public class FutureTaskJobCallable implements Callable<String>{
public String call() throws Exception {
System.out.println("FutureTaskJobCallable已经执行了哦");
Thread.sleep(1000);
return "返回结果";
}
}
public class FutureTaskJobRunnable implements Runnable {
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println("FutureTaskJobRunnable已经执行了哦");
}
}
根据上面的代码我们从ExecutorService接口中submit方法入手，看下AbstractExecutorService类对submit方法的具体实现。
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;
}
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
return new FutureTask<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
return new FutureTask<T>(callable);
}
可以看到当你使用submit方法提交任务时，都会通过newTaskFor方法转换成FutureTask对象，所以我们具体分析下上面代码中的三种情况：
1、如果你传入的是自己实现的Runaable类或者Callable类，那么sumbit方法自然会帮你自动封装为FutureTask对象，运行后通过Future对象获取结果。
2、你传入的已经是个自己构造的FutureTask对象，由于FutureTask其实是实现了Runnable接口的，它本身就是个Runaable实现类， sumbit方法还是会将它视为Runnable类来进行封装，并最终会执行FutureTask自己的run方法，一系列实现都在你传入的FutureTask对象内完成，所以你可以直接通过自己构建的FutureTask获取结果；
3、自己单独声明线程运行，跟第2点类似，FutureTask本身就是个Runnabel实现类，自然可以做为参数传入Thread运行；
那么我们把自定义的Runnable、Callable实现类做为参数构造FutureTask后，FuttureTask是如何运行的呢，我们可以看下FuttureTask中具体的代码实现
//你传入的Runnable与Callable实现类都会在构造函数中转化为Callable
private Callable<V> callable;
public void run() {
if (state != NEW ||
!UNSAFE.compareAndSwapObject(this, runnerOffset,
null, Thread.currentThread()))
return;
try {
Callable<V> c = callable;//你传入的实现类
if (c != null && state == NEW) {
V result;//返回值
boolean ran;
try {
result = c.call();//运行后返回结果
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);
}
}
可以看到FutureTask类本身的run方法，就是执行Runnable、Callable的实现类并获取返回结果的过程。