线程屏障源码.java源码CyclicBarrier.java
cyclicbarrier就最下面那么一点点.事实上cyclicbarrier操作很简单.只需要两步就能达到目地.
1.new 一个对象,告诉构造函数最大的支持数量
2.然后就是await提交事务了,就这两步
使用简单,那么实现起来的话思路也应该很简单.其实现的主要代码在dowait里面.
这代码一目了然,讲解的网上一大把.
我在看代码时候,一直没搞清的是lock.lock();这个锁的调用.因为根据我对AQS的理解,这里使用ReentrantLock不公平锁的重入锁来lock的.
而不公平重入锁,只对本线程才会支持可重入.别的线程还是老实的到队列中去等待.
所以我们使用不同线程来提交事务的时候,除了第一次.剩下就都应该阻塞在lock.lock()这个函数调用.因为我们这时候并没有unlock.
但事实上,从网上找的实例运行打印,证明根本没这阻塞在这里.为此我把实现的renntrantLock代码看了一遍又一遍,仍然没有发现没什么没阻塞.最终没办法,我把java源码实现的细节部分,能抠的全抠出来.自己新建class来调用接口看问题出在什么地方
最终调用的打印结果如上.结果就是发现没有unlock的时候,有人主动调用了tryRelease接口.把state变成0了.所以线程调用只阻塞了非常短的时间就获取了锁,导致事实上并没有明显的阻塞.那么这个调用是谁调用的呢,看AQS条件列队的提交实现
看见没.就是上面高亮函数干的事.看代码不仔细,之前看到这里直接跳过去了.没了解原因,导致我为了搞清楚这个问题.浪费了整整大半天.下在把示例代码贴上来.大家有兴趣可以自己试试
BarrierTest.java
package barrier;
import barrier.ThreadTest;
import java.util.concurrent.CyclicBarrier;
public class BarrierTest {
public static void main(String[] args){
ThreadTest cyclicBarrier = new ThreadTest(3);
Thread thread1 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("线程1正在执行");
try {
// 等待
cyclicBarrier.await();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("线程1运行结束，时间： " + System.currentTimeMillis());
}
});
Thread thread2 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("线程2正在执行");
try {
// 等待
cyclicBarrier.await();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("线程2运行结束，时间： " + System.currentTimeMillis());
}
});
Thread thread3 = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("线程3正在执行");
try {
//线程3阻塞2秒，测试效果
// Thread.sleep(2000);
// 等待
cyclicBarrier.await();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("线程3运行结束，时间： " + System.currentTimeMillis());
}
});
thread1.start();
thread2.start();
//thread3.start();
try {
Thread.sleep(2000);
}catch(Exception e) {
}
//thread3.start();
thread2.start();
// thread1.start();
}
}
NonfairSync.java
package barrier;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
public class NonfairSync extends AbstractQueuedSynchronizer{
private static final long serialVersionUID = 7316153563782823691L;
/**
* Performs lock. Try immediate barge, backing up to normal
* acquire on failure.
*/
final void lock() {
if (compareAndSetState(0, 1)) {
System.out.println("compareAndSetState state="+getState());
setExclusiveOwnerThread(Thread.currentThread());
}
else
acquire(1);
}
protected final boolean tryAcquire(int acquires) {
return nonfairTryAcquire(acquires);
}
final boolean nonfairTryAcquire(int acquires) {
final Thread current = Thread.currentThread();
int c = getState();
if (c == 0) {
System.out.println("++++++幽幽");
if (compareAndSetState(0, acquires)) {
setExclusiveOwnerThread(current);
return true;
}
}
else if (current == getExclusiveOwnerThread()) {
System.out.println("++++++current");
int nextc = c + acquires;
if (nextc < 0) // overflow
throw new Error("Maximum lock count exceeded");
setState(nextc);
return true;
}
System.out.println("nonfairTryAcquire +++++++++");
return false;
}
final ConditionObject newCondition() {
return new ConditionObject();
}
public void unlock() {
System.out.println("unlock");
release(1);
}
protected final boolean tryRelease(int releases) {
int c = getState() - releases;
if (Thread.currentThread() != getExclusiveOwnerThread())
throw new IllegalMonitorStateException();
boolean free = false;
if (c == 0) {
free = true;
setExclusiveOwnerThread(null);
}
System.out.println("setState c="+c);
setState(c);
return free;
}
protected final boolean isHeldExclusively() {
// While we must in general read state before owner,
// we don't need to do so to check if current thread is owner
return getExclusiveOwnerThread() == Thread.currentThread();
}
}
ThreadTest.java
package barrier;
import java.util.concurrent.BrokenBarrierException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
public class ThreadTest {
/**
* Each use of the barrier is represented as a generation instance.
* The generation changes whenever the barrier is tripped, or
* is reset. There can be many generations associated with threads
* using the barrier - due to the non-deterministic way the lock
* may be allocated to waiting threads - but only one of these
* can be active at a time (the one to which {@code count} applies)
* and all the rest are either broken or tripped.
* There need not be an active generation if there has been a break
* but no subsequent reset.
*/
private static class Generation {
boolean broken = false;
}
/** The lock for guarding barrier entry */
private final NonfairSync lock = new NonfairSync();
/** Condition to wait on until tripped */
private final Condition trip = lock.newCondition();
/** The number of parties */
private final int parties;
/* The command to run when tripped */
private final Runnable barrierCommand;
/** The current generation */
private Generation generation = new Generation();
/**
* Number of parties still waiting. Counts down from parties to 0
* on each generation. It is reset to parties on each new
* generation or when broken.
*/
private int count;
private transient Thread exclusiveOwnerThread = null;
/**
* Updates state on barrier trip and wakes up everyone.
* Called only while holding lock.
*/
private void nextGeneration() {
// signal completion of last generation
trip.signalAll();
// set up next generation
count = parties;
generation = new Generation();
}
/**
* Sets current barrier generation as broken and wakes up everyone.
* Called only while holding lock.
*/
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
/**
* Main barrier code, covering the various policies.
*/
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException,
TimeoutException {
final NonfairSync lock = this.lock;
lock.lock();
if(exclusiveOwnerThread == null)
exclusiveOwnerThread = Thread.currentThread();
System.out.println(Thread.currentThread()+" xx "+exclusiveOwnerThread + " is equal" + (exclusiveOwnerThread == Thread.currentThread()));
try {
final Generation g = generation;
if (g.broken)
throw new BrokenBarrierException();
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
int index = --count;
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
try {
trip.await();
}catch(Exception e) {
System.out.println("dfdfxxxxx");
}
// loop until tripped, broken, interrupted, or timed out
// for (;;) {
// try {
// System.out.println(" sdsd");
// if (!timed)
// //trip.await();
// // else if (nanos > 0L)
// // nanos = trip.awaitNanos(nanos);
//
// System.out.println(" +++sdsd");
// } catch (InterruptedException ie) {
//
// System.out.println(" 1122");
// if (g == generation && ! g.broken) {
// breakBarrier();
// throw ie;
// } else {
// // We're about to finish waiting even if we had not
// // been interrupted, so this interrupt is deemed to
// // "belong" to subsequent execution.
// Thread.currentThread().interrupt();
// }
// }
//
// if (g.broken)
// throw new BrokenBarrierException();
//
// if (g != generation)
// return index;
//
// if (timed && nanos <= 0L) {
// breakBarrier();
// throw new TimeoutException();
// }
// System.out.println("33344");
// }
} finally {
System.out.println("release loc");
lock.unlock();
}
return 0;
}
/**
* Creates a new {@code CyclicBarrier} that will trip when the
* given number of parties (threads) are waiting upon it, and which
* will execute the given barrier action when the barrier is tripped,
* performed by the last thread entering the barrier.
*
* @param parties the number of threads that must invoke {@link #await}
* before the barrier is tripped
* @param barrierAction the command to execute when the barrier is
* tripped, or {@code null} if there is no action
* @throws IllegalArgumentException if {@code parties} is less than 1
*/
public ThreadTest(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
/**
* Creates a new {@code CyclicBarrier} that will trip when the
* given number of parties (threads) are waiting upon it, and
* does not perform a predefined action when the barrier is tripped.
*
* @param parties the number of threads that must invoke {@link #await}
* before the barrier is tripped
* @throws IllegalArgumentException if {@code parties} is less than 1
*/
public ThreadTest(int parties) {
this(parties, null);
}
/**
* Returns the number of parties required to trip this barrier.
*
* @return the number of parties required to trip this barrier
*/
public int getParties() {
return parties;
}
/**
* Waits until all {@linkplain #getParties parties} have invoked
* {@code await} on this barrier.
*
* <p>If the current thread is not the last to arrive then it is
* disabled for thread scheduling purposes and lies dormant until
* one of the following things happens:
* <ul>
* <li>The last thread arrives; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* one of the other waiting threads; or
* <li>Some other thread times out while waiting for barrier; or
* <li>Some other thread invokes {@link #reset} on this barrier.
* </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
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the barrier is {@link #reset} while any thread is waiting,
* or if the barrier {@linkplain #isBroken is broken} when
* {@code await} is invoked, or while any thread is waiting, then
* {@link BrokenBarrierException} is thrown.
*
* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
* then all other waiting threads will throw
* {@link BrokenBarrierException} and the barrier is placed in the broken
* state.
*
* <p>If the current thread is the last thread to arrive, and a
* non-null barrier action was supplied in the constructor, then the
* current thread runs the action before allowing the other threads to
* continue.
* If an exception occurs during the barrier action then that exception
* will be propagated in the current thread and the barrier is placed in
* the broken state.
*
* @return the arrival index of the current thread, where index
* {@code getParties() - 1} indicates the first
* to arrive and zero indicates the last to arrive
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws BrokenBarrierException if <em>another</em> thread was
* interrupted or timed out while the current thread was
* waiting, or the barrier was reset, or the barrier was
* broken when {@code await} was called, or the barrier
* action (if present) failed due to an exception
*/
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
/**
* Waits until all {@linkplain #getParties parties} have invoked
* {@code await} on this barrier, or the specified waiting time elapses.
*
* <p>If the current thread is not the last to arrive then it is
* disabled for thread scheduling purposes and lies dormant until
* one of the following things happens:
* <ul>
* <li>The last thread arrives; or
* <li>The specified timeout elapses; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* one of the other waiting threads; or
* <li>Some other thread times out while waiting for barrier; or
* <li>Some other thread invokes {@link #reset} on this barrier.
* </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
* </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.
*
* <p>If the barrier is {@link #reset} while any thread is waiting,
* or if the barrier {@linkplain #isBroken is broken} when
* {@code await} is invoked, or while any thread is waiting, then
* {@link BrokenBarrierException} is thrown.
*
* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
* waiting, then all other waiting threads will throw {@link
* BrokenBarrierException} and the barrier is placed in the broken
* state.
*
* <p>If the current thread is the last thread to arrive, and a
* non-null barrier action was supplied in the constructor, then the
* current thread runs the action before allowing the other threads to
* continue.
* If an exception occurs during the barrier action then that exception
* will be propagated in the current thread and the barrier is placed in
* the broken state.
*
* @param timeout the time to wait for the barrier
* @param unit the time unit of the timeout parameter
* @return the arrival index of the current thread, where index
* {@code getParties() - 1} indicates the first
* to arrive and zero indicates the last to arrive
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the specified timeout elapses.
* In this case the barrier will be broken.
* @throws BrokenBarrierException if <em>another</em> thread was
* interrupted or timed out while the current thread was
* waiting, or the barrier was reset, or the barrier was broken
* when {@code await} was called, or the barrier action (if
* present) failed due to an exception
*/
public int await(long timeout, TimeUnit unit)
throws InterruptedException,
BrokenBarrierException,
TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
/**
* Queries if this barrier is in a broken state.
*
* @return {@code true} if one or more parties broke out of this
* barrier due to interruption or timeout since
* construction or the last reset, or a barrier action
* failed due to an exception; {@code false} otherwise.
*/
public boolean isBroken() {
final NonfairSync lock = this.lock;
lock.lock();
try {
return generation.broken;
} finally {
lock.unlock();
}
}
/**
* Resets the barrier to its initial state. If any parties are
* currently waiting at the barrier, they will return with a
* {@link BrokenBarrierException}. Note that resets <em>after</em>
* a breakage has occurred for other reasons can be complicated to
* carry out; threads need to re-synchronize in some other way,
* and choose one to perform the reset. It may be preferable to
* instead create a new barrier for subsequent use.
*/
public void reset() {
final NonfairSync lock = this.lock;
lock.lock();
try {
breakBarrier(); // break the current generation
nextGeneration(); // start a new generation
} finally {
lock.unlock();
}
}
/**
* Returns the number of parties currently waiting at the barrier.
* This method is primarily useful for debugging and assertions.
*
* @return the number of parties currently blocked in {@link #await}
*/
public int getNumberWaiting() {
final NonfairSync lock = this.lock;
lock.lock();
try {
return parties - count;
} finally {
lock.unlock();
}
}
}
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