Callable可以看成是一个增强版的Runnable, 带返回结果, 需要通过Future或者FutureTask来提交任务或运行线程, 然后通过Future/FutureTask的get方法得到返回结果.
Callable在子线程中运行, 在主线程中异步得到执行结果(get()方法是阻塞的), 或者检查是否已取消, 是否已完成(检查取消和完成的方法是非阻塞的)
通过Thread子线程启动
这种方式, 需要创建一个FutureTask对象, 再用这个FutureTask对象创建一个Thread来运行. 后续操作都通过FutureTask进行.
public class DemoCallableFuture {
public static void main(String[] args) {
FutureTask<String> task = new FutureTask<>(()->{
System.out.println("task start");
Thread.sleep(1000);
System.out.println("task done");
return "task get";
});
new Thread(task).start();
FutureTask<String> task2 = new FutureTask<>(()->{
System.out.println("task2 start");
Thread.sleep(1000);
System.out.println("task2 done");
return "task2 get";
});
new Thread(task2).start();
if (task.isCancelled()) {
System.out.println("task cancelled yes");
} else {
System.out.println("task cancelled no");
}
if (task.isDone()) {
System.out.println("task done yes");
} else {
System.out.println("task done no");
}
try {
System.out.println(task.get());
} catch (InterruptedException|ExecutionException e) {
e.printStackTrace();
}
if (task2.isCancelled()) {
System.out.println("task2 cancelled yes");
} else {
System.out.println("task2 cancelled no");
}
if (task2.isDone()) {
System.out.println("task2 done yes");
} else {
System.out.println("task2 done no");
}
try {
System.out.println(task2.get());
} catch (InterruptedException|ExecutionException e) {
e.printStackTrace();
}
}
}
运行结果
task start task cancelled no task done no task2 start task2 done task done task get task2 cancelled no task2 done yes task2 get
通过ExecutorService线程池启动
这种方式, 通过线程池submit一个Callable对象, 就会得到一个Future对象, 根据这个Future对象做后续操作
public class DemoCallableFuture2 {
public static void main(String[] args) {
ExecutorService service = Executors.newFixedThreadPool(4);
Future<String> future = service.submit(()->{
System.out.println("task start");
Thread.sleep(1000);
System.out.println("task done");
return "task get";
});
if (future.isCancelled()) {
System.out.println("task cancelled yes");
} else {
System.out.println("task cancelled no");
}
if (future.isDone()) {
System.out.println("task done yes");
} else {
System.out.println("task done no");
}
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (future.isCancelled()) {
System.out.println("task cancelled yes");
} else {
System.out.println("task cancelled no");
}
if (future.isDone()) {
System.out.println("task done yes");
} else {
System.out.println("task done no");
}
try {
System.out.println(future.get());
} catch (InterruptedException|ExecutionException e) {
e.printStackTrace();
}
}
}
运行结果
task cancelled no task done no task start task cancelled no task done no task done task get
.
FutureCallback
FutureCallback是Google Guava中的一个类, 解决的是Future中get阻塞的问题, 让全过程异步. 需要使用ListeningExecutorService的线程池提交.
代码例子
public class DemoFutureCallback {
public static void main(String[] args) {
ListeningExecutorService service = MoreExecutors.listeningDecorator(Executors.newFixedThreadPool(10));
for (int i = 0; i < 5; i++) {
int j = i;
ListenableFuture<String> future = service.submit(()->{
System.out.println("task start");
Thread.sleep(1000);
System.out.println("task done");
return "task return " + j;
});
Futures.addCallback(future, new FutureCallback<String>() {
@Override
public void onSuccess(String s) {
System.out.println("callback success: " + s);
}
@Override
public void onFailure(Throwable throwable) {
throwable.printStackTrace();
}
});
}
System.out.println("thread ongoing");
service.shutdown();
}
}
运行结果
task start task start task start thread ongoing task start task start task done callback success: task return 0 task done callback success: task return 1 task done callback success: task return 3 task done callback success: task return 4 task done callback success: task return 2 Process finished with exit code 0