Java多線程之異步Future機制的道理和完成。本站提示廣大學習愛好者：（Java多線程之異步Future機制的道理和完成）文章只能為提供參考，不一定能成為您想要的結果。以下是Java多線程之異步Future機制的道理和完成正文
項目中常常有些義務須要異步(提交到線程池中)去履行，而主線程常常須要曉得異步履行發生的成果，這時候我們要怎樣做呢？用runnable是沒法完成的，我們須要用callable看上面的代碼：
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今朝為止前往的都是FutureTask的實例
Future<Integer> future = executor.submit(new AddTask(1, 2));
Integer result = future.get;// 只要當future的狀況是已完成時(future.isDone = true),get辦法才會前往
}
}
固然可以完成獲得異步履行成果的需求，然則我們發明這個Future其實很欠好用，由於它沒有供給告訴的機制，也就是說我們不曉得future甚麼時刻完成（假如我們須要輪詢isDone()來斷定的話感到就沒有效這個的需要了）。看下java.util.concurrent.future.Future 的接口辦法：
public interface Future<V> {
boolean cancel(boolean mayInterruptIfRunning);
boolean isCancelled;
boolean isDone;
V get throws InterruptedException, ExecutionException;
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
因而可知JDK的Future機制其實其實不好用，假如能給這個future加個監聽器，讓它在完成時告訴監聽器的話就比擬好用了，就像上面這個IFuture：
package future;
import java.util.concurrent.CancellationException;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;
/**
* The result of an asynchronous operation.
*
* @author lixiaohui
* @param <V> 履行成果的類型參數
*/
public interface IFuture<V> extends Future<V> {
boolean isSuccess; // 能否勝利
V getNow; //立刻前往成果(不論Future能否處於完成狀況)
Throwable cause; //若履行掉敗時的緣由
boolean isCancellable; //能否可以撤消
IFuture<V> await throws InterruptedException; //期待future的完成
boolean await(long timeoutMillis) throws InterruptedException; // 超時期待future的完成
boolean await(long timeout, TimeUnit timeunit) throws InterruptedException;
IFuture<V> awaitUninterruptibly; //期待future的完成，不呼應中止
boolean awaitUninterruptibly(long timeoutMillis);//超時期待future的完成，不呼應中止
boolean awaitUninterruptibly(long timeout, TimeUnit timeunit);
IFuture<V> addListener(IFutureListener<V> l); //當future完成時，會告訴這些加出去的監聽器
IFuture<V> removeListener(IFutureListener<V> l);
}
接上去就一路來完成這個IFuture，在這之前要解釋下Object.wait,Object.notifyAll辦法，由於全部Future完成的原���的焦點就是這兩個辦法.看看JDK外面的說明:
public class Object {
/**
* Causes the current thread to wait until another thread invokes the
* {@link java.lang.Object#notify} method or the
* {@link java.lang.Object#notifyAll} method for this object.
* In other words, this method behaves exactly as if it simply
* performs the call {@code wait(0)}.
* 挪用該辦法後，以後線程會釋放對象監督器鎖，並讓出CPU應用權。直到其余線程挪用notify/notifyAll
*/
public final void wait throws InterruptedException {
wait(0);
}
/**
* Wakes up all threads that are waiting on this object's monitor. A
* thread waits on an object's monitor by calling one of the
* {@code wait} methods.
* <p>
* The awakened threads will not be able to proceed until the current
* thread relinquishes the lock on this object. The awakened threads
* will compete in the usual manner with any other threads that might
* be actively competing to synchronize on this object; for example,
* the awakened threads enjoy no reliable privilege or disadvantage in
* being the next thread to lock this object.
*/
public final native void notifyAll;
}
曉得這個後，我們要本身完成Future也就有了思緒，當線程挪用了IFuture.await等一系列的辦法時，假如Future還未完成，那末就挪用future.wait 辦法使線程進入WAITING狀況。而當其余線程設置Future為完成狀況（留意這裡的完成狀況包含正常停止和異常停止）時，就須要挪用future.notifyAll辦法來叫醒之前由於挪用過wait辦法而處於WAITING狀況的那些線程。完全的完成以下(代碼應當沒有很難懂得的處所，我是參考netty的Future機制的。有興致的可以去看看netty的源碼)：
package future;
import java.util.Collection;
import java.util.concurrent.CancellationException;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
/**
* <pre>
* 正常停止時, 若履行的成果不為null, 則result為履行成果; 若履行成果為null, 則result = {@link AbstractFuture#SUCCESS_SIGNAL}
* 異常停止時, result為 {@link CauseHolder} 的實例;若是被撤消而招致的異常停止, 則result為 {@link CancellationException} 的實例, 不然為其它異常的實例
* 以下情形會使異步操作由未完成狀況轉至已完成狀況, 也就是在以下情形產生時挪用notifyAll辦法:
* <ul>
* <li>異步操作被撤消時(cancel辦法)</li>
* <li>異步操作正常停止時(setSuccess辦法)</li>
* <li>異步操作異常停止時(setFailure辦法)</li>
* </ul>
* </pre>
*
* @author lixiaohui
*
* @param <V>
* 異步履行成果的類型
*/
public class AbstractFuture<V> implements IFuture<V> {
protected volatile Object result; // 須要包管其可見性
/**
* 監聽器集
*/
protected Collection<IFutureListener<V>> listeners = new CopyOnWriteArrayList<IFutureListener<V>>;
/**
* 當義務正常履行成果為null時, 即客戶端挪用{@link AbstractFuture#setSuccess(null)}時,
* result援用該對象
*/
private static final SuccessSignal SUCCESS_SIGNAL = new SuccessSignal;
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
if (isDone) { // 已完成了不克不及撤消
return false;
}
synchronized (this) {
if (isDone) { // double check
return false;
}
result = new CauseHolder(new CancellationException);
notifyAll; // isDone = true, 告訴期待在該對象的wait的線程
}
notifyListeners; // 告訴監聽器該異步操作已完成
return true;
}
@Override
public boolean isCancellable {
return result == null;
}
@Override
public boolean isCancelled {
return result != null && result instanceof CauseHolder && ((CauseHolder) result).cause instanceof CancellationException;
}
@Override
public boolean isDone {
return result != null;
}
@Override
public V get throws InterruptedException, ExecutionException {
await; // 期待履行成果
Throwable cause = cause;
if (cause == null) { // 沒有產生異常，異步操作正常停止
return getNow;
}
if (cause instanceof CancellationException) { // 異步操作被撤消了
throw (CancellationException) cause;
}
throw new ExecutionException(cause); // 其他異常
}
@Override
public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
if (await(timeout, unit)) {// 超時期待履行成果
Throwable cause = cause;
if (cause == null) {// 沒有產生異常，異步操作正常停止
return getNow;
}
if (cause instanceof CancellationException) {// 異步操作被撤消了
throw (CancellationException) cause;
}
throw new ExecutionException(cause);// 其他異常
}
// 時光到了異步操作還沒有停止, 拋出超時異常
throw new TimeoutException;
}
@Override
public boolean isSuccess {
return result == null ? false : !(result instanceof CauseHolder);
}
@SuppressWarnings("unchecked")
@Override
public V getNow {
return (V) (result == SUCCESS_SIGNAL ? null : result);
}
@Override
public Throwable cause {
if (result != null && result instanceof CauseHolder) {
return ((CauseHolder) result).cause;
}
return null;
}
@Override
public IFuture<V> addListener(IFutureListener<V> listener) {
if (listener == null) {
throw new NullPointerException("listener");
}
if (isDone) { // 若已完成直接告訴該監聽器
notifyListener(listener);
return this;
}
synchronized (this) {
if (!isDone) {
listeners.add(listener);
return this;
}
}
notifyListener(listener);
return this;
}
@Override
public IFuture<V> removeListener(IFutureListener<V> listener) {
if (listener == null) {
throw new NullPointerException("listener");
}
if (!isDone) {
listeners.remove(listener);
}
return this;
}
@Override
public IFuture<V> await throws InterruptedException {
return await0(true);
}
private IFuture<V> await0(boolean interruptable) throws InterruptedException {
if (!isDone) { // 若已完造詣直接前往了
// 若許可終端且被中止了則拋出中止異常
if (interruptable && Thread.interrupted) {
throw new InterruptedException("thread " + Thread.currentThread.getName + " has been interrupted.");
}
boolean interrupted = false;
synchronized (this) {
while (!isDone) {
try {
wait; // 釋放鎖進入waiting狀況，期待其它線程挪用本對象的notify/notifyAll辦法
} catch (InterruptedException e) {
if (interruptable) {
throw e;
} else {
interrupted = true;
}
}
}
}
if (interrupted) {
// 為何這裡要設中止標記位？由於從wait辦法前往後, 中止標記是被clear了的,
// 這裡從新設置以便讓其它代碼曉得這裡被中止了。
Thread.currentThread.interrupt;
}
}
return this;
}
@Override
public boolean await(long timeoutMillis) throws InterruptedException {
return await0(TimeUnit.MILLISECONDS.toNanos(timeoutMillis), true);
}
@Override
public boolean await(long timeout, TimeUnit unit) throws InterruptedException {
return await0(unit.toNanos(timeout), true);
}
private boolean await0(long timeoutNanos, boolean interruptable) throws InterruptedException {
if (isDone) {
return true;
}
if (timeoutNanos <= 0) {
return isDone;
}
if (interruptable && Thread.interrupted) {
throw new InterruptedException(toString);
}
long startTime = timeoutNanos <= 0 ? 0 : System.nanoTime;
long waitTime = timeoutNanos;
boolean interrupted = false;
try {
synchronized (this) {
if (isDone) {
return true;
}
if (waitTime <= 0) {
return isDone;
}
for (;;) {
try {
wait(waitTime / 1000000, (int) (waitTime % 1000000));
} catch (InterruptedException e) {
if (interruptable) {
throw e;
} else {
interrupted = true;
}
}
if (isDone) {
return true;
} else {
waitTime = timeoutNanos - (System.nanoTime - startTime);
if (waitTime <= 0) {
return isDone;
}
}
}
}
} finally {
if (interrupted) {
Thread.currentThread.interrupt;
}
}
}
@Override
public IFuture<V> awaitUninterruptibly {
try {
return await0(false);
} catch (InterruptedException e) { // 這裡若拋異常了就沒法處置了
throw new java.lang.InternalError;
}
}
@Override
public boolean awaitUninterruptibly(long timeoutMillis) {
try {
return await0(TimeUnit.MILLISECONDS.toNanos(timeoutMillis), false);
} catch (InterruptedException e) {
throw new java.lang.InternalError;
}
}
@Override
public boolean awaitUninterruptibly(long timeout, TimeUnit unit) {
try {
return await0(unit.toNanos(timeout), false);
} catch (InterruptedException e) {
throw new java.lang.InternalError;
}
}
protected IFuture<V> setFailure(Throwable cause) {
if (setFailure0(cause)) {
notifyListeners;
return this;
}
throw new IllegalStateException("complete already: " + this);
}
private boolean setFailure0(Throwable cause) {
if (isDone) {
return false;
}
synchronized (this) {
if (isDone) {
return false;
}
result = new CauseHolder(cause);
notifyAll;
}
return true;
}
protected IFuture<V> setSuccess(Object result) {
if (setSuccess0(result)) { // 設置勝利後告訴監聽器
notifyListeners;
return this;
}
throw new IllegalStateException("complete already: " + this);
}
private boolean setSuccess0(Object result) {
if (isDone) {
return false;
}
synchronized (this) {
if (isDone) {
return false;
}
if (result == null) { // 異步操作正常履行終了的成果是null
this.result = SUCCESS_SIGNAL;
} else {
this.result = result;
}
notifyAll;
}
return true;
}
private void notifyListeners {
for (IFutureListener<V> l : listeners) {
notifyListener(l);
}
}
private void notifyListener(IFutureListener<V> l) {
try {
l.operationCompleted(this);
} catch (Exception e) {
e.printStackTrace;
}
}
private static class SuccessSignal {
}
private static final class CauseHolder {
final Throwable cause;
CauseHolder(Throwable cause) {
this.cause = cause;
}
}
}
那末要怎樣應用這個呢，有了下面的骨架完成，我們便可以定制各類各樣的異步成果了。上面模仿一下一個延時的義務：
package future.test;
import future.IFuture;
import future.IFutureListener;
/**
* 延時加法
* @author lixiaohui
*
*/
public class DelayAdder {
public static void main(String[] args) {
new DelayAdder.add(3 * 1000, 1, 2).addListener(new IFutureListener<Integer> {
@Override
public void operationCompleted(IFuture<Integer> future) throws Exception {
System.out.println(future.getNow);
}
});
}
/**
* 延遲加
* @param delay 延不時長 milliseconds
* @param a 加數
* @param b 加數
* @return 異步成果
*/
public DelayAdditionFuture add(long delay, int a, int b) {
DelayAdditionFuture future = new DelayAdditionFuture;
new Thread(new DelayAdditionTask(delay, a, b, future)).start;
return future;
}
private class DelayAdditionTask implements Runnable {
private long delay;
private int a, b;
private DelayAdditionFuture future;
public DelayAdditionTask(long delay, int a, int b, DelayAdditionFuture future) {
super;
this.delay = delay;
this.a = a;
this.b = b;
this.future = future;
}
@Override
public void run {
try {
Thread.sleep(delay);
Integer i = a + b;
// TODO 這裡設置future為完成狀況(正常履行終了)
future.setSuccess(i);
} catch (InterruptedException e) {
// TODO 這裡設置future為完成狀況(異常履行終了)
future.setFailure(e.getCause);
}
}
}
} package future.test;
import future.AbstractFuture;
import future.IFuture;
//只是把兩個辦法對外裸露
public class DelayAdditionFuture extends AbstractFuture<Integer> {
@Override
public IFuture<Integer> setSuccess(Object result) {
return super.setSuccess(result);
}
@Override
public IFuture<Integer> setFailure(Throwable cause) {
return super.setFailure(cause);
}
}
可以看到客戶端不消自動去訊問future能否完成，而是future完成時主動回調operationcompleted辦法，客戶端只需在回調裡完成邏輯便可。
以上就是本文的全體內容，願望對年夜家的進修有所贊助，也願望年夜家多多支撐。
