Runnable，Callable，Future和FutureTask是什么？
我们上一篇讲了Thread的使用，发现可以继承Thread或实现Runnable来创建一个线程，两者都与Runnable脱不了关系。同时从下图可以发现，线程池的使用还可以执行Callable任务，这几和有什么关系呢？Future和FurureTask又是什么呢，下面看看吧。
首先我们发现线程池和线程可提交的任务都是Runnable和Callable：
我们先粘出Runnable和Callable和Future的代码看看
public interface Runnable {
public abstract void run();
}
public interface Callable<V> {
V call() throws Exception;
}
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 {@code cancel} is called,
* this task should never run. If the task has already started,
* then the {@code mayInterruptIfRunning} parameter determines
* whether the thread executing this task should be interrupted in
* an attempt to stop the task.
*
* <p>After this method returns, subsequent calls to {@link #isDone} will
* always return {@code true}. Subsequent calls to {@link #isCancelled}
* will always return {@code true} if this method returned {@code true}.
*
* @param mayInterruptIfRunning {@code true} if the thread executing this
* task should be interrupted; otherwise, in-progress tasks are allowed
* to complete
* @return {@code false} if the task could not be cancelled,
* typically because it has already completed normally;
* {@code true} otherwise
*/
boolean cancel(boolean mayInterruptIfRunning);
/**
* Returns {@code true} if this task was cancelled before it completed
* normally.
*
* @return {@code true} if this task was cancelled before it completed
*/
boolean isCancelled();
/**
* Returns {@code true} if this task completed.
*
* Completion may be due to normal termination, an exception, or
* cancellation -- in all of these cases, this method will return
* {@code true}.
*
* @return {@code true} if this task completed
*/
boolean isDone();
/**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
* @return the computed result
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
*/
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
* @throws CancellationException if the computation was cancelled
* @throws ExecutionException if the computation threw an
* exception
* @throws InterruptedException if the current thread was interrupted
* while waiting
* @throws TimeoutException if the wait timed out
*/
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
都是接口，而且只有需要实现的方法，不同的是 run()没有返回值，而call()可以有返回值还能抛出异常
Future没得什么run方法call方法，但却有一大堆任务的检查方法。方法名字都很能体现方法作用了，至于详细的，自己看下注释就能知道了。
所以说
- 单独实现Runnable的任务只是埋头做，也不管什么时候做完，做没做完
- 单独实现Callable的任务可以做完回一声你，如果做错了还能告诉你。
- Future因为没有run方法call方法，所以会与Runnable和Callable配合，增强功能。
FutureTask
public class FutureTask<V> implements RunnableFuture<V> {
实现了RunnableFuture
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继承了Future，和Runnable，所以说它和FutureTask都能够用作线程任务，而且验证了我们上面的说法。
我们看到RunnableFuture是个接口所以并没有实现run方法，交给我们的FutureTask实现了：
public void run() {
if (state != NEW ||
!U.compareAndSwapObject(this, RUNNER, 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);
}
}
可以发现它又跑去执行Callable的call()方法,这个Callable是通过构造方法传递进来的(当然也可以传runnable，人家自己会帮你转成callable)，所以说这个call()方法是我们需要手动实现的方法
看看我们平常怎么用FutureTask的吧
public class MyClass {
public static void main(String[] args) {
ExecutorService es = Executors.newSingleThreadScheduledExecutor();
MyCallable myCallable = new MyCallable();
FutureTask<String> myFutureTask = new FutureTask<String>(myCallable) {
@Override
public boolean cancel(boolean b) {
return false;
}
@Override
public boolean isCancelled() {
return false;
}
@Override
protected void done() {
super.done();
}
@Override
public boolean isDone() {
return false;
}
@Override
public String get() throws InterruptedException, ExecutionException {
return null;
}
@Override
public String get(long l, TimeUnit timeUnit) throws InterruptedException, ExecutionException, TimeoutException {
return null;
}
};
es.submit(myFutureTask);
myFutureTask.cancel(true);
}
}
class MyCallable implements Callable<String> {
@Override
public String call() throws Exception {
System.out.println("正在执行");
return "执行完毕";
}
}
FutureTask代码就几百行可以很快看完，大家去了解以下吧，这样更能加深记忆哦。有什么不好的地方请指出哦。
