Future接口是Java标准API的一部分，在java.util.concurrent包中。Future接口是Java线程Future模式的实现，可以来进行异步计算。
有了Future就可以进行三段式的编程了，1.启动多线程任务2.处理其他事3.收集多线程任务结果。从而实现了非阻塞的任务调用。在途中遇到一个问题，那就是虽然能异步获取结果，但是Future的结果需要通过isdone来判断是否有结果，或者使用get()函数来阻塞式获取执行结果。这样就不能实时跟踪其他线程的结果状态了，所以直接使用get还是要慎用，最好配合isdone来使用。
这里有一种更好的方式来实现对任意一个线程运行完成后的结果都能及时获取的办法：使用CompletionService，它内部添加了阻塞队列，从而获取future中的值，然后根据返回值做对应的处理。一般future使用和CompletionService使用的两个测试案例如下：
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class AsyncThread {
public static void main(String[] args) {
AsyncThread t = new AsyncThread();
List> futureList = new ArrayList>();
t.generate(3, futureList);
t.doOtherThings();
t.getResult(futureList);
}
public void generate(int threadNum, List> fList) {
ExecutorService service = Executors.newFixedThreadPool(threadNum);
for (int i = 0; i < threadNum; i++) {
Future f = service.submit(getJob(i));
fList.add(f);
}
service.shutdown();
}
public void doOtherThings() {
try {
for (int i = 0; i < 3; i++) {
System.out.println("do thing no:" + i);
Thread.sleep(1000 * (new Random().nextInt(10)));
}
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public void getResult(List> fList) {
ExecutorService service = Executors.newSingleThreadExecutor();
service.execute(getCollectJob(fList));
service.shutdown();
}
public Callable getJob(final int i) {
final int time = new Random().nextInt(10);
return new Callable() {
@Override
public String call() throws Exception {
Thread.sleep(1000 * time);
return "thread-" + i;
}
};
}
public Runnable getCollectJob(final List> fList) {
return new Runnable() {
public void run() {
for (Future future : fList) {
try {
while (true) {
if (future.isDone() && !future.isCancelled()) {
System.out.println("Future:" + future
+ ",Result:" + future.get());
break;
} else {
Thread.sleep(1000);
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
};
}
}
运行结果打印和future放入列表时的顺序一致，为0，1，2：
do thing no:0 do thing no:1 do thing no:2 Future:java.util.concurrent.FutureTask@68e1ca74,Result:thread-0 Future:java.util.concurrent.FutureTask@3fb2bb77,Result:thread-1 Future:java.util.concurrent.FutureTask@6f31a24c,Result:thread-2
下面是先执行完的线程先处理的方案：
import java.util.Random;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
import java.util.concurrent.linkedBlockingDeque;
public class testCallable {
public static void main(String[] args) {
try {
completionServiceCount();
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
}
public static void completionServiceCount() throws InterruptedException, ExecutionException {
ExecutorService executorService = Executors.newCachedThreadPool();
CompletionService completionService = new ExecutorCompletionService(
executorService);
int threadNum = 5;
for (int i = 0; i < threadNum; i++) {
completionService.submit(getTask(i));
}
int sum = 0;
int temp = 0;
for(int i=0;i getTask(final int no) {
final Random rand = new Random();
Callable task = new Callable() {
@Override
public Integer call() throws Exception {
int time = rand.nextInt(100)*100;
System.out.println("thead:"+no+" time is:"+time);
Thread.sleep(time);
return no;
}
};
return task;
}
}
运行结果为最先结束的线程结果先被处理：
thead:0 time is:4200 thead:1 time is:6900 thead:2 time is:2900 thead:3 time is:9000 thead:4 time is:7100 0 1 4 3 CompletionService all is : 10
总结
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