前言
ThreadPoolExecutor置信大家都很相熟：线程池的实现类。明天咱们就来看看它外部是怎么实现的。
实现原理
先来看看它的类构造：
public class ThreadPoolExecutor extends AbstractExecutorService {}public abstract class AbstractExecutorService implements ExecutorService {}public interface ExecutorService extends Executor { void shutdown(); <T> Future<T> submit(Callable<T> task); // ...}public interface Executor { void execute(Runnable command);}再来看看它的要害属性：
// ctl高3位示意线程池的运行状态，低29位示意线程个数private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));// 线程个数掩码，Integer位数-3，与具体平台Integer位数无关，大部分是32-3=29private static final int COUNT_BITS = Integer.SIZE - 3;// 线程最大个数private static final int CAPACITY = (1 << COUNT_BITS) - 1;// runState is stored in the high-order bits// 线程池状态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;// 互斥锁private final ReentrantLock mainLock = new ReentrantLock();// 工作线程汇合private final HashSet<Worker> workers = new HashSet<Worker>();// 线程池终止条件private final Condition termination = mainLock.newCondition();// 线程池外围参数// 阻塞队列private final BlockingQueue<Runnable> workQueue;// 线程工厂private volatile ThreadFactory threadFactory;// 回绝策略private volatile RejectedExecutionHandler handler;// 线程闲置时长private volatile long keepAliveTime;// 外围线程数private volatile int corePoolSize;// 最大线程数private volatile int maximumPoolSize;// 默认回绝策略：AbortPolicy抛出异样private static final RejectedExecutionHandler defaultHandler = new AbortPolicy();其中Worker是它的外部类，代表工作线程。
private final class Worker extends AbstractQueuedSynchronizer implements Runnable {}它继承了AQS，实现了Runnable接口
再来看要害办法：
execute()
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);}其中addWorker()办法如下：
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 } } 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;}我滴个乖乖，addWorker()这么简单，今天再来吧。
晚安全世界！