一、什么是线程池？为什么要用线程池？
1、 降低资源的消耗。降低线程创建和销毁的资源消耗；
2、 提高响应速度：线程的创建时间为T1，执行时间T2,销毁时间T3，免去T1和T3的时间
3、 提高线程的可管理性。
二、线程池的创建
ThreadPoolExecutor，jdk所有线程池实现的父类
三、参数介绍
先看构造函数：
/** * Creates a new {@code ThreadPoolExecutor} with the given initial * parameters. * * @param corePoolSize the number of threads to keep in the pool, even * if they are idle, unless {@code allowCoreThreadTimeOut} is set * @param maximumPoolSize the maximum number of threads to allow in the * pool * @param keepAliveTime when the number of threads is greater than * the core, this is the maximum time that excess idle threads * will wait for new tasks before terminating. * @param unit the time unit for the {@code keepAliveTime} argument * @param workQueue the queue to use for holding tasks before they are * executed. This queue will hold only the {@code Runnable} * tasks submitted by the {@code execute} method. * @param threadFactory the factory to use when the executor * creates a new thread * @param handler the handler to use when execution is blocked * because the thread bounds and queue capacities are reached * @throws IllegalArgumentException if one of the following holds:<br> * {@code corePoolSize < 0}<br> * {@code keepAliveTime < 0}<br> * {@code maximumPoolSize <= 0}<br> * {@code maximumPoolSize < corePoolSize} * @throws NullPointerException if {@code workQueue} * or {@code threadFactory} or {@code handler} is null */ public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler) { if (corePoolSize < 0 || maximumPoolSize <= 0 || maximumPoolSize < corePoolSize || keepAliveTime < 0) throw new IllegalArgumentException(); if (workQueue == null || threadFactory == null || handler == null) throw new NullPointerException(); this.acc = System.getSecurityManager() == null ? null : AccessController.getContext(); this.corePoolSize = corePoolSize; this.maximumPoolSize = maximumPoolSize; this.workQueue = workQueue; this.keepAliveTime = unit.toNanos(keepAliveTime); this.threadFactory = threadFactory; this.handler = handler; }
再看看参数的具体解释：
// 阻塞队列 private final BlockingQueue<Runnable> workQueue; // 互斥锁 private final ReentrantLock mainLock = new ReentrantLock(); // 线程集合。一个Worker对应一个线程。 private final HashSet<Worker> workers = new HashSet<Worker>(); // “终止条件”，与“mainLock”绑定。 private final Condition termination = mainLock.newCondition(); // 线程池中线程数量曾经达到过的最大值。 private int largestPoolSize; // 已完成任务数量 private long completedTaskCount; // ThreadFactory对象，用于创建线程。 private volatile ThreadFactory threadFactory; // 拒绝策略的处理句柄。 private volatile RejectedExecutionHandler handler; // 保持线程存活时间。 private volatile long keepAliveTime; private volatile boolean allowCoreThreadTimeOut; // 核心池大小 private volatile int corePoolSize; // 最大池大小 private volatile int maximumPoolSize;
参数详解：
1.核心线程 int corePoolSize：核心池大小
the number of threads to keep in the pool, even if they are idle, unless {@code allowCoreThreadTimeOut} is set
线程池在new 出来后是没有线程的，仅当执行任务的时候才会创建线程；
当然也可以调用线程池的prestartAllCoreThreads()方法，让线程池在创建时就创建corePoolSize数目的线程；
核心线程在任务完成时，会new 新的线程，并且不会销毁，除非设置allowCoreThreadTimeOut
public void allowCoreThreadTimeOut(boolean value) { if (value && keepAliveTime <= 0) throw new IllegalArgumentException("Core threads must have nonzero keep alive times"); if (value != allowCoreThreadTimeOut) { allowCoreThreadTimeOut = value; if (value) interruptIdleWorkers(); } }
2 最大线程数 int maximuxPoolSize：最大线程池大小
the maximum number of threads to allow in the pool 线程池所允许创建的最大线程数;
3 存活时间 long keepAliveTime：线程存活时间
when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
当线程池中的线程数量大于核心池大小后，超过核心数量的线程在keepAliveTime的时间内可以等待一个新任务，超过时间就会销毁;
4. TimeUnit timeUnit：keepAliveTime的单位，
TimeUnit 枚举值有：DAYS、HOURS、MINUTES、SECONDS、MILLISECONDS(毫秒)、MICROSECONDS(微秒)、NANOSECONDS（纳秒);
5.工作队列 BlockingQueue workQueue：阻塞任务队列
执行前的任务，存储在队列中（线程池会先使用核心线程，当核心线程用完才会使用队列）；
线程池只会execute Runnable任务，Callable任务也会包装成Runnable任务
主要实现类有：
1）LinkedBlockingQueue：基于链表的无界（默认构造函数为：最大值Integer.MAX_VALUE容量）阻塞队列，按FIFO(先进先出)的规则存取任务
2）ArrayBlockingQueue：基于数组的有界阻塞队列，按FIFO的规则对任务进行存取，必须传入参数来定义队列大小
3）DelayedWorkQueue：基于堆的延迟队列，Executors.newScheduledThreadPool(...)中使用了该队列
4）PriorityBlockingQueue：具有优先级的阻塞队列
5）SynchronousQueue：不存储任务的阻塞队列，每一个存入对应一个取出，串行化队列
吞吐量：SynchronousQueue > LinkedBlockingQueue > ArrayBlockingQueue
6 线程创建工厂 ThreadFactory threadFactory：线程工厂
用来创建线程，可以通过自定义线程工厂给新创建的线程设置更合理的名字、设置优先级和是否守护线程。可以根据需要自定义线程工厂。
static class DefaultThreadFactory implements ThreadFactory { private static final AtomicInteger poolNumber = new AtomicInteger(1); private final ThreadGroup group; private final AtomicInteger threadNumber = new AtomicInteger(1); private final String namePrefix; DefaultThreadFactory() { SecurityManager s = System.getSecurityManager(); //线程组 group = (s != null) ? s.getThreadGroup() : Thread.currentThread().getThreadGroup(); //名称前缀，这就是我们打印的线程池中线程名称前缀 //可以修改的，如果自定义线程工厂 namePrefix = "pool-" + poolNumber.getAndIncrement() + "-thread-"; } //工厂创建线程 public Thread newThread(Runnable r) { Thread t = new Thread(group, r, namePrefix + threadNumber.getAndIncrement(), 0); if (t.isDaemon()) //非守护线程 t.setDaemon(false); if (t.getPriority() != Thread.NORM_PRIORITY) //常规优先级 t.setPriority(Thread.NORM_PRIORITY); return t; } }
7 拒绝策略 RejectedExecutionHandler handler：拒绝任务的接口处理器
达到队列和线程池的最大线程数限制，就执行拒绝策略
拒绝策略有：
1）AbortPolicy：拒绝任务并抛出异常，默认的策略
2）DiscardPolicy：直接拒绝不抛出异常
3）DiscardOldestPolicy：丢弃队列中最远的一个任务（最先进入队列的，FIFO），并执行当前任务；
4）CallerRunsPolicy：只用调用者所在的线程来执行任务，不管其他线程的事。
四、我们看线程池的主要方法
1.execute（）方法
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. */ //AtomicInteger存 int c = ctl.get(); //工作线程小于corePoolSize if (workerCountOf(c) < corePoolSize) { //添加一个core线程，此处参数为true，表示添加的线程是core容量下的线程 if (addWorker(command, true)) return; //刷新数据，乐观锁就是没有锁 c = ctl.get(); } //判断线程池运行状态，工作队列是否有空间 if (isRunning(c) && workQueue.offer(command)) { //再次检测 int recheck = ctl.get(); //线程池已停止，就移除队列 if (! isRunning(recheck) && remove(command)) //执行拒绝策略 reject(command); //线程池在运行，有效线程数为0 else if (workerCountOf(recheck) == 0) //添加一个空线程进线程池，使用非core容量线程 //仅有一种情况，会走这步，core线程数为0，max线程数>0,队列容量>0 //创建一个非core容量的线程，线程池会将队列的command执行 addWorker(null, false); } //线程池停止了或者队列已满，添加maximumPoolSize容量工作线程，如果失败，执行拒绝策略 else if (!addWorker(command, false)) reject(command); } final void reject(Runnable command) { handler.rejectedExecution(command, this); }
看上面的英文注释，有3步：
1） 检查core线程池数量<corePoolSize数量，是，可以提交任务或新建线程执行任务 。
2）如果corePoolSize线程数量已使用，如果队列容量未满，则加入队列。
3）队列已满，创建maximumPoolSize线程数量执行；如果失败则执行关闭线程池或者拒绝策略。
继续跟踪 private boolean addWorker(Runnable firstTask, boolean core)
rivate final ReentrantLock mainLock = new ReentrantLock(); private final HashSet<Worker> workers = new HashSet<Worker>(); /** * Checks if a new worker can be added with respect to current * pool state and the given bound (either core or maximum). If so, * the worker count is adjusted accordingly, and, if possible, a * new worker is created and started, running firstTask as its * first task. This method returns false if the pool is stopped or * eligible to shut down. It also returns false if the thread * factory fails to create a thread when asked. If the thread * creation fails, either due to the thread factory returning * null, or due to an exception (typically OutOfMemoryError in * Thread.start()), we roll back cleanly. * * @param firstTask the task the new thread should run first (or * null if none). Workers are created with an initial first task * (in method execute()) to bypass queuing when there are fewer * than corePoolSize threads (in which case we always start one), * or when the queue is full (in which case we must bypass queue). * Initially idle threads are usually created via * prestartCoreThread or to replace other dying workers. * * @param core if true use corePoolSize as bound, else * maximumPoolSize. (A boolean indicator is used here rather than a * value to ensure reads of fresh values after checking other pool * state). * @return true if successful */ private boolean addWorker(Runnable firstTask, boolean core) { //自旋检查 retry: for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. //如果线程池已关闭；线程池正在关闭，提交的任务为null，任务队列不为空，则直接返回失败 if (rs >= SHUTDOWN && ! (rs == SHUTDOWN && firstTask == null && ! workQueue.isEmpty())) return false; for (;;) { int wc = workerCountOf(c); //工作线程数达到或超过最大容量，或者添加core线程达到最大容量或者添加max线程达到最大容量，直接失败 if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize)) return false; //线程数+1，CAS原子操作，成功后跳出循环 if (compareAndIncrementWorkerCount(c)) break retry; c = ctl.get(); // Re-read ctl //原子操作失败，判断线程池运行状态是否已改变 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 { //所有代码的核心，new Worker，创建了线程，或者复用线程 w = new Worker(firstTask); final Thread t = w.thread; if (t != null) { //线程池是操作多线程，加锁 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()); //再检查一次，线程池在运行或正在停止 if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) { //添加的线程不能是活跃线程 if (t.isAlive()) // precheck that t is startable throw new IllegalThreadStateException(); //添加线程存储，HashSet存储 workers.add(w); //重入锁最大池数量 int s = workers.size(); if (s > largestPoolSize) largestPoolSize = s; workerAdded = true; } } finally { mainLock.unlock(); } if (workerAdded) { //上面的标记，启动线程 t.start(); workerStarted = true; } } } finally { if (! workerStarted) //失败处理 addWorkerFailed(w); } return workerStarted; }
Worker 是啥
private final class Worker extends AbstractQueuedSynchronizer implements Runnable { /** * This class will never be serialized, but we provide a * serialVersionUID to suppress a javac warning. */ 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; /** * 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; //此处创建了线程，线程工厂，使用this this.thread = getThreadFactory().newThread(this); } /** Delegates main run loop to outer runWorker */ public void run() { runWorker(this); }
Worker实现AQS和Runnable接口 ，是线程接口实现
追踪 run方法
/** * Main worker run loop. Repeatedly gets tasks from queue and * executes them, while coping with a number of issues: * * 1. We may start out with an initial task, in which case we * don't need to get the first one. Otherwise, as long as pool is * running, we get tasks from getTask. If it returns null then the * worker exits due to changed pool state or configuration * parameters. Other exits result from exception throws in * external code, in which case completedAbruptly holds, which * usually leads processWorkerExit to replace this thread. * * 2. Before running any task, the lock is acquired to prevent * other pool interrupts while the task is executing, and then we * ensure that unless pool is stopping, this thread does not have * its interrupt set. * * 3. Each task run is preceded by a call to beforeExecute, which * might throw an exception, in which case we cause thread to die * (breaking loop with completedAbruptly true) without processing * the task. * * 4. Assuming beforeExecute completes normally, we run the task, * gathering any of its thrown exceptions to send to afterExecute. * We separately handle RuntimeException, Error (both of which the * specs guarantee that we trap) and arbitrary Throwables. * Because we cannot rethrow Throwables within Runnable.run, we * wrap them within Errors on the way out (to the thread's * UncaughtExceptionHandler). Any thrown exception also * conservatively causes thread to die. * * 5. After task.run completes, we call afterExecute, which may * also throw an exception, which will also cause thread to * die. According to JLS Sec 14.20, this exception is the one that * will be in effect even if task.run throws. * * The net effect of the exception mechanics is that afterExecute * and the thread's UncaughtExceptionHandler have as accurate * information as we can provide about any problems encountered by * user code. * * @param w the worker */ final void runWorker(Worker w) { Thread wt = Thread.currentThread(); Runnable task = w.firstTask; w.firstTask = null; //任务线程的锁状态默认为-1（构造函数设置的），此时解锁+1，变为0，是锁打开状态，允许中断。 w.unlock(); // allow interrupts boolean completedAbruptly = true; try { //如果添加的任务存在或者队列取 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 //线程池正在停止，当前线程未中断 if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted()) //中断当前线程 wt.interrupt(); try { //准备，空方法，可自定义实现 beforeExecute(wt, task); Throwable thrown = null; try { //本质，直接调用run方法 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 { //结束，空方法，可自定义实现 afterExecute(task, thrown); } } finally { task = null; //记录任务数 w.completedTasks++; w.unlock(); } } completedAbruptly = false; } finally { //worker结束处理 processWorkerExit(w, completedAbruptly); } }
跟踪 getTask
private Runnable getTask() { boolean timedOut = false; // Did the last poll() time out? //自旋锁 for (;;) { int c = ctl.get(); int rs = runStateOf(c); // Check if queue empty only if necessary. //检查线程池是否关闭，队列为空 if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) { //减少工作线程数量 decrementWorkerCount(); return null; } int wc = workerCountOf(c); // Are workers subject to culling? boolean timed = allowCoreThreadTimeOut || wc > corePoolSize; //工作线程数超过core或者max，或者队列为空，工作线程存在 if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { //减少任务数 if (compareAndDecrementWorkerCount(c)) return null; continue; } try { //超时机制控制队列取元素 //take 移除并返回队列头部的元素 如果队列为空，则阻塞 //poll 移除并返问队列头部的元素 如果队列为空，则返回null Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
跟踪 processWorkerExit(w, completedAbruptly);
/** * Performs cleanup and bookkeeping for a dying worker. Called * only from worker threads. Unless completedAbruptly is set, * assumes that workerCount has already been adjusted to account * for exit. This method removes thread from worker set, and * possibly terminates the pool or replaces the worker if either * it exited due to user task exception or if fewer than * corePoolSize workers are running or queue is non-empty but * there are no workers. * * @param w the worker * @param completedAbruptly if the worker died due to user exception */ private void processWorkerExit(Worker w, boolean completedAbruptly) { //正常执行，此处设置为法false if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted decrementWorkerCount(); final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //任务数增加 completedTaskCount += w.completedTasks; //移除HashSet的线程 workers.remove(w); } finally { mainLock.unlock(); } //尝试停止线程池 tryTerminate(); int c = ctl.get(); //如果没有停止线程池 if (runStateLessThan(c, STOP)) { if (!completedAbruptly) { int min = allowCoreThreadTimeOut ? 0 : corePoolSize; //core线程数量为0或者队列为空，默认1 if (min == 0 && ! workQueue.isEmpty()) min = 1; //工作线程比core线程多，直接返回 if (workerCountOf(c) >= min) return; // replacement not needed } //当前线程运行结束，增加空线程，容量maximumPoolSize addWorker(null, false); } }
addWorker 失败处理
/** * Rolls back the worker thread creation. * - removes worker from workers, if present * - decrements worker count * - rechecks for termination, in case the existence of this * worker was holding up termination */ private void addWorkerFailed(Worker w) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { if (w != null) //工作线程移除任务HashSet workers.remove(w); //工作线程数量-1 decrementWorkerCount(); //尝试停止线程池 tryTerminate(); } finally { mainLock.unlock(); } }
tryTerminate
/** * Transitions to TERMINATED state if either (SHUTDOWN and pool * and queue empty) or (STOP and pool empty). If otherwise * eligible to terminate but workerCount is nonzero, interrupts an * idle worker to ensure that shutdown signals propagate. This * method must be called following any action that might make * termination possible -- reducing worker count or removing tasks * from the queue during shutdown. The method is non-private to * allow access from ScheduledThreadPoolExecutor. */ final void tryTerminate() { for (;;) { int c = ctl.get(); //线程池在运行或者状态在变化中，或者正在停止但队列有任务，直接返回 if (isRunning(c) || runStateAtLeast(c, TIDYING) || (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty())) return; //有任务工作线程 if (workerCountOf(c) != 0) { // Eligible to terminate //中断所有线程 interruptIdleWorkers(ONLY_ONE); return; } final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //线程池状态改变 if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) { try { //结束线程池，空方法，does nothing terminated(); } finally { //设置线程池状态，结束 ctl.set(ctlOf(TERMINATED, 0)); //唤醒所有wait线程 termination.signalAll(); } return; } } finally { mainLock.unlock(); } // else retry on failed CAS } } private void interruptIdleWorkers(boolean onlyOne) { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { for (Worker w : workers) { Thread t = w.thread; if (!t.isInterrupted() && w.tryLock()) { try { t.interrupt(); } catch (SecurityException ignore) { } finally { w.unlock(); } } if (onlyOne) break; } } finally { mainLock.unlock(); } } /** * Method invoked when the Executor has terminated. Default * implementation does nothing. Note: To properly nest multiple * overridings, subclasses should generally invoke * {@code super.terminated} within this method. */ protected void terminated() { }
大量的CAS和自旋锁；位运算算法和重入锁AQS。
看看销毁线程池方法
public void shutdown() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //检查权限 checkShutdownAccess(); //CAS 更新线程池状态 advanceRunState(SHUTDOWN); //中断所有线程 interruptIdleWorkers(); //关闭，此处是do nothing onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } //尝试结束，上面代码已分析 tryTerminate(); }
2. submit方法
/** * @throws RejectedExecutionException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public Future<?> submit(Runnable task) { if (task == null) throw new NullPointerException(); RunnableFuture<Void> ftask = newTaskFor(task, null); execute(ftask); return ftask; } /** * @throws RejectedExecutionException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ 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; } /** * @throws RejectedExecutionException {@inheritDoc} * @throws NullPointerException {@inheritDoc} */ public <T> Future<T> submit(Callable<T> task) { if (task == null) throw new NullPointerException(); RunnableFuture<T> ftask = newTaskFor(task); execute(ftask); return ftask; }
执行图解：
五、预定义的线程池
FixedThreadPool
创建固定线程数量的，适用于负载较重的服务器，使用了无界队列
SingleThreadExecutor
创建单个线程，需要顺序保证执行任务，不会有多个线程活动，使用了无界队列
CachedThreadPool
会根据需要来创建新线程的，执行很多短期异步任务的程序，使用了SynchronousQueue
WorkStealingPool（JDK7以后）
基于ForkJoinPool实现
ScheduledThreadPoolExecutor
需要定期执行周期任务，Timer不建议使用了。
newSingleThreadScheduledExecutor：只包含一个线程，只需要单个线程执行周期任务，保证顺序的执行各个任务
newScheduledThreadPool 可以包含多个线程的，线程执行周期任务，适度控制后台线程数量的时候
方法说明：
schedule：只执行一次，任务还可以延时执行
scheduleAtFixedRate：提交固定时间间隔的任务
scheduleWithFixedDelay：提交固定延时间隔执行的任务