Java并发编程之异步Future机制的原理和实现
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看下面的代码：
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
}
}
虽然可以实现获取异步执行结果的需求，但是我们发现这个Future其实很不好用，因为它没有提供通知的机制，也就是说我们不知道future什么时候完成（如果我们需要轮询isDone()来判断的话感觉就没有用这个的必要了）。看下java.util.concurrent.future.Future 的接口方法：
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;
}
由此可见JDK的Future机制其实并不好用，如果能给这个future加个监听器，让它在完成时通知监听器的话就比较好用了，就像下面这个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> 执行结果的类型参数
*/
public interface IFuture<V> extends Future<V> {
boolean isSuccess(); // 是否成功
V getNow(); //立即返回结果(不管Future是否处于完成状态)
Throwable cause(); //若执行失败时的原因
boolean isCancellable(); //是否可以取消
IFuture<V> await() throws InterruptedException; //等待future的完成
boolean await(long timeoutMillis) throws InterruptedException; // 超时等待future的完成
boolean await(long timeout, TimeUnit timeunit) throws InterruptedException;
IFuture<V> awaitUninterruptibly(); //等待future的完成，不响应中断
boolean awaitUninterruptibly(long timeoutMillis);//超时等待future的完成，不响应中断
boolean awaitUninterruptibly(long timeout, TimeUnit timeunit);
IFuture<V> addListener(IFutureListener<V> l); //当future完成时，会通知这些加进来的监听器
IFuture<V> removeListener(IFutureListener<V> l);
}
接下来就一起来实现这个IFuture，在这之前要说明下Object.wait(),Object.notifyAll()方法，因为整个Future实现的原理的核心就是这两个方法.看看JDK里面的解释:
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)}.
* 调用该方法后，当前线程会释放对象监视器锁，并让出CPU使用权。直到别的线程调用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();
}
知道这个后，我们要自己实现Future也就有了思路，当线程调用了IFuture.await()等一系列的方法时，如果Future还未完成，那么就调用future.wait() 方法使线程进入WAITING状态。而当别的线程设置Future为完成状态（注意这里的完成状态包括正常结束和异常结束）时，就需要调用future.notifyAll()方法来唤醒之前因为调用过wait()方法而处于WAITING状态的那些线程。完整的实现如下(代码应该没有很难理解的地方，我是参考netty的Future机制的。有兴趣的可以去看看netty的源码)：
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>
* 正常结束时, 若执行的结果不为null, 则result为执行结果; 若执行结果为null, 则result = {@link AbstractFuture#SUCCESS_SIGNAL}
* 异常结束时, result为 {@link CauseHolder} 的实例;若是被取消而导致的异常结束, 则result为 {@link CancellationException} 的实例, 否则为其它异常的实例
* 以下情况会使异步操作由未完成状态转至已完成状态, 也就是在以下情况发生时调用notifyAll()方法:
* <ul>
* <li>异步操作被取消时(cancel方法)</li>
* <li>异步操作正常结束时(setSuccess方法)</li>
* <li>异步操作异常结束时(setFailure方法)</li>
* </ul>
* </pre>
*
* @author lixiaohui
*
* @param <V>
* 异步执行结果的类型
*/
public class AbstractFuture<V> implements IFuture<V> {
protected volatile Object result; // 需要保证其可见性
/**
* 监听器集
*/
protected Collection<IFutureListener<V>> listeners = new CopyOnWriteArrayList<IFutureListener<V>>();
/**
* 当任务正常执行结果为null时, 即客户端调用{@link AbstractFuture#setSuccess(null)}时,
* result引用该对象
*/
private static final SuccessSignal SUCCESS_SIGNAL = new SuccessSignal();
@Override
public boolean cancel(boolean mayInterruptIfRunning) {
if (isDone()) { // 已完成了不能取消
return false;
}
synchronized (this) {
if (isDone()) { // double check
return false;
}
result = new CauseHolder(new CancellationException());
notifyAll(); // isDone = true, 通知等待在该对象的wait()的线程
}
notifyListeners(); // 通知监听器该异步操作已完成
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(); // 等待执行结果
Throwable cause = cause();
if (cause == null) { // 没有发生异常，异步操作正常结束
return getNow();
}
if (cause instanceof CancellationException) { // 异步操作被取消了
throw (CancellationException) cause;
}
throw new ExecutionException(cause); // 其他异常
}
@Override
public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException {
if (await(timeout, unit)) {// 超时等待执行结果
Throwable cause = cause();
if (cause == null) {// 没有发生异常，异步操作正常结束
return getNow();
}
if (cause instanceof CancellationException) {// 异步操作被取消了
throw (CancellationException) cause;
}
throw new ExecutionException(cause);// 其他异常
}
// 时间到了异步操作还没有结束, 抛出超时异常
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()) { // 若已完成直接通知该监听器
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()) { // 若已完成就直接返回了
// 若允许终端且被中断了则抛出中断异常
if (interruptable && Thread.interrupted()) {
throw new InterruptedException("thread " + Thread.currentThread().getName() + " has been interrupted.");
}
boolean interrupted = false;
synchronized (this) {
while (!isDone()) {
try {
wait(); // 释放锁进入waiting状态，等待其它线程调用本对象的notify()/notifyAll()方法
} catch (InterruptedException e) {
if (interruptable) {
throw e;
} else {
interrupted = true;
}
}
}
}
if (interrupted) {
// 为什么这里要设中断标志位？因为从wait方法返回后, 中断标志是被clear了的,
// 这里重新设置以便让其它代码知道这里被中断了。
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) { // 这里若抛异常了就无法处理了
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)) { // 设置成功后通知监听器
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) { // 异步操作正常执行完毕的结果是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;
}
}
}
那么要怎么使用这个呢，有了上面的骨架实现，我们就可以定制各种各样的异步结果了。下面模拟一下一个延时的任务：
package future.test;
import future.IFuture;
import future.IFutureListener;
/**
* 延时加法
* @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());
}
});
}
/**
* 延迟加
* @param delay 延时时长 milliseconds
* @param a 加数
* @param b 加数
* @return 异步结果
*/
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 这里设置future为完成状态(正常执行完毕)
future.setSuccess(i);
} catch (InterruptedException e) {
// TODO 这里设置future为完成状态(异常执行完毕)
future.setFailure(e.getCause());
}
}
}
}
package future.test;
import future.AbstractFuture;
import future.IFuture;
//只是把两个方法对外暴露
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);
}
}
可以看到客户端不用主动去询问future是否完成，而是future完成时自动回调operationcompleted方法，客户端只需在回调里实现逻辑即可。