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/* |
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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/publicdomain/zero/1.0/ |
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*/ |
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|
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package java.util.concurrent; |
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import java.util.concurrent.atomic.*; |
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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* A synchronization point at which threads can pair and swap elements |
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* within pairs. Each thread presents some object on entry to the |
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* {@link #exchange exchange} method, matches with a partner thread, |
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* and receives its partner's object on return. An Exchanger may be |
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* viewed as a bidirectional form of a {@link SynchronousQueue}. |
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* Exchangers may be useful in applications such as genetic algorithms |
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* and pipeline designs. |
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* |
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* <p><b>Sample Usage:</b> |
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* Here are the highlights of a class that uses an {@code Exchanger} |
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* to swap buffers between threads so that the thread filling the |
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* buffer gets a freshly emptied one when it needs it, handing off the |
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* filled one to the thread emptying the buffer. |
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* <pre> {@code |
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* class FillAndEmpty { |
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* Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>(); |
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* DataBuffer initialEmptyBuffer = ... a made-up type |
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* DataBuffer initialFullBuffer = ... |
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* |
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* class FillingLoop implements Runnable { |
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* public void run() { |
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* DataBuffer currentBuffer = initialEmptyBuffer; |
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* try { |
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* while (currentBuffer != null) { |
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* addToBuffer(currentBuffer); |
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* if (currentBuffer.isFull()) |
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* currentBuffer = exchanger.exchange(currentBuffer); |
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* } |
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* } catch (InterruptedException ex) { ... handle ... } |
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* } |
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* } |
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* |
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* class EmptyingLoop implements Runnable { |
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* public void run() { |
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* DataBuffer currentBuffer = initialFullBuffer; |
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* try { |
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* while (currentBuffer != null) { |
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* takeFromBuffer(currentBuffer); |
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* if (currentBuffer.isEmpty()) |
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* currentBuffer = exchanger.exchange(currentBuffer); |
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* } |
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* } catch (InterruptedException ex) { ... handle ...} |
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* } |
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* } |
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* |
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* void start() { |
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* new Thread(new FillingLoop()).start(); |
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* new Thread(new EmptyingLoop()).start(); |
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* } |
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* }}</pre> |
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* |
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* <p>Memory consistency effects: For each pair of threads that |
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* successfully exchange objects via an {@code Exchanger}, actions |
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* prior to the {@code exchange()} in each thread |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* those subsequent to a return from the corresponding {@code exchange()} |
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* in the other thread. |
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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 <V> The type of objects that may be exchanged |
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*/ |
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public class Exchanger<V> { |
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/* |
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* Algorithm Description: |
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* |
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* The basic idea is to maintain a "slot", which is a reference to |
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* a Node containing both an Item to offer and a "hole" waiting to |
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* get filled in. If an incoming "occupying" thread sees that the |
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* slot is null, it CAS'es (compareAndSets) a Node there and waits |
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* for another to invoke exchange. That second "fulfilling" thread |
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* sees that the slot is non-null, and so CASes it back to null, |
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* also exchanging items by CASing the hole, plus waking up the |
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* occupying thread if it is blocked. In each case CAS'es may |
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* fail because a slot at first appears non-null but is null upon |
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* CAS, or vice-versa. So threads may need to retry these |
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* actions. |
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* |
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* This simple approach works great when there are only a few |
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* threads using an Exchanger, but performance rapidly |
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* deteriorates due to CAS contention on the single slot when |
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* there are lots of threads using an exchanger. So instead we use |
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* an "arena"; basically a kind of hash table with a dynamically |
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* varying number of slots, any one of which can be used by |
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* threads performing an exchange. Incoming threads pick slots |
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* based on a hash of their Thread ids. If an incoming thread |
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* fails to CAS in its chosen slot, it picks an alternative slot |
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* instead. And similarly from there. If a thread successfully |
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* CASes into a slot but no other thread arrives, it tries |
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* another, heading toward the zero slot, which always exists even |
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* if the table shrinks. The particular mechanics controlling this |
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* are as follows: |
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* |
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* Waiting: Slot zero is special in that it is the only slot that |
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* exists when there is no contention. A thread occupying slot |
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* zero will block if no thread fulfills it after a short spin. |
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* In other cases, occupying threads eventually give up and try |
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* another slot. Waiting threads spin for a while (a period that |
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* should be a little less than a typical context-switch time) |
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* before either blocking (if slot zero) or giving up (if other |
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* slots) and restarting. There is no reason for threads to block |
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* unless there are unlikely to be any other threads present. |
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* Occupants are mainly avoiding memory contention so sit there |
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* quietly polling for a shorter period than it would take to |
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* block and then unblock them. Non-slot-zero waits that elapse |
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* because of lack of other threads waste around one extra |
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* context-switch time per try, which is still on average much |
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* faster than alternative approaches. |
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* |
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* Sizing: Usually, using only a few slots suffices to reduce |
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* contention. Especially with small numbers of threads, using |
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* too many slots can lead to just as poor performance as using |
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* too few of them, and there's not much room for error. The |
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* variable "max" maintains the number of slots actually in |
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* use. It is increased when a thread sees too many CAS |
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* failures. (This is analogous to resizing a regular hash table |
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* based on a target load factor, except here, growth steps are |
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* just one-by-one rather than proportional.) Growth requires |
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* contention failures in each of three tried slots. Requiring |
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* multiple failures for expansion copes with the fact that some |
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* failed CASes are not due to contention but instead to simple |
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* races between two threads or thread pre-emptions occurring |
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* between reading and CASing. Also, very transient peak |
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* contention can be much higher than the average sustainable |
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* levels. An attempt to decrease the max limit is usually made |
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* when a non-slot-zero wait elapses without being fulfilled. |
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* Threads experiencing elapsed waits move closer to zero, so |
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* eventually find existing (or future) threads even if the table |
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* has been shrunk due to inactivity. The chosen mechanics and |
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* thresholds for growing and shrinking are intrinsically |
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* entangled with indexing and hashing inside the exchange code, |
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* and can't be nicely abstracted out. |
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* |
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* Hashing: Each thread picks its initial slot to use in accord |
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* with a simple hashcode. The sequence is the same on each |
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* encounter by any given thread, but effectively random across |
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* threads. Using arenas encounters the classic cost vs quality |
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* tradeoffs of all hash tables. Here, we use a one-step FNV-1a |
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* hash code based on the current thread's Thread.getId(), along |
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* with a cheap approximation to a mod operation to select an |
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* index. The downside of optimizing index selection in this way |
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* is that the code is hardwired to use a maximum table size of |
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* 32. But this value more than suffices for known platforms and |
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* applications. |
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* |
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* Probing: On sensed contention of a selected slot, we probe |
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* sequentially through the table, analogously to linear probing |
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* after collision in a hash table. (We move circularly, in |
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* reverse order, to mesh best with table growth and shrinkage |
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* rules.) Except that to minimize the effects of false-alarms |
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* and cache thrashing, we try the first selected slot twice |
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* before moving. |
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* |
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* Padding: Even with contention management, slots are heavily |
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* contended, so use cache-padding to avoid poor memory |
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* performance. Because of this, slots are lazily constructed |
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* only when used, to avoid wasting this space unnecessarily. |
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* While isolation of locations is not much of an issue at first |
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* in an application, as time goes on and garbage-collectors |
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* perform compaction, slots are very likely to be moved adjacent |
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* to each other, which can cause much thrashing of cache lines on |
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* MPs unless padding is employed. |
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* |
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* This is an improvement of the algorithm described in the paper |
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* "A Scalable Elimination-based Exchange Channel" by William |
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* Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05 |
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* workshop. Available at: http://hdl.handle.net/1802/2104 |
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*/ |
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|
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/** The number of CPUs, for sizing and spin control */ |
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private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
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|
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/** |
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* The capacity of the arena. Set to a value that provides more |
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* than enough space to handle contention. On small machines |
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* most slots won't be used, but it is still not wasted because |
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* the extra space provides some machine-level address padding |
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* to minimize interference with heavily CAS'ed Slot locations. |
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* And on very large machines, performance eventually becomes |
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* bounded by memory bandwidth, not numbers of threads/CPUs. |
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* This constant cannot be changed without also modifying |
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* indexing and hashing algorithms. |
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*/ |
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private static final int CAPACITY = 32; |
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|
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/** |
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* The value of "max" that will hold all threads without |
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* contention. When this value is less than CAPACITY, some |
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* otherwise wasted expansion can be avoided. |
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*/ |
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private static final int FULL = |
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Math.max(0, Math.min(CAPACITY, NCPU / 2) - 1); |
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|
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/** |
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* The number of times to spin (doing nothing except polling a |
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* memory location) before blocking or giving up while waiting to |
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* be fulfilled. Should be zero on uniprocessors. On |
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* multiprocessors, this value should be large enough so that two |
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* threads exchanging items as fast as possible block only when |
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* one of them is stalled (due to GC or preemption), but not much |
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* longer, to avoid wasting CPU resources. Seen differently, this |
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* value is a little over half the number of cycles of an average |
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* context switch time on most systems. The value here is |
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* approximately the average of those across a range of tested |
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* systems. |
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*/ |
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private static final int SPINS = (NCPU == 1) ? 0 : 2000; |
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|
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/** |
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* The number of times to spin before blocking in timed waits. |
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* Timed waits spin more slowly because checking the time takes |
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* time. The best value relies mainly on the relative rate of |
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* System.nanoTime vs memory accesses. The value is empirically |
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* derived to work well across a variety of systems. |
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*/ |
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private static final int TIMED_SPINS = SPINS / 20; |
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|
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/** |
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* Sentinel item representing cancellation of a wait due to |
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* interruption, timeout, or elapsed spin-waits. This value is |
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* placed in holes on cancellation, and used as a return value |
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* from waiting methods to indicate failure to set or get hole. |
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*/ |
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private static final Object CANCEL = new Object(); |
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|
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/** |
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* Value representing null arguments/returns from public |
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* methods. This disambiguates from internal requirement that |
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* holes start out as null to mean they are not yet set. |
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*/ |
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private static final Object NULL_ITEM = new Object(); |
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|
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/** |
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* Nodes hold partially exchanged data. This class |
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* opportunistically subclasses AtomicReference to represent the |
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* hole. So get() returns hole, and compareAndSet CAS'es value |
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* into hole. This class cannot be parameterized as "V" because |
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* of the use of non-V CANCEL sentinels. |
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*/ |
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@SuppressWarnings("serial") |
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private static final class Node extends AtomicReference<Object> { |
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/** The element offered by the Thread creating this node. */ |
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public final Object item; |
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|
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/** The Thread waiting to be signalled; null until waiting. */ |
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public volatile Thread waiter; |
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|
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/** |
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* Creates node with given item and empty hole. |
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* @param item the item |
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*/ |
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public Node(Object item) { |
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this.item = item; |
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} |
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} |
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|
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/** |
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* A Slot is an AtomicReference with heuristic padding to lessen |
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* cache effects of this heavily CAS'ed location. While the |
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* padding adds noticeable space, all slots are created only on |
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* demand, and there will be more than one of them only when it |
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* would improve throughput more than enough to outweigh using |
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* extra space. |
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*/ |
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@SuppressWarnings("serial") |
278 |
private static final class Slot extends AtomicReference<Object> { |
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// Improve likelihood of isolation on <= 128 byte cache lines. |
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// We used to target 64 byte cache lines, but some x86s (including |
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// i7 under some BIOSes) actually use 128 byte cache lines. |
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long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe; |
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} |
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|
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/** |
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* Slot array. Elements are lazily initialized when needed. |
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* Declared volatile to enable double-checked lazy construction. |
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*/ |
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private volatile Slot[] arena = new Slot[CAPACITY]; |
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|
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/** |
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* The maximum slot index being used. The value sometimes |
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* increases when a thread experiences too many CAS contentions, |
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* and sometimes decreases when a spin-wait elapses. Changes |
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* are performed only via compareAndSet, to avoid stale values |
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* when a thread happens to stall right before setting. |
297 |
*/ |
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private final AtomicInteger max = new AtomicInteger(); |
299 |
|
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/** |
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* Main exchange function, handling the different policy variants. |
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* Uses Object, not "V" as argument and return value to simplify |
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* handling of sentinel values. Callers from public methods decode |
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* and cast accordingly. |
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* |
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* @param item the (non-null) item to exchange |
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* @param timed true if the wait is timed |
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* @param nanos if timed, the maximum wait time |
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* @return the other thread's item, or CANCEL if interrupted or timed out |
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*/ |
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private Object doExchange(Object item, boolean timed, long nanos) { |
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Node me = new Node(item); // Create in case occupying |
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int index = hashIndex(); // Index of current slot |
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int fails = 0; // Number of CAS failures |
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|
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for (;;) { |
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Object y; // Contents of current slot |
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Slot slot = arena[index]; |
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if (slot == null) // Lazily initialize slots |
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createSlot(index); // Continue loop to reread |
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else if ((y = slot.get()) != null && // Try to fulfill |
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slot.compareAndSet(y, null)) { |
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Node you = (Node)y; // Transfer item |
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if (you.compareAndSet(null, item)) { |
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LockSupport.unpark(you.waiter); |
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return you.item; |
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} // Else cancelled; continue |
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} |
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else if (y == null && // Try to occupy |
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slot.compareAndSet(null, me)) { |
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if (index == 0) // Blocking wait for slot 0 |
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return timed ? |
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awaitNanos(me, slot, nanos) : |
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await(me, slot); |
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Object v = spinWait(me, slot); // Spin wait for non-0 |
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if (v != CANCEL) |
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return v; |
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me = new Node(item); // Throw away cancelled node |
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int m = max.get(); |
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if (m > (index >>>= 1)) // Decrease index |
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max.compareAndSet(m, m - 1); // Maybe shrink table |
342 |
} |
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else if (++fails > 1) { // Allow 2 fails on 1st slot |
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int m = max.get(); |
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if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1)) |
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index = m + 1; // Grow on 3rd failed slot |
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else if (--index < 0) |
348 |
index = m; // Circularly traverse |
349 |
} |
350 |
} |
351 |
} |
352 |
|
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/** |
354 |
* Returns a hash index for the current thread. Uses a one-step |
355 |
* FNV-1a hash code (http://www.isthe.com/chongo/tech/comp/fnv/) |
356 |
* based on the current thread's Thread.getId(). These hash codes |
357 |
* have more uniform distribution properties with respect to small |
358 |
* moduli (here 1-31) than do other simple hashing functions. |
359 |
* |
360 |
* <p>To return an index between 0 and max, we use a cheap |
361 |
* approximation to a mod operation, that also corrects for bias |
362 |
* due to non-power-of-2 remaindering (see {@link |
363 |
* java.util.Random#nextInt}). Bits of the hashcode are masked |
364 |
* with "nbits", the ceiling power of two of table size (looked up |
365 |
* in a table packed into three ints). If too large, this is |
366 |
* retried after rotating the hash by nbits bits, while forcing new |
367 |
* top bit to 0, which guarantees eventual termination (although |
368 |
* with a non-random-bias). This requires an average of less than |
369 |
* 2 tries for all table sizes, and has a maximum 2% difference |
370 |
* from perfectly uniform slot probabilities when applied to all |
371 |
* possible hash codes for sizes less than 32. |
372 |
* |
373 |
* @return a per-thread-random index, 0 <= index < max |
374 |
*/ |
375 |
private final int hashIndex() { |
376 |
long id = Thread.currentThread().getId(); |
377 |
int hash = (((int)(id ^ (id >>> 32))) ^ 0x811c9dc5) * 0x01000193; |
378 |
|
379 |
int m = max.get(); |
380 |
int nbits = (((0xfffffc00 >> m) & 4) | // Compute ceil(log2(m+1)) |
381 |
((0x000001f8 >>> m) & 2) | // The constants hold |
382 |
((0xffff00f2 >>> m) & 1)); // a lookup table |
383 |
int index; |
384 |
while ((index = hash & ((1 << nbits) - 1)) > m) // May retry on |
385 |
hash = (hash >>> nbits) | (hash << (33 - nbits)); // non-power-2 m |
386 |
return index; |
387 |
} |
388 |
|
389 |
/** |
390 |
* Creates a new slot at given index. Called only when the slot |
391 |
* appears to be null. Relies on double-check using builtin |
392 |
* locks, since they rarely contend. This in turn relies on the |
393 |
* arena array being declared volatile. |
394 |
* |
395 |
* @param index the index to add slot at |
396 |
*/ |
397 |
private void createSlot(int index) { |
398 |
// Create slot outside of lock to narrow sync region |
399 |
Slot newSlot = new Slot(); |
400 |
Slot[] a = arena; |
401 |
synchronized (a) { |
402 |
if (a[index] == null) |
403 |
a[index] = newSlot; |
404 |
} |
405 |
} |
406 |
|
407 |
/** |
408 |
* Tries to cancel a wait for the given node waiting in the given |
409 |
* slot, if so, helping clear the node from its slot to avoid |
410 |
* garbage retention. |
411 |
* |
412 |
* @param node the waiting node |
413 |
* @param the slot it is waiting in |
414 |
* @return true if successfully cancelled |
415 |
*/ |
416 |
private static boolean tryCancel(Node node, Slot slot) { |
417 |
if (!node.compareAndSet(null, CANCEL)) |
418 |
return false; |
419 |
if (slot.get() == node) // pre-check to minimize contention |
420 |
slot.compareAndSet(node, null); |
421 |
return true; |
422 |
} |
423 |
|
424 |
// Three forms of waiting. Each just different enough not to merge |
425 |
// code with others. |
426 |
|
427 |
/** |
428 |
* Spin-waits for hole for a non-0 slot. Fails if spin elapses |
429 |
* before hole filled. Does not check interrupt, relying on check |
430 |
* in public exchange method to abort if interrupted on entry. |
431 |
* |
432 |
* @param node the waiting node |
433 |
* @return on success, the hole; on failure, CANCEL |
434 |
*/ |
435 |
private static Object spinWait(Node node, Slot slot) { |
436 |
int spins = SPINS; |
437 |
for (;;) { |
438 |
Object v = node.get(); |
439 |
if (v != null) |
440 |
return v; |
441 |
else if (spins > 0) |
442 |
--spins; |
443 |
else |
444 |
tryCancel(node, slot); |
445 |
} |
446 |
} |
447 |
|
448 |
/** |
449 |
* Waits for (by spinning and/or blocking) and gets the hole |
450 |
* filled in by another thread. Fails if interrupted before |
451 |
* hole filled. |
452 |
* |
453 |
* When a node/thread is about to block, it sets its waiter field |
454 |
* and then rechecks state at least one more time before actually |
455 |
* parking, thus covering race vs fulfiller noticing that waiter |
456 |
* is non-null so should be woken. |
457 |
* |
458 |
* Thread interruption status is checked only surrounding calls to |
459 |
* park. The caller is assumed to have checked interrupt status |
460 |
* on entry. |
461 |
* |
462 |
* @param node the waiting node |
463 |
* @return on success, the hole; on failure, CANCEL |
464 |
*/ |
465 |
private static Object await(Node node, Slot slot) { |
466 |
Thread w = Thread.currentThread(); |
467 |
int spins = SPINS; |
468 |
for (;;) { |
469 |
Object v = node.get(); |
470 |
if (v != null) |
471 |
return v; |
472 |
else if (spins > 0) // Spin-wait phase |
473 |
--spins; |
474 |
else if (node.waiter == null) // Set up to block next |
475 |
node.waiter = w; |
476 |
else if (w.isInterrupted()) // Abort on interrupt |
477 |
tryCancel(node, slot); |
478 |
else // Block |
479 |
LockSupport.park(node); |
480 |
} |
481 |
} |
482 |
|
483 |
/** |
484 |
* Waits for (at index 0) and gets the hole filled in by another |
485 |
* thread. Fails if timed out or interrupted before hole filled. |
486 |
* Same basic logic as untimed version, but a bit messier. |
487 |
* |
488 |
* @param node the waiting node |
489 |
* @param nanos the wait time |
490 |
* @return on success, the hole; on failure, CANCEL |
491 |
*/ |
492 |
private Object awaitNanos(Node node, Slot slot, long nanos) { |
493 |
int spins = TIMED_SPINS; |
494 |
long lastTime = 0; |
495 |
Thread w = null; |
496 |
for (;;) { |
497 |
Object v = node.get(); |
498 |
if (v != null) |
499 |
return v; |
500 |
long now = System.nanoTime(); |
501 |
if (w == null) |
502 |
w = Thread.currentThread(); |
503 |
else |
504 |
nanos -= now - lastTime; |
505 |
lastTime = now; |
506 |
if (nanos > 0) { |
507 |
if (spins > 0) |
508 |
--spins; |
509 |
else if (node.waiter == null) |
510 |
node.waiter = w; |
511 |
else if (w.isInterrupted()) |
512 |
tryCancel(node, slot); |
513 |
else |
514 |
LockSupport.parkNanos(node, nanos); |
515 |
} |
516 |
else if (tryCancel(node, slot) && !w.isInterrupted()) |
517 |
return scanOnTimeout(node); |
518 |
} |
519 |
} |
520 |
|
521 |
/** |
522 |
* Sweeps through arena checking for any waiting threads. Called |
523 |
* only upon return from timeout while waiting in slot 0. When a |
524 |
* thread gives up on a timed wait, it is possible that a |
525 |
* previously-entered thread is still waiting in some other |
526 |
* slot. So we scan to check for any. This is almost always |
527 |
* overkill, but decreases the likelihood of timeouts when there |
528 |
* are other threads present to far less than that in lock-based |
529 |
* exchangers in which earlier-arriving threads may still be |
530 |
* waiting on entry locks. |
531 |
* |
532 |
* @param node the waiting node |
533 |
* @return another thread's item, or CANCEL |
534 |
*/ |
535 |
private Object scanOnTimeout(Node node) { |
536 |
Object y; |
537 |
for (int j = arena.length - 1; j >= 0; --j) { |
538 |
Slot slot = arena[j]; |
539 |
if (slot != null) { |
540 |
while ((y = slot.get()) != null) { |
541 |
if (slot.compareAndSet(y, null)) { |
542 |
Node you = (Node)y; |
543 |
if (you.compareAndSet(null, node.item)) { |
544 |
LockSupport.unpark(you.waiter); |
545 |
return you.item; |
546 |
} |
547 |
} |
548 |
} |
549 |
} |
550 |
} |
551 |
return CANCEL; |
552 |
} |
553 |
|
554 |
/** |
555 |
* Creates a new Exchanger. |
556 |
*/ |
557 |
public Exchanger() { |
558 |
} |
559 |
|
560 |
/** |
561 |
* Waits for another thread to arrive at this exchange point (unless |
562 |
* the current thread is {@linkplain Thread#interrupt interrupted}), |
563 |
* and then transfers the given object to it, receiving its object |
564 |
* in return. |
565 |
* |
566 |
* <p>If another thread is already waiting at the exchange point then |
567 |
* it is resumed for thread scheduling purposes and receives the object |
568 |
* passed in by the current thread. The current thread returns immediately, |
569 |
* receiving the object passed to the exchange by that other thread. |
570 |
* |
571 |
* <p>If no other thread is already waiting at the exchange then the |
572 |
* current thread is disabled for thread scheduling purposes and lies |
573 |
* dormant until one of two things happens: |
574 |
* <ul> |
575 |
* <li>Some other thread enters the exchange; or |
576 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
577 |
* the current thread. |
578 |
* </ul> |
579 |
* <p>If the current thread: |
580 |
* <ul> |
581 |
* <li>has its interrupted status set on entry to this method; or |
582 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
583 |
* for the exchange, |
584 |
* </ul> |
585 |
* then {@link InterruptedException} is thrown and the current thread's |
586 |
* interrupted status is cleared. |
587 |
* |
588 |
* @param x the object to exchange |
589 |
* @return the object provided by the other thread |
590 |
* @throws InterruptedException if the current thread was |
591 |
* interrupted while waiting |
592 |
*/ |
593 |
public V exchange(V x) throws InterruptedException { |
594 |
if (!Thread.interrupted()) { |
595 |
Object o = doExchange((x == null) ? NULL_ITEM : x, false, 0); |
596 |
if (o == NULL_ITEM) |
597 |
return null; |
598 |
if (o != CANCEL) { |
599 |
@SuppressWarnings("unchecked") V v = (V)o; |
600 |
return v; |
601 |
} |
602 |
Thread.interrupted(); // Clear interrupt status on IE throw |
603 |
} |
604 |
throw new InterruptedException(); |
605 |
} |
606 |
|
607 |
/** |
608 |
* Waits for another thread to arrive at this exchange point (unless |
609 |
* the current thread is {@linkplain Thread#interrupt interrupted} or |
610 |
* the specified waiting time elapses), and then transfers the given |
611 |
* object to it, receiving its object in return. |
612 |
* |
613 |
* <p>If another thread is already waiting at the exchange point then |
614 |
* it is resumed for thread scheduling purposes and receives the object |
615 |
* passed in by the current thread. The current thread returns immediately, |
616 |
* receiving the object passed to the exchange by that other thread. |
617 |
* |
618 |
* <p>If no other thread is already waiting at the exchange then the |
619 |
* current thread is disabled for thread scheduling purposes and lies |
620 |
* dormant until one of three things happens: |
621 |
* <ul> |
622 |
* <li>Some other thread enters the exchange; or |
623 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
624 |
* the current thread; or |
625 |
* <li>The specified waiting time elapses. |
626 |
* </ul> |
627 |
* <p>If the current thread: |
628 |
* <ul> |
629 |
* <li>has its interrupted status set on entry to this method; or |
630 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
631 |
* for the exchange, |
632 |
* </ul> |
633 |
* then {@link InterruptedException} is thrown and the current thread's |
634 |
* interrupted status is cleared. |
635 |
* |
636 |
* <p>If the specified waiting time elapses then {@link |
637 |
* TimeoutException} is thrown. If the time is less than or equal |
638 |
* to zero, the method will not wait at all. |
639 |
* |
640 |
* @param x the object to exchange |
641 |
* @param timeout the maximum time to wait |
642 |
* @param unit the time unit of the <tt>timeout</tt> argument |
643 |
* @return the object provided by the other thread |
644 |
* @throws InterruptedException if the current thread was |
645 |
* interrupted while waiting |
646 |
* @throws TimeoutException if the specified waiting time elapses |
647 |
* before another thread enters the exchange |
648 |
*/ |
649 |
public V exchange(V x, long timeout, TimeUnit unit) |
650 |
throws InterruptedException, TimeoutException { |
651 |
if (!Thread.interrupted()) { |
652 |
Object o = doExchange((x == null) ? NULL_ITEM : x, |
653 |
true, unit.toNanos(timeout)); |
654 |
if (o == NULL_ITEM) |
655 |
return null; |
656 |
if (o != CANCEL) { |
657 |
@SuppressWarnings("unchecked") V v = (V)o; |
658 |
return v; |
659 |
} |
660 |
if (!Thread.interrupted()) |
661 |
throw new TimeoutException(); |
662 |
} |
663 |
throw new InterruptedException(); |
664 |
} |
665 |
} |