Java线程池：简明易懂的源码分析java,多线程,并发编程zycxnanwang的博客-

Java线程池：简明易懂的源码分析

Java线程池的一些基础知识，可以参考博客

Java线程池，你五分钟讲完，而我和面试官聊了半小时

本文将从源码角度分析线程池原理，加深对线程池原理的理解，简单背几个原理知识，其实很难得到面试官的青睐，了解源码知识，可以由内而外得征服面试官。

Java线程池状态转换

使用Integer类型(32bit) ctl中高3位记录Java线程池的状态,低29位记录线程数量

RUNNING:111、SHUTDOWN:000、STOP:001

TIDYING:010、TERMINATED:011.后面均又29位零

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); private static final int COUNT_BITS = Integer.SIZE - 3; // 29 //线程池最大容量2^29-1 private static final int CAPACITY   = (1 << COUNT_BITS) - 1; private static final int RUNNING    = -1 << COUNT_BITS; private static final int SHUTDOWN   = 0 << COUNT_BITS; private static final int STOP       = 1 << COUNT_BITS; private static final int TIDYING    = 2 << COUNT_BITS; private static final int TERMINATED = 3 << COUNT_BITS; //取得线程池运行状态 private static int runStateOf(int c) { return c & ~CAPACITY; } //取得线程池线程数量 private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; } 

下面从线程池的submit方法作为出发点，从源码角度分析，线程池的实现原理

submit方法

submit方法将提交的任务task,包装成RunnableFuture对象,既实现了Runnable接口，又实现了Future接口。之后会调用execute方法

public Future<?> submit(Runnable task) { if (task == null) throw new NullPointerException();     RunnableFuture<Void> ftask = newTaskFor(task, null); execute(ftask); return ftask; } 

execute方法

当提交一个新任务，线程池的处理流程如下：

• 判断线程池中核心线程数是否达到corePoolSize,若否，则创建一个新核心线程执行任务

• 若核心线程数已达阈值,判断workQueue是否已满，若未满，则将新任务添加进阻塞队列

• 若满，再判断，线程池中线程数是否达到阈值maximumPoolSize,若否，则新建一个非核心线程执行任务。若达到阈值，则执行线程池饱和策略。

execute方法，各种参数

public void execute(Runnable command) { if (command == null) throw new NullPointerException(); int c = ctl.get(); //线程数小于corePoolSize if (workerCountOf(c) < corePoolSize) { if (addWorker(command, true)) return;         c = ctl.get(); } //线程数大于等于corePoolSize,尝试添加进workQueue if (isRunning(c) && workQueue.offer(command)) { //再次检查 int recheck = ctl.get(); //如果状态不为Running，则从队列中移除任务 if (!isRunning(recheck) && remove(command)) reject(command); //如果线程池数量为0，则添加一个非核心线程执行任务 else if (workerCountOf(recheck) == 0) addWorker(null, false); } //队列满，尝试新建非核心线程，执行任务 else if (!addWorker(command, false)) //否则执行拒绝策略 reject(command); } 

addWorker方法

首先通过自旋操作，将线程总数加1.之后用独占锁锁住，构建一个Worker对象，将Worker对象添加进workers(HashSet<Worker>),释放锁，启动线程。

private boolean addWorker(Runnable firstTask, boolean core) {     retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); //以下情况直接返回 //线程池处于STOP、TYDING、TERMINATED状态 //线程池处于SHUTDOWN状态，firstTask!=null //线程池处于SHUTDOWN状态，firstTask==null, workQueue为空 if (rs >= SHUTDOWN && ! (rs == SHUTDOWN &&                firstTask == null && ! workQueue.isEmpty())) return false; //循环CAS操作，增加元素个数 for (;;) { int wc = workerCountOf(c); if (wc >= CAPACITY ||                 wc >= (core ? corePoolSize : maximumPoolSize)) return false; if (compareAndIncrementWorkerCount(c)) break retry; //CAS失败，查看线程池状态是否改变，变化则跳到最外层循环             c = ctl.get(); if (runStateOf(c) != rs) continue retry; } } 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; //独占锁进行同步             mainLock.lock(); try { int rs = runStateOf(ctl.get()); //重新check一下线程池状态 if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { if (t.isAlive()) throw new IllegalThreadStateException(); //添加任务                     workers.add(w); int s = workers.size(); if (s > largestPoolSize)                         largestPoolSize = s;                     workerAdded = true; } } finally {                 mainLock.unlock(); } //添加成功，执行任务 if (workerAdded) {                 t.start();                 workerStarted = true; } } } finally { if (!workerStarted) addWorkerFailed(w); } return workerStarted; } 

Worker类( 实现了 AQS)

Worker对象，包括thread和firstTask成员变量。thread成员变量的构造函数的参数是自身的Worker对象。

所以调用thread成员变量的run方法，其实就是调用本身Worker对象的run方法。

本身Worker对象的run方法，会调用runWorker方法，参数为本身Worker对象

private final class Worker extends AbstractQueuedSynchronizer implements Runnable { final Thread thread;     Runnable firstTask; volatile long completedTasks; Worker(Runnable firstTask) { this.firstTask = firstTask; this.thread = getThreadFactory().newThread(this); } public void run() { runWorker(this); } } 

runWorker方法

final void runWorker(Worker w) {     Thread wt = Thread.currentThread();     Runnable task = w.firstTask;     w.firstTask = null;     w.unlock(); //允许中断 boolean completedAbruptly = true; try { //循环过程，执行完第一个任务后，一直从队列中拿取任务 while (task != null || (task = getTask()) != null) {             w.lock(); //处于SHUTDOWN状态，并不会中断正在运行的任务 if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted())                 wt.interrupt(); try { //执行任务前干一些事 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 { //执行任务后干一些事 afterExecute(task, thrown); } } finally {                 task = null;                 w.completedTasks++;                 w.unlock(); } }         completedAbruptly = false; } finally { processWorkerExit(w, completedAbruptly); } } 

getTask方法

从工作队列中，拿取任务，如果满足某几个条件(线程超时与否、线程池状态、工作队列是否为空)，直接返回null.

private Runnable getTask() { boolean timedOut = false; for (;;) { int c = ctl.get(); int rs = runStateOf(c); //线程池状态为SHUTDOWN，并且workQueue为空 //线程池处于STOP、TYDING、TERMINATED状态 //以上两种情况,返回null if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { //线程总数减一 decrementWorkerCount(); return null; } int wc = workerCountOf(c); //allowCoreThreadTimeOut为true,运行核心线程受超时机制影响 boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; //空闲线程超时，直接返回null if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try { //从队列中取任务 //如果空闲线程超时，workQueue.poll方法返回null             Runnable r = timed ?                 workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :                 workQueue.take(); if (r != null) return r;             timedOut = true; } catch (InterruptedException retry) {             timedOut = false; } } } 

参考文章

Java线程池源码分析（一)

Java 线程池 ThreadPoolExecutor 源码分析