java线程——ThreadPoolExecutor详解

来源：互联网发布：煤炭建设工程预算软件编辑：程序博客网时间：2024/06/06 23:14

转自http://blog.csdn.net/java2000_wl/article/details/22097059

ThreadPoolExecutor是JDK并发包提供的一个线程池服务，基于ThreadPoolExecutor可以很容易将一个Runnable接口的任务放入线程池中。

ThreadPoolExecutor的构建参数：

    public ThreadPoolExecutor(int corePoolSize,                                int maximumPoolSize,                                long keepAliveTime,                                TimeUnit unit,                                BlockingQueue<Runnable> workQueue,                                RejectedExecutionHandler handler) {          this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,               Executors.defaultThreadFactory(), handler);      }

1. 参数解释
corePoolSize：核心线程数，会一直存活，即使没有任务，线程池也会维护线程的最少数量
maximumPoolSize：线程池维护线程的最大数量
keepAliveTime：线程池维护线程所允许的空闲时间，当线程空闲时间达到keepAliveTime，该线程会退出，直到线程数量等于corePoolSize。如果allowCoreThreadTimeout设置为true，则所有线程均会退出直到线程数量为0。
unit：线程池维护线程所允许的空闲时间的单位、可选参数值为：TimeUnit中的几个静态属性：NANOSECONDS、MICROSECONDS、MILLISECONDS、SECONDS。
workQueue：线程池所使用的缓冲队列，常用的是：java.util.concurrent.ArrayBlockingQueue、LinkedBlockingQueue、SynchronousQueue
handler：线程池中的数量大于maximumPoolSize，对拒绝任务的处理策略，默认值ThreadPoolExecutor.AbortPolicy()。

2. execute方法JDK 实现

    public void execute(Runnable command) {          if (command == null)              throw new NullPointerException();          if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {              if (runState == RUNNING && workQueue.offer(command)) {                  if (runState != RUNNING || poolSize == 0)                      ensureQueuedTaskHandled(command);              }              else if (!addIfUnderMaximumPoolSize(command))                  reject(command); // is shutdown or saturated          }      }

一个任务通过 execute(Runnable)方法被添加到线程池，任务就是一个Runnable类型的对象，任务的执行方法就是run()方法，如果传入的为null，侧抛出NullPointerException。

如果当前线程数小于corePoolSize，调用addIfUnderCorePoolSize方法，addIfUnderCorePoolSize方法首先调用mainLock加锁，再次判断当前线程数小于corePoolSize并且线程池处于RUNNING状态，则调用addThread增加线程

addIfUnderCorePoolSize方法实现：

    private boolean addIfUnderCorePoolSize(Runnable firstTask) {          Thread t = null;          final ReentrantLock mainLock = this.mainLock;          mainLock.lock();          try {              if (poolSize < corePoolSize && runState == RUNNING)                  t = addThread(firstTask);          } finally {              mainLock.unlock();          }          if (t == null)              return false;          t.start();          return true;      }

addThread方法首先创建Work对象，然后调用threadFactory创建新的线程，如果创建的线程不为null，将Work对象的thread属性设置为此创建出来的线程，并将此Work对象放入workers中，然后在增加当前线程池的中线程数，增加后回到addIfUnderCorePoolSize方法，释放mainLock，最后启动这个新创建的线程来执行新传入的任务。

addThread方法实现：

 private Thread addThread(Runnable firstTask) {          Worker w = new Worker(firstTask);          Thread t = threadFactory.newThread(w);<span style="color:#ff0000;"></span>          if (t != null) {              w.thread = t;              workers.add(w);              int nt = ++poolSize;              if (nt > largestPoolSize)                  largestPoolSize = nt;          }          return t;      }

ThreadFactory 接口默认实现DefaultThreadFactory

public Thread newThread(Runnable r) {      Thread t = new Thread(group, r,                            namePrefix + threadNumber.getAndIncrement(),                            0);      if (t.isDaemon())          t.setDaemon(false);      if (t.getPriority() != Thread.NORM_PRIORITY)          t.setPriority(Thread.NORM_PRIORITY);      return t;  }

从addThread方法看得出，Worker对象包装了参数传入的任务，threadFactory新创建的线程包装了Worker对象，在执行新创建线程的run方法时，调用到了Worker对象的run方法.

Worker的run方法

public void run() {      try {          Runnable task = firstTask;          firstTask = null;          while (task != null || (task = getTask()) != null) {              runTask(task);              task = null;          }      } finally {          workerDone(this);      }  }

从以上方法可以看出，Worker所在的线程启动后，首先执行创建其时传入的Runnable任务，执行完成后，循环调用getTask来获取新的任务，在没有任务的情况下，退出此线程。

getTask方法实现：

    Runnable getTask() {          for (;;) {              try {                  int state = runState;                  if (state > SHUTDOWN)                      return null;                  Runnable r;                  if (state == SHUTDOWN)  // Help drain queue                      r = workQueue.poll();                  else if (poolSize > corePoolSize || allowCoreThreadTimeOut)                      r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);                  else                      r = workQueue.take();                  if (r != null)                      return r;                  if (workerCanExit()) {                      if (runState >= SHUTDOWN) // Wake up others                          interruptIdleWorkers();                      return null;                  }                  // Else retry              } catch (InterruptedException ie) {                  // On interruption, re-check runState              }          }      }

getTask就是通过WorkQueue的poll或task方法来获取下一个要执行的任务。

回到execute方法，execute 方法部分实现：

    if (runState == RUNNING && workQueue.offer(command)) {                     if (runState != RUNNING || poolSize == 0)                         ensureQueuedTaskHandled(command);                 }                 else if (!addIfUnderMaximumPoolSize(command))                     reject(command); // is shutdown or saturated

如果当前线程池数量大于corePoolSize或addIfUnderCorePoolSize方法执行失败，则执行后续操作；如果线程池处于运行状态并且workQueue中成功加入任务，再次判断如果线程池的状态不为运行状态或当前线程池数为0，则调用ensureQueuedTaskHandled方法

ensureQueuedTaskHandled方法实现：

    private void ensureQueuedTaskHandled(Runnable command) {          final ReentrantLock mainLock = this.mainLock;          mainLock.lock();          boolean reject = false;          Thread t = null;          try {              int state = runState;              if (state != RUNNING && workQueue.remove(command))                  reject = true;              else if (state < STOP &&                       poolSize < Math.max(corePoolSize, 1) &&                       !workQueue.isEmpty())                  t = addThread(null);          } finally {              mainLock.unlock();          }          if (reject)              reject(command);          else if (t != null)              t.start();      }

ensureQueuedTaskHandled方法判断线程池运行，如果状态不为运行状态，从workQueue中删除，并调用reject做拒绝处理。

reject方法实现:

    void reject(Runnable command) {          handler.rejectedExecution(command, this);      }

再次回到execute方法，

    if (runState == RUNNING && workQueue.offer(command)) {                     if (runState != RUNNING || poolSize == 0)                         ensureQueuedTaskHandled(command);                 }                 else if (!addIfUnderMaximumPoolSize(command))                     reject(command); // is shutdown or saturated

如线程池workQueue offer失败或不处于运行状态，调用addIfUnderMaximumPoolSize，addIfUnderMaximumPoolSize方法基本和addIfUnderCorePoolSize实现类似，不同点在于根据最大线程数（maximumPoolSize）进行比较，如果超过最大线程数，返回false，调用reject方法，下面是addIfUnderMaximumPoolSize方法实现：

private boolean addIfUnderMaximumPoolSize(Runnable firstTask) {         Thread t = null;         final ReentrantLock mainLock = this.mainLock;         mainLock.lock();         try {             if (poolSize < maximumPoolSize && runState == RUNNING)                 t = addThread(firstTask);         } finally {             mainLock.unlock();         }         if (t == null)             return false;         t.start();         return true;     }

3. 添加任务处理流程
当一个任务通过execute(Runnable)方法欲添加到线程池时：
如果当前线程池中的数量小于corePoolSize，并线程池处于Running状态，创建并添加的任务。
如果当前线程池中的数量等于corePoolSize，并线程池处于Running状态，缓冲队列 workQueue未满，那么任务被放入缓冲队列、等待任务调度执行。
如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量小于maximumPoolSize，新提交任务会创建新线程执行任务。

如果当前线程池中的数量大于corePoolSize，缓冲队列workQueue已满，并且线程池中的数量等于maximumPoolSize，新提交任务由Handler处理。

当线程池中的线程大于corePoolSize时，多余线程空闲时间超过keepAliveTime时，会关闭这部分线程。

4. RejectedExecutionHandler 默认有四个选择：

ThreadPoolExecutor.AbortPolicy() 当线程池中的数量等于最大线程数时、直接抛出抛出Java.util.concurrent.RejectedExecutionException异常

    public static class AbortPolicy implements RejectedExecutionHandler {          /**          * Creates an {@code AbortPolicy}.          */          public AbortPolicy() { }                /**          * Always throws RejectedExecutionException.          *          * @param r the runnable task requested to be executed          * @param e the executor attempting to execute this task          * @throws RejectedExecutionException always.          */          public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {              throw new RejectedExecutionException("Task " + r.toString() +                                                   " rejected from " +                                                   e.toString());          }      }

ThreadPoolExecutor.CallerRunsPolicy() 当线程池中的数量等于最大线程数时、重试执行当前的任务，交由调用者线程来执行任务

    public static class CallerRunsPolicy implements RejectedExecutionHandler {           /**           * Creates a {@code CallerRunsPolicy}.           */           public CallerRunsPolicy() { }                 /**           * Executes task r in the caller's thread, unless the executor           * has been shut down, in which case the task is discarded.           *           * @param r the runnable task requested to be executed           * @param e the executor attempting to execute this task           */           public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {               if (!e.isShutdown()) {                   r.run();               }           }       }

ThreadPoolExecutor.DiscardOldestPolicy() 当线程池中的数量等于最大线程数时、抛弃线程池中最后一个要执行的任务，并执行新传入的任务

public static class DiscardOldestPolicy implements RejectedExecutionHandler {        /**        * Creates a {@code DiscardOldestPolicy} for the given executor.        */        public DiscardOldestPolicy() { }          /**        * Obtains and ignores the next task that the executor        * would otherwise execute, if one is immediately available,        * and then retries execution of task r, unless the executor        * is shut down, in which case task r is instead discarded.        *        * @param r the runnable task requested to be executed        * @param e the executor attempting to execute this task        */        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {            if (!e.isShutdown()) {                e.getQueue().poll();                e.execute(r);            }        }    }

ThreadPoolExecutor.DiscardPolicy() 当线程池中的数量等于最大线程数时，不做任何动作

    public static class DiscardPolicy implements RejectedExecutionHandler {          /**          * Creates a {@code DiscardPolicy}.          */          public DiscardPolicy() { }                /**          * Does nothing, which has the effect of discarding task r.          *          * @param r the runnable task requested to be executed          * @param e the executor attempting to execute this task          */          public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {          }      }

0 0