本文已参加「新人创作礼」活动，一起敞开创作之路。
Future就是对于详细的Runnable或者Callable使命的履行成果进行撤销、查询是否完结、获取成果等操作。必要时能够经过get办法获取履行成果，该办法会阻塞直到使命回来成果。
Future类位于java.util.concurrent包下，它是一个接口：
/**
* @see FutureTask
* @see Executor
* @since 1.5
* @author Doug Lea
* @param <V> The result type returned by this Future's <tt>get</tt> method
*/
public interface Future<V> {
/**
* Attempts to cancel execution of this task. This attempt will
* fail if the task has already completed, has already been cancelled,
* or could not be cancelled for some other reason. If successful,
* and this task has not started when <tt>cancel</tt> is called,
* this task should never run. If the task has already started,
* then the <tt>mayInterruptIfRunning</tt> parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task. *
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns <tt>true</tt> if this task was cancelled before it completed
* normally.
*/
boolean isCancelled();
/**
* Returns <tt>true</tt> if this task completed.
*
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
*/
V get() throws InterruptedException, ExecutionException;
/**
* Waits if necessary for at most the given time for the computation
* to complete, and then retrieves its result, if available.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
* @return the computed result
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
在Future接口中声明了5个办法，下面依次解释每个办法的效果：
cancel（）
办法用来撤销使命，假如撤销使命成功则回来true，假如撤销使命失利则回来false。参数mayInterruptIfRunning表明是否允许撤销正在履行却没有履行结束的使命，假如设置true，则表明能够撤销正在履行过程中的使命。假如使命现已完结，则无论mayInterruptIfRunning为true仍是false，此办法必定回来false，即假如撤销现已完结的使命会回来false；假如使命正在履行，若mayInterruptIfRunning设置为true，则回来true，若mayInterruptIfRunning设置为false，则回来false；假如使命还没有履行，则无论mayInterruptIfRunning为true仍是false，必定回来true。
isCancelled（）
办法表明使命是否被撤销成功，假如在使命正常完结前被撤销成功，则回来 true。
isDone（）
办法表明使命是否现已完结，若使命完结，则回来true；
get()
办法用来获取履行成果，这个办法会产生阻塞，会一直比及使命履行结束才回来；
get(long timeout, TimeUnit unit)
用来获取履行成果，假如在指定时间内，还没获取到成果，就直接回来null。
也就是说Future提供了三种功能：
1）判断使命是否完结；
2）能够中止使命；
3）能够获取使命履行成果。
由于Future仅仅一个接口，所以是无法直接用来创建对象运用的，因而就有了下面的FutureTask。
FutureTask
FutureTask的完成：
public class FutureTask<V> implements RunnableFuture<V>
FutureTask类完成了RunnableFuture接口，RunnableFuture接口：
public interface RunnableFuture<V> extends Runnable, Future<V> {
/**
* Sets this Future to the result of its computation
* unless it has been cancelled.
*/
void run();
}
能够看出RunnableFuture继承了Runnable接口和Future接口，而FutureTask完成了RunnableFuture接口。所以它既能够作为Runnable被线程履行，又能够作为Future得到Callable的回来值。
FutureTask提供了2个构造器：
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
}
能够看到，Runnable注入会被Executors.callable()函数转换为Callable类型，即FutureTask终究都是履行Callable类型的使命。该适配函数的完成如下:
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, result);
}
RunnableAdapter适配器
/**
* A callable that runs given task and returns given result
*/
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;
}
}
FutureTask是Future接口的一个仅有完成类。
FutureTask完成了Runnable，因而它既能够经过Thread包装来直接履行，也能够提交给ExecuteService来履行。
FutureTask完成了Futrue能够直接经过get()函数获取履行成果，该函数会阻塞，直到成果回来。
实例：
Callable+Future获取履行成果
public class Test {
public static void main(String[] args) {
ExecutorService executor = Executors.newCachedThreadPool();
Task task = new Task();
Future<Integer> result = executor.submit(task);
executor.shutdown();
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println("主线程在履行使命");
try {
System.out.println("task运转成果"+result.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("一切使命履行结束");
}
}
class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {
System.out.println("子线程在进行核算");
Thread.sleep(3000);
int sum = 0;
for(int i=0;i<100;i++)
sum += i;
return sum;
}
}
履行成果:
子线程在进行核算
主线程在履行使命
task运转成果4950
一切使命履行结束
Callable+FutureTask获取履行成果
public class Test {
public static void main(String[] args) {
//第一种方法
ExecutorService executor = Executors.newCachedThreadPool();
Task task = new Task();
FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
executor.submit(futureTask);
executor.shutdown();
//第二种方法，注意这种方法和第一种方法效果是类似的，只不过一个运用的是ExecutorService，一个运用的是Thread
/*Task task = new Task();
FutureTask<Integer> futureTask = new FutureTask<Integer>(task);
Thread thread = new Thread(futureTask);
thread.start();*/
try {
Thread.sleep(1000);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
System.out.println("主线程在履行使命");
try {
System.out.println("task运转成果"+futureTask.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("一切使命履行结束");
}
}
class Task implements Callable<Integer>{
@Override
public Integer call() throws Exception {
System.out.println("子线程在进行核算");
Thread.sleep(3000);
int sum = 0;
for(int i=0;i<100;i++)
sum += i;
return sum;
}
}
