接口CompletionService的功能是以异步的方式一边生产新的任务，一边处理已完成任务的结果，这样可以将执行任务与处理任务分离。使用submit()执行任务，使用take取得已完成的任务，并按照这些任务的时间顺序处理他们的结果。

使用CompletionService解决Future的缺点

public class MyCallable implements Callable {

private String username;

private long sleepValue;

public MyCallable(String username, long sleepValue) {

super();

this.username = username;

this.sleepValue = sleepValue;

}

@Override

public String call() throws Exception {

System.out.println(username + " " + Thread.currentThread().getName() + System.currentTimeMillis());

Thread.sleep(sleepValue);

return "return " + username;

}

public static void main(String[] args) {

try {

MyCallable callable1 = new MyCallable("username1", 5000);

MyCallable callable2 = new MyCallable("username2", 4000);

MyCallable callable3 = new MyCallable("username3", 3000);

MyCallable callable4 = new MyCallable("username4", 2000);

MyCallable callable5 = new MyCallable("username5", 1000);

List> callables = new ArrayList<>();

callables.add(callable1);

callables.add(callable2);

callables.add(callable3);

callables.add(callable4);

callables.add(callable5);

int corePoolSize = 5;

int maximumPoolSize = 10;

int keepAliveTime = 5;

TimeUnit unit = TimeUnit.SECONDS;

LinkedBlockingDeque workQueue = new LinkedBlockingDeque<>();

ThreadPoolExecutor executor = new ThreadPoolExecutor(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);

CompletionService completionService = new ExecutorCompletionService<>(executor);

for (int i = 0; i < 5; i++) {

completionService.submit(callables.get(i));

}

for (int i = 0; i < 5; i++) {

//take方法：获取并移除表示下一个已完成任务的Future，如果目前不存在这样的任务，则等待。

System.out.println(completionService.take().get() + " -- " + System.currentTimeMillis());

}

} catch (Exception e) {

e.printStackTrace();

}

}

}

控制台打印结果如下：

username2 pool-1-thread-21470920687933

username4 pool-1-thread-41470920687934

username3 pool-1-thread-31470920687933

username5 pool-1-thread-51470920687934

username1 pool-1-thread-11470920687933

return username5 -- 1470920688939

return username4 -- 1470920689937

return username3 -- 1470920690938

return username2 -- 1470920691938

return username1 -- 1470920692938

从打印结果来看，CompletionService解决了Future阻塞的缺点，哪个任务先执行完，哪个任务就先返回值。在CompletionService接口中如果当前没有任务执行完，则completionService.take().get()方法还是呈阻塞特性。

take()方法

public class Run {

public static void main(String[] args) {

try {

ExecutorService executorService = Executors.newCachedThreadPool();

CompletionService completionService = new ExecutorCompletionService(executorService);

for (int i = 0; i < 10; i++) {

completionService.submit(new Callable() {

@Override

public String call() throws Exception {

long sleepValue = (int)(Math.random() * 1000);

System.out.println("sleep=" + sleepValue + " " + Thread.currentThread().getName());

Thread.sleep(sleepValue);

return sleepValue + "-" + Thread.currentThread().getName();

}

});

}

for (int i = 0; i < 10; i++) {

//take()方法是按照任务执行的速度，从快到慢获得Future对象

System.out.println(completionService.take().get());

}

} catch (Exception e) {

e.printStackTrace();

}

}

}

打印结果如下：

sleep=662 pool-1-thread-2

sleep=476 pool-1-thread-6

sleep=977 pool-1-thread-5

sleep=175 pool-1-thread-7

sleep=461 pool-1-thread-4

sleep=836 pool-1-thread-3

sleep=267 pool-1-thread-1

sleep=82 pool-1-thread-8

sleep=714 pool-1-thread-9

sleep=946 pool-1-thread-10

82-pool-1-thread-8

175-pool-1-thread-7

267-pool-1-thread-1

461-pool-1-thread-4

476-pool-1-thread-6

662-pool-1-thread-2

714-pool-1-thread-9

836-pool-1-thread-3

946-pool-1-thread-10

977-pool-1-thread-5

take()方法是按照任务执行的速度，从快到慢获得Future对象

poll()方法

public class Run1 {

public static void main(String[] args) {

ExecutorService executorService = Executors.newCachedThreadPool();

CompletionService completionService = new ExecutorCompletionService(executorService);

completionService.submit(new Callable() {

@Override

public String call() throws Exception {

TimeUnit.SECONDS.sleep(3);

System.out.println("3秒过了");

return "3S";

}

});

//poll()方法获取并移除表示下一个已完成任务的Future，

//如果不存在这样的任务则返回null，无阻塞效果

System.out.println(completionService.poll());

}

}

打印结果如下：

null

3秒过了

方法poll()返回的Future为null，因为当前没有任何已完成任务的Future对象。poll()不像take()方法具有阻塞的效果。

类CompletionService与异常

public class MyCalableA implements Callable {

@Override

public String call() throws Exception {

System.out.println("MyCalableA begin," + System.currentTimeMillis());

TimeUnit.SECONDS.sleep(1);

System.out.println("MyCalableA end," + System.currentTimeMillis());

return "MyCalableA";

}

}

public class MyCalableB implements Callable {

@Override

public String call() throws Exception {

System.out.println("MyCalableB begin," + System.currentTimeMillis());

TimeUnit.SECONDS.sleep(2);

int i = 0;

if(i == 0){

throw new Exception("异常...");

}

System.out.println("MyCalableB end," + System.currentTimeMillis());

return "MyCalableB";

}

}

public class Main {

public static void main(String[] args) {

try {

MyCalableA myCalableA = new MyCalableA();

MyCalableB myCalableB = new MyCalableB();

Executor executor = Executors.newSingleThreadExecutor();

CompletionService completionService = new ExecutorCompletionService<>(executor);

completionService.submit(myCalableA);

completionService.submit(myCalableB);

for (int i = 0; i < 2; i++) {

System.out.println("----" + completionService.take());

}

System.out.println("main end");

} catch (Exception e) {

e.printStackTrace();

}

}

}

控制台打印结果如下：

MyCalableA begin,1471006680464

MyCalableA end,1471006682467

MyCalableB begin,1471006682467

----java.util.concurrent.FutureTask@3918d722

----java.util.concurrent.FutureTask@dd41677

main end

MyCalableB类中虽然跑出了异常，但是并没有调用GutureTask类的get()方法，所以不出现异常。

对以上代码做如下修改：

for (int i = 0; i < 2; i++) {

System.out.println("----" + completionService.take().get());

}

此时运行结果如下：

MyCalableA begin,1471007865642

MyCalableA end,1471007866647

MyCalableB begin,1471007866647

----MyCalableA

java.util.concurrent.ExecutionException: java.lang.Exception: 异常...

at java.util.concurrent.FutureTask.report(FutureTask.java:122)

at java.util.concurrent.FutureTask.get(FutureTask.java:188)

at com.concurrent.chapter6.concurrent02.Main.main(Main.java:19)

Caused by: java.lang.Exception: 异常...

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:14)

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:1)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at java.lang.Thread.run(Thread.java:745)

MyCalableA先执行完，未抛出异常。MyCalableB线程抛出异常。

对以上代码做如下修改：

public class Main {

public static void main(String[] args) {

try {

MyCalableA myCalableA = new MyCalableA();

MyCalableB myCalableB = new MyCalableB();

Executor executor = Executors.newSingleThreadExecutor();//单线程

CompletionService completionService = new ExecutorCompletionService<>(executor);

completionService.submit(myCalableB);//先执行B任务

completionService.submit(myCalableA);

for (int i = 0; i < 2; i++) {

System.out.println("----" + completionService.take().get());

}

System.out.println("main end");

} catch (Exception e) {

e.printStackTrace();

}

}

}

此时控制台输出结果如下：

MyCalableB begin,1471008229367

MyCalableA begin,1471008231370

java.util.concurrent.ExecutionException: java.lang.Exception: 异常...

at java.util.concurrent.FutureTask.report(FutureTask.java:122)

at java.util.concurrent.FutureTask.get(FutureTask.java:188)

at com.concurrent.chapter6.concurrent02.Main.main(Main.java:19)

Caused by: java.lang.Exception: 异常...

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:14)

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:1)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at java.lang.Thread.run(Thread.java:745)

MyCalableA end,1471008232373

此时B任务抛出异常，A任务执行完但未返回值。

对以上代码继续做如下修改：

public class Main {

public static void main(String[] args) {

try {

MyCalableA myCalableA = new MyCalableA();

MyCalableB myCalableB = new MyCalableB();

Executor executor = Executors.newSingleThreadExecutor();//单线程

CompletionService completionService = new ExecutorCompletionService<>(executor);

completionService.submit(myCalableA);//先执行A任务

completionService.submit(myCalableB);

for (int i = 0; i < 2; i++) {

System.out.println("----" + completionService.poll());

}

TimeUnit.SECONDS.sleep(5);

System.out.println("A处：" + completionService.poll());

System.out.println("B处：" + completionService.poll());

System.out.println("main end");

} catch (Exception e) {

e.printStackTrace();

}

}

}

此时运行结果如下：

----null

----null

MyCalableA begin,1471009161165

MyCalableA end,1471009162166

MyCalableB begin,1471009162166

A处：java.util.concurrent.FutureTask@5f0ee5b8

B处：java.util.concurrent.FutureTask@4b0bc3c9

main end

未调用get()方法，未抛出异常。

继续修改以上代码：

public class Main {

public static void main(String[] args) {

try {

MyCalableA myCalableA = new MyCalableA();

MyCalableB myCalableB = new MyCalableB();

Executor executor = Executors.newSingleThreadExecutor();//单线程

CompletionService completionService = new ExecutorCompletionService<>(executor);

completionService.submit(myCalableA);//先执行A任务

completionService.submit(myCalableB);

for (int i = 0; i < 2; i++) {

System.out.println("----" + completionService.poll());

}

TimeUnit.SECONDS.sleep(5);

System.out.println("A处：" + completionService.poll().get());

System.out.println("B处：" + completionService.poll().get());

System.out.println("main end");

} catch (Exception e) {

e.printStackTrace();

}

}

}

控制台打印结果如下：

----null

----null

MyCalableA begin,1471009511872

MyCalableA end,1471009512876

MyCalableB begin,1471009512877

A处：MyCalableA

java.util.concurrent.ExecutionException: java.lang.Exception: 异常...

at java.util.concurrent.FutureTask.report(FutureTask.java:122)

at java.util.concurrent.FutureTask.get(FutureTask.java:188)

at com.concurrent.chapter6.concurrent02.Main.main(Main.java:24)

Caused by: java.lang.Exception: 异常...

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:14)

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:1)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at java.lang.Thread.run(Thread.java:745)

此时A任务有返回值。B任务抛出异常，无返回值。

继续修改以上代码：

public class Main {

public static void main(String[] args) {

try {

MyCalableA myCalableA = new MyCalableA();

MyCalableB myCalableB = new MyCalableB();

Executor executor = Executors.newSingleThreadExecutor();//单线程

CompletionService completionService = new ExecutorCompletionService<>(executor);

completionService.submit(myCalableB);//先执行B任务

completionService.submit(myCalableA);

for (int i = 0; i < 2; i++) {

System.out.println("----" + completionService.poll());

}

TimeUnit.SECONDS.sleep(5);

System.out.println("A处：" + completionService.poll().get());

System.out.println("B处：" + completionService.poll().get());

System.out.println("main end");

} catch (Exception e) {

e.printStackTrace();

}

}

}

运行结果如下：

----null

----null

MyCalableB begin,1471009732036

MyCalableA begin,1471009734037

MyCalableA end,1471009735037

java.util.concurrent.ExecutionException: java.lang.Exception: 异常...

at java.util.concurrent.FutureTask.report(FutureTask.java:122)

at java.util.concurrent.FutureTask.get(FutureTask.java:188)

at com.concurrent.chapter6.concurrent02.Main.main(Main.java:23)

Caused by: java.lang.Exception: 异常...

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:14)

at com.concurrent.chapter6.concurrent02.MyCalableB.call(MyCalableB.java:1)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)

at java.util.concurrent.FutureTask.run(FutureTask.java:262)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at java.lang.Thread.run(Thread.java:745)

此时任务B抛出异常，任务A未打印。