Java 并发编程——Callable+Future+FutureTask
Java 并发编程系列文章
Java 并发编程——Callable+Future+FutureTask
java并发编程——通过ReentrantLock,Condition实现银行存取款
项目中经常有些任务需要异步(提交到线程池中)去执行，而主线程往往需要知道异步执行产生的结果，这时我们要怎么做呢？用runnable是无法实现的，我们需要用callable实现。
import java.util.concurrent.Callable; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; public class AddTask implements Callable<Integer> { private int a,b; public AddTask(int a, int b) { this.a = a; this.b = b; } @Override public Integer call() throws Exception { Integer result = a + b; return result; } public static void main(String[] args) throws InterruptedException, ExecutionException { ExecutorService executor = Executors.newSingleThreadExecutor(); //JDK目前为止返回的都是FutureTask的实例 Future<Integer> future = executor.submit(new AddTask(1, 2)); Integer result = future.get();// 只有当future的状态是已完成时(future.isDone() = true),get()方法才会返回 } }
Callable接口
Callable接口Runable接口可谓是兄弟关系，只不过Callable是带返回值的。
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; }
Future 接口
接口函数及含义 ：public interface Future<V>
boolean cancel(boolean mayInterruptIfRunning)
取消当前执行的任务，如果已经执行完毕或者已经被取消/由于某种原因不能被取消 则取消任务失败。
参数mayInterruptIfRunning： 当任务正在执行，如果参数为true ，则尝试中断任务，否则让任务继续执行知道结束。
boolean isCancelled()
Returns {@code true} if this task was cancelled before it completed
* normally.
boolean isDone();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
由注释可以看出，当没有执行完成时，需要等待任务执行完成了才会将计算结果返回。
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
如果等待的时间超过设置的时间则会报 TimeoutException异常
FutureTask
public class FutureTask<V> implements RunnableFuture<V>
由定义可以看出它实现了RunnableFuture接口，那么这个接口又是什么呢？看下面的接口定义，其实很简单
public interface RunnableFuture<V> extends Runnable, Future<V> { /** * Sets this Future to the result of its computation * unless it has been cancelled. */ void run(); }
再回到FutureTask,它其实就是实现了Runnable和Future接口，FutureTask的执行是 状态转换的过程，源码中有七种状态如下：
* 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;
当FutureTask刚刚被创建时，它的状态是NEW，其它状态查看源码。
其它成员变量：
/** The underlying callable; nulled out after running */ private Callable<V> callable; /** The result to return or exception to throw from get() */ private Object outcome; // non-volatile, protected by state reads/writes /** The thread running the callable; CASed during run() */ private volatile Thread runner; /** Treiber stack of waiting threads */ private volatile WaitNode waiters;
callable是待执行的任务，FutureTask 的 run（）函数中执行callable中的任务。
outcome ： 是callable的执行结果，当正常执行完成后会将结果set到outcome中
runner：是执行callable 的线程
WaitNode : 是的受阻塞的线程链表，当cancel一个任务后，阻塞的线程会被唤醒。
构造函数：
public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable } public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable }
从构造函数可以看出，不光可以通过callable构造FutureTask还可以通过Runnable接口转化为callable来构造。关键函数为黄色标记部分，Executors中的实现源码如下：
/** * A callable that runs given task and returns given result. */ private static final class RunnableAdapter<T> implements Callable<T> { private final Runnable task; private final T result; RunnableAdapter(Runnable task, T result) { this.task = task; this.result = result; } public T call() { task.run(); return result; } }
这里面不懂result到底有什么意义，明明就是预先设置好的。
其它具体的方法说明这里不再细说，里面用到了很多sun.misc.Unsafe中的方法以及其他SDK底层接口，后续有时间再学习。下面贴出了整个源码及说明
public class FutureTask<V> implements RunnableFuture<V> { /* * Revision notes: This differs from previous versions of this * class that relied on AbstractQueuedSynchronizer, mainly to * avoid surprising users about retaining interrupt status during * cancellation races. Sync control in the current design relies * on a "state" field updated via CAS to track completion, along * with a simple Treiber stack to hold waiting threads. * * Style note: As usual, we bypass overhead of using * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics. */ /** * 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 */ 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 */ private Callable<V> callable; /** 用来存储任务执行结果或者异常对象,根据任务state在get时候选择返回执行结果还是抛出异常 */ private Object outcome; // non-volatile, protected by state reads/writes /** 当前运行Run方法的线程 */ private volatile Thread runner; /** Treiber stack of waiting threads */ private volatile WaitNode waiters; /** * Returns result or throws exception for completed task. * * @param s completed state value */ @SuppressWarnings("unchecked") 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); } /** * Creates a {@code FutureTask} that will, upon running, execute the * given {@code Callable}. * * @param callable the callable task * @throws NullPointerException if the callable is null */ public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable } /** * Creates a {@code FutureTask} that will, upon running, execute the * given {@code Runnable}, and arrange that {@code get} will return the * given result on successful completion. * * @param runnable the runnable task * @param result the result to return on successful completion. If * you don't need a particular result, consider using * constructions of the form: * {@code Future<?> f = new FutureTask<Void>(runnable, null)} * @throws NullPointerException if the runnable is null */ public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable } //判断任务是否已取消(异常中断、取消等) public boolean isCancelled() { return state >= CANCELLED; } /** 判断任务是否已结束（取消、异常、完成、NORMAL都等于结束） ** public boolean isDone() { return state != NEW; } /** mayInterruptIfRunning用来决定任务的状态。 true : 任务状态= INTERRUPTING = 5。如果任务已经运行，则强行中断。如果任务未运行，那么则不会再运行 false：CANCELLED = 4。如果任务已经运行，则允许运行完成（但不能通过get获取结果）。如果任务未运行，那么则不会再运行 **/ public boolean cancel(boolean mayInterruptIfRunning) { if (state != NEW) return false; if (mayInterruptIfRunning) { if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, INTERRUPTING)) return false; Thread t = runner; if (t != null) t.interrupt(); UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED); // final state } else if (!UNSAFE.compareAndSwapInt(this, stateOffset, NEW, CANCELLED)) return false; finishCompletion(); return true; } /** * @throws CancellationException {@inheritDoc} */ public V get() throws InterruptedException, ExecutionException { int s = state; //如果任务未彻底完成，那么则阻塞直至任务完成后唤醒该线程 if (s <= COMPLETING) s = awaitDone(false, 0L); return report(s); } /** * @throws CancellationException {@inheritDoc} */ 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); } /** * Protected method invoked when this task transitions to state * {@code isDone} (whether normally or via cancellation). The * default implementation does nothing. Subclasses may override * this method to invoke completion callbacks or perform * bookkeeping. Note that you can query status inside the * implementation of this method to determine whether this task * has been cancelled. */ protected void done() { } /** 该方法在FutureTask里只有run方法在任务完成后调用。 主要保存任务执行结果到成员变量outcome 中，和切换任务执行状态。 由该方法可以得知： COMPLETING ： 任务已执行完成（也可能是异常完成），但还未设置结果到成员变量outcome中，也意味着还不能get NORMAL ： 任务彻底执行完成 **/ protected void set(V v) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = v; UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state finishCompletion(); } } /** * Causes this future to report an {@link ExecutionException} * with the given throwable as its cause, unless this future has * already been set or has been cancelled. * * <p>This method is invoked internally by the {@link #run} method * upon failure of the computation. * * @param t the cause of failure */ protected void setException(Throwable t) { if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) { outcome = t; UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state finishCompletion(); } } /** 由于实现了Runnable接口的缘故，该方法可由执行线程所调用。 **/ public void run() { //只有当任务状态=new时才被运行继续执行 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 { //调用Callable的Call方法 result = c.call(); ran = true; } catch (Throwable ex) { result = null; ran = false; setException(ex); } 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; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } } /** 如果该任务在执行过程中不被取消或者异常结束，那么该方法不记录任务的执行结果，且不修改任务执行状态。 所以该方法可以重复执行N次。不过不能直接调用，因为是protected权限。 **/ 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 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 s = state; if (s >= INTERRUPTING) handlePossibleCancellationInterrupt(s); } return ran && s == NEW; } /** * Ensures that any interrupt from a possible cancel(true) is only * delivered to a task while in run or runAndReset. */ 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) while (state == INTERRUPTING) Thread.yield(); // wait out pending interrupt // assert state == INTERRUPTED; // We want to clear any interrupt we may have received from // cancel(true). However, it is permissible to use interrupts // as an independent mechanism for a task to communicate with // its caller, and there is no way to clear only the // cancellation interrupt. // // Thread.interrupted(); } /** * Simple linked list nodes to record waiting threads in a Treiber * stack. See other classes such as Phaser and SynchronousQueue * for more detailed explanation. */ static final class WaitNode { volatile Thread thread; volatile WaitNode next; WaitNode() { thread = Thread.currentThread(); } } /** 该方法在任务完成（包括异常完成、取消）后调用。删除所有正在get获取等待的节点且唤醒节点的线程。和调用done方法和置空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 } /** 阻塞等待任务执行完成（中断、正常完成、超时） **/ private int awaitDone(boolean timed, long nanos) throws InterruptedException { final long deadline = timed ? System.nanoTime() + nanos : 0L; WaitNode q = null; boolean queued = false; for (;;) { /** 这里的if else的顺序也是有讲究的。 1.先判断线程是否中断，中断则从队列中移除(也可能该线程不存在于队列中) 2.判断当前任务是否执行完成，执行完成则不再阻塞，直接返回。 3.如果任务状态=COMPLETING，证明该任务处于已执行完成，正在切换任务执行状态，CPU让出片刻即可 4.q==null，则证明还未创建节点，则创建节点 5.q节点入队 6和7.阻塞 **/ 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); } } /** * Tries to unlink a timed-out or interrupted wait node to avoid * accumulating garbage. Internal nodes are simply unspliced * without CAS since it is harmless if they are traversed anyway * by releasers. To avoid effects of unsplicing from already * removed nodes, the list is retraversed in case of an apparent * race. This is slow when there are a lot of nodes, but we don't * expect lists to be long enough to outweigh higher-overhead * schemes. */ 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简单应用：
public class FutureTaskTest {
public static void main(String[] args) {
test();
}
private static void test() {
Task task = new Task();
FutureTask futureTask = new FutureTask(task);
//step3:将FutureTask提交给Thread执行
Thread thread1 = new Thread(futureTask);
thread1.setName("task thread 1");
thread1.start();
//step4:获取执行结果，由于get()方法可能会阻塞当前调用线程，如果子任务执行时间不确定，最好在子线程中获取执行结果
try {
// boolean result = (boolean) futureTask.get();
boolean result = (boolean) futureTask.get(5, TimeUnit.SECONDS);
System.out.println("result:" + result);
} catch (InterruptedException e) {
System.out.println("守护线程阻塞被打断...");
e.printStackTrace();
} catch (ExecutionException e) {
System.out.println("执行任务时出错...");
e.printStackTrace();
} catch (TimeoutException e) {
System.out.println("执行超时...");
futureTask.cancel(true);
e.printStackTrace();
} catch (CancellationException e) {
//如果线程已经cancel了，再执行get操作会抛出这个异常
System.out.println("future已经cancel了...");
e.printStackTrace();
}
}
private static final long SLEEP_TIME = 100;
static class Task implements Callable<Boolean> {
@Override
public Boolean call() throws Exception {
try {
for (int i = 0; i < 10; i++) {
System.out.println("curr threadName=" + Thread.currentThread().getName() + " i=" + i);
//模拟耗时操作
Thread.sleep(SLEEP_TIME);
}
} catch (InterruptedException e) {
System.out.println(" is interrupted when calculating, will stop...");
return false; // 注意这里如果不return的话，线程还会继续执行，所以任务超时后在这里处理结果然后返回
}
return true;
}
}
}
1. 上述代码的执行结果为：
curr threadName=task thread 1 i=0 curr threadName=task thread 1 i=1 curr threadName=task thread 1 i=2 curr threadName=task thread 1 i=3 curr threadName=task thread 1 i=4 result:true
上述结果可以看出，get方法为阻塞执行，需要等到任务执行才会有返回值。
2. 当把SLEEP_TIME改为1500时，get方法回超时，进入timeout的异常处理分支，其结果如下。
curr threadName=task thread 1 i=0 curr threadName=task thread 1 i=1 curr threadName=task thread 1 i=2 curr threadName=task thread 1 i=3 执行超时... is interrupted when calculating, will stop... java.util.concurrent.TimeoutException at java.util.concurrent.FutureTask.get(FutureTask.java:205) at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.test(FutureTaskTest.java:30) at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.main(FutureTaskTest.java:16)
可以看出超时后，任务并不会继续执行，因为cancel方法传了true。（这里能够被cancel，是因为runable处于sleep状态，如果是一直执行的任务则无法被interrupt）
3. 当调用cancel传值为false时，执行结果如下：
curr threadName=task thread 1 i=0
curr threadName=task thread 1 i=1
curr threadName=task thread 1 i=2
curr threadName=task thread 1 i=3
执行超时...
java.util.concurrent.TimeoutException
at java.util.concurrent.FutureTask.get(FutureTask.java:205)
at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.test(FutureTaskTest.java:30)
at com.iflytek.drip.selflearn2.ConcurrentTest.FutureTaskTest.main(FutureTaskTest.java:16)
curr threadName=task thread 1 i=4
虽然执行了cancel，但是任务并没有被中断。
参考：
梦想不是浮躁,而是沉淀和积累
