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java源码-ThreadPoolExecutor(2)

日期:2018-08-22点击:310

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这篇文章的主要目标是为了讲解清楚ThreadPoolExecutor的提交任务的过程,非常推荐静下心来仔细阅读。
java源码-ThreadPoolExecutor(1)
java源码-ThreadPoolExecutor(2)
java源码-ThreadPoolExecutor(3)


ThreadPoolExecutor状态介绍

ThreadPoolExecutor针对线程池一共维护了五种状态,实现上用用高3位表示ThreadPoolExecutor的执行状态,低29位维持线程池线程个数,分别是:

  • RUNNING = -1 << COUNT_BITS = -1<<29 高三位为111
  • SHUTDOWN = 0 << COUNT_BITS = 0<<29 高三位为000
  • STOP = 1 << COUNT_BITS = 1<<29 高三位为001
  • TIDYING = 2 << COUNT_BITS = 2<<29 高三位为010
  • TERMINATED = 3 << COUNT_BITS = 3<<29 高三位为011
public class ThreadPoolExecutor extends AbstractExecutorService { private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); // Integer.SIZE=32,Integer.SIZE-3=29,COUNT_BITS=29 private static final int COUNT_BITS = Integer.SIZE - 3; // 线程池最大线程数=536870911(2^29-1),CAPACITY二进制中低29为为1,高3位为0 private static final int CAPACITY = (1 << COUNT_BITS) - 1; // 用高3位表示ThreadPoolExecutor的执行状态 // RUNNING=111 private static final int RUNNING = -1 << COUNT_BITS; // SHUTDOWN=000 private static final int SHUTDOWN = 0 << COUNT_BITS; // STOP=001 private static final int STOP = 1 << COUNT_BITS; // TIDYING=010 private static final int TIDYING = 2 << COUNT_BITS; // TERMINATED=110 private static final int TERMINATED = 3 << COUNT_BITS; // Packing and unpacking ctl // runStateOf通过获取高3位来对比 private static int runStateOf(int c) { return c & ~CAPACITY; } // workerCountOf通过比较低29位来获取线程数 private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; } private static boolean runStateLessThan(int c, int s) { return c < s; } private static boolean runStateAtLeast(int c, int s) { return c >= s; } private static boolean isRunning(int c) { return c < SHUTDOWN; } 


ThreadPoolExecutor任务提交过程

ThreadPoolExecutor提交任务代码是在AbstractExecutorService当中通过submit()方法实现的,按照两个步骤来实现:

  • 通过newTaskFor()方法创建待提交任务,该方法内部的实现后面再分析。
  • 通过execute()方法提交task,execute的在ThreadPoolExecutor类中实现重写。
  • 进一步跟进ThreadPoolExecutor的execute方法
public abstract class AbstractExecutorService implements ExecutorService { public <T> Future<T> submit(Runnable task, T result) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task, result); execute(ftask); return ftask; } public <T> Future<T> submit(Callable<T> task) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task); execute(ftask); return ftask; } } 



整个ThreadPoolExecutor的execute其实在源码自带的注释中已经写的很清楚了,怕自己翻译的不是特别所以这次直接把注释也贴在代码当中了,整个过程分为三个过程:

  • 1、当前的线程数是否小于corePoolSize,新建core线程并运行第一个任务。
  • 2、如果第一步不满足条件,那么就把任务提交到workQueue代表的队列当中。
  • 3、如果第二步不满足条件,那么就就新建不属于corePoolSize计数的线程(也就是新建core以外的线程)来进行处理。
  • 4、如果都失败那么就直接通过rejectHandler拒绝任务,步骤123当中任何检测到线程池关闭的情况直接执行任务拒绝。
public class ThreadPoolExecutor extends AbstractExecutorService { public void execute(Runnable command) { if (command == null) throw new NullPointerException(); /* * Proceed in 3 steps: * * 1. If fewer than corePoolSize threads are running, try to * start a new thread with the given command as its first * task. The call to addWorker atomically checks runState and * workerCount, and so prevents false alarms that would add * threads when it shouldn't, by returning false. * * 2. If a task can be successfully queued, then we still need * to double-check whether we should have added a thread * (because existing ones died since last checking) or that * the pool shut down since entry into this method. So we * recheck state and if necessary roll back the enqueuing if * stopped, or start a new thread if there are none. * * 3. If we cannot queue task, then we try to add a new * thread. If it fails, we know we are shut down or saturated * and so reject the task. */ 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); } } 



ThreadPoolExecutor的addWorker方法有两个参数,Runnable firstTask代表待执行任务, boolean core代表是否启动核心线程,整个启动过程主要分为三个步骤:

  • 前置检查:检查线程池是否处于关闭状态,在正常运行的情况下增加工作线程计数。
  • 正常处理:创建Worker对象并在加锁的条件下将新建worker添加到workers集合当中,并通过调用t.start()方法启动线程。
  • 后置处理:判断启动线程是否失败,如果失败那么就尝试中止线程池。
public class ThreadPoolExecutor extends AbstractExecutorService { 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); // 判断是否超出线程限制,corePoolSize和core线程数, // maximumPoolSize代表超出core部分的线程数 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 } } boolean workerStarted = false; 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()); 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(); if (s > largestPoolSize) largestPoolSize = s; workerAdded = true; } } finally { mainLock.unlock(); } if (workerAdded) { t.start(); workerStarted = true; } } } finally { if (! workerStarted) addWorkerFailed(w); } return workerStarted; } private void addWorkerFailed(Worker w) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (w != null) workers.remove(w); decrementWorkerCount(); tryTerminate(); } finally { mainLock.unlock(); } } } 


ThreadPoolExecutor的worker介绍

ThreadPoolExecutor的worker实现Runnable接口,在worker的内部run()方法中通过执行runWorker()方法来启动task,启动方式会调用task.run()方法,所以从这个角度来看,task的执行线程其实ThreadPoolExecutor线程池中的worker。

  • Worker类内部包含:Thread thread工作线程用于执行task、Runnable firstTask标识待执行任务。
  • runWorker()方法内部负责执行来自提交的firstTask或者阻塞从任务队列通过getTask()方法取得待执行任务
  • runWorker()方法内部通过执行task.run()负责真正执行任务。
public class ThreadPoolExecutor extends AbstractExecutorService { private final class Worker extends AbstractQueuedSynchronizer implements Runnable { private static final long serialVersionUID = 6138294804551838833L; /** 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; 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); } } 



runWorker内部主要做两件事情,分别是:

  • 获取任务:通过直接传进来firstTask或者通过getTask从任务队列中获取任务
  • 执行任务:task.run()执行真正的task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); } } 



getTask()方法外层是一个for循环,然后内部从workQueue获取任务,区分设置超时或者阻塞等待。

  • 阻塞等待直至线程获取到可消费任务。
  • 超时等待使用的是keepAliveTime,用于超时后设置线程超时标记然后线程退出工作。
  • 线程退出循环是通过返回task=null,外层循环直接结束实现。
 private Runnable getTask() { 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. if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; } int wc = workerCountOf(c); // Are workers subject to culling? boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; // 标记线程退出工作部分的逻辑,通过返回task=null,从而在外层调用方实现退出while循环 if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } } } 


ThreadPoolExecutor的task介绍

ThreadPoolExecutor的newTaskFor()方法负责创建task,创建的FutureTask的实例本身实现了Runnable、Future的接口。

  • FutureTask内部可以创建入参为Runnable的对象的时候会创建一个代理器
  • RunnableAdapter,创建入参为Callable的对象就比较直接了。
  • FutureTask的运行函数run()负责执行Callable对象的call()方法并将返回值通过set()方法设置到outcome对象。
  • FutureTask的get()方法负责获取返回值,就是我们submit()后返回的future的get()调用。
public abstract class AbstractExecutorService implements ExecutorService { protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) { return new FutureTask<T>(runnable, value); } protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) { return new FutureTask<T>(callable); } } 
public class Executors { public static <T> Callable<T> callable(Runnable task, T result) { if (task == null) throw new NullPointerException(); return new RunnableAdapter<T>(task, result); } static final class RunnableAdapter<T> implements Callable<T> { final Runnable task; final T result; RunnableAdapter(Runnable task, T result) { this.task = task; this.result = result; } public T call() { task.run(); return result; } } } public interface RunnableFuture<V> extends Runnable, Future<V> { void run(); } public class FutureTask<V> implements RunnableFuture<V> { /** * Possible state transitions: * NEW -> COMPLETING -> NORMAL * NEW -> COMPLETING -> EXCEPTIONAL * NEW -> CANCELLED * NEW -> INTERRUPTING -> INTERRUPTED */ private volatile int state; private static final int NEW = 0; private static final int COMPLETING = 1; private static final int NORMAL = 2; private static final int EXCEPTIONAL = 3; private static final int CANCELLED = 4; private static final int INTERRUPTING = 5; private static final int INTERRUPTED = 6; private Callable<V> callable; private Object outcome; // non-volatile, protected by state reads/writes private volatile Thread runner; private volatile WaitNode waiters; private V report(int s) throws ExecutionException { Object x = outcome; if (s == NORMAL) return (V)x; if (s >= CANCELLED) throw new CancellationException(); throw new ExecutionException((Throwable)x); } public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable } public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable } public boolean isCancelled() { return state >= CANCELLED; } public boolean isDone() { return state != NEW; } public V get() throws InterruptedException, ExecutionException { int s = state; if (s <= COMPLETING) s = awaitDone(false, 0L); return report(s); } public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { if (unit == null) throw new NullPointerException(); int s = state; if (s <= COMPLETING && (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING) throw new TimeoutException(); return report(s); } protected void set(V v) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = v; UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state finishCompletion(); } } // 核心的逻辑,负责调用对象的call方法并赋值返回值 public void run() { if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread())) return; try { Callable<V> c = callable; if (c != null && state == NEW) { V result; boolean ran; try { result = c.call(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } if (ran) set(result); } } finally { runner = null; int s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE; private static final long stateOffset; private static final long runnerOffset; private static final long waitersOffset; static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class<?> k = FutureTask.class; stateOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("state")); runnerOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("runner")); waitersOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("waiters")); } catch (Exception e) { throw new Error(e); } } } 


参考文章

ThreadPoolExecutor解析-主要源码研究
ThreadPoolExecutor(五)——线程池关闭相关操作
ThreadPoolExecutor(六)——线程池关闭之后

原文链接:https://yq.aliyun.com/articles/666320
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