Future接口是Java標准API的一部分，在java.util.concurrent包中。Future接口是Java線程Future模式的實現，可以來進行異步計算。
有了Future就可以進行三段式的編程了，1.啟動多線程任務2.處理其他事3.收集多線程任務結果。從而實現了非阻塞的任務調用。在途中遇到一個問題，那就是雖然能異步獲取結果，但是Future的結果需要通過isdone來判斷是否有結果，或者使用get()函數來阻塞式獲取執行結果。這樣就不能實時跟蹤其他線程的結果狀態了，所以直接使用get還是要慎用，最好配合isdone來使用。如果直接使用get來獲取結果, get是阻塞操作, 相當於獲取結果仍然是同步.因此在獲取結果的時候需要注意.
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;
/**
* 多線程執行，異步獲取結果
*
* @author i-clarechen
*
*/
public class AsyncThread {
public static void main(String[] args) {
AsyncThread t = new AsyncThread();
List<Future<String>> futureList = new ArrayList<Future<String>>();
t.generate(3, futureList);
t.doOtherThings();
t.getResult(futureList);
}
/**
* 生成指定數量的線程，都放入future數組
*
* @param threadNum
* @param fList
*/
public void generate(int threadNum, List<Future<String>> fList) {
ExecutorService service = Executors.newFixedThreadPool(threadNum);
for (int i = 0; i < threadNum; i++) {
Future<String> f = service.submit(getJob(i));
fList.add(f);
}
service.shutdown();
}
/**
* other things
*/
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();
}
}
/**
* 從future中獲取線程結果，打印結果
*
* @param fList
*/
public void getResult(List<Future<String>> fList) {
ExecutorService service = Executors.newSingleThreadExecutor();
service.execute(getCollectJob(fList));
service.shutdown();
}
/**
* 生成指定序號的線程對象
*
* @param i
* @return
*/
public Callable<String> getJob(final int i) {
final int time = new Random().nextInt(10);
return new Callable<String>() {
@Override
public String call() throws Exception {
Thread.sleep(1000 * time);
return "thread-" + i;
}
};
}
/**
* 生成結果收集線程對象
*
* @param fList
* @return
*/
public Runnable getCollectJob(final List<Future<String>> fList) {
return new Runnable() {
public void run() {
for (Future<String> 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();
}
}
}
};
}
}
優化獲取方式:
List<Object> results = null; int retryLimit = 3; long delay = 10; int tryTime = 0; for(Future future : futures) do { try { //此處原理: results = future.get(); } catch (Exception e) { results = null; } tryTime++; if (results == null) { try { Thread.sleep(delay << tryTime); } catch (InterruptedException e) { } } } while (results == null && tryTime < retryLimit); } return results;運行結果打印和future放入列表時的順序一致，為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
使用CompletionService，它內部添加了阻塞隊列，從而獲取future中的值，然后根據返回值做對應的處理
下面是先執行完的線程先處理的方案：
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();
}
}
/**
* 使用completionService收集callable結果
* @throws ExecutionException
* @throws InterruptedException
*/
public static void completionServiceCount() throws InterruptedException, ExecutionException {
ExecutorService executorService = Executors.newCachedThreadPool();
CompletionService<Integer> completionService = new ExecutorCompletionService<Integer>(
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<threadNum;i++){
temp = completionService.take().get();
sum += temp;
System.out.print(temp + "\t");
}
System.out.println("CompletionService all is : " + sum);
executorService.shutdown();
}
public static Callable<Integer> getTask(final int no) {
final Random rand = new Random();
Callable<Integer> task = new Callable<Integer>() {
@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;
}
}
運行結果為最先結束的線程結果先被處理：
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 2 0 1 4 3 CompletionService all is : 10