设计模式（一）单例模式：4-静态代码块模式

 1 public final class StaticBlockSingleton {
 2 
 3     private static final StaticBlockSingleton instance;
 4 
 5     private StaticBlockSingleton() {
 6         if (instance != null) {
 7             throw new IllegalStateException();
 8         }
 9     }
10 
11     static {
12         instance = new StaticBlockSingleton();
13     }
14 
15     public static final StaticBlockSingleton getInstance() {
16         return instance;
17     }
18 
19 }