Java从源码看异步任务计算FutureTask
目录
- 了解一下什么是FutureTask?
- FutureTask 是如何实现的呢?
- FutureTask 运行流程
- FutureTask 的使用
前言:
大家是否熟悉FutureTask呢?或者说你有没有异步计算的需求呢?FutureTask就能够很好的帮助你实现异步计算,并且可以实现同步获取异步任务的计算结果。下面我们就一起从源码分析一下FutureTask。
了解一下什么是FutureTask?
FutureTask 是一个可取消的异步计算。
FutureTask
提供了对Future的基本实现,可以调用方法去开始和取消一个计算,可以查询计算是否完成,并且获取计算结果。
FutureTask只能在计算完成后获取到计算结果,一旦计算完成,将不能重启或者取消,除非调用runAndReset方法。
FutureTask除了实现了Future接口以外,还实现了Runnable
接口,因此FutureTask是可以交由线程池的Executor执行,也可以直接使用一个异步线程调用执行(futureTask.run())。
FutureTask 是如何实现的呢?
首先,我们看一下FutureTask
类的继承结构,如下图,它实现的是RunnableFuture
接口,而RunnableFuture
继承自Future和函数式接口Runnable
,所以说FutureTask本质就是一个可运行的Future。
Future 接口约定了一些异步计算类必须要实现的功能,源码如下:
package java.util.concurrent; public interface Future<V> { /** * 尝试取消任务的执行,并返回取消结果。 * 参数mayInterruptIfRunning:是否中断线程。 */ boolean cancel(boolean mayInterruptIfRunning); /** * 判断任务是否被取消(正常结束之前被被取消返回true) */ boolean isCancelled(); /** * 判断当前任务是否执行完毕,包括正常执行完毕、执行异常或者任务取消。 */ boolean isDone(); /** * 获取任务执行结果,任务结束之前会阻塞。 */ V get() throws InterruptedException, ExecutionException; /** * 在指定时间内尝试获取执行结果。若超时则抛出超时异常TimeoutException */ V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException; }
Runnable
接口我们都很熟悉,他就是一个函数式接口,我们常用其创建一个线程。
package java.lang; ? @FunctionalInterface public interface Runnable { ? ? ? ?public abstract void run(); }
FutureTask就是一个将要被执行的任务,它包含了以上接口具体的实现,FutureTask内部定义了任务的状态state和一些状态的常量,它的内部核心是一个Callable callable,我们通过构造函数可以传入callable或者是runnable,最后都会内部转为callable,因为我们需要获取异步任务的执行结果,只有通过Callable创建的线程才会返回结果。
我们可以通过此时的状态判断Future中isCancelled()
,isDone()
的返回结果。
以下为FutureTask源码,内含核心源码分析注释
package java.util.concurrent; import java.util.concurrent.locks.LockSupport; public class FutureTask<V> implements RunnableFuture<V> { /** * 任务的运行状态 */ private volatile int state; private static final int NEW = 0; // 新建 private static final int COMPLETING = 1; // 完成 private static final int NORMAL = 2; // 正常 private static final int EXCEPTIONAL = 3; // 异常 private static final int CANCELLED = 4; // 取消 private static final int INTERRUPTING = 5; // 中断中 private static final int INTERRUPTED = 6; // 中断的 private Callable<V> callable; /** * 返回结果 */ private Object outcome; private volatile Thread runner; private volatile WaitNode waiters; ... public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; } public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; } public boolean isCancelled() { return state >= CANCELLED; } public boolean isDone() { return state != NEW; } /* * 取消任务实现 * 如果任务还没有启动就调用了cancel(true),任务将永远不会被执行。 * 如果任务已经启动,参数mayInterruptIfRunning将决定任务是否应该中断执行该任务的线程,以尝试中断该任务。 * 如果任务任务已经取消、已经完成或者其他原因不能取消,尝试将失败。 */ public boolean cancel(boolean mayInterruptIfRunning) { if (!(state == NEW && UNSAFE.compareAndSwapInt(this, stateOffset, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED))) return false; try { // in case call to interrupt throws exception if (mayInterruptIfRunning) { try { Thread t = runner; if (t != null) t.interrupt(); } finally { // final state UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); } } } finally { finishCompletion(); } return true; } /* * 等待获取结果 * 获取当前状态,判断是否执行完成。并且判断时间是否超时 * 如果任务没有执行完成,就阻塞等待完成,若超时抛出超时等待异常。 */ public V get() throws InterruptedException, ExecutionException { int s = state; if (s <= COMPLETING) s = awaitDone(false, 0L); return report(s); } /* * 等待获取结果 * 获取当前状态,判断是否执行完成。 * 如果任务没有执行完成,就阻塞等待完成。 */ public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { if (unit == null) throw new NullPointerException(); int s = state; if (s <= COMPLETING && (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING) throw new TimeoutException(); return report(s); } /** * 根据状态判断返回结果还是异常 */ 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); } protected void done() { } /** * 设置结果借助CAS确认状态是否完成状态 */ protected void set(V v) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = v; UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state finishCompletion(); } } /** * 设置异常,当运行完成出现异常,设置异常状态 */ protected void setException(Throwable t) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = t; UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state finishCompletion(); } } /* * 执行callable获取结果,或者异常 * 判断状态是不是启动过的,如果是新建才可以执行run方法 */ 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(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } if (ran) set(result); } } finally { runner = null; int s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } } /** * 重新执行 */ protected boolean runAndReset() { if (state != NEW || !UNSAFE.compareAndSwapObject(this, runnerOffset, null, Thread.currentThread())) return false; boolean ran = false; int s = state; try { Callable<V> c = callable; if (c != null && s == NEW) { try { c.call(); // don't set result ran = true; } catch (Throwable ex) { setException(ex); } } } finally { runner = null; s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } return ran && s == NEW; } /* * 处理可能取消的中断 */ private void handlePossibleCancellationInterrupt(int s) { if (s == INTERRUPTING) while (state == INTERRUPTING) Thread.yield(); } static final class WaitNode { volatile Thread thread; volatile WaitNode next; WaitNode() { thread = Thread.currentThread(); } } /** * 移除并唤醒所有等待线程,执行done,置空callable */ 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; 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 } /** * 等待完成 * 首先判断是否超时 * 处理中断的,然后处理异常状态的,处理完成的... */ 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 yet Thread.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); } else LockSupport.park(this); } } /** * 去除等待 */ private void removeWaiter(WaitNode node) { if (node != null) { node.thread = null; retry: for (;;) { // restart on removeWaiter race for (WaitNode pred = null, q = waiters, s; q != null; q = s) { s = q.next; if (q.thread != null) pred = q; else if (pred != null) { pred.next = s; if (pred.thread == null) // check for race continue retry; } else if (!UNSAFE.compareAndSwapObject(this, waitersOffset, q, s)) continue retry; } break; } } } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE; private static final long stateOffset; private static final long runnerOffset; private static final long waitersOffset; static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class<?> k = FutureTask.class; stateOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("state")); runnerOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("runner")); waitersOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("waiters")); } catch (Exception e) { throw new Error(e); } } }
FutureTask 运行流程
一般来说,我们可以认为FutureTask具有以下三种状态:
未启动:新建的FutureTask,在run()没执行之前,FutureTask处于未启动状态。
private static final int NEW = 0; // 新建
已启动:FutureTask
对象的run方法启动并执行的过程中,FutureTask处于已启动状态。
已完成:FutureTask正常执行结束,或者FutureTask
执行被取消(FutureTask对象cancel方法),或者FutureTask对象run方法执行抛出异常而导致中断而结束,FutureTask都处于已完成状态。
private static final int COMPLETING = 1; // 完成 private static final int NORMAL = 2; // 完成后正常设置结果 private static final int EXCEPTIONAL = 3; // 完成后异常设置异常 private static final int CANCELLED = 4; // 执行取消 private static final int INTERRUPTING = 5; // 中断中 private static final int INTERRUPTED = 6; // 中断的
FutureTask 的使用
使用一(直接新建一个线程调用):
FutureTask<Integer> task = new FutureTask<>(new Callable() { @Override public Integer call() throws Exception { return sum(); } }); new Thread(task).stat(); Integer result = task.get();
使用二(结合线程池使用)
FutureTask<Integer> task = new FutureTask<>(new Callable() { @Override public Integer call() throws Exception { return sum(); } }); Executors.newCachedThreadPool().submit(task); Integer result = task.get();
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