首先分析内部类:ThreadPoolExecutor$Worker
//Worker对线程和任务做了一个封装,同时它又实现了Runnable接口, //所以Worker类的线程跑的是自身的run方法 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. * @param firstTask the first task (null if none) */ Worker(Runnable firstTask) { setState(-1); // inhibit interrupts until runWorker this.firstTask = firstTask; //创建一个Thread对象,它的Runnable对象是当前Worker对象 //创建了线程,但是还没启动,在外部start //Executors.DefaultThreadFactory this.thread = getThreadFactory().newThread(this); } /** Delegates main run loop to outer runWorker */ public void run() { runWorker(this); } // Lock methods // // The value 0 represents the unlocked state. // The value 1 represents the locked state. protected boolean isHeldExclusively() { return getState() != 0; } protected boolean tryAcquire(int unused) { if (compareAndSetState(0, 1)) { setExclusiveOwnerThread(Thread.currentThread()); return true; } return false; } protected boolean tryRelease(int unused) { setExclusiveOwnerThread(null); setState(0); return true; } public void lock() { acquire(1); } public boolean tryLock() { return tryAcquire(1); } public void unlock() { release(1); } public boolean isLocked() { return isHeldExclusively(); } void interruptIfStarted() { Thread t; if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) { try { t.interrupt(); } catch (SecurityException ignore) { } } } } final void runWorker(Worker w) { Thread wt = Thread.currentThread(); //调用pool.execute()时传入任务时,如果addWorker返回为true,表示创建了worker,则任务也放在worker对象中了。 //如果addWorker返回为false,则把任务放入队列 Runnable task = w.firstTask; w.firstTask = null; w.unlock(); // allow interrupts boolean completedAbruptly = true; try { //第二个task是从队列中取得的 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 { //任务执行完后,置空 task = null; w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { processWorkerExit(w, completedAbruptly); } } private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? retry: 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; } boolean timed; // Are workers subject to culling? for (;;) { int wc = workerCountOf(c); timed = allowCoreThreadTimeOut || wc > corePoolSize; if (wc <= maximumPoolSize && ! (timedOut && timed)) break; if (compareAndDecrementWorkerCount(c)) return null; c = ctl.get(); // Re-read ctl if (runStateOf(c) != rs) continue retry; // else CAS failed due to workerCount change; retry inner loop } try { Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
接着分析ThreadPoolExecutor
public class ThreadPoolExecutor extends AbstractExecutorService { //状态变量,保存了workerCount和runState的值 //线程池的初始状态是RUNNING private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); private static int ctlOf(int rs, int wc) { return rs | wc; } //状态值从小到大排列 // runState is stored in the high-order bits 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; // 默认为 false,当线程池中已经有了 corePoolSize 个线程,即使这些线程不干活,也不会回收。 // 但是如果线程池中的线程数量超过了 corePoolSize,则会回收 private volatile boolean allowCoreThreadTimeOut; private volatile int corePoolSize; private volatile int maximumPoolSize; public void execute(Runnable command) { if (command == null) throw new NullPointerException(); int c = ctl.get(); //worker数量小于最小线程数,创建一个worker,并启动 //如果addWorker返回true,表示创建了一个worker对象,任务也放在worker对象中了。 //如果addWorker返回false,则随后把任务放入队列 if (workerCountOf(c) < corePoolSize) { if (addWorker(command, true)) return; c = ctl.get(); } //如果线程池处于运行状态,往队列投任务 //workQueue.offer(command) 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); } //如果workQueue.offer(command)返回false呢? //当队列中积压的任务太多时,就会返回false //这时传给addWorker的是false else if (!addWorker(command, false)) reject(command); } //core决定worker数量以corePoolSize和maximumPoolSize中哪一个值为上限 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 { final ReentrantLock mainLock = this.mainLock; w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { mainLock.lock(); try { // Recheck while holding lock. // Back out on ThreadFactory failure or if // shut down before lock acquired. int c = ctl.get(); int rs = runStateOf(c); 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; } }
线程池接收任务的流程图:
关闭线程池有 shutdown 和 shutdownNow 2种方法:
shutdown 不再接收新任务,但会把队列中的任务执行完,shutdownNow 不会执行队列中的任务。
public void shutdown() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(SHUTDOWN); interruptIdleWorkers(); onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } tryTerminate(); } public List<Runnable> shutdownNow() { List<Runnable> tasks; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); interruptWorkers(); // 删除队列中的任务 tasks = drainQueue(); } finally { mainLock.unlock(); } tryTerminate(); return tasks; }
shutdown 方法置线程池状态为 SHUTDOWN,shutdownNow 方法置为 STOP,线程池的线程一旦启动,会不停地从队列中取任务
getTask 的部分逻辑
int c = ctl.get(); // 获取线程池的状态 int rs = runStateOf(c); // Check if queue empty only if necessary. // 状态如果为 SHUTDOWN,则当队列没有任务时,返回 null,即线程执行完 run 方法,执行结束;如果队列不空,则继续执行队列中的任务 // 状态如果为 STOP,则直接返回 null,不管队列是否有任务 if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { decrementWorkerCount(); return null; }
但是,shutdown 和 shutdownNow 一定会关闭线程池吗?这两个方法均是设置了状态,interrupt 了 worker,但是如果 worker 的 run 方法是一个死循环,而且它不关心这个 interrupt 标志位的话,那么线程是无法关闭的。当然正常的业务逻辑中,不会有这种情况。使用了 shutdown 后,因为这只是置标志,所以需要调用 awaitTermination 等线程池真正关闭或者超时。
假如一个 jvm 进程,有一个前台线程,多个 daemon 线程,当前台线程退出后,jvm 进程退出。
线程池创建线程时,daemon 属性默认为 false,即默认前台线程。
// java.util.concurrent.Executors.DefaultThreadFactory#newThread public Thread newThread(Runnable r) { Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0); if (t.isDaemon()) t.setDaemon(false); if (t.getPriority() != Thread.NORM_PRIORITY) t.setPriority(Thread.NORM_PRIORITY); return t; }
Thread 对象初始化时跟随当前线程的 daemon 属性
Thread parent = currentThread();
this.daemon = parent.isDaemon();