王大咩的图书馆资料

本文主要介绍王大咩的图书馆资料 方法和在新技术下所面对的“挑战”,方便大家深入理解王大咩的图书馆资料 过程。本文也将分享王大咩的图书馆资料 所遇到的问题和应对策略,怎么解决怎么做的问题。
通过深入本文可以理解代码原理,进行代码文档的下载,也可以查看相应 Demo 部署效果。

背景

  本文基于JDK 11,主要介绍FutureTask类中的run()、get()和cancel() 方法,没有过多解析相应interface中的注释,但阅读源码时建议先阅读注释,明白方法的主要的功能,再去看源码会更快。

  文中若有不正确的地方欢迎大伙留言指出,谢谢了!

1、FutureTask类图

  1.1 FutureTask简介

  FutureTask类图如下(使用IDEA生成)。如图所示,FutureTask实现了Future接口的所有方法,并且实现了Runnable接口,其中,Runnable接口的现实类用于被线程执行,而Future代表的是异步计算的结果。因此,FutureTask类可以理解为,执行run()(实现Runnable接口中的方法),通过Future的get()方法获取结果。

王大咩的图书馆

  1.2 FutureTask的属性

 //任务线程总共有七中状态如下:     * Possible state transitions:      * NEW -> COMPLETING -> NORMAL      * NEW -> COMPLETING -> EXCEPTIONAL      * NEW -> CANCELLED      * NEW -> INTERRUPTING -> INTERRUPTED      */     private volatile int state;     private static final int NEW          = 0;     private static final int COMPLETING   = 1;     private static final int NORMAL       = 2;     private static final int EXCEPTIONAL  = 3;     private static final int CANCELLED    = 4;     private static final int INTERRUPTING = 5;     private static final int INTERRUPTED  = 6;      /** The underlying callable; nulled out after running */     //在run()方法中调用     private Callable<V> callable;     /** The result to return or exception to throw from get() */     //任务执行结果,callable.call()正常执行的返回值     private Object outcome; // non-volatile, protected by state reads/writes     /** The thread running the callable; CASed during run() */     //任务线程     private volatile Thread runner;     /** Treiber stack of waiting threads */     //等待任务结果的线程组成的节点,放在链表对列中     private volatile WaitNode waiters;

 2、源码解析

  2.1 run()方法

public void run() {         //1、若是任务的状态不是NEW,且使用CAS将runner置为当前线程则直接返回         if (state != NEW ||             !RUNNER.compareAndSet(this, null, Thread.currentThread()))             return;         try {             Callable<V> c = callable;             //2、任务不为null,且state的状态为NEW的情况下才执行任务             if (c != null && state == NEW) {                 V result;                 boolean ran;                 try {                     //执行任务并接收执行结果                     result = c.call();                     //正常执行结果则将标识置为true                     ran = true;                 } catch (Throwable ex) {                     //3、任务发生异常,执行或cancel(),则结果置为null,并记录异常信息                     result = null;                     ran = false;                     setException(ex);                 }                 //4、任务正常结束,则设置返回结果                 if (ran)                     set(result);             }         } finally {             // runner must be non-null until state is settled to             // prevent concurrent calls to run()             runner = null;             // state must be re-read after nulling runner to prevent             // leaked interrupts             int s = state;             //5、若是异常导致,走另一个流程             if (s >= INTERRUPTING)                 handlePossibleCancellationInterrupt(s);         }     }

  1)若任务的状态不是NEW,或者使用CAS将runner置为当前线程失败,则直接返回的原因是防止多线程调用;

  2)再度确认任务执行的前置条件;

  3)任务执行异常,将result置为null,并记录异常,setException()源码如下:

protected void setException(Throwable t) {         //使用CAS将状态置为中间态COMPLETING         if (STATE.compareAndSet(this, NEW, COMPLETING)) {             outcome = t;             STATE.setRelease(this, EXCEPTIONAL); // final state             //任务处于结束态时,遍历唤醒等待result的线程             finishCompletion();         }     }

  任务的状态变化为NEW  – >  COMPLETING  ->  EXCEPTIONAL

  4)任务正常结果则会设置result之后,唤醒waitNode的链表对列中等待任务结果的线程;

  5)异常后的调用逻辑如下:

 //保证调用cancel在run方法返回之前中断执行任务     private void handlePossibleCancellationInterrupt(int s) {         // It is possible for our interrupter to stall before getting a         // chance to interrupt us.  Let's spin-wait patiently.         if (s == INTERRUPTING)             //自旋等待             while (state == INTERRUPTING)             //当前线程让出CPU执行权                 Thread.yield(); // wait out pending interrupt     }

   2.2  get()方法

  源码分析如下:

public V get() throws InterruptedException, ExecutionException {         int s = state;         if (s <= COMPLETING)             //等待任务完成             s = awaitDone(false, 0L);         //返回结果         return report(s);     }

  其中,等待过程分析如下:

private int awaitDone(boolean timed, long nanos)         throws InterruptedException {         // The code below is very delicate, to achieve these goals:         // - call nanoTime exactly once for each call to park         // - if nanos <= 0L, return promptly without allocation or nanoTime         // - if nanos == Long.MIN_VALUE, don't underflow         // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic         //   and we suffer a spurious wakeup, we will do no worse than         //   to park-spin for a while         long startTime = 0L;    // Special value 0L means not yet parked         WaitNode q = null;         boolean queued = false;         for (;;) {             int s = state;             //1、任务的状态已经处于最终的状态,则将任务线程的引用置为null,直接返回状态             if (s > COMPLETING) {                 if (q != null)                     q.thread = null;                 return s;             }             //2、任务的状态为COMPLETING说明任务已经接近完成,则当前线程让出CPU权限以便任务执行线程获取到CPU执行权             else if (s == COMPLETING)                 // We may have already promised (via isDone) that we are done                 // so never return empty-handed or throw InterruptedException                 Thread.yield();             //3、当前线程被中断,则将当前线程从等待任务结果的对列中移除,并抛出异常             else if (Thread.interrupted()) {                 removeWaiter(q);                 throw new InterruptedException();             }             //4、任务线程的状态小于COMPLETING,则将当前调用get()方法的线程新建一个Node             else if (q == null) {                 if (timed && nanos <= 0L)                     return s;                 q = new WaitNode();             }             //5、若由当前线程构成的Node未加入链表中,则加入             else if (!queued)                 queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);             //6、是否开启了超时获取结果             else if (timed) {                 final long parkNanos;                 if (startTime == 0L) { // first time                     startTime = System.nanoTime();                     if (startTime == 0L)                         startTime = 1L;                     parkNanos = nanos;                 } else {                     long elapsed = System.nanoTime() - startTime;                     //7、超时则从栈中移除当前线程                     if (elapsed >= nanos) {                         removeWaiter(q);                         return state;                     }                     parkNanos = nanos - elapsed;                 }                 // nanoTime may be slow; recheck before parking                 //当前线程挂起                 if (state < COMPLETING)                     LockSupport.parkNanos(this, parkNanos);             }             else                 LockSupport.park(this);         }     }

  获取到返回的状态值后,根据其状态值判断是返回结果还是抛出异常。

  2.2 cancel()方法

public boolean cancel(boolean mayInterruptIfRunning) {         //1、若任务线程的状态为NEW,则将其状态从NEW置为INTERRUPTING、CANCELLED         if (!(state == NEW && STATE.compareAndSet               (this, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))             //CAS改变任务线程的状态失败,则直接返回false,表示cancel失败             return false;         try {    // in case call to interrupt throws exception             //2、改变任务线程的状态成功后,根据是否中断running的任务线程的标识位,决定是否中断正在运行的任务线程             if (mayInterruptIfRunning) {                 try {                     Thread t = runner;                     //任务线程不为null,则使用interrupt()中断                     if (t != null)                         t.interrupt();                 } finally { // final state                     //设置状态                     STATE.setRelease(this, INTERRUPTED);                 }             }         } finally {             //3、清理等待任务结果的等待线程             finishCompletion();         }         return true;     }

 3、总结

  1)执行run()方法,是在调用在Callable的call()方法,其实在初始化时被指定;

  2)调用get()方法,若是任务线程还在执行,则会把调用get的线程封装成waitNode塞入到FutureTask类内部的阻塞链表对列中,可以有多个线程同时调用get()方法;

  3)cancel()方法是通过对任务线程调用interrupt()实现;

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