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笔记文章，没有废话，句句关键 线程池的优点重用线程池里的线程，避免创建和销毁线程所带来的性能开销有效控制最大并发数，避免造成线程间抢占系统资源而造成阻塞提高线程可管理性，可以统一进行分配，调优和监控的能力Android中的线程池复用Java中的Executor接口，具体实现类为ThreadPoolExecutor，它有以下几个参数： 参数 说明 注释 corePoolSize 线程池中核心线程数量 一直存活，即使处于闲置状态 maximumPoolSize 最大能创建的线程数量(非核心线程，包含核心线程个数) 达到这个值后，后续任务会阻塞 keepAliveTime 非核心线程最大存活时间 当设置allowCoreThreadTimeOut=true 同样会做用于核心线程，但通常不会这么做 unit keepAliveTime的时间单位 TimeUnit中的枚举(时间单位) workQueue 等待队列。execute 方法提交的Runnable存储在其中 如果线程池中的线程数量大于等于corePoolSize的时候，把该任务放入等待队列 threadFactory 线程创建工程厂(用来创建线程的) 默认使用Executors.defaultThreadFactory() 来创建线程，线程具有相同的NORM_PRIORITY优先级并且是非守护线程 handler 线程池的饱和拒绝策略(不常用) 阻塞队列已且没有空闲的线程，此时继续提交任务，就需要采取一种策略处理该任务，默认会抛出异常 常用与关键方法void execute(Runnable run)//提交任务,交由线程池调度void shutdown()//关闭线程池,等待任务执行完成void shutdownNow()//关闭线程池，不等待任务执行完成int getTaskCount()//返回线程池找中所有任务的数量 (已完成的任务+阻塞队列中的任务) int getCompletedTaskCount()//返回线程池中已执行完成的任务数量 (已完成的任务) int getPoolSize()//返回线程池中已创建线程数量int getActiveCount()//返回当前正在运行的线程数量void terminated() 线程池终止时执行的策略线程池的运行状态线程存在运行状态，如下： 状态 说明 NEW 初始状态,线程被新建,还没调用start方法 RUNNABLE 运行状态,把"运行中"和"就绪"统称为运行状态 BLOCKED 阻塞状态，表示线程阻塞于锁 WAITING 等待状态,需要其他线程通知唤醒 TIME_WAITING 超时等待状态，表示可以在指定的时间超时后自行返回 TERMINATED 终止状态，表示当前线程已经执行完毕  在这里插入图片描述 根据线程的运行状态思想，我们来展开线程池的运行状态： 状态 说明 RUNNABLE 表示当前线程池，存在正在运行的线程 SHUTDOWN 关闭线程池，不在执行新的任务，但会执行完线程池正在运行的任务，和添加到队列中的任务，对应shutDown()方法 STOP 立即关闭线程池，打断正在运行的任务，且不再处理等待队列中已添加的任务，对应shutDownNow()方法 TIDYING shutDown() / shutDownNow()后进入此状态，表示队列和线程池为空，之后进入TERMINATED状态 TERMINATED 终止状态，表示当前线程已经执行完毕，并调用 terminated() 通知外界  在这里插入图片描述 安卓中常用的四种线程池Executors.newFixedThreadPool() 代码语言：javascript复制public static ExecutorService newFixedThreadPool(int nThreads) { return new ThreadPoolExecutor(nThreads, nThreads, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue()); }线程数量固定的线程池核心线程与非核心线程数量相等，意味着只有核心线程。 keepAliveTime = 0L，即使线程池中的线程空闲，也不会被回收。除非调用shutDown()或shutDownNow()去关闭线程池。可以更快响应外界的请求，且任务阻塞队列为无边界队列(LinkedBlockingQueue()链表结构的阻塞队列)，意味着任务可以无限加入线程池Executors.newScheduledThreadPool() 代码语言：javascript复制 public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) { return new ScheduledThreadPoolExecutor(corePoolSize); } public ScheduledThreadPoolExecutor(int corePoolSize) { super(corePoolSize, Integer.MAX_VALUE, 10L, MILLISECONDS, new DelayedWorkQueue()); }核心线程数量固定，非核心线程是无限的，但非核心线程存活时间非常短(10毫秒)，闲置后会被回收适合执行定时任务和固定周期的重复任务DelayedWorkQueue()优先级队列(堆结构)，会根据定时/延时时间进行排序，延时少，先执行Executors.newSingleThreadExecutor() 常用 代码语言：javascript复制 public static ExecutorService newSingleThreadExecutor() { return new FinalizableDelegatedExecutorService (new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue())); }核心线程数为1的线程池。确保所有任务在同一线程执行，因此不用考虑线程同步问题 阻塞队列是无界的，可以一直接收任务 、Executors.newCachedThreadPool() 代码语言：javascript复制 public static ExecutorService newCachedThreadPool() { return new ThreadPoolExecutor(0, Integer.MAX_VALUE, 60L, TimeUnit.SECONDS, new SynchronousQueue()); }没有核心线程，非核心线程无限，但执行完任务后，线程存活60秒SynchronousQueue() 是个特殊的队列，可以理解为无法存储元素的队列，因此线程池接收到任务就会创建线程去执行。当整个线程都处于空闲状态，60秒后，线程池中的线程都会因超时而被停止，不占用系统资源。根据以上两点，此线程池适合执行耗时较少但频繁的任务执行线程池源码分析在分析源码前，先把线程池的工作流程图放出来。这样再去阅读源码会更加深刻  在这里插入图片描述 阅读源码的目的： 了解线程池如何复用如何巧妙的使用线程池，根据需求去自定义线程池，以达到暂停/恢复，任务具有优先级，监控，分配，调优等学习线程池优秀的设计思想面试吹牛逼源码分析 分析源码前看下ThreadPoolExecutor类的头注释： 代码语言：javascript复制public class ThreadPoolExecutor extends AbstractExecutorService { /** * 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 * SHUTDOWN: Don't accept new tasks, but process queued tasks * STOP: Don't accept new tasks, don't process queued tasks, * 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 * TERMINATED: terminated() has completed * * 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: * * 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). */这里面有几段比较重要 ： 代码语言：javascript复制 * 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.线程池控制状态：clt是一个int类型的原子数，它表示两个概念，workerCount表示有效线程，笼统来讲，相当于线程池运行的线程个数，runState，表示线程池运行的状态，如RUNNING SHUTDOWN等等。 将这两个字段打包成一个int，int为4字节，有32位，后29位表示线程池的数量，最大可为5个亿，如果不够可以使用AtomicLong代替。当然，运行线程池根本达不到这个数量。 代码语言：javascript复制 * RUNNING: Accept new tasks and process queued tasks * SHUTDOWN: Don't accept new tasks, but process queued tasks * STOP: Don't accept new tasks, don't process queued tasks, * 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 * TERMINATED: terminated() has completed五种状态的含义，上面提到过。 代码语言：javascript复制 * 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线程池状态间的变化。等于上面我们画的图：  在这里插入图片描述 结论：阅读源码可以适当看下类头说明，尤其是Android源码的类头。可以帮我们更好的理解源码 从线程池入口进入，理解前面提到ctl是什么？ 代码语言：javascript复制 private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); private static final int COUNT_BITS = Integer.SIZE - 3; private static final int CAPACITY = (1 |
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