图中 Futures 就好比居民楼一层的信箱（每个信箱有房间编号），左侧的 t0，t2，t4 就好比等待邮件的居民，右侧的 t1，t3，t5 就好比邮递员
    如果需要在多个类之间使用 GuardedObject 对象，作为参数传递不是很方便，因此设计一个用来解耦的中间类，这样不仅能够解耦【结果等待者】和【结果生产者】，还能够同时支持多个任务的管理

    image.png

    1. public class TestGuardedObjectV3 {
    2.     public static void main(String[] args) {
    3.         for (int i = 0; i < 3; i++) {
    4.             GuardedObjectV3 v3 = Fetures.createFeture();
    6.             new Thread(() -> {
    7.                 log.debug("waiting id({})...", v3.getId());
    8.                 log.debug("get response id({}): [{}] lines", v3.getId(), ((List<String>) v3.get()).size());
    9.             }).start();
    11.             new Thread(() -> {
    12.                 try {
    13.                     List<String> lines = download();
    14.                     log.debug("download complete id({})...", v3.getId());
    15.                     Fetures.complete(v3.getId(), lines);
    16.                 } catch (IOException e) {
    17.                     e.printStackTrace();
    18.                 }
    19.             }).start();
    21.         }
    22.     }
    23. }
    25. class Fetures {
    26.     private static final ConcurrentHashMap<Integer, GuardedObjectV3> FETURES = new ConcurrentHashMap<>();
    27.     private static final AtomicInteger ID_GENERATOR = new AtomicInteger();
    29.     public static GuardedObjectV3 createFeture() {
    30.         // 为每个 GuardedObject 分配一个 id
    31.         int id = ID_GENERATOR.incrementAndGet();
    32.         GuardedObjectV3 v3 = new GuardedObjectV3(id);
    33.         // 放入公共位置，将来异步响应返回时，根据编号获取
    34.         FETURES.put(id, v3);
    35.         return v3;
    36.     }
    38.     public static void complete(int id, Object response) {
    39.         // 异步响应完成，根据编号获取并移除
    40.         GuardedObjectV3 v3 = FETURES.remove(id);
    41.         if (v3 != null) {
    42.             v3.complete(response);
    43.         }
    44.     }
    45. }
    48. /**
    49.  * 添加多任务处理
    50.  */
    51. class GuardedObjectV3 {
    53.     private int id;
    54.     private Object response;
    55.     private final Object lock = new Object();
    58.     public GuardedObjectV3(int id) {
    59.         this.id = id;
    60.     }
    62.     public int getId() {
    63.         return id;
    64.     }
    66.     public Object get() {
    67.         synchronized (lock) {
    68.             while (response == null) {
    69.                 try {
    70.                     lock.wait();
    71.                 } catch (InterruptedException e) {
    72.                     e.printStackTrace();
    73.                 }
    74.             }
    75.             return response;
    76.         }
    77.     }
    79.     public void complete(Object response) {
    80.         synchronized (lock) {
    81.             this.response = response;
    82.             lock.notifyAll();
    83.         }
    84.     }
    85. }