ThreadPoolExecutor源码分析

ThreadPoolExecutor是Java自带线程池FixedThreadPool（固定大小）、 SingleThreadExecutor（单线程）、CacheThreadPool （无限大）的具体实现。我们也可以继承此类来实现自己的线程池。

其内部主要实现是通过队列保存需要执行的任务，并通过coreSize和maxSize控制线程池内线程的个数。

ThreadPoolExecutor的关键属性

ctl是存储runState（运行状态）和workerCount（工作线程数量）的一个AtomicInteger。

其中runState存储在ctl（共32位）的高3位，workerCount存储在低29位。

 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));//初始为0个workerCount
 private static final int COUNT_BITS = Integer.SIZE - 3;//29位
 private static final int CAPACITY   = (1 << COUNT_BITS) - 1;//前3位为0，后29位为1

 private static final int RUNNING    = -1 << COUNT_BITS;//运行
 private static final int SHUTDOWN   =  0 << COUNT_BITS;//不接受新任务，继续接受队列任务，线程池内任务继续执行
 private static final int STOP       =  1 << COUNT_BITS;//不接受新任务，不接受队列任务，且线程池内任务停止
 private static final int TIDYING    =  2 << COUNT_BITS;//全部任务已经停止，运行终止代码
 private static final int TERMINATED =  3 << COUNT_BITS;//终止代码运行完毕

 private static int runStateOf(int c)     { return c & ~CAPACITY; }//根据ctl获取runState
 private static int workerCountOf(int c)  { return c & CAPACITY; }//根据ctl获取workerCount
 private static int ctlOf(int rs, int wc) { return rs | wc; }//根据runState、workerCount获取ctl

ctl

ThreadPoolExecutor运行流程

光是看属性比较难理解，我们可以模拟平常使用线程池的方法，看看其内部是怎么运行的。

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

 public static ExecutorService newCachedThreadPool() {
         return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                       60L, TimeUnit.SECONDS,
                                       new SynchronousQueue<Runnable>());
     }

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

 public ThreadPoolExecutor(int corePoolSize,
                               int maximumPoolSize,
                               long keepAliveTime,
                               TimeUnit unit,
                               BlockingQueue<Runnable> workQueue,
                               ThreadFactory threadFactory,
                               RejectedExecutionHandler handler) {
         if (corePoolSize < 0 ||
             maximumPoolSize <= 0 ||
             maximumPoolSize < corePoolSize ||
             keepAliveTime < 0)
             throw new IllegalArgumentException();
         if (workQueue == null || threadFactory == null || handler == null)
             throw new NullPointerException();
         this.corePoolSize = corePoolSize;
         this.maximumPoolSize = maximumPoolSize;
         this.workQueue = workQueue;
         this.keepAliveTime = unit.toNanos(keepAliveTime);
         this.threadFactory = threadFactory;
         this.handler = handler;
     }

构造

FixedThreadPool传入的coreSize ==  maxSize，线程存活时间为无限，工作队列为有缓存（放入时未达到队列缓存值则返回）的无界阻塞队列。

CachedThreadPool传入的coreSize = 0， maxSize为无限大，线程存活时间为60秒，工作队列为无缓存（放入时必须要等待取出后才能返回、即缓存值为1）的阻塞队列。

一般我们调用excute(Runnable r)的方法来使用线程池。

 public void execute(Runnable command) {
         if (command == null)
             throw new NullPointerException();

         int c = ctl.get();
         if (workerCountOf(c) < corePoolSize) {//当前工作线程小于coreSize
             if (addWorker(command, true))//新增一个工作线程，并把其第一个任务设为传入的command
                 return;//成功直接返回
             c = ctl.get();
         }
         if (isRunning(c) && workQueue.offer(command)) {//工作线程达到coreSize，将任务放入队列中，等待工作线程获取
             int recheck = ctl.get();
             if (! isRunning(recheck) && remove(command))//再次确认是否运行中，若无则移除该线程
                 reject(command);//拒绝
             else if (workerCountOf(recheck) == 0)//workerCount在上面语句执行途中变为0
                 addWorker(null, false);//增加worker
         }
         else if (!addWorker(command, false))//添加失败，可能线程池关闭或饱和
             reject(command);//拒绝
     }

     private boolean addWorker(Runnable firstTask, boolean core) {
         retry:
         for (;;) {
             int c = ctl.get();
             int rs = runStateOf(c);

             // 是否运行中
             if (rs >= SHUTDOWN &&
                 ! (rs == SHUTDOWN &&
                    firstTask == null &&
                    ! workQueue.isEmpty()))
                 return false;

             for (;;) {
                 int wc = workerCountOf(c);//
                 if (wc >= CAPACITY ||
                     wc >= (core ? corePoolSize : maximumPoolSize))//工作线程数量已达最大值
                     return false;//返回增加失败
                 if (compareAndIncrementWorkerCount(c))//尝试CAS增加workerCount
                     break retry;//成功返回
                 c = ctl.get();  // Re-read ctl
                 if (runStateOf(c) != rs)//可能终止
                     continue retry;
                 //增加失败，原因可能为workerCount被其他线程改变，重试内部循环
             }
         }

         boolean workerStarted = false;
         boolean workerAdded = false;
         Worker w = null;
         try {
             w = new Worker(firstTask);//将该任务作为首任务
             final Thread t = w.thread;
             if (t != null) {
                 final ReentrantLock mainLock = this.mainLock;
                 mainLock.lock();
                 try {
                     int rs = runStateOf(ctl.get());//在有锁的情况下重新检查runState

                     if (rs < SHUTDOWN ||
                         (rs == SHUTDOWN && firstTask == null)) {
                         if (t.isAlive()) //t可用
                             throw new IllegalThreadStateException();
                         workers.add(w);//放入集合中
                         int s = workers.size();
                         if (s > largestPoolSize)
                             largestPoolSize = s;//largestPoolSize供外部调用，仅在有锁情况下返回
                         workerAdded = true;
                     }
                 } finally {
                     mainLock.unlock();
                 }
                 if (workerAdded) {
                     t.start();//开启工作线程
                     workerStarted = true;
                 }
             }
         } finally {
             if (! workerStarted)
                 addWorkerFailed(w);
         }
         return workerStarted;
     }

excute

除了我在代码中另加的注释，jdk官方本身也有对excute执行顺序的解释，我尝试翻译如下：

1.如果小于corePoolSize的线程正在运行，尝试启动新的线程并把传入的command作为其第一个任务。addWorker方法检查runState和workerCount，通过返回false防止错误添加线程。
2.如果任务可以被成功入队，我们依然需要去双重检测在进入方法后，是否已经增加了一个线程（因为存在可能在第一次检测后一个线程结束了）或者线程池结束了。所以我们再次检测state，并当发现若线程池停止了，我们会撤回入队，或者当发现没有线程在执行了，会开启一个新线程。
3.如果我们不能把一个任务成功入队，我们就尝试增加一个新的线程，如果增加线程也失败了，我们就得知线程被终止或者饱和了，拒绝该任务。

任务执行的核心代码显然是在内部类Worker的run方法中，我们去看看。

 public void run() {
             runWorker(this);//调用外部的runWorker并把当前的worker传进去
         }

 final void runWorker(Worker w) {
         Thread wt = Thread.currentThread();
         Runnable task = w.firstTask;
         w.firstTask = null;
         w.unlock(); // 解除锁定以允许中断
         boolean completedAbruptly = true;
         try {
             while (task != null || (task = getTask()) != null) {//第一个任务为null，或者从队列取任务时返回null
                 w.lock();
         //如果线程池正在停止，确保线程中断，否则确保线程不中断。
                 if ((runStateAtLeast(ctl.get(), STOP) ||
                      (Thread.interrupted() &&
                       runStateAtLeast(ctl.get(), STOP))) &&
                     !wt.isInterrupted())
                     wt.interrupt();//当前线程应当中断
                 try {
                     beforeExecute(wt, task);
                     Throwable thrown = null;
                     try {
                         task.run();//执行任务
                     } 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);//处理回收的worker
         }
     }

run

其中getTask中允许线程的复用，我们去看一下。

 private Runnable getTask() {
         boolean timedOut = false; // Did the last poll() time out?

         for (;;) {
             int c = ctl.get();
             int rs = runStateOf(c);

             // 检测线程池状态
             if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                 decrementWorkerCount();
                 return null;//返回null由worker自己结束
             }

             int wc = workerCountOf(c);

             // workerCount大于corePoolSize时候、为true
             boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

             //大于maximum或者上次获取已经超时
             if ((wc > maximumPoolSize || (timed && timedOut))
                 && (wc > 1 || workQueue.isEmpty())) {
                 if (compareAndDecrementWorkerCount(c))
                     return null;//返回null由worker自己结束
                 continue;//cas失败，继续尝试获取task
             }

             try {
                 Runnable r = timed ?
                     workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) ://大于core，在keepAliveTime时间内尝试从队列获取任务
                     workQueue.take();//小于或等于，阻塞获取任务
                 if (r != null)
                     return r;
                 timedOut = true;//r==null，超时，进入下次循环
             } catch (InterruptedException retry) {
                 timedOut = false;
             }
         }
     }

getTask

主要就是判断了超时时间，CachedThreadPool空闲线程的回收也就是通过这个来判断的。

而FixedThreadPool的线程则一直在队列获取时阻塞。

另外还有个processWorkerExit比较简单，主要是从worker集合中删除，这里就不再说明了。