关于java:FutureTask源码分析

咱们都晓得线程创立的形式有以下几种

• Thread
• Runnable
• Callable
• Executors

其中 Callable 是能获取到返回值或者异样的，callable 接口如下

@FunctionalInterface
public interface Callable<V> {
    /**
 * Computes a result, or throws an exception if unable to do so. * * @return computed result
 * @throws Exception if unable to compute a result
 */ V call() throws Exception;}

但 callable 必须要和线程池搭配应用，或者应用 FutureTask，具体用法如下

public interface Future<V> {
    // 勾销工作
    boolean cancel(boolean mayInterruptIfRunning);
    // 是否被勾销
    boolean isCancelled();
       // 是否实现，已实现返回 true
    boolean isDone();
      // 阻塞获取返回值，如果有异样则抛出异样
    V get() throws InterruptedException, ExecutionException;
       // 指定阻塞工夫获取返回值
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

实现类 RunnableFuture, 该类实现了 Runnable 接口和 Future 接口

public interface RunnableFuture<V> extends Runnable, Future<V> {
    // 将此 Future 设置为其计算的后果，除非工作勾销
    void run();}

实现类 FutureTask（重点 )

future 接口实现个别应用线程池或者 FutureTask 实现线程调用

// 构造方法传入 callable 赋值给成员变量 callable, 并设置 state 为新建
public FutureTask(Callable<V> callable) {if (callable == null)
        throw new NullPointerException();
    this.callable = callable;
    this.state = NEW;       // ensure visibility of callable
}

state 有以下几个状态

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;

FutureTask 有下几个成员变量

构造方法 1

// 构造方法传入的 callable 对象
private Callable<V> callable;
// 线程执行完的返回后果
private Object outcome; 
// 以后运行的线程
private volatile Thread runner;
//waiters 示意如果多个线程执行一个 callable 对象，则会存在一个单向链表中
private volatile WaitNode waiters;

构造方法 2

// 带返回值的构造方法
public FutureTask(Runnable runnable, V result) {this.callable = Executors.callable(runnable, result);
    this.state = NEW;      
}

因为 FutureTask 实现了 Runnable 接口，所以线程调度时执行 FutureTask 的 run 办法

public void run() {
      // 如果状态不为 NEW 或无奈将以后线程设置进去，为 runnerOffeset，则返回
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
          // 如果 FutureTask 内的 Callable 不为空且为新建状态，则执行 if 外部办法
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                  // 执行 callale 的 call 办法，且设置 ran=true 示意执行胜利
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                  // 设置 ran=false 示意执行失败
                ran = false;
                  // 如果捕捉到异样，则传入异样
                setException(ex);
            }
              // 如果是执行胜利，通过 set 办法设置后果
            if (ran)
                  // 将后果设置给 set 办法
                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;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

将 callable 的返回值通过 cas 设置进去，并

protected void set(V v) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = v;
          // 将状态设置失常 NORMAL
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
          // 实现赋值后唤醒 UnPack get 的线程
        finishCompletion();}
}

如果 call 办法出现异常，则通过 cas 设置胜利后进行唤醒

protected void setException(Throwable t) {if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();}
}

实现后进行唤醒

private void finishCompletion() {
    // assert state > COMPLETING;
    for (WaitNode q; (q = waiters) != null;) {if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {for (;;) {
                Thread t = q.thread;
                if (t != null) {
                    q.thread = null;
                      // 唤醒 get 线程，且将前面所有执行的 waitNode 节点
                    LockSupport.unpark(t);
                }
                WaitNode next = q.next;
                if (next == null)
                    break;
                q.next = null; // unlink to help gc
                q = next;
            }
            break;
        }
    }

    done();

    callable = null;        // to reduce footprint
}

// 是否勾销, 当 state>= 勾销状态, 即等于 CANCELLED,INTERRUPTING 或者 INTERRUPTED 时返回 true
public boolean isCancelled() {return state >= CANCELLED;}
// 是否实现, 当线程不为 NEW 状态都视为已实现 (包含失常实现, 异样实现, 以及中断等)
public boolean isDone() {return state != NEW;}

// 阻塞获取返回后果
public V get() throws InterruptedException, ExecutionException {
    int s = state;
     // 当线程小于等于 COMPLETING(NEW 新建或者 COMPLETING 筹备实现) 时, 进行无工夫闲置的阻塞
    if (s <= COMPLETING)
        s = awaitDone(false, 0L);
  // 此处只有当后果返回时下面的阻塞才会被唤醒, 否则始终阻塞在下面的 if 中
    return report(s);
}

// 阻塞指定工夫获取返回后果, 当指定工夫仍旧未获取到后果, 抛出 TimeoutException 异样
public V get(long timeout, TimeUnit unit)
    throws InterruptedException, ExecutionException, TimeoutException {if (unit == null)
          // 传入的工夫单位如果为空，则抛空指针异样
        throw new NullPointerException();
    int s = state;
  // 当后果未返回且等待时间大于指定的 timeout 工夫, 抛出 timeout 异样
    if (s <= COMPLETING &&
        (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
        throw new TimeoutException();
  
  // 此处只有当后果返回时下面的阻塞才会被唤醒, 否则始终阻塞在下面的 if 中
    return report(s);
}

report 是依据以后的 state 判断 call 办法是执行失常还是失败，失常则返回泛型的 result, 如果是勾销状态，则抛出勾销异样，否则抛出 ExecutionException 异样示意执行异样

private V report(int s) throws ExecutionException {
    Object x = outcome;
    if (s == NORMAL)
        return (V)x;
    if (s >= CANCELLED)
        throw new CancellationException();
    throw new ExecutionException((Throwable)x);
}

阻塞办法