Java多线程之异步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���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。

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。