Java并发编程核心方法与框架-CompletionService的使用

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<String> {
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<Callable<String>> 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<Runnable> 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<String> completionService = new ExecutorCompletionService<String>(executorService);
for (int i = 0; i < 10; i++) {
completionService.submit(new Callable<String>() {

@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<String> completionService = new ExecutorCompletionService<String>(executorService);
completionService.submit(new Callable<String>() {

@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<String> {

@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<String> {

@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<String> 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<String> 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<String> 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<String> 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<String> 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。