### 1.内部静态类
内部静态类即使存在多个类加载器的情况下也只会初始化一次，因此是线程安全的
```java
public class StaticHolderSingleton {  
  
private StaticHolderSingleton() {  
}  
  
private static class ClassHolder {  
static final StaticHolderSingleton INSTANCE = new StaticHolderSingleton();  
}  
  
public static StaticHolderSingleton getInstance() {  
return ClassHolder.INSTANCE;  
}  
  
public static void main(String[] args) {  
StaticHolderSingleton clz = StaticHolderSingleton.getInstance();  
}  
}
```
### 2.基于枚举
```java
public enum EnumSingleton {  
INSTANCE;  
  
EnumSingleton() {  
  
}  
}
```
当EnumSingleton.INSTANCE首次被引用时才会被初始化