an "arena"; basically a kind of hash table with a dynamically |
| * varying number of slots, any one of which can be used by |
| * threads performing an exchange. Incoming threads pick slots |
| * based on a hash of their Thread ids. If an incoming thread |
| * fails to CAS in its chosen slot, it picks an alternative slot |
| * instead. And similarly from there. If a thread successfully |
| * CASes into a slot but no other thread arrives, it tries |
| * another, heading toward the zero slot, which always exists even |
| * if the table shrinks. The particular mechanics controlling this |
| * are as follows: |
| * |
| * Waiting: Slot zero is special in that it is the only slot that |
| * exists when there is no contention. A thread occupying slot |
| * zero will block if no thread fulfills it after a short spin. |
| * In other cases, occupying threads eventually give up and try |
| * another slot. Waiting threads spin for a while (a period that |
| * should be a little less than a typical context-switch time) |
| * before either blocking (if slot zero) or giving up (if other |
| * slots) and restarting. There is no reason for threads to block |
| * unless there are unlikely to be any other threads present. |
| * Occupants are mainly avoiding memory contention so sit there |
| * quietly polling for a shorter period than it would take to |
| * block and then unblock them. Non-slot-zero waits that elapse |
| * because of lack of other threads waste around one extra |
| * context-switch time per try, which is still on average much |
| * faster than alternative approaches. |
| * |
| * Sizing: Usually, using only a few slots suffices to reduce |
| * contention. Especially with small numbers of threads, using |
| * too many slots can lead to just as poor performance as using |
| * too few of them, and there's not much room for error. The |
| * variable "max" maintains the number of slots actually in |
| * use. It is increased when a thread sees too many CAS |
| * failures. (This is analogous to resizing a regular hash table |
| * based on a target load factor, except here, growth steps are |
| * just one-by-one rather than proportional.) Growth requires |
| * contention failures in each of three tried slots. Requiring |
| * multiple failures for expansion copes with the fact that some |
| * failed CASes are not due to contention but instead to simple |
| * races between two threads or thread pre-emptions occurring |
| * between reading and CASing. Also, very transient peak |
| * contention can be much higher than the average sustainable |
| * levels. The max limit is decreased on average 50% of the times |
| * that a non-slot-zero wait elapses without being fulfilled. |
| * Threads experiencing elapsed waits move closer to zero, so |
| * eventually find existing (or future) threads even if the table |
| * has been shrunk due to inactivity. The chosen mechanics and |
| * thresholds for growing and shrinking are intrinsically |
| * entangled with indexing and hashing inside the exchange code, |
| * and can't be nicely abstracted out. |
| * |
| * Hashing: Each thread picks its initial slot to use in accord |
| * with a simple hashcode. The sequence is the same on each |
| * encounter by any given thread, but effectively random across |
| * threads. Using arenas encounters the classic cost vs quality |
| * tradeoffs of all hash tables. Here, we use a one-step FNV-1a |
| * hash code based on the current thread's Thread.getId(), along |
| * with a cheap approximation to a mod operation to select an |
| * index. The downside of optimizing index selection in this way |
| * is that the code is hardwired to use a maximum table size of |
| * 32. But this value more than suffices for known platforms and |
| * applications. |
| * |
| * Probing: On sensed contention of a selected slot, we probe |
| * sequentially through the table, analogously to linear probing |
| * after collision in a hash table. (We move circularly, in |
| * reverse order, to mesh best with table growth and shrinkage |
| * rules.) Except that to minimize the effects of false-alarms |
| * and cache thrashing, we try the first selected slot twice |
| * before moving. |
| * |
| * Padding: Even with contention management, slots are heavily |
| * contended, so use cache-padding to avoid poor memory |
| * performance. Because of this, slots are lazily constructed |
| * only when used, to avoid wasting this space unnecessarily. |
| * While isolation of locations is not much of an issue at first |
| * in an application, as time goes on and garbage-collectors |
| * perform compaction, slots are very likely to be moved adjacent |
| * to each other, which can cause much thrashing of cache lines on |
| * MPs unless padding is employed. |
| * |
| * This is an improvement of the algorithm described in the paper |
| * "A Scalable Elimination-based Exchange Channel" by William |
| * Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05 |
| * workshop. Available at: http://hdl.handle.net/1802/2104 |
| */ |
| |
| /** The number of CPUs, for sizing and spin control */ |
| private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
| |
| /** |
| * The capacity of the arena. Set to a value that provides more |
| * than enough space to handle contention. On small machines |
| * most slots won't be used, but it is still not wasted because |
| * the extra space provides some machine-level address padding |
| * to minimize interference with heavily CAS'ed Slot locations. |
| * And on very large machines, performance eventually becomes |
| * bounded by memory bandwidth, not numbers of threads/CPUs. |
| * This constant cannot be changed without also modifying |
| * indexing and hashing algorithms. |
| */ |
| private static final int CAPACITY = 32; |
| |
| /** |
| * The value of "max" that will hold all threads without |
| * contention. When this value is less than CAPACITY, some |
| * otherwise wasted expansion can be avoided. |
| */ |
| private static final int FULL = |
| Math.max(0, Math.min(CAPACITY, NCPU / 2) - 1); |
| |
| /** |
| * The number of times to spin (doing nothing except polling a |
| * memory location) before blocking or giving up while waiting to |
| * be fulfilled. Should be zero on uniprocessors. On |
| * multiprocessors, this value should be large enough so that two |
| * threads exchanging items as fast as possible block only when |
| * one of them is stalled (due to GC or preemption), but not much |
| * longer, to avoid wasting CPU resources. Seen differently, this |
| * value is a little over half the number of cycles of an average |
| * context switch time on most systems. The value here is |
| * approximately the average of those across a range of tested |
| * systems. |
| */ |
| private static final int SPINS = (NCPU == 1) ? 0 : 2000; |
| |
| /** |
| * The number of times to spin before blocking in timed waits. |
| * Timed waits spin more slowly because checking the time takes |
| * time. The best value relies mainly on the relative rate of |
| * System.nanoTime vs memory accesses. The value is empirically |
| * derived to work well across a variety of systems. |
| */ |
| private static final int TIMED_SPINS = SPINS / 20; |
| |
| /** |
| * Sentinel item representing cancellation of a wait due to |
| * interruption, timeout, or elapsed spin-waits. This value is |
| * placed in holes on cancellation, and used as a return value |
| * from waiting methods to indicate failure to set or get hole. |
| */ |
| private static final Object CANCEL = new Object(); |
| |
| /** |
| * Value representing null arguments/returns from public |
| * methods. This disambiguates from internal requirement that |
| * holes start out as null to mean they are not yet set. |
| */ |
| private static final Object NULL_ITEM = new Object(); |
| |
| /** |
| * Nodes hold partially exchanged data. This class |
| * opportunistically subclasses AtomicReference to represent the |
| * hole. So get() returns hole, and compareAndSet CAS'es value |
| * into hole. This class cannot be parameterized as "V" because |
| * of the use of non-V CANCEL sentinels. |
| */ |
| private static final class Node extends AtomicReference<Object> { |
| /** The element offered by the Thread creating this node. */ |
| public final Object item; |
| |
| /** The Thread waiting to be signalled; null until waiting. */ |
| public volatile Thread waiter; |
| |
| /** |
| * Creates node with given item and empty hole. |
| * @param item the item |
| */ |
| public Node(Object item) { |
| this.item = item; |
| } |
| } |
| |
| /** |
| * A Slot is an AtomicReference with heuristic padding to lessen |
| * cache effects of this heavily CAS'ed location. While the |
| * padding adds noticeable space, all slots are created only on |
| * demand, and there will be more than one of them only when it |
| * would improve throughput more than enough to outweigh using |
| * extra space. |
| */ |
| private static final class Slot extends AtomicReference<Object> { |
| // Improve likelihood of isolation on <= 64 byte cache lines |
| long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe; |
| } |
| |
| /** |
| * Slot array. Elements are lazily initialized when needed. |
| * Declared volatile to enable double-checked lazy construction. |
| */ |
| private volatile Slot[] arena = new Slot[CAPACITY]; |
| |
| /** |
| * The maximum slot index being used. The value sometimes |
| * increases when a thread experiences too many CAS contentions, |
| * and sometimes decreases when a spin-wait elapses. Changes |
| * are performed only via compareAndSet, to avoid stale values |
| * when a thread happens to stall right before setting. |
| */ |
| private final AtomicInteger max = new AtomicInteger(); |
| |
| /** |
| * Main exchange function, handling the different policy variants. |
| * Uses Object, not "V" as argument and return value to simplify |
| * handling of sentinel values. Callers from public methods decode |
| * and cast accordingly. |
| * |
| * @param item the (non-null) item to exchange |
| * @param timed true if the wait is timed |
| * @param nanos if timed, the maximum wait time |
| * @return the other thread's item, or CANCEL if interrupted or timed out |
| */ |
| private Object doExchange(Object item, boolean timed, long nanos) { |
| Node me = new Node(item); // Create in case occupying |
| int index = hashIndex(); // Index of current slot |
| int fails = 0; // Number of CAS failures |
| |
| for (;;) { |
| Object y; // Contents of current slot |
| Slot slot = arena[index]; |
| if (slot == null) // Lazily initialize slots |
| createSlot(index); // Continue loop to reread |
| else if ((y = slot.get()) != null && // Try to fulfill |
| slot.compareAndSet(y, null)) { |
| Node you = (Node)y; // Transfer item |
| if (you.compareAndSet(null, item)) { |
| LockSupport.unpark(you.waiter); |
| return you.item; |
| } // Else cancelled; continue |
| } |
| else if (y == null && // Try to occupy |
| slot.compareAndSet(null, me)) { |
| if (index == 0) // Blocking wait for slot 0 |
| return timed? awaitNanos(me, slot, nanos): await(me, slot); |
| Object v = spinWait(me, slot); // Spin wait for non-0 |
| if (v != CANCEL) |
| return v; |
| me = new Node(item); // Throw away cancelled node |
| int m = max.get(); |
| if (m > (index >>>= 1)) // Decrease index |
| max.compareAndSet(m, m - 1); // Maybe shrink table |
| } |
| else if (++fails > 1) { // Allow 2 fails on 1st slot |
| int m = max.get(); |
| if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1)) |
| index = m + 1; // Grow on 3rd failed slot |
| else if (--index < 0) |
| index = m; // Circularly traverse |
| } |
| } |
| } |
| |
| /** |
| * Returns a hash index for the current thread. Uses a one-step |
| * FNV-1a hash code (http://www.isthe.com/chongo/tech/comp/fnv/) |
| * based on the current thread's Thread.getId(). These hash codes |
| * have more uniform distribution properties with respect to small |
| * moduli (here 1-31) than do other simple hashing functions. |
| * |
| * <p>To return an index between 0 and max, we use a cheap |
| * approximation to a mod operation, that also corrects for bias |
| * due to non-power-of-2 remaindering (see {@link |
| * java.util.Random#nextInt}). Bits of the hashcode are masked |
| * with "nbits", the ceiling power of two of table size (looked up |
| * in a table packed into three ints). If too large, this is |
| * retried after rotating the hash by nbits bits, while forcing new |
| * top bit to 0, which guarantees eventual termination (although |
| * with a non-random-bias). This requires an average of less than |
| * 2 tries for all table sizes, and has a maximum 2% difference |
| * from perfectly uniform slot probabilities when applied to all |
| * possible hash codes for sizes less than 32. |
| * |
| * @return a per-thread-random index, 0 <= index < max |
| */ |
| private final int hashIndex() { |
| long id = Thread.currentThread().getId(); |
| int hash = (((int)(id ^ (id >>> 32))) ^ 0x811c9dc5) * 0x01000193; |
| |
| int m = max.get(); |
| int nbits = (((0xfffffc00 >> m) & 4) | // Compute ceil(log2(m+1)) |
| ((0x000001f8 >>> m) & 2) | // The constants hold |
| ((0xffff00f2 >>> m) & 1)); // a lookup table |
| int index; |
| while ((index = hash & ((1 << nbits) - 1)) > m) // May retry on |
| hash = (hash >>> nbits) | (hash << (33 - nbits)); // non-power-2 m |
| return index; |
| } |
| |
| /** |
| * Creates a new slot at given index. Called only when the slot |
| * appears to be null. Relies on double-check using builtin |
| * locks, since they rarely contend. This in turn relies on the |
| * arena array being declared volatile. |
| * |
| * @param index the index to add slot at |
| */ |
| private void createSlot(int index) { |
| // Create slot outside of lock to narrow sync region |
| Slot newSlot = new Slot(); |
| Slot[] a = arena; |
| synchronized (a) { |
| if (a[index] == null) |
| a[index] = newSlot; |
| } |
| } |
| |
| /** |
| * Tries to cancel a wait for the given node waiting in the given |
| * slot, if so, helping clear the node from its slot to avoid |
| * garbage retention. |
| * |
| * @param node the waiting node |
| * @param slot the slot it is waiting in |
| * @return true if successfully cancelled |
| */ |
| private static boolean tryCancel(Node node, Slot slot) { |
| if (!node.compareAndSet(null, CANCEL)) |
| return false; |
| if (slot.get() == node) // pre-check to minimize contention |
| slot.compareAndSet(node, null); |
| return true; |
| } |
| |
| // Three forms of waiting. Each just different enough not to merge |
| // code with others. |
| |
| /** |
| * Spin-waits for hole for a non-0 slot. Fails if spin elapses |
| * before hole filled. Does not check interrupt, relying on check |
| * in public exchange method to abort if interrupted on entry. |
| * |
| * @param node the waiting node |
| * @return on success, the hole; on failure, CANCEL |
| */ |
| private static Object spinWait(Node node, Slot slot) { |
| int spins = SPINS; |
| for (;;) { |
| Object v = node.get(); |
| if (v != null) |
| return v; |
| else if (spins > 0) |
| --spins; |
| else |
| tryCancel(node, slot); |
| } |
| } |
| |
| /** |
| * Waits for (by spinning and/or blocking) and gets the hole |
| * filled in by another thread. Fails if interrupted before |
| * hole filled. |
| * |
| * When a node/thread is about to block, it sets its waiter field |
| * and then rechecks state at least one more time before actually |
| * parking, thus covering race vs fulfiller noticing that waiter |
| * is non-null so should be woken. |
| * |
| * Thread interruption status is checked only surrounding calls to |
| * park. The caller is assumed to have checked interrupt status |
| * on entry. |
| * |
| * @param node the waiting node |
| * @return on success, the hole; on failure, CANCEL |
| */ |
| private static Object await(Node node, Slot slot) { |
| Thread w = Thread.currentThread(); |
| int spins = SPINS; |
| for (;;) { |
| Object v = node.get(); |
| if (v != null) |
| return v; |
| else if (spins > 0) // Spin-wait phase |
| --spins; |
| else if (node.waiter == null) // Set up to block next |
| node.waiter = w; |
| else if (w.isInterrupted()) // Abort on interrupt |
| tryCancel(node, slot); |
| else // Block |
| LockSupport.park(node); |
| } |
| } |
| |
| /** |
| * Waits for (at index 0) and gets the hole filled in by another |
| * thread. Fails if timed out or interrupted before hole filled. |
| * Same basic logic as untimed version, but a bit messier. |
| * |
| * @param node the waiting node |
| * @param nanos the wait time |
| * @return on success, the hole; on failure, CANCEL |
| */ |
| private Object awaitNanos(Node node, Slot slot, long nanos) { |
| int spins = TIMED_SPINS; |
| long lastTime = 0; |
| Thread w = null; |
| for (;;) { |
| Object v = node.get(); |
| if (v != null) |
| return v; |
| long now = System.nanoTime(); |
| if (w == null) |
| w = Thread.currentThread(); |
| else |
| nanos -= now - lastTime; |
| lastTime = now; |
| if (nanos > 0) { |
| if (spins > 0) |
| --spins; |
| else if (node.waiter == null) |
| node.waiter = w; |
| else if (w.isInterrupted()) |
| tryCancel(node, slot); |
| else |
| LockSupport.parkNanos(node, nanos); |
| } |
| else if (tryCancel(node, slot) && !w.isInterrupted()) |
| return scanOnTimeout(node); |
| } |
| } |
| |
| /** |
| * Sweeps through arena checking for any waiting threads. Called |
| * only upon return from timeout while waiting in slot 0. When a |
| * thread gives up on a timed wait, it is possible that a |
| * previously-entered thread is still waiting in some other |
| * slot. So we scan to check for any. This is almost always |
| * overkill, but decreases the likelihood of timeouts when there |
| * are other threads present to far less than that in lock-based |
| * exchangers in which earlier-arriving threads may still be |
| * waiting on entry locks. |
| * |
| * @param node the waiting node |
| * @return another thread's item, or CANCEL |
| */ |
| private Object scanOnTimeout(Node node) { |
| Object y; |
| for (int j = arena.length - 1; j >= 0; --j) { |
| Slot slot = arena[j]; |
| if (slot != null) { |
| while ((y = slot.get()) != null) { |
| if (slot.compareAndSet(y, null)) { |
| Node you = (Node)y; |
| if (you.compareAndSet(null, node.item)) { |
| LockSupport.unpark(you.waiter); |
| return you.item; |
| } |
| } |
| } |
| } |
| } |
| return CANCEL; |
| } |
| |
| /** |
| * Creates a new Exchanger. |
| */ |
| public Exchanger() { |
| } |
| |
| /** |
| * Waits for another thread to arrive at this exchange point (unless |
| * the current thread is {@linkplain Thread#interrupt interrupted}), |
| * and then transfers the given object to it, receiving its object |
| * in return. |
| * |
| * <p>If another thread is already waiting at the exchange point then |
| * it is resumed for thread scheduling purposes and receives the object |
| * passed in by the current thread. The current thread returns immediately, |
| * receiving the object passed to the exchange by that other thread. |
| * |
| * <p>If no other thread is already waiting at the exchange then the |
| * current thread is disabled for thread scheduling purposes and lies |
| * dormant until one of two things happens: |
| * <ul> |
| * <li>Some other thread enters the exchange; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current |
| * thread. |
| * </ul> |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * for the exchange, |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * @param x the object to exchange |
| * @return the object provided by the other thread |
| * @throws InterruptedException if the current thread was |
| * interrupted while waiting |
| */ |
| public V exchange(V x) throws InterruptedException { |
| if (!Thread.interrupted()) { |
| Object v = doExchange(x == null? NULL_ITEM : x, false, 0); |
| if (v == NULL_ITEM) |
| return null; |
| if (v != CANCEL) |
| return (V)v; |
| Thread.interrupted(); // Clear interrupt status on IE throw |
| } |
| throw new InterruptedException(); |
| } |
| |
| /** |
| * Waits for another thread to arrive at this exchange point (unless |
| * the current thread is {@linkplain Thread#interrupt interrupted} or |
| * the specified waiting time elapses), and then transfers the given |
| * object to it, receiving its object in return. |
| * |
| * <p>If another thread is already waiting at the exchange point then |
| * it is resumed for thread scheduling purposes and receives the object |
| * passed in by the current thread. The current thread returns immediately, |
| * receiving the object passed to the exchange by that other thread. |
| * |
| * <p>If no other thread is already waiting at the exchange then the |
| * current thread is disabled for thread scheduling purposes and lies |
| * dormant until one of three things happens: |
| * <ul> |
| * <li>Some other thread enters the exchange; or |
| * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| * the current thread; or |
| * <li>The specified waiting time elapses. |
| * </ul> |
| * <p>If the current thread: |
| * <ul> |
| * <li>has its interrupted status set on entry to this method; or |
| * <li>is {@linkplain Thread#interrupt interrupted} while waiting |
| * for the exchange, |
| * </ul> |
| * then {@link InterruptedException} is thrown and the current thread's |
| * interrupted status is cleared. |
| * |
| * <p>If the specified waiting time elapses then {@link |
| * TimeoutException} is thrown. If the time is less than or equal |
| * to zero, the method will not wait at all. |
| * |
| * @param x the object to exchange |
| * @param timeout the maximum time to wait |
| * @param unit the time unit of the <tt>timeout</tt> argument |
| * @return the object provided by the other thread |
| * @throws InterruptedException if the current thread was |
| * interrupted while waiting |
| * @throws TimeoutException if the specified waiting time elapses |
| * before another thread enters the exchange |
| */ |
| public V exchange(V x, long timeout, TimeUnit unit) |
| throws InterruptedException, TimeoutException { |
| if (!Thread.interrupted()) { |
| Object v = doExchange(x == null? NULL_ITEM : x, |
| true, unit.toNanos(timeout)); |
| if (v == NULL_ITEM) |
| return null; |
| if (v != CANCEL) |
| return (V)v; |
| if (!Thread.interrupted()) |
| throw new TimeoutException(); |
| } |
| throw new InterruptedException(); |
| } |
| } |
