本文主要概述java.util.concurrent包下的相关类和使用方法

Package java.util.concurrent

原子性操作类

java.util.concurrent.atomic包下的类:

Package java.util.concurrent.atomic

线程池

java.util.concurrent:Class Executors

常用线程池

几种常用的的生成线程池的方法：

  • newCachedThreadPool
  • newFixedThreadPool
  • newScheduledThreadPool
  • newSingleThreadExecutor
  • newSingleThreadScheduledExecutor

例子：newFixedThreadPool

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ExecutorService threadPool = Executors.newFixedThreadPool(3);
for(int i=0;i<10;i++){
    threadPool.execute(new Runnable() {
        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName());
        }
    });
}

单线程newSingleThreadExecutor可用于重启

用线程池启动定时器

例子：类似Timer的定时执行

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Executors.newScheduledThreadPool(3).scheduleAtFixedRate(
            new Runnable() {
                @Override
                public void run() {
                    System.out.println("ScheduledThreadPool "+Thread.currentThread().getName());
                }
            },3,1, TimeUnit.SECONDS
    );

Callable&Future

ExecutorService在Executor的基础上增加了一些方法，其中有两个核心的方法：

  • Future<?> submit(Runnable task)
  • <T> Future<T> submit(Callable<T> task)

这两个方法都是向线程池中提交任务，它们的区别在于Runnable在执行完毕后没有结果，Callable执行完毕后有一个结果。这在多个线程中传递状态和结果是非常有用的。另外他们的相同点在于都返回一个Future对象。Future对象可以阻塞线程直到运行完毕（获取结果，如果有的话），也可以取消任务执行，当然也能够检测任务是否被取消或者是否执行完毕。

Lock&Condition

Lock

Lock功能类似传统多线程技术里的synchronized，实现线程互斥，但更加面向对象。将需要互斥的代码片段放到lock.lock();和lock.unlock();之间。

例子

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class A{
    private Lock lock = new ReentrantLock();

    public void function(){
        lock.lock();
        try{
            //功能代码
        }finally{
            lock.unlock();
        }
    }
}
  • 读写锁

java.util.concurrent.locks:Class ReentrantReadWriteLock

javaDoc文档读写锁例子,缓存:

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class CachedData {
   Object data;
   volatile boolean cacheValid;
   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

   void processCachedData() {
     rwl.readLock().lock();
     if (!cacheValid) {
       // Must release read lock before acquiring write lock
       rwl.readLock().unlock();
       rwl.writeLock().lock();
       try {
         // Recheck state because another thread might have
         // acquired write lock and changed state before we did.
         if (!cacheValid) {
           data = ...
           cacheValid = true;
         }
         // Downgrade by acquiring read lock before releasing write lock
         rwl.readLock().lock();
       } finally {
         rwl.writeLock().unlock(); // Unlock write, still hold read
       }
     }

     try {
       use(data);
     } finally {
       rwl.readLock().unlock();
     }
   }
 }

重点注意在释放写锁前加读锁那部分代码，注释为// Downgrade by acquiring read lock before releasing write lock。自己挂了写锁，再挂读锁是可以的，这面涉及的技巧以后再研究。

Condition

Condition类似于传统多线程技术中的Object.wait和Object.notify,实现线程间同步。

javaDoc文档例子，可阻塞队列

class BoundedBuffer例子

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class BoundedBuffer {
   final Lock lock = new ReentrantLock();
   final Condition notFull  = lock.newCondition();
   final Condition notEmpty = lock.newCondition();

   final Object[] items = new Object[100];
   int putptr, takeptr, count;

   public void put(Object x) throws InterruptedException {
     lock.lock();
     try {
       while (count == items.length)
         notFull.await();
       items[putptr] = x;
       if (++putptr == items.length) putptr = 0;
       ++count;
       notEmpty.signal();
     } finally {
       lock.unlock();
     }
   }

   public Object take() throws InterruptedException {
     lock.lock();
     try {
       while (count == 0)
         notEmpty.await();
       Object x = items[takeptr];
       if (++takeptr == items.length) takeptr = 0;
       --count;
       notFull.signal();
       return x;
     } finally {
       lock.unlock();
     }
   }
 }

使用了两个condition

同步工具

  • Semaphore

类似占坑

  • CyclicBarrier

阶段性使进度一致

  • CountDownLatch

一人通知多人/多人通知一人

  • Exchanger

线程间数据交换，都到达则自然交换

参考资料