线程池实现原理-2

我欲只争朝夕

发布于 2023-11-7 11:45

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前言

线程池实现原理-1

addWorker实现

在看addWorker方法之前，我们先看一个例子，了解一下retry的使用

break retry 跳到retry处，且不再进入循环
continue retry 跳到retry处，且再次进入循环

public static void main(String[] args) {
   breakRetry();
   continueRetry();
}

private static void breakRetry() {
   int i = 0;
   retry:
   for (; ; ) {
       System.out.println("start");
       for (; ; ) {
           i++;
           if (i == 4)
               break retry;
       }
   }
   //start 进入外层循环
   //4
   System.out.println(i);
}

private static void continueRetry() {
   int i = 0;
   retry:
   for(;;) {
       System.out.println("start");
       for(;;) {
           i++;
           if (i == 3)
               continue retry;
           System.out.println("end");
           if (i == 4)
               break retry;
       }
   }
   //start 第一次进入外层循环
   //end i=1输出
   //end i=2输出
   //start 再次进入外层循环
   //end i=4输出
   //4 最后输出
   System.out.println(i);
}1.
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这里说一下Runnable 参数的含义

firstTask != null 说明任务被添加了，我们需要启动一个线程去执行它
fistTask == null 说明我只想启动一个线程去消费阻塞队列中的任务

// core为ture表示是核心线程，否则非核心线程
private boolean addWorker(Runnable firstTask, boolean core) {
   retry:
   for (;;) {
       int c = ctl.get();
       int rs = runStateOf(c);

       // Check if queue empty only if necessary.
       /**
        * 将条件改为如下形式，方便理解
        * rs >= SHUTDOWN && (rs != SHUTDOWN || fistTask != null || workQueue.isEmpty)
        * 1.如果当前线程池的状态>SHUTDOWN，addWorker返回false，添加任务失败
        * 2.如果当前线程池的状态=SHUTDOWN，分为如下2种情况
        * (1)workQueue为空，fistTask == null 和fistTask != null的任务都不能
        * (2)workQueue不为空，可以添加fistTask != null的任务
        */
       if (rs >= SHUTDOWN &&
           ! (rs == SHUTDOWN &&
              firstTask == null &&
              ! workQueue.isEmpty()))
           return false;

       for (;;) {
           int wc = workerCountOf(c);
           if (wc >= CAPACITY ||
               // 1.是核心线程 >= corePoolSize
               // 2.非核心线程 >= maximumPoolSize
               wc >= (core ? corePoolSize : maximumPoolSize))
               return false;
           // 成功将线程数+1，跳到retry处，并且不再进入死循环
           if (compareAndIncrementWorkerCount(c))
               break retry;
           // 否则重新读取ctl
           c = ctl.get();  // Re-read ctl
           // 线程状态发生改变，跳到retry处，并且进入死循环
           if (runStateOf(c) != rs)
               continue retry;
           // else CAS failed due to workerCount change; retry inner loop
       }
   }

   // 线程是否启动的标志位
   boolean workerStarted = false;
   // 线程封装成Worker对象，是否添加到线程池中的标志位
   boolean workerAdded = false;
   Worker w = null;
   try {
       w = new Worker(firstTask);
       final Thread t = w.thread;
       if (t != null) {
           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 rs = runStateOf(ctl.get());

               // 1.rs < SHUTDOWN 即 rs = RUNNING
               // 2.rs == SHUTDOWN && firstTask == null
               if (rs < SHUTDOWN ||
                   (rs == SHUTDOWN && firstTask == null)) {
                   if (t.isAlive()) // precheck that t is startable
                       throw new IllegalThreadStateException();
                   workers.add(w);
                   int s = workers.size();
                   // 刷新了largestPoolSize
                   if (s > largestPoolSize)
                       largestPoolSize = s;
                   workerAdded = true;
               }
           } finally {
               mainLock.unlock();
           }
           if (workerAdded) {
               // 留心一下这里，后面会从这里开始讲起
               t.start();
               workerStarted = true;
           }
       }
   } finally {
       if (! workerStarted)
           addWorkerFailed(w);
   }
   return workerStarted;
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仔细理解一下这段代码，其实就能理解，当线程池处于RUNNING 接受新任务，并且处理进入队列的任务，处于SHUTDOWN 不接受新任务，处理进入队列的任务，剩余状态都不会处理任务，上面代码中的注释有详细解释

if (rs >= SHUTDOWN &&
   ! (rs == SHUTDOWN &&
      firstTask == null &&
      ! workQueue.isEmpty()))
   return false;1.
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线程池在执行任务的时候，会把任务对象包装成一个Worker对象，Worker对象是ThreadPoolExecutor的一个内部类，继承了AbstractQueuedSynchronizer，实现了一个独占锁，status值为0表示未锁定状态，status值为1表示锁定状态，实现了Runnable接口，在执行run方法的时候，它执行完初始化的firstTask后，还会从workQueue中取出任务执行，这样就不用新建一个线程执行任务，而是在一个线程中执行了好几个任务

Worker内部类

// 省略了一部分对锁的操作，简单的对AQS的一个实现
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.
    */
   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);
   }

}1.
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runWorker实现

当t.start()被执行后，run方法会执行runWorker方法，来看runWorker方法

final void runWorker(Worker w) {
   Thread wt = Thread.currentThread();
   Runnable task = w.firstTask;
   w.firstTask = null;
   // 允许中断
   w.unlock(); // allow interrupts
   // 标识线程是不是异常终止的
   boolean completedAbruptly = true;
   try {
       // 先执行初始化的fistTask，执行完成后还会无限循环获取workQueue里的任务来执行
       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
           // 配合shutdownNow 方法
           if ((runStateAtLeast(ctl.get(), STOP) ||
                (Thread.interrupted() &&
                 runStateAtLeast(ctl.get(), STOP))) &&
               !wt.isInterrupted())
               wt.interrupt();
           try {
               // 线程开始开始执行之前执行此方法
               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 {
                   // 线程执行后执行
                   afterExecute(task, thrown);
               }
           } finally {
               // 运行过的置为null
               task = null;
               w.completedTasks++;
               w.unlock();
           }
       }
       completedAbruptly = false;
   } finally {
       processWorkerExit(w, completedAbruptly);
   }
}1.
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这个方法需要注意的就是

getTask()从阻塞队列中获取任务，如果队列中没有任务会被阻塞，并不会占用CPU资源
可以根据业务需要自定义beforeExecute和afterExecute方法

getTask实现

private Runnable getTask() {
   // 标记获取头结点并且移除头结点的方法是否超时
   boolean timedOut = false; // Did the last poll() time out?

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

       // Check if queue empty only if necessary.
       // 1.rs >= STOP
       // 2.rs == SHUTDOWN && workQueue.isEmpty()
       if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
           // 用CAS将线程池中的数量-1，直到成功才会退出
           decrementWorkerCount();
           return null;
       }

       int wc = workerCountOf(c);

       // Are workers subject to culling?
       // 1.核心线程允许被销毁
       // 2.核心线程数 > corePoolSize
       boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

       // 1.timeOut为true，表示超时获取，workQueue没有任务，说明线程应该被销毁，但是还是要 && timed
       // 2.wc > maximumPoolSize肯定要删除线程了
       // 3.workQueue为空可以销毁线程，此时有可能所有线程都被销毁了
       // 4.workQueue不为空，只有wc > 1才能被删除
       if ((wc > maximumPoolSize || (timed && timedOut))
           && (wc > 1 || workQueue.isEmpty())) {
           if (compareAndDecrementWorkerCount(c))
               return null;
           continue;
       }

       try {
           // timed为true，超过keepAliveTime还是没有任务，返回null
           // timed为false，则一直阻塞等待任务
           Runnable r = timed ?
               workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
               workQueue.take();
           if (r != null)
               return r;
           timedOut = true;
       } catch (InterruptedException retry) {
           timedOut = false;
       }
   }
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processWorkerExit实现

线程执行完毕执行的方法

// processWorkerExit在runWorker结束之后被调用
private void processWorkerExit(Worker w, boolean completedAbruptly) {
   // 如果是异常终止，或者被中断，减少workerCount
   if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
       decrementWorkerCount();

   final ReentrantLock mainLock = this.mainLock;
   mainLock.lock();
   try {
       completedTaskCount += w.completedTasks;
       workers.remove(w);
   } finally {
       mainLock.unlock();
   }

   // Transitions to TERMINATED state if either (SHUTDOWN and pool
   // and queue empty) or (STOP and pool empty)
   tryTerminate();

   int c = ctl.get();
   // 状态为RUNNING或者SHUTDOWN
   if (runStateLessThan(c, STOP)) {
       if (!completedAbruptly) {
           int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
           if (min == 0 && ! workQueue.isEmpty())
               min = 1;
           // 目前核心线程已经够用了，不用再创建
           if (workerCountOf(c) >= min)
               return; // replacement not needed
       }
       // 增加一个消费的线程
       addWorker(null, false);
   }
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shutdown实现

public void shutdown() {
   final ReentrantLock mainLock = this.mainLock;
   mainLock.lock();
   try {
       // 检查能否操作线程
       checkShutdownAccess();
       // 确保状态 >= SHUTDOWN
       advanceRunState(SHUTDOWN);
       // 中断所有的空闲线程
       interruptIdleWorkers();
       // ScheduledThreadPoolExecutor会重写这个方法，做一些其他的运算
       onShutdown(); // hook for ScheduledThreadPoolExecutor
   } finally {
       mainLock.unlock();
   }
   tryTerminate();
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// 中断空闲线程
private void interruptIdleWorkers() {
   interruptIdleWorkers(false);
}1.
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// onlyOne为true则只中断一个空闲线程，否则全部中断
private void interruptIdleWorkers(boolean onlyOne) {
   final ReentrantLock mainLock = this.mainLock;
   mainLock.lock();
   try {
       for (Worker w : workers) {
           Thread t = w.thread;
           // 遍历Worker并执行中断操作，w.tryLock()保证了正在执行的Worker不会被中断
           // 因为正在运行的Worker再次获取锁会失败
           if (!t.isInterrupted() && w.tryLock()) {
               try {
                   t.interrupt();
               } catch (SecurityException ignore) {
               } finally {
                   w.unlock();
               }
           }
           if (onlyOne)
               break;
       }
   } finally {
       mainLock.unlock();
   }
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这里需要注意的是不会中断正在运行的线程，因为正在运行的线程w.tryLock()会返回false

shutdownNow实现

public List<Runnable> shutdownNow() {
   List<Runnable> tasks;
   final ReentrantLock mainLock = this.mainLock;
   mainLock.lock();
   try {
       checkShutdownAccess();
       // 确保状态 >= STOP
       advanceRunState(STOP);
       // 中断所有线程
       interruptWorkers();
       // 获取所有没有执行完成的task
       // 即将阻塞队列中的任务放到tasks中
       tasks = drainQueue();
   } finally {
       mainLock.unlock();
   }
   tryTerminate();
   return tasks;
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private void interruptWorkers() {
   final ReentrantLock mainLock = this.mainLock;
   mainLock.lock();
   try {
       for (Worker w : workers)
           w.interruptIfStarted();
   } finally {
       mainLock.unlock();
   }
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// 这个是Worker内部类的方法
void interruptIfStarted() {
   Thread t;
 // state的初始值为-1，运行到runWorker才允许中断
   if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
       try {
           t.interrupt();
       } catch (SecurityException ignore) {
       }
   }
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shutdownNow会中断所有的线程，因为和shutdown相比在中断之前，不用获取锁

tryTerminate实现

// 将状态转换到TERMINATED
final void tryTerminate() {
   for (;;) {
       int c = ctl.get();
       // 以下几种状态不能转换为TERMINATED
       // 1.RUNNING状态
       // 2.TIDYING或TERMINATED
       // 3.SHUTDOWN状态，但是workQueue不为空
       if (isRunning(c) ||
           runStateAtLeast(c, TIDYING) ||
           (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
           return;
       if (workerCountOf(c) != 0) { // Eligible to terminate
           interruptIdleWorkers(ONLY_ONE);
           return;
       }

       final ReentrantLock mainLock = this.mainLock;
       mainLock.lock();
       try {
           if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
               try {
                   // 让子类去实现，做一些操作
                   terminated();
               } finally {
                   ctl.set(ctlOf(TERMINATED, 0));
                   termination.signalAll();
               }
               return;
           }
       } finally {
           mainLock.unlock();
       }
       // else retry on failed CAS
   }
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从上面可看出状态转换的条件

SHUTDOWN想转化为TIDYING，需要workQueue为空，同时workerCount为0
STOP转化为TIDYING，需要workerCount为0

文章转载自公众号：Java识堂

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已于2023-11-7 11:45:27修改

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