几个Java并发工具类解析

CountDownLatch

简介

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

只有当N个线程执行完毕，并且进行countDown操作时，才允许await的线程继续执行。否则该线程挂起。

适用情况：一个线程需要等待其他N个线程执行完毕，再继续执行，是join的替代。

构造方法

参数count为计数值，传入AQS的实现类Sync设置成AQS的state。

public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
}

Sync

通过继承AQS从而完成同步的核心功能。

private static final class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 4982264981922014374L;
		
    	//构造方法
        Sync(int count) {
            setState(count);
        }

        int getCount() {
            return getState();
        }

        protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }

        protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
    }

核心方法

• countDown：将count值减1
• await：调用await的线程会被挂起，直到count为0才继续执行，允许中断

public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
}

public void countDown() {
        sync.releaseShared(1);
}

public boolean await(long timeout, TimeUnit unit)
        throws InterruptedException {
        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}

使用案例

class Driver { // ...
   void main() throws InterruptedException {
     CountDownLatch startSignal = new CountDownLatch(1);
     CountDownLatch doneSignal = new CountDownLatch(N);

     for (int i = 0; i < N; ++i) // create and start threads
       new Thread(new Worker(startSignal, doneSignal)).start();

     doSomethingElse();            // don't let run yet
     startSignal.countDown();      // let all threads proceed
     doSomethingElse();
     doneSignal.await();           // wait for all to finish
   }
 }

 class Worker implements Runnable {
   private final CountDownLatch startSignal;
   private final CountDownLatch doneSignal;
   Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
      this.startSignal = startSignal;
      this.doneSignal = doneSignal;
   }
   public void run() {
      try {
        startSignal.await();
        doWork();
        doneSignal.countDown();
      } catch (InterruptedException ex) {} // return;
   }

   void doWork() { ... }
 }

CyclicBarrier

简介

A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point. CyclicBarriers are useful in programs involving a fixed sized party of threads that must occasionally wait for each other. The barrier is called cyclic because it can be re-used after the waiting threads are released.

一组线程到达barrier时会被阻塞，直到最后一个线程到达barrier，被阻塞的线程才会继续执行。

与CountDownLatch的作用类似，CyclicBarrier可以执行reset方法进行重用。

构造方法

参数含义如下：

• parties：拦截的线程数量
• barrierAction：所有线程到达barrier后执行的任务

public CyclicBarrier(int parties, Runnable barrierAction) {
        if (parties <= 0) throw new IllegalArgumentException();
        this.parties = parties;
        this.count = parties;
        this.barrierCommand = barrierAction;
}

成员属性

• lock：可重入锁，用于进行dowait时锁定
• parties：参与的线程数量
• trip：实际进行await()的condition
• barrierCommand：最后一个线程到达时执行的任务
• count：等待进入屏障的线程数量
• generation：当前的generation
  • broken，表示当前屏障是否被破坏。

private static class Generation {
        boolean broken = false;
    }

    /** The lock for guarding barrier entry */
    private final ReentrantLock lock = new ReentrantLock();
    /** 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();

    private int count;

核心方法

await

可响应中断，通过调用dowait(false, 0L)实现

public int await() throws InterruptedException, BrokenBarrierException {
        try {
            return dowait(false, 0L);
        } catch (TimeoutException toe) {
            throw new Error(toe); // cannot happen
        }
}

dowait

await的具体实现。

private int dowait(boolean timed, long nanos)
        throws InterruptedException, BrokenBarrierException,
               TimeoutException {
        final ReentrantLock lock = this.lock;
        lock.lock();
        try {
            final Generation g = generation;
			//屏障被破坏，抛出异常
            if (g.broken)
                throw new BrokenBarrierException();
			//检查中断
            if (Thread.interrupted()) {
                //损坏屏障，唤醒所有线程
                breakBarrier();
                throw new InterruptedException();
            }
			//减少等待进入屏障的线程数量
            int index = --count;
            //index == 0表示 所有进程都已经进入
            if (index == 0) {  // tripped
                //运行的动作标识
                boolean ranAction = false;
                try {
                    //运行任务
                    final Runnable command = barrierCommand;
                    if (command != null)
                        command.run();
                    ranAction = true;
                    //进入下一代
                    nextGeneration();
                    return 0;
                } finally {
                    //如果没有改成功，损坏当前屏障
                    if (!ranAction)
                        breakBarrier();
                }
            }

            // loop until tripped, broken, interrupted, or timed out
            for (;;) {
                try {
                    //如果没有设置等待时间
                    //调用condition.await()进行等待
                    if (!timed)
                        trip.await();
                    //否则调用awaitNanos()进行等待
                    else if (nanos > 0L)
                        nanos = trip.awaitNanos(nanos);
                } catch (InterruptedException ie) {
                    //如果被中断，并且当前代的屏障没有被损坏
                    if (g == generation && ! g.broken) {
                        //损坏当前屏障
                        breakBarrier();
                        throw ie;
                    } else {
                        //不是当前代，进行中断
                        Thread.currentThread().interrupt();
                    }
                }
				//检查损坏标识
                if (g.broken)
                    throw new BrokenBarrierException();
				//不等于当前代损坏表示
                if (g != generation)
                    return index;

                if (timed && nanos <= 0L) {
                    breakBarrier();
                    throw new TimeoutException();
                }
            }
        } finally {
            lock.unlock();
        }
    }

nextGeneration

线程进入屏障后会进行调用。

private void nextGeneration() {
        // signal completion of last generation
    	//唤醒所有线程
        trip.signalAll();
        // set up next generation
    	//恢复正在等待进入屏障的线程数量
        count = parties;
        generation = new Generation();
}

breakBarrier

损坏当前屏障，会唤醒所有在屏障中的线程。

private void breakBarrier() {
    	//设置损坏标志
        generation.broken = true;
    	//恢复正在等待进入屏障的线程
        count = parties;
    	//唤醒所有线程
        trip.signalAll();
}

使用案例

class Solver {
   final int N;
   final float[][] data;
   final CyclicBarrier barrier;

   class Worker implements Runnable {
     int myRow;
     Worker(int row) { myRow = row; }
     public void run() {
       while (!done()) {
         processRow(myRow);

         try {
           barrier.await();
         } catch (InterruptedException ex) {
           return;
         } catch (BrokenBarrierException ex) {
           return;
         }
       }
     }
   }

   public Solver(float[][] matrix) {
     data = matrix;
     N = matrix.length;
     barrier = new CyclicBarrier(N,
                                 new Runnable() {
                                   public void run() {
                                     mergeRows(...);
                                   }
                                 });
     for (int i = 0; i < N; ++i)
       new Thread(new Worker(i)).start();

     waitUntilDone();
   }
 }

Semaphore

简介

A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each acquire() blocks if necessary until a permit is available, and then takes it. Each release() adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.

线程执行acquire()后，会判断permit是否可用，不可用则阻塞，可用则permit - 1。线程执行release()后，则permit + 1，并且释放一个阻塞线程。

适用场景：控制同时访问特定资源的线程数量。保证公共资源合理使用，与OS的信号量核心理念相同。

Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource. For example, here is a class that uses a semaphore to control access to a pool of items:

class Pool {
  private static final int MAX_AVAILABLE = 100;
  private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

  public Object getItem() throws InterruptedException {
    available.acquire();
    return getNextAvailableItem();
  }

  public void putItem(Object x) {
    if (markAsUnused(x))
      available.release();
  }

  // Not a particularly efficient data structure; just for demo

  protected Object[] items = ... whatever kinds of items being managed
  protected boolean[] used = new boolean[MAX_AVAILABLE];

  protected synchronized Object getNextAvailableItem() {
    for (int i = 0; i < MAX_AVAILABLE; ++i) {
      if (!used[i]) {
         used[i] = true;
         return items[i];
      }
    }
    return null; // not reached
  }

  protected synchronized boolean markAsUnused(Object item) {
    for (int i = 0; i < MAX_AVAILABLE; ++i) {
      if (item == items[i]) {
         if (used[i]) {
           used[i] = false;
           return true;
         } else
           return false;
      }
    }
    return false;
  }
}

构造方法

两个构造方法：默认创建非公平策略的信号量，另一个构造方法可以选择公平策略的信号量。

public Semaphore(int permits) {
    sync = new NonfairSync(permits);
}

public Semaphore(int permits, boolean fair) {
     sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}

成员属性

Semaphore主要通过sync（AQS的实现类）来实现核心功能。

private final Sync sync;

Sync

Sync代码如下：

abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = 1192457210091910933L;
		//构造方法
        Sync(int permits) {
            setState(permits);
        }
		//返回permit
        final int getPermits() {
            return getState();
        }
		//共享模式下的非公平策略获取
        final int nonfairTryAcquireShared(int acquires) {
            for (;;) {
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
		//共享模式下的释放
        protected final boolean tryReleaseShared(int releases) {
            for (;;) {
                int current = getState();
                int next = current + releases;
                if (next < current) // overflow
                    throw new Error("Maximum permit count exceeded");
                if (compareAndSetState(current, next))
                    return true;
            }
        }
		//根据指定数量减少可用许可数量
        final void reducePermits(int reductions) {
            for (;;) {
                int current = getState();
                int next = current - reductions;
                if (next > current) // underflow
                    throw new Error("Permit count underflow");
                if (compareAndSetState(current, next))
                    return;
            }
        }
		//permit不为0则更新permit，并返回permit
        final int drainPermits() {
            for (;;) {
                int current = getState();
                if (current == 0 || compareAndSetState(current, 0))
                    return current;
            }
        }
    }

NonfairSync

非公平策略直接调用tryAcquireShared完成获取资源的操作。

static final class NonfairSync extends Sync {
        private static final long serialVersionUID = -2694183684443567898L;

        NonfairSync(int permits) {
            super(permits);
        }

        protected int tryAcquireShared(int acquires) {
            return nonfairTryAcquireShared(acquires);
        }
    }

FairSync

公平策略中，获取共享状态时，会判断Sync Queue中是否有前驱元素。

static final class FairSync extends Sync {
        private static final long serialVersionUID = 2014338818796000944L;

        FairSync(int permits) {
            super(permits);
        }

        protected int tryAcquireShared(int acquires) {
            for (;;) {
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
}

核心方法

acquire

获取一个permit，在permit有效之前，将会阻塞，响应中断。

public void acquire() throws InterruptedException {
    sync.acquireSharedInterruptibly(1);
}

acquireUninterruptibly

不接受中断的acquire().

public void acquireUninterruptibly() {
    sync.acquireShared(1);
}

release

释放一个permits。通过AQS.releaseShared()。

public void release(int permits) {
    if (permits < 0) throw new IllegalArgumentException();
    sync.releaseShared(permits);
}

参考

Java SE 8 Docs API