Java并发编程之FutureTask源码解析

上次总结一下AQS的一些相关知识，这次总结了一下FutureTask的东西，相对于AQS来说简单好多呀

之前提到过一个LockSupport的工具类，也了解一下这个工具类的用法，这里也巩固一下吧

    /*** Makes available the permit for the given thread, if it* was not already available.  If the thread was blocked on* {@code park} then it will unblock.  Otherwise, its next call* to {@code park} is guaranteed not to block. This operation* is not guaranteed to have any effect at all if the given* thread has not been started.** @param thread the thread to unpark, or {@code null}, in which case*        this operation has no effect*///将指定线程唤醒，继续执行指定线程public static void unpark(Thread thread) {if (thread != null)UNSAFE.unpark(thread);}    /*** Disables the current thread for thread scheduling purposes unless the* permit is available.** <p>If the permit is available then it is consumed and the call* returns immediately; otherwise the current thread becomes disabled* for thread scheduling purposes and lies dormant until one of three* things happens:** <ul>** <li>Some other thread invokes {@link #unpark unpark} with the* current thread as the target; or** <li>Some other thread {@linkplain Thread#interrupt interrupts}* the current thread; or** <li>The call spuriously (that is, for no reason) returns.* </ul>** <p>This method does <em>not</em> report which of these caused the* method to return. Callers should re-check the conditions which caused* the thread to park in the first place. Callers may also determine,* for example, the interrupt status of the thread upon return.*///阻塞当前线程，等待调用unpark()唤醒当前线程public static void park() {UNSAFE.park(false, 0L);}// Hotspot implementation via intrinsics APIprivate static final sun.misc.Unsafe UNSAFE;

就是阻塞线程以及唤醒指定线程，在FutureTask的源码中能用到

RunnableFuture<V>

FutureTask继承自这个接口，这个接口有继承了Runnable以及Future接口，所以FutureTask对象可以用new Thread().start()去启动，所以之前提到了创建线程的三种方式，采用Callable+FutureTask的形式创建，依旧还是依赖于Runnable创建线程

/*** A {@link Future} that is {@link Runnable}. Successful execution of* the {@code run} method causes completion of the {@code Future}* and allows access to its results.* @see FutureTask* @see Executor* @since 1.6* @author Doug Lea* @param <V> The result type returned by this Future's {@code get} method*/
public interface RunnableFuture<V> extends Runnable, Future<V> {/*** Sets this Future to the result of its computation* unless it has been cancelled.*/void run();
}

源码解析

既然继承了Runnable接口就必然执行run()方法，我们先看下主要成员变量

    /*** The run state of this task, initially NEW.  The run state* transitions to a terminal state only in methods set,* setException, and cancel.  During completion, state may take on* transient values of COMPLETING (while outcome is being set) or* INTERRUPTING (only while interrupting the runner to satisfy a* cancel(true)). Transitions from these intermediate to final* states use cheaper ordered/lazy writes because values are unique* and cannot be further modified.** Possible state transitions:* NEW -> COMPLETING -> NORMAL* NEW -> COMPLETING -> EXCEPTIONAL* NEW -> CANCELLED* NEW -> INTERRUPTING -> INTERRUPTED*///记录当前线程执行的状态，是否正常、结束、异常、中断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 *///Callable对象private Callable<V> callable;/** The result to return or exception to throw from get() *///结果集private Object outcome; // non-volatile, protected by state reads/writes/** The thread running the callable; CASed during run() *///当前执行的线程private volatile Thread runner;/** Treiber stack of waiting threads *///等待线程节点private volatile WaitNode waiters;//单向链表static final class WaitNode {volatile Thread thread;//记录当前线程volatile WaitNode next;//下一个节点WaitNode() { thread = Thread.currentThread(); }}

看一下执行主体，这个方法主要是将Callable对象的那个业务逻辑执行完毕，只有执行完成之后采用将值返回，并且将当前线程通过LockSupport.unpark()进行唤醒。

public void run() {if (state != NEW ||!UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))return;try {Callable<V> c = callable;if (c != null && state == NEW) {V result;boolean ran;try {result = c.call();//调用Callable对象并执行call()方法中的变量ran = true;} catch (Throwable ex) {result = null;ran = false;setException(ex);//发生异常则将结果设置成异常}if (ran)set(result);//设置正常结果}} 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
//如果是中断结束的，则调用线程中断方法int s = state;if (s >= INTERRUPTING)handlePossibleCancellationInterrupt(s);}}/*** Removes and signals all waiting threads, invokes done(), and* nulls out callable.*///无论结果是否正常，都会执行，主要是为了唤醒线程，避免死锁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;//唤醒当前对象的线程LockSupport.unpark(t);}WaitNode next = q.next;if (next == null)break;q.next = null; // unlink to help gcq = next;}break;}}done();callable = null;        // to reduce footprint}

看一下Future的结果值的方法，每步方法在代码中都有讲解

    /*** @throws CancellationException {@inheritDoc}*/public V get() throws InterruptedException, ExecutionException {int s = state;//先判断当前线程的执行状态是否执行完毕，未执行完的则调用等待方法if (s <= COMPLETING)s = awaitDone(false, 0L);return report(s);}/*** Awaits completion or aborts on interrupt or timeout.** @param timed true if use timed waits* @param nanos time to wait, if timed* @return state upon completion*///方法就是用过LockSupport.park()进入线程等待方法，等待调用unpark然后在次判断是否执行完，执行完后将改方法结束，进入下一阶段private int awaitDone(boolean timed, long nanos)throws InterruptedException {final long deadline = timed ? System.nanoTime() + nanos : 0L;WaitNode q = null;boolean queued = false;for (;;) {if (Thread.interrupted()) {removeWaiter(q);throw new InterruptedException();}int s = state;if (s > COMPLETING) {if (q != null)q.thread = null;return s;}else if (s == COMPLETING) // cannot time out yetThread.yield();else if (q == null)q = new WaitNode();else if (!queued)queued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);else if (timed) {nanos = deadline - System.nanoTime();if (nanos <= 0L) {removeWaiter(q);return state;}LockSupport.parkNanos(this, nanos);}elseLockSupport.park(this);}}/*** Returns result or throws exception for completed task.** @param s completed state value*/@SuppressWarnings("unchecked")//在等待完之后，再次判断是否正常完成执行，正常的话将值返回，否则抛出异常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);}

通过上面的讲解，应该对FutureTask为什么能有返回值以及基本运行机制应该有个初步的了解，可以自行的去多看几遍。

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Java并发编程之FutureTask源码解析

RunnableFuture<V>

源码解析

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