Future介面是Java標準API的一部分，在java.util.concurrent包中。Future介面是Java執行緒Future模式的實現，可以來進行非同步計算。

有了Future就可以進行三段式的程式設計了，1.啟動多執行緒任務2.處理其他事3.收集多執行緒任務結果。從而實現了非阻塞的任務呼叫。在途中遇到一個問題，那就是雖然能非同步獲取結果，但是Future的結果需要通過isdone來判斷是否有結果，或者使用get()函式來阻塞式獲取執行結果。這樣就不能實時跟蹤其他執行緒的結果狀態了，所以直接使用get還是要慎用，最好配合isdone來使用。

這裡有一種更好的方式來實現對任意一個執行緒執行完成後的結果都能及時獲取的辦法：使用CompletionService，它內部添加了阻塞佇列，從而獲取future中的值，然後根據返回值做對應的處理。一般future使用和CompletionService使用的兩個測試案例如下：

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;

/**
 * 多執行緒執行，非同步獲取結果
 * 
 * @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);
    }

    /**
     * 生成指定數量的執行緒，都放入future陣列
     * 
     * @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中獲取執行緒結果，列印結果
     * 
     * @param fList
     */
    public void getResult(List<Future<String>> fList) {
        ExecutorService service = Executors.newSingleThreadExecutor();
        service.execute(getCollectJob(fList));
        service.shutdown();
    }

    /**
     * 生成指定序號的執行緒物件
     * 
     * @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;
            }
        };
    }

    /**
     * 生成結果收集執行緒物件
     * 
     * @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();
                    }
                }
            }
        };
    }

}

執行結果列印和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:[email protected],Result:thread-0
Future:[email protected],Result:thread-1
Future:[email protected],Result:thread-2

下面是先執行完的執行緒先處理的方案：

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;
    }
}

執行結果為最先結束的執行緒結果先被處理：

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