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[Java教程]ThreadPoolExecutor源码解析


LZ目前正在做一个批量生成报表的系统,需要定时批量生成多张报表,便考虑使用线程池来完成。JDK自带的Executors工具类只提供创建固定线程和可伸展但无上限的两个静态方法,并不能满足LZ想自定制线程池大小的要求。于是就直接深入了解下ThreadPoolExecutor类,以方便在工作中灵活使用以及为以后的扩展打下基础。

 

java doc中对ThreadPoolExecutor的说明是:

An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using Executors factory methods.
  一个使用线程池来执行提交的任务的ExecutorService子类,正常通过Executors工具类中的工厂方法进行配置。
 
 
那我们就先看一下比较熟悉的Executors中的几个方法的实现代码:
 
1.Executors.newCachedThreadPool (threadFactory);
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,                   60L, TimeUnit.SECONDS,                   new SynchronousQueue<Runnable>(),                   threadFactory);}

 


2.Executors.newFixedThreadPool(nThreads , threadFactory);
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {    return new ThreadPoolExecutor(nThreads, nThreads,                   0L, TimeUnit.MILLISECONDS,                   new LinkedBlockingQueue<Runnable>(),                   threadFactory);}

 


 


3.Executors.newSingleThreadExecutor(threadFactory);
public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {    return new FinalizableDelegatedExecutorService      (new ThreadPoolExecutor(1, 1,                  0L, TimeUnit.MILLISECONDS,                  new LinkedBlockingQueue<Runnable>(),                  threadFactory));}

 





可以看到其实这些方法都是通过构造方法创建了ThreadPoolExecutor对象,我们来看下具体的构造方法实现

public ThreadPoolExecutor(int corePoolSize,               int maximumPoolSize,               long keepAliveTime,               TimeUnit unit,               BlockingQueue<Runnable> workQueue,               ThreadFactory threadFactory) {    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,       threadFactory, defaultHandler);}

 





 这里我们可以看到ThreadPoolExecutor中比较重要的一些参数,这些参数都是可以通过外部传入,对ThreadPoolExecutor内部进行控制。而ThreadPoolExecutor内部的工作机制究竟是怎样进行的呢?下面我们就揭开它的外衣,深入其中仔细探究。

 
 
1.ThreadPoolExecutor继承了AbstractExecutorService类
public class ThreadPoolExecutor extends AbstractExecutorService

 

 




2. ThreadPoolExecutor的重要变量参数

    ctl:    用来标识线程池状态的重要参数,很多操作执行前都需要对线程池状态进行前置判断,以确定线程池状态是否正常
    workQueue:    任务队列,用来在全部当前线程正在处理任务时存储提交来的任务
    works:    存储所有工作线程
    corePoolSize:    核心线程数
    maximumPoolSize:    最大线程数
    keepAliveTime:    空闲线程等待任务时间
    threadFactory:    线程创建工厂
    handler:    因线程池饱和或关闭触发的拒绝异常处理器


 
   //标识线程池控制状态  private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));    //线程池状态类型  //接受新的任务并处理队列中的任务  private static final int RUNNING  = -1 << COUNT_BITS;  //不接受新任务但处理队列中的任务  private static final int SHUTDOWN  = 0 << COUNT_BITS;  //不接受新任务也不处理队列中的任务,且中断正在进行的任务  private static final int STOP    = 1 << COUNT_BITS;  //所有任务已经完结,工作线程数为0,并调用terminated方法  private static final int TIDYING  = 2 << COUNT_BITS;  //terminated方法执行完成  private static final int TERMINATED = 3 << COUNT_BITS;  //任务队列,储存任务以提供给工作线程  private final BlockingQueue<Runnable> workQueue;  //主要锁,设置workers和相关数据记录调用  private final ReentrantLock mainLock = new ReentrantLock();  //存储所有工作线程,设置时需要加mainLock锁  private final HashSet<Worker> workers = new HashSet<Worker>();  //线程池已达到的最大数,设置时需要加mainLock锁  private int largestPoolSize;  //已完成任务数,设置时需要加mainLock锁  private long completedTaskCount;  //线程创建工厂  private volatile ThreadFactory threadFactory;  //因饱和或线程池关闭触发的拒绝异常处理器  private volatile RejectedExecutionHandler handler;  //空闲线程等待任务时间(单位:纳秒),到时则会被销毁  private volatile long keepAliveTime;  //默认为false,核心线程在空闲时一直存活  //如果为true,核心线程使用keepAliveTime参数来等待任务  private volatile boolean allowCoreThreadTimeOut;  //核心线程数  private volatile int corePoolSize;  //最大线程数  private volatile int maximumPoolSize;  //默认拒绝异常处理器  private static final RejectedExecutionHandler defaultHandler =    new AbortPolicy();

 

 




3.execute方法,用户通过该方法提交任务给线程池。处理任务分四种种情况:

     (1)如果当前工作线程数小于核心线程数,则创建新的线程来处理任务
    (2)如果当前工作线程等于核心线程数,新提交的任务存储到工作队列中

        重新检测线程池状态是否正常,如果不是运行状态,则移除任务,并处理拒绝异常

        如果线程池正常,工作线程数等于0,则增加工作线程
    (3)当工作队列达到最大容量,工作线程数没有达到最大线程数,增加新的工作线程,并处理任务

    (4)当工作线程数达到最大线程数,则使用拒绝异常处理器对任务进行处理

 
public void execute(Runnable command) {    if (command == null)      throw new NullPointerException();        int c = ctl.get();    if (workerCountOf(c) < corePoolSize) {      if (addWorker(command, true))        return;      c = ctl.get();    }    if (isRunning(c) && workQueue.offer(command)) {      int recheck = ctl.get();      if (! isRunning(recheck) && remove(command))        reject(command);      else if (workerCountOf(recheck) == 0)        addWorker(null, false);    }    else if (!addWorker(command, false))      reject(command);}

 





4.线程池是怎么增加一个新的线程的呢?接下来我们来看addWorker方法

    (1)双重for循环检查线程池是否适合增加新的线程
      (2)创建Worker对象并获得mainLock锁

    (3)再次检查状态,防止线程工厂失败或线程池关闭

    (4)works增加worker对象,并更新largestPoolSize,释放锁

    (5)启用worker对象中的线程

    (6)由于并发原因,可能会出现线程尚未执行,但线程池正在关闭,因此可能会出现线程池关闭时,错过中断当前线程,因此再进行一次判断,如果线程池状态为关闭且当前线程未被中断,则手动中断它
 
private boolean addWorker(Runnable firstTask, boolean core) {    retry:    for (;;) {      int c = ctl.get();      int rs = runStateOf(c);      // Check if queue empty only if necessary.      if (rs >= SHUTDOWN &&        ! (rs == SHUTDOWN &&          firstTask == null &&          ! workQueue.isEmpty()))        return false;      for (;;) {        int wc = workerCountOf(c);        if (wc >= CAPACITY ||          wc >= (core ? corePoolSize : maximumPoolSize))          return false;        if (compareAndIncrementWorkerCount(c))          break retry;        c = ctl.get(); // Re-read ctl        if (runStateOf(c) != rs)          continue retry;        // else CAS failed due to workerCount change; retry inner loop      }    }    Worker w = new Worker(firstTask);    Thread t = w.thread;    final ReentrantLock mainLock = this.mainLock;    mainLock.lock();    try {      // Recheck while holding lock.      // Back out on ThreadFactory failure or if      // shut down before lock acquired.      int c = ctl.get();      int rs = runStateOf(c);      if (t == null ||        (rs >= SHUTDOWN &&         ! (rs == SHUTDOWN &&          firstTask == null))) {        decrementWorkerCount();        tryTerminate();        return false;      }      workers.add(w);      int s = workers.size();      if (s > largestPoolSize)        largestPoolSize = s;    } finally {      mainLock.unlock();    }    t.start();    // It is possible (but unlikely) for a thread to have been    // added to workers, but not yet started, during transition to    // STOP, which could result in a rare missed interrupt,    // because Thread.interrupt is not guaranteed to have any effect    // on a non-yet-started Thread (see Thread#interrupt).    if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())      t.interrupt();    return true;}

 





5.在addWorker方法中,我们并没有看到任务具体执行的操作,但是可以很明显地猜测到应该是在调用t.start()方法时进行调用。而线程t是来自于Worker对象,我们来看下内部类Worker(删除了部分代码)。

    (1)Worker类继承自AbstractQueuedSynchronizer,实现了Runnable接口
    (2)new Worker()时,通过ThreadFactory的newThread方法创建了一个新的线程

    (3)当调用addWorker中的t.start()时,其实触发的是run方法中的runWorker(this)

 
private final class Worker    extends AbstractQueuedSynchronizer    implements Runnable  {        /** Thread this worker is running in. Null if factory fails. */    final Thread thread;    /** Initial task to run. Possibly null. */    Runnable firstTask;    /** Per-thread task counter */    volatile long completedTasks;    /**     * Creates with given first task and thread from ThreadFactory.     * @param firstTask the first task (null if none)     */    Worker(Runnable firstTask) {      setState(-1); // inhibit interrupts until runWorker      this.firstTask = firstTask;      this.thread = getThreadFactory().newThread(this);    }    /** Delegates main run loop to outer runWorker */    public void run() {      runWorker(this);    }  }

 




6.我们再来看runWorker究竟做了什么操作

    (1)while循环保证了线程可以重复执行任务,如果firstTask执行完成后,通过getTask方法从任务队列中获取新的任务继续执行
    (2)执行前和执行后分别调用beforExecute和afterExecute两个钩子方法,可以用来在子类中自己实现,比如用于线程池监控

    (3)如果处理过程中出现意外情况,在finally中调用processWorkerExit进行处理,主要是对线程记录相关变量进行恢复,且处理当核心线程全部超时而任务队列中有新的任务时,重新增加新线程来处理任务

 
final void runWorker(Worker w) {    Thread wt = Thread.currentThread();    Runnable task = w.firstTask;    w.firstTask = null;    w.unlock(); // allow interrupts    boolean completedAbruptly = true;    try {      while (task != null || (task = getTask()) != null) {        w.lock();        // If pool is stopping, ensure thread is interrupted;        // if not, ensure thread is not interrupted. This        // requires a recheck in second case to deal with        // shutdownNow race while clearing interrupt        if ((runStateAtLeast(ctl.get(), STOP) ||           (Thread.interrupted() &&           runStateAtLeast(ctl.get(), STOP))) &&          !wt.isInterrupted())          wt.interrupt();        try {          beforeExecute(wt, task);          Throwable thrown = null;          try {            task.run();          } catch (RuntimeException x) {            thrown = x; throw x;          } catch (Error x) {            thrown = x; throw x;          } catch (Throwable x) {            thrown = x; throw new Error(x);          } finally {            afterExecute(task, thrown);          }        } finally {          task = null;          w.completedTasks++;          w.unlock();        }      }      completedAbruptly = false;    } finally {      processWorkerExit(w, completedAbruptly);    }  }

 




7.最后来看下getTask方法中是怎么获取任务队列中的任务的

    (1)判断线程池状态是否正常,根据timed = allowCoreThreadTimeout || wc > corePoolSize来决定队列获取任务的方式是指定keepAliveTime时间进行等待还是阻塞式等待
    (2)如果keepAliveTime超时,允许核心线程超时销毁或者当前线程池总量大于核心线程数,则getTask()返回null,回溯到runWorker方法中,则while循环结束,即线程执行完成,此线程将被销毁。

 
private Runnable getTask() {    boolean timedOut = false; // Did the last poll() time out?    retry:    for (;;) {      int c = ctl.get();      int rs = runStateOf(c);      // Check if queue empty only if necessary.      if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {        decrementWorkerCount();        return null;      }      boolean timed;   // Are workers subject to culling?      for (;;) {        int wc = workerCountOf(c);        timed = allowCoreThreadTimeOut || wc > corePoolSize;        if (wc <= maximumPoolSize && ! (timedOut && timed))          break;        if (compareAndDecrementWorkerCount(c))          return null;        c = ctl.get(); // Re-read ctl        if (runStateOf(c) != rs)          continue retry;        // else CAS failed due to workerCount change; retry inner loop      }      try {        Runnable r = timed ?          workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :          workQueue.take();        if (r != null)          return r;        timedOut = true;      } catch (InterruptedException retry) {        timedOut = false;      }    }  }

 




以上对ThreadPoolExecutor中的主要部分都进行了解析。相信大家应该对该类的实现有了大概的了解,下面对一些细节方面补充说明:

1.RejectExceptionHandler四种方式
    (1)默认为AbortPolicy,任务直接被抛弃,抛出RejectedExecutionException异常
    (2)DiscardPolicy,同AbortPolicy一样,只是不抛出异常
    (3)DiscardOldestPolicy,将队列中最早的任务抛弃,然后执行当前任务

    (4)CallerRunsPolicy,使用主线程执行任务,减缓任务提交,以等待线程池中的线程执行

 
 
来源: <http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/ThreadPoolExecutor.html>