本篇就是個小的demo，看到了覺得有意思就敲了敲代碼

問題描述：

a,b,c,d四個盤，分別統計出大小，最後要求得四個盤總的大小的值。

首先想到的是CountDownLatch這個類，在所有單個盤的大小統計完成之後，再進行總和的計算。之後，我們需要一個DiskMemory類，抽象盤的大小的統計。有個size和totalSize，大概就這些。啟動線程時，啟動4次，每次分別計算單個的size，最後匯總。

代碼如下：

 1 import java.util.Random;
 2 
 3 public class DiskMemory {
 4 
 5     private static
 
 int totalSize;
 6     
 7     public int getSize() {
 8         return new Random().nextInt(5) + 1;
 9     }
10     
11     public void setSize(int size) {
12         // synchronized method is must
13         synchronized (this) {
14             System.out.println("-------first totalSize: " + totalSize);
 
15             totalSize = totalSize + size;
16             System.out.println("-------end totalSize: " + totalSize);
17         }
18         
19     }
20     
21     public int getTotalSize() {
22         return totalSize;
23     }
24 }

 1 import java.util.concurrent.CountDownLatch;
 2 import
 
 java.util.concurrent.ExecutorService;
 3 import java.util.concurrent.Executors;
 4 import java.util.concurrent.TimeUnit;
 5 
 6 public class CountSizeDemo {
 7 
 8     public static void main(String[] args) {
 9 
10         CountDownLatch countDownLatch = new CountDownLatch(4);
11         DiskMemory diskMemory = new DiskMemory();
12         ExecutorService executorService = Executors.newCachedThreadPool();
13         for (int i = 0; i < 4; i++) {
14 
15             executorService.execute(() -> {
16                 try {
17                     TimeUnit.MICROSECONDS.sleep(1000);
18                 } catch (InterruptedException ex) {
19                     ex.printStackTrace();
20                 }
21                 int size = diskMemory.getSize();
22                 System.out.println("get size: " + size);
23                 diskMemory.setSize(size);
24                 countDownLatch.countDown();
25             });
26         }
27         try {
28             countDownLatch.await();
29         } catch (InterruptedException e) {
30             // TODO Auto-generated catch block
31             e.printStackTrace();
32         }
33         System.out.println(diskMemory.getTotalSize());
34         executorService.shutdownNow();
35     }
36 
37 }

執行結果：

 1 get size: 3
 2 get size: 4
 3 get size: 5
 4 get size: 5
 5 -------first totalSize: 0
 6 -------end totalSize: 3
 7 -------first totalSize: 3
 8 -------end totalSize: 8
 9 -------first totalSize: 8
10 -------end totalSize: 12
11 -------first totalSize: 12
12 -------end totalSize: 17
13 17

並發（三）