Java 併發編程——Callable+Future+FutureTask
Java 併發編程系列文章
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Java 併發編程——Callable+Future+FutureTaskjava
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項目中常常有些任務須要異步(提交到線程池中)去執行，而主線程每每須要知道異步執行產生的結果，這時咱們要怎麼作呢？用runnable是沒法實現的，咱們須要用callable實現。app
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目前爲止返回的都是FutureTask的實例 Future<Integer> future = executor.submit(new AddTask(1, 2)); Integer result = future.get();// 只有當future的狀態是已完成時(future.isDone() = true),get()方法纔會返回 } }
Callable接口
Callable接口Runable接口可謂是兄弟關係，只不過Callable是帶返回值的。框架
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
接口函數及含義 ：public interface Future<V>less
boolean cancel(boolean mayInterruptIfRunning)異步
取消當前執行的任務，若是已經執行完畢或者已經被取消/因爲某種緣由不能被取消 則取消任務失敗。
參數mayInterruptIfRunning： 當任務正在執行，若是參數爲true ，則嘗試中斷任務，不然讓任務繼續執行知道結束。
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
*/
由註釋能夠看出，當沒有執行完成時，須要等待任務執行完成了纔會將計算結果返回。
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.
若是等待的時間超過設置的時間則會報 TimeoutException異常
FutureTask
public class FutureTask<V> implements RunnableFuture<V>
由定義能夠看出它實現了RunnableFuture接口，那麼這個接口又是什麼呢？看下面的接口定義，其實很簡單
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,它其實就是實現了Runnable和Future接口，FutureTask的執行是 狀態轉換的過程，源碼中有七種狀態以下：
* 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;
當FutureTask剛剛被建立時，它的狀態是NEW，其它狀態查看源碼。
其它成員變量：
/** 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是待執行的任務，FutureTask 的 run（）函數中執行callable中的任務。
outcome ： 是callable的執行結果，當正常執行完成後會將結果set到outcome中
runner：是執行callable 的線程
WaitNode : 是的受阻塞的線程鏈表，當cancel一個任務後，阻塞的線程會被喚醒。
構造函數：
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 }
從構造函數能夠看出，不光能夠經過callable構造FutureTask還能夠經過Runnable接口轉化爲callable來構造。關鍵函數爲黃色標記部分，Executors中的實現源碼以下：
/** * 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; } }
這裏面不懂result到底有什麼意義，明明就是預先設置好的。
其它具體的方法說明這裏再也不細說，裏面用到了不少sun.misc.Unsafe中的方法以及其餘SDK底層接口，後續有時間再學習。下面貼出了整個源碼及說明
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; /** 用來存儲任務執行結果或者異常對象,根據任務state在get時候選擇返回執行結果仍是拋出異常 */ private Object outcome; // non-volatile, protected by state reads/writes /** 當前運行Run方法的線程 */ 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 } //判斷任務是否已取消(異常中斷、取消等) public boolean isCancelled() { return state >= CANCELLED; } /** 判斷任務是否已結束（取消、異常、完成、NORMAL都等於結束） ** public boolean isDone() { return state != NEW; } /** mayInterruptIfRunning用來決定任務的狀態。 true : 任務狀態= INTERRUPTING = 5。若是任務已經運行，則強行中斷。若是任務未運行，那麼則不會再運行 false：CANCELLED = 4。若是任務已經運行，則容許運行完成（但不能經過get獲取結果）。若是任務未運行，那麼則不會再運行 **/ 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; //若是任務未完全完成，那麼則阻塞直至任務完成後喚醒該線程 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() { } /** 該方法在FutureTask裏只有run方法在任務完成後調用。 主要保存任務執行結果到成員變量outcome 中，和切換任務執行狀態。 由該方法能夠得知： COMPLETING ： 任務已執行完成（也多是異常完成），但還未設置結果到成員變量outcome中，也意味着還不能get NORMAL ： 任務完全執行完成 **/ 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(); } } /** 因爲實現了Runnable接口的緣故，該方法可由執行線程所調用。 **/ public void run() { //只有當任務狀態=new時才被運行繼續執行 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 { //調用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); } } /** 若是該任務在執行過程當中不被取消或者異常結束，那麼該方法不記錄任務的執行結果，且不修改任務執行狀態。 因此該方法能夠重複執行N次。不過不能直接調用，由於是protected權限。 **/ 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(); } } /** 該方法在任務完成（包括異常完成、取消）後調用。刪除全部正在get獲取等待的節點且喚醒節點的線程。和調用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 } /** 阻塞等待任務執行完成（中斷、正常完成、超時） **/ private int awaitDone(boolean timed, long nanos) throws InterruptedException { final long deadline = timed ? System.nanoTime() + nanos : 0L; WaitNode q = null; boolean queued = false; for (;;) { /** 這裏的if else的順序也是有講究的。 1.先判斷線程是否中斷，中斷則從隊列中移除(也可能該線程不存在於隊列中) 2.判斷當前任務是否執行完成，執行完成則再也不阻塞，直接返回。 3.若是任務狀態=COMPLETING，證實該任務處於已執行完成，正在切換任務執行狀態，CPU讓出片刻便可 4.q==null，則證實還未建立節點，則建立節點 5.q節點入隊 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簡單應用：
public class FutureTaskTest {
public static void main(String[] args) {
test();
}
private static void test() {
Task task = new Task();
FutureTask futureTask = new FutureTask(task);
//step3:將FutureTask提交給Thread執行
Thread thread1 = new Thread(futureTask);
thread1.setName("task thread 1");
thread1.start();
//step4:獲取執行結果，因爲get()方法可能會阻塞當前調用線程，若是子任務執行時間不肯定，最好在子線程中獲取執行結果
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("守護線程阻塞被打斷...");
e.printStackTrace();
} catch (ExecutionException e) {
System.out.println("執行任務時出錯...");
e.printStackTrace();
} catch (TimeoutException e) {
System.out.println("執行超時...");
futureTask.cancel(true);
e.printStackTrace();
} catch (CancellationException e) {
//若是線程已經cancel了，再執行get操做會拋出這個異常
System.out.println("future已經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);
//模擬耗時操做
Thread.sleep(SLEEP_TIME);
}
} catch (InterruptedException e) {
System.out.println(" is interrupted when calculating, will stop...");
return false; // 注意這裏若是不return的話，線程還會繼續執行，因此任務超時後在這裏處理結果真後返回
}
return true;
}
}
}
1. 上述代碼的執行結果爲：
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
上述結果能夠看出，get方法爲阻塞執行，須要等到任務執行纔會有返回值。
2. 當把SLEEP_TIME改成1500時，get方法回超時，進入timeout的異常處理分支，其結果以下。
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 執行超時... 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)
能夠看出超時後，任務並不會繼續執行，由於cancel方法傳了true。（這裏可以被cancel，是由於runable處於sleep狀態，若是是一直執行的任務則沒法被interrupt）
3. 當調用cancel傳值爲false時，執行結果以下：
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
執行超時...
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
雖然執行了cancel，可是任務並無被中斷。
參考：
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