java并发设计模式：Future与JDK8的CompletableFuture异步编程详解
一. Future
JDK 5引入了Future模式。Future接口是Java多线程Future模式的实现，在java.util.concurrent包中，可以来进行异步计算。
Future模式是多线程设计常用的一种设计模式。Future模式可以理解成：我有一个任务，提交给了Future，Future替我完成这个任务。期间我自己可以去做任何想做的事情。一段时间之后，我就便可以从Future那儿取出结果。
Future的接口很简单，只有五个方法，当然最核心的就是get, 我们便是通过get方法从Future的实体对象中获取执行结果。
首先我们来看一下Future的编程实例：
public static void main(String[] args) throws Exception{
// 使用线程池，返回的是ThreadPoolExecutor执行器
ExecutorService executor = Executors.newFixedThreadPool(1);
//Lambda 是一个 callable， 提交后便立即执行，这里返回的是 FutureTask 实例
Future future = executor.submit(()->{
System.out.println("running task");
Thread.sleep(1000);
return "hello world";
});
// 以上提交不阻塞主线程，立即返回，这里可用来处理其它业务逻辑，用一个sleep代替
Thread.sleep(2000);
//等待 future 的执行结果，执行完毕之后打印出来
System.out.println("返回执行结果：" + future.get());
//关闭线程池
executor.shutdown();
}可以看到submit是核心的提交任务的方法，我们从ThreadPoolExecutor找到submit方法：
protected RunnableFuture newTaskFor(Callable callable) {
return new FutureTask(callable);
}
public Future submit(Callable task) {
if (task == null) throw new NullPointerException();
RunnableFuture ftask = newTaskFor(task);
execute(ftask);
return ftask;
} 可以看到，FutureTask任务ftask还是会跟随execute执行,定位到ThreadPoolExecutor下的execute：
public void execute(Runnable command) {
if (command == null)
throw new NullPointerException();
int c = ctl.get();
if (workerCountOf(c) < corePoolSize) {
if (addWorker(command, true))
return;
c = ctl.get();
}
if (isRunning(c) && workQueue.offer(command)) {
int recheck = ctl.get();
if (! isRunning(recheck) && remove(command))
reject(command);
else if (workerCountOf(recheck) == 0)
addWorker(null, false);
}
else if (!addWorker(command, false))
reject(command);
}又调用到内部关键接口addWorker：
private boolean addWorker(Runnable firstTask, boolean core) {
retry:
for (;;) {
int c = ctl.get();
int rs = runStateOf(c);
// Check if queue empty only if necessary.
if (rs >= SHUTDOWN &&
! (rs == SHUTDOWN &&
firstTask == null &&
! workQueue.isEmpty()))
return false;
for (;;) {
int wc = workerCountOf(c);
if (wc >= CAPACITY ||
wc >= (core ? corePoolSize : maximumPoolSize))
return false;
if (compareAndIncrementWorkerCount(c))
break retry;
c = ctl.get(); // Re-read ctl
if (runStateOf(c) != rs)
continue retry;
// else CAS failed due to workerCount change; retry inner loop
}
}
boolean workerStarted = false;
boolean workerAdded = false;
Worker w = null;
try {
w = new Worker(firstTask);
final Thread t = w.thread;
if (t != null) {
final ReentrantLock mainLock = this.mainLock;
mainLock.lock();
try {
// Recheck while holding lock.
// Back out on ThreadFactory failure or if
// shut down before lock acquired.
int rs = runStateOf(ctl.get());
if (rs < SHUTDOWN ||
(rs == SHUTDOWN && firstTask == null)) {
if (t.isAlive()) // precheck that t is startable
throw new IllegalThreadStateException();
workers.add(w);
int s = workers.size();
if (s > largestPoolSize)
largestPoolSize = s;
workerAdded = true;
}
} finally {
mainLock.unlock();
}
if (workerAdded) {
t.start();
workerStarted = true;
}
}
} finally {
if (! workerStarted)
addWorkerFailed(w);
}
return workerStarted;
}可以看到，它会先判断队列的当前状态以及队列能够容纳的最大任务数，条件允许，则会往下创建工作线程workers, 然后把firstTask添加到工作线程组当中workers.add(w)，接下来可以看到工作线程Worker会调用runWorker方法：
final void runWorker(Worker w) {
Thread wt = Thread.currentThread();
Runnable task = w.firstTask;
w.firstTask = null;
w.unlock(); // allow interrupts
boolean completedAbruptly = true;
try {
while (task != null || (task = getTask()) != null) {
w.lock();
// If pool is stopping, ensure thread is interrupted;
// if not, ensure thread is not interrupted. This
// requires a recheck in second case to deal with
// shutdownNow race while clearing interrupt
if ((runStateAtLeast(ctl.get(), STOP) ||
(Thread.interrupted() &&
runStateAtLeast(ctl.get(), STOP))) &&
!wt.isInterrupted())
wt.interrupt();
try {
beforeExecute(wt, task);
Throwable thrown = null;
try {
task.run();
} catch (RuntimeException x) {
thrown = x; throw x;
} catch (Error x) {
thrown = x; throw x;
} catch (Throwable x) {
thrown = x; throw new Error(x);
} finally {
afterExecute(task, thrown);
}
} finally {
task = null;
w.completedTasks++;
w.unlock();
}
}
completedAbruptly = false;
} finally {
processWorkerExit(w, completedAbruptly);
}
}注意看，这里绕了一大圈，最终主要还是调用到task的run方法里去了，前面就追踪过，task是FutureTask的实例那么看FutureTask的run方法：
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))
return;
try {
Callable c = callable;
if (c != null && state == NEW) {
V result;
boolean ran;
try {
result = c.call();
ran = true;
} catch (Throwable ex) {
result = null;
ran = false;
setException(ex);
}
if (ran)
set(result);
}
} finally {
...
}
} 我们注意到，最终还是会调用FutureTask的set(V v)方法将result装载到成员变量outcome, 最后通过get方法拿到类对象里面的outcome变量的值，即可获取到执行结果。
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;//这里将result的值放到成员变量outcome中
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);//get的时候会阻塞await等待任务执行完
return report(s);
}
private V report(int s) throws ExecutionException {
//注意看这里x取到outcome的值，并作为后面返回值
Object x = outcome;
if (s == NORMAL)
return (V)x;//返回取到的x
if (s >= CANCELLED)
throw new CancellationException();
throw new ExecutionException((Throwable)x);
}以上便是整个Future的执行过程，如何将结果保存到类成员，以及如何get获取执行结果。这里展示一下类/接口的依赖关系：
二.CompletableFuture
在jdk1.8之后我们引入了一种全新的Future可以弥补之前FutureTask的get阻塞获取结果的不足，这个时候我们的主角CompletableFuture就闪亮登场了。
我们直接先看下它的正确打开方式，上用例代码：
public static void main(String[] args) throws InterruptedException {
CompletableFuture.supplyAsync(()->{
//to do thread task
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
//return task execute result
return "hello world";
}).thenAccept(result->{
//这里我们监听任务的执行结果，并对其做响应的处理
System.out.println("print async task result:"+result);
});
//由于以上是异步线程执行的，在这里我们主线程等待3秒，保证我们的任务线程能执行完
Thread.sleep(3000);
}好了，我们仅仅数十行代码就展示清楚了它的基本使用方法了。