单元素的枚举类型已经成为实现Singleton的最佳方法

传统的实现singleton的方法

1，提供一个已经被实例化的对象，并且将构造方法私有化

 

public static final SingleTon INSTANCE = new SingleTon();
private SingleTon() {}

 

 

2，提供一个static方法

 

private static final SingleTon INSTANCE = new SingleTon();
private SingleTon() {}
public static SingleTon getInstance() {
	return INSTANCE;
}

 

 

但是这两种方式都可以用反射来生成新对象

SingleTon s1 = SingleTon.INSTANCE;

Class<SingleTon> cls = SingleTon.class;
Constructor<SingleTon> constructor = cls.getDeclaredConstructor(new Class[] {});
constructor.setAccessible(true);
SingleTon s2 = constructor.newInstance(new Object[] {});

System.out.println(s1 == s2);

结果是：false 

从java1.5版本开始，实现singleton出现了第三种方式

public enum SingleTon {
	INSTANCE;
	public void speak() {
		System.out.println(this + " is speaking! ");
	}
}

 测试代码

public class SingleTonTest {

	public static void main(String[] args) throws Exception {
		SingleTon s1 = SingleTon.INSTANCE;
		s1.speak();
		SingleTon s2 = SingleTon.INSTANCE;
		s2.speak();
		System.out.println(s1 == s2);
	}
}

测试结果：
INSTANCE is speaking! 
INSTANCE is speaking! 
true