应用层面：
ThreadPoolExecutor：
创建多线程池执行器：new ThreadPoolExecutor(),创建方法最终都是走的以下这个构造方法：
/** * 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,//核心线程数 int maximumPoolSize,//核心线程最大数量 long keepAliveTime,//超出核心线程数的其他空闲线程保留时间 TimeUnit unit,//空闲时间单位 BlockingQueue<Runnable> workQueue,//对列，当线程数量大于等于核心线程数时，将任务works保存进对列 ThreadFactory threadFactory,//创建线程的工厂 RejectedExecutionHandler handler) {//超出最大核心线程数的拒绝策略 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; }
创建线程池的其他方式：(返回的实际对象仍然是ThreadPoolExecutor，只不过是对构造函数的参数进行的特殊规定)
1、Executors.newFixedThreadPool(int nThreads)
public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>()); }
Executors.newFixedThreadPool(int nThreads, ThreadFactory threadFactory)//自动以创建线程的工厂
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(), threadFactory); }
2、Executors.newSingleThreadExecutor()
public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>())); }
3、Executor.newCachedThreadPool()
public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>()); }
public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue<Runnable>(), threadFactory); }
源码：
ThreadPoolExecutor
构造方法：
ThreadPoolExecutor(int corePoolSize,//核心线程数
int maximumPoolSize,//核心线程最大数量
long keepAliveTime,//超出核心线程数的其他空闲线程保留时间
TimeUnit unit,//空闲时间单位
BlockingQueue<Runnable> workQueue,//对列，当线程数量大于等于核心线程数时，将任务works保存进对列
ThreadFactory threadFactory,//创建线程的工厂
RejectedExecutionHandler handler) {//超出最大核心线程数的拒绝策略
corePoolSize：线程池的核心线程数，当线程池中的工作线程数小于核心线程数的时候，只要向线程池指派任务，线程池就会创建工作线程。
maximumPoolSize:线程池最大工作线程数，当线程池中的工作线程达到最大数的时候，即使再向线程池指派任务，线程池不会创建工作线程，回执行对应的拒绝策略。
keepAliveTime:当线程池的工作线程数大于核心线程数的时候，多余的核心线程数的部分线程（空闲的）可以保持keepAliveTime的空闲时间，当keepAliveTime时间内还没有获取到任务，这些线程后就会被回收。
unit：保持空闲时间的时间单位。
workQueue:任务队列，当线程池里面核心线程都在工作的时候，再向线程池指派任务，线程池会将任务放入任务队列里，工作线程在执行完任务后会再向任务队列里取出任务来执行。
threadFactory：创建执行任务的工作线程的线程工厂。
handler：拒绝任务加入线程池的策越，当线程池里的线程已经达到最大数后，再向线程池里加派任务时，线程池会决绝执行这些任务，handler就是具体执行拒绝的对象。
线程池的大体工作思路
1.当线程池小于corePoolSize时，新提交任务将创建一个新线程执行任务，即使此时线程池中存在空闲线程。
2.当线程池达到corePoolSize时，新提交任务将被放入workQueue中，等待线程池中任务调度执行
3.当workQueue已满，且maximumPoolSize>corePoolSize时，新提交任务会创建新线程执行任务
4.当提交任务数超过maximumPoolSize时，新提交任务由RejectedExecutionHandler处理
5.当线程池中超过corePoolSize数的线程，空闲时间达到keepAliveTime时，关闭空闲线程
6.当设置allowCoreThreadTimeOut(true)时，线程池中核心线程空闲时间达到keepAliveTime也将关闭
/** * The main pool control state, ctl, is an atomic integer packing * two conceptual fields * workerCount, indicating the effective number of threads * runState, indicating whether running, shutting down etc * * In order to pack them into one int, we limit workerCount to * (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2 * billion) otherwise representable. If this is ever an issue in * the future, the variable can be changed to be an AtomicLong, * and the shift/mask constants below adjusted. But until the need * arises, this code is a bit faster and simpler using an int. * * The workerCount is the number of workers that have been * permitted to start and not permitted to stop. The value may be * transiently different from the actual number of live threads, * for example when a ThreadFactory fails to create a thread when * asked, and when exiting threads are still performing * bookkeeping before terminating. The user-visible pool size is * reported as the current size of the workers set. * * The runState provides the main lifecycle control, taking on values: * * RUNNING: Accept new tasks and process queued tasks
running状态是可以接受和处理任务 * SHUTDOWN: Don't accept new tasks, but process queued tasks
shutdown状态时不能接受新的任务，但是仍可以处理对列中的任务 * STOP: Don't accept new tasks, don't process queued tasks,
stop状态，不接受新任务，也不执行对列中的任务，同事中断正在执行的任务 * and interrupt in-progress tasks * TIDYING: All tasks have terminated, workerCount is zero, * the thread transitioning to state TIDYING * will run the terminated() hook method
tidying状态，所有的工作线程全部停止，并工作线程数量为0，将调用terminated方法，进入到terninated状态 * TERMINATED: terminated() has completed
终止状态 * * The numerical order among these values matters, to allow * ordered comparisons. The runState monotonically increases over * time, but need not hit each state. The transitions are: *各种状态的转换----- * RUNNING -> SHUTDOWN * On invocation of shutdown(), perhaps implicitly in finalize() * (RUNNING or SHUTDOWN) -> STOP * On invocation of shutdownNow() * SHUTDOWN -> TIDYING * When both queue and pool are empty * STOP -> TIDYING * When pool is empty * TIDYING -> TERMINATED * When the terminated() hook method has completed * * Threads waiting in awaitTermination() will return when the * state reaches TERMINATED. * * Detecting the transition from SHUTDOWN to TIDYING is less * straightforward than you'd like because the queue may become * empty after non-empty and vice versa during SHUTDOWN state, but * we can only terminate if, after seeing that it is empty, we see * that workerCount is 0 (which sometimes entails a recheck -- see * below). */ private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); private static final int COUNT_BITS = Integer.SIZE - 3; private static final int CAPACITY = (1 << COUNT_BITS) - 1;//默认的容量2^29 -1 // 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; // Packing and unpacking ctl private static int runStateOf(int c) { return c & ~CAPACITY; } private static int workerCountOf(int c) { return c & CAPACITY; } private static int ctlOf(int rs, int wc) { return rs | wc; }//rs:状态 ws：数量
转： 为什么线程池的状态简单的定义为 -1,0,1,2,3不就得了，为什么还要用移位操作呢？ 原来这样的，ThreadPool ctl的这个变量的设计哲学是用int的高3位 + 29个0代表状态，，用高位000+低29位来表示线程池中工作线程的数量，太佩服了。 首先CAPACITY的值为workCount的最大容量，该值为 000 11111 11111111 11111111 11111111,29个1(默认的出事容量536870911) 我们来看一下 private static int runStateOf(int c) { return c & ~CAPACITY; } 用ctl里面的值与容量取反的方式获取状态值。由于CAPACITY的值为000 11111 11111111 11111111 11111111,
那取反后为111 00000 00000000 00000000 00000000, 用 c 与 该值进行与运算，这样就直接保留了c的高三位，
然后将c的低29位设置为0,这不就是线程池状态的存放规则吗，绝。 根据此方法，不难得出计算workCount的方法。 private static int ctlOf(int rs, int wc) { return rs | wc; } 该方法，主要是用来更新运行状态的。确保工作线程数量不丢失。
--------->
理解： ctl初始化：1110 0000 0000 0000 0000 0000 0000 0000 （该值也就是running状态值）-536870912 capacity： 0001 1111 1111 1111 1111 1111 1111 1111 536870911 当addworker()添加任务是，ctl中的value（也就是通过ctl.get()取到的值）就会加1， 即： 1110 0000 0000 0000 0000 0000 0000 0001 该值 & 初始容量capacity，即workerCountOf(c)方法：结果就是0000 0000 0000 0000 0000 0000 0000 0001（1），也就是线程数量为1个，同理 getTask()的时候回进行-1操作
线程池设计原理：
1）线程池的工作线程为ThreadPoolExecutors的Worker线程，无论是submit还是executor方法中传入的Callable task,Runable参数，只是实现了Runnable接口，在线程池的调用过程，不会调用其start方法，只会调用Worker线程的start方法，然后在Worker线程的run方法中会调用入参的run方法。
2）线程的生命周期在run方法运行结束后（包括异常退出）就结束。要想重复利用线程，就要确保工作线程Worker的run方法运行在一个
无限循环中，然后从任务队列中一个一个获取对象，如果任务队列为空，则阻塞，当然需要提供一些控制，结束无限循环，来销毁线程。在源码 runWorker方法与getTask来实现。
大概的实现思路是 如果getTask返回null,则该worker线程将被销毁。
那getTask在什么情况下会返回false呢？
1、如果线程池的状态为SHUTDOWN并且队列不为空
2、如果线程池的状态大于STOP
3、如果当前运行的线程数大于核心线程数，会返回null,已销毁该worker线程
对keepAliveTime的理解，如果allowCoreThreadTimeOut为真，那么keepAliveTime其实就是从任务队列获取任务等待的超时时间，也就是workerQueue.poll(keepALiveTime, TimeUnit.NANOSECONDS)
/** * Executes the given task sometime in the future. The task * may execute in a new thread or in an existing pooled thread. * * If the task cannot be submitted for execution, either because this * executor has been shutdown or because its capacity has been reached, * the task is handled by the current {@code RejectedExecutionHandler}. * * @param command the task to execute * @throws RejectedExecutionException at discretion of * {@code RejectedExecutionHandler}, if the task * cannot be accepted for execution * @throws NullPointerException if {@code command} is null */ 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();//从ctl中取值，该值包含状态和数量 if (workerCountOf(c) < corePoolSize) {//调用workCountOf方法得到当前的线程数量，和核心线程数比较 if (addWorker(command, true))//符合，则调用addworker直接创建线程来执行（这里就是表示，当小于核心线程数时，不管有无空闲线程，都会创建新的线程） return;//创建成功直接return c = ctl.get(); }
//没有创建成功则会进行拒绝策略方面的方法判断 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); }
addWorder():
/** * 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) { retry://重复执行的标记，下边代码有break retry（结束）和continue retry（返回周之前标记为重新执行） for (;;) { int c = ctl.get();//取码 int rs = runStateOf(c);//状态码 // 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))//进行ctl.value加1操作，成功则结束retry 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);//new worker的时候，内部类中会调用工厂来新建一个线程 final Thread t = w.thread; if (t != null) { final ReentrantLock mainLock = this.mainLock;//重复锁 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);//workers，set集合，保存着所有的worker 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; }
getTask()：
/** * Performs blocking or timed wait for a task, depending on * current configuration settings, or returns null if this worker * must exit because of any of: * 1. There are more than maximumPoolSize workers (due to * a call to setMaximumPoolSize). * 2. The pool is stopped. * 3. The pool is shutdown and the queue is empty. * 4. This worker timed out waiting for a task, and timed-out * workers are subject to termination (that is, * {@code allowCoreThreadTimeOut || workerCount > corePoolSize}) * both before and after the timed wait, and if the queue is * non-empty, this worker is not the last thread in the pool. * * @return task, or null if the worker must exit, in which case * workerCount is decremented */ 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();//处于stop、tidying、terminate状态时，循环减线程数量，回去返回对象 return null; } int wc = workerCountOf(c); // Are workers subject to culling? boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;// if ((wc > maximumPoolSize || (timed && timedOut)) && (wc > 1 || workQueue.isEmpty())) { if (compareAndDecrementWorkerCount(c)) return null; continue; } try {
//下边这一块代码控制着线程超时时间 Runnable r = timed ? workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : workQueue.take(); if (r != null) return r; timedOut = true; } catch (InterruptedException retry) { timedOut = false; } } }
ThreadPoolExecutor的执行：
当第一次submit或者execute添加任务的时候，如果添加成功会调Thread.start()方法，想线程得到CPU的使用位置的时候，就会走Worker的
run()方法，该run方法会走ThreadPoolExecutor中的runWorker()方法，在这个方法中会走Runnable的run()方法。
关于多线程的blog
https://blog.csdn.net/hounanjsj/article/details/73822998
https://blog.csdn.net/wangbiao007/article/details/78196413