今天碰到一个需要获取线程返回结果的业务场景，所以了解到了Callable接口。
先来看下下面这个例子：
public class ThreadTest {
public static void main(String[] args) throws Exception {
ExecutorService exc = Executors.newCachedThreadPool();
try {
String result = null;
FutureTask<String> task = (FutureTask<String>) exc.submit(new Runnable() {
@Override
public void run() {
for (int i = 0; i <; i++) {
try {
Thread.sleep(100L);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(this.getClass() + "::线程执行中.." + i);
}
}
}, result);
System.out.println("task return value:" + task.get());
FutureTask<String> callableTask = (FutureTask<String>) exc.submit(new Callable<String>() {
@Override
public String call() throws InterruptedException {
for (int i = 0; i <; i++) {
Thread.sleep(100L);
System.out.println(this.getClass() + "::线程执行中.." + i);
}
return "success";
}
});
System.out.println("提前出结果了 task return value:" + task.get());
System.out.println("callableTask return value:" + callableTask.get());
} finally {
exc.shutdown();
}
}
}
运行结果如下：
class thread.ThreadTest$1::线程执行中..0
class thread.ThreadTest$1::线程执行中..1
class thread.ThreadTest$1::线程执行中..2
class thread.ThreadTest$1::线程执行中..3
class thread.ThreadTest$1::线程执行中..4
class thread.ThreadTest$1::线程执行中..5
class thread.ThreadTest$1::线程执行中..6
class thread.ThreadTest$1::线程执行中..7
class thread.ThreadTest$1::线程执行中..8
class thread.ThreadTest$1::线程执行中..9
task return value:null
提前出结果了 task return value:null
class thread.ThreadTest$2::线程执行中..0
class thread.ThreadTest$2::线程执行中..1
class thread.ThreadTest$2::线程执行中..2
class thread.ThreadTest$2::线程执行中..3
class thread.ThreadTest$2::线程执行中..4
class thread.ThreadTest$2::线程执行中..5
class thread.ThreadTest$2::线程执行中..6
class thread.ThreadTest$2::线程执行中..7
class thread.ThreadTest$2::线程执行中..8
class thread.ThreadTest$2::线程执行中..9
callableTask return value:success
可以得到以下几点：
1 Runnable，Callable两个接口方法体不一样，前者为run，后者为call，且返回值也不一样；
2 Runnable接口由于run方法返回void所以无法解决线程成功后返回相应结果的问题；但是实现Callable接口的线程类可以，因为Callable的执行方法体call方法
可以返回对象。
3 由于runnable接口没有返回值，所以FutureTask为了解决此问题将runnable线程类通过支配器转换为callable线程。
4 当通过task对象调用get方法时，已经执行完成的现成可以立刻得到返回结果，但是还没执行完的线程一直在等待。
下面进入源码看看：
线程池执行submit方法时进入AbstractExecutorService类中的submit
public <T> Future<T> submit(Callable<T> task) {
if (task == null) throw new NullPointerException();
RunnableFuture<T> ftask = newTaskFor(task);
execute(ftask);
return ftask;
}
这里好理解，将线程放入任务，由线程池的execute方法去执行。
执行完成后，当调用get方法时，会进入FutureTask的get方法：
public V get() throws InterruptedException, ExecutionException {
int s = state;
//当线程状态为新建活着执行中时一直调用awaitDone方法
if (s <= COMPLETING)
//循环判断线程状态是否已经执行成功，如果执行成功返回线程状态；其中还包括线程取消，中断等情况的判断。可参见下方源码。
//所以这里便是上面例子中为什么线程执行成功后即可立即得到结果，如果还没有执行成功
s = awaitDone(false, 0L);
//线程状态正常返回结果
return report(s);
}
awaitDone源码
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);
}
}
@SuppressWarnings("unchecked")
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);
}
然后我们来看看FutureTask是如何对runnable线程进行转换的。代码也很简单：
public FutureTask(Runnable runnable, V result) {
this.callable = Executors.callable(runnable, result);
this.state = NEW; // ensure visibility of callable
}
public static <T> Callable<T> callable(Runnable task, T result) {
if (task == null)
throw new NullPointerException();
return new RunnableAdapter<T>(task, 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;
}
}