一、前言
通常，java中创建多线程的两种方式：
- 直接继承Thread;
- 实现Runnable接口。
考虑到一些逻辑需要一定的先后顺序，如果直接用这两种方式都会有共同的缺点：
- 通常为阻塞式（通过join等待一个线程结束，但这样就失去了多线程的意义），或者通过wait、notify、notifyAll并结合状态变量等来进行并发设计，设计起来相当复杂;
- 线程执行完成后难以获取线程执行结果（需要通过共享变量、线程间通信等方式来获取, 比较复杂）
由此，我们想到了多线程开发中常见的Future模式。开发中经常有一些操作可能比较耗时，但又不想阻塞式的等待，这时可以先执行一些其它操作，等其它操作完成后再去获取耗时操作的结果，这就是Future模式的描述。对应于生活中例子比比皆是：比如，打开电饭煲烧米饭后继续炒菜，等炒菜完了去看下米饭有没有煲熟，过程中无需死等电饭煲把饭煲熟，只有在炒完菜后这个时间点，我们才尝试去看电饭煲煲饭的结果，这就是Future模式的一个生活原型。
java从1.5开始，在并发包中提供了Future模式的设计，我们这要结合Callable、Future/FutureTask就能很容易的使用Future模式。
二、Future模式的一个简单示例
我们来看一个简单示例：
public static void main(String[] args) throws InterruptedException, ExecutionException
{
ExecutorService executor = Executors.newCachedThreadPool();
Future<Integer> future = executor.submit(new Callable<Integer>(){
@Override
public Integer call()
throws Exception
{
int total = 0;
for(int i = 5001; i<=10000; i++){
total += i;
}
return total;
}
});
System.out.print("Submit future task now...");
executor.shutdown();
int total = 0;
for(int i = 1; i<=5000; i++){
total += i;
}
total += future.get();
System.out.print("1+2+...+10000 = " + total);
}
示例中计算了1~10000且步长为1的等比数列之和，将数列拆均分成两部分分别求和，最后进行累计。Future模式通常需要配合ExecutorService和Callable一起使用，代码中采用ExecutorService的submit方法提交Callable线程，在主线程任务完成后获取Callable线程的结果。
三、源码分析
-
我们先直接看下Future类型的源码：
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;
}
Future接口的5个方法含义如下：
- cancel(boolean mayInterruptIfRunning) : 取消任务, 取消成功返回true；入参mayInterruptIfRunning表示是否允许取消正在执行中的任务。
- isCancelled() : 返回是否取消成功
- isDone() : 返回任务是否已经完成
- get() : 返回执行结果，如果任务没有完成会阻塞到任务完成再返回
- get(long timeout, TimeUnit unit) 获取执行结果并设置超时时间，如果超时返回null
-
Future模式通常需要配合ExecutorService和Callable一起使用，通过ExecutorService的submit方法提交Callable线程。我们知道，execute()方法在Executor接口中定义，而submit()方法在ExecutorService接口中定义，ExecutorService接口继承Executor接口：
public interface Executor {
void execute(Runnable command);
}
public interface ExecutorService extends Executor {
...
<T> Future<T> submit(Callable<T> task);
<T> Future<T> submit(Runnable task, T result);
Future<?> submit(Runnable task);
...
}
-
ExecutorService只是一个接口，我们以上一节的newCachedThreadPool为例，看下它的源码：
public static ExecutorService newCachedThreadPool() {
return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
60L, TimeUnit.SECONDS,
new SynchronousQueue<Runnable>());
}
-
上面结果返回的是一个ThreadPoolExecutor，它是ExecutorService的一个子类，看ThreadPoolExecutor源码可以发现，ThreadPoolExecutor没有实现submit方法，它的submit方法由其直接父类AbstractExecutorService实现：
...
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
...
public Future<?> submit(Runnable task) {
if (task == null) throw new NullPointerException();
RunnableFuture<Void> ftask = newTaskFor(task, null);
execute(ftask);
return ftask;
}
...
public <T> Future<T> submit(Runnable task, T result) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task, result);
execute(ftask);
return ftask;
}
...
-
在上面三个submit方法中，无论是Callable接口还是Runnable接口，均是转化成了RunnableFuture实例，看下RunnableFuture的实现：
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
-
RunnableFuture接口同时继承了Runnable接口和Future接口。再看下上面讲Callable或Runnable转化成RunnableFuture实例的实现：
...
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
...
-
通过两个newTaskFor方法分别将Callable和Runnable实例转化成FutureTask实例，FutureTask是RunnableFuture的实现，上述源码中涉及FutureTask的两种构造函数：
...
private Callable<V> callable;
private volatile int state;
private static final int NEW = 0;
...
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
}
...
-
对于Callable实例，直接将入参Callable对象赋值给this.callable属性，并设置this.state属性为NEW; 而对于Funnable实例，需要通过Executors类的callable(runnable, result)方法转化成Callable实例：
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
-
Executors类的callable(runnable, result)方法实际生成了一个RunnableAdapter对象，看下其源码：
static final class RunnableAdapter<T> implements Callable<T> {
final Runnable task;
final T result;
RunnableAdapter(Runnable task, T result) {
this.task = task;
this.result = result;
}
public T call() {
task.run();
return result;
}
}
显而易见，RunnableAdapter类实现了Callable接口的call()方法，内部调用了Runnable实例的run()方法，并返回预先传过来的result值。
-
回过头来看下，FutureTask类实现了RunnableFuture接口，进而实现了Runnable接口和Future接口的统一，那么它是如何实现Runnable接口的run()方法的呢？看下其源码：
public void run() {
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 {
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);
}
}
...
protected void set(V v) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = v;
UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
finishCompletion();
}
}
...
protected void setException(Throwable t) {
if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
outcome = t;
UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
finishCompletion();
}
}
可以看到FutureTask的run()方法内部调用了Runnable实例的call()方法，并且如果运行成功，将call()方法的返回值赋值给outcome，否则将异常赋值给outcome。
这样也就容易理解ExecutorService的submit方法实现中是如何调用execute(Runnable command)方法的了，它将Runnable或者Callable实例统一转换成了RunnableFuture实例，由于RunnableFuture继承了Runnable接口，所以线程池可以通过execute(Runnable command)方法来进行处理。
- 回过来看下FutureTask类的get()方法实现：
public V get() throws InterruptedException, ExecutionException {
int s = state;
if (s <= COMPLETING)
s = awaitDone(false, 0L);
return report(s);
}
...
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);
}
...
private int awaitDone(boolean timed, long nanos)
throws InterruptedException {
final long deadline = timed ? System.nanoTime() + nanos : 0L;
WaitNode q = null;
boolean queued = false;
for (;;) {
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);
}
}
- 如果状态state为任务执行中，则阻塞等待，否则通过report(s)返回结果。返回结果时：如果状态正常，则直接返回outcome；如果取消或者中断，则返回CancellationException异常；如果执行异常，则返回ExecutionException。
- 上述源码可以看出，get()方法通过awaitDone方法进行阻塞等待，awaitDone方法实现上采用LockSupport.park()进行线程阻塞，在FutureTasl的run()方法执行完成或异常发生，会执行set(V v)方法或setException(Throwable t)方法，两者的实现中都会调用finishCompletion()方法，并在finishCompletion()方法中采用LockSupport.unpark方法进行了线程唤醒：
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
}
四、总结
通过上述举例和源码分析我们理解了java中Future模式的原理和使用，Future模式对于一些耗时操作（比如网络请求等）的性能提升还是比较有用的，实际开发中可以灵活运用。
