单例模式的写法

出现多例的原因

1. 线程不安全, 多线程同时创建
2. 单例由不同的类装载器装入，那便有可能存在多个单例类的实例
3. 类现了java.io.Serializable接口，就可能被序列化和反序列化。
   反序列化一个单例类的对象,就会有多个单例类的实例

1.懒汉(线程不安全)

public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  

    public static Singleton getInstance() {  
	    if (instance == null) {  
	        instance = new Singleton();  
	    }  
	    return instance;  
    }  
} 

2.懒汉

线程安全, 效率很低

public class Singleton {  
    private static Singleton instance;  
    private Singleton (){}  
    
    public static synchronized Singleton getInstance() {  
	    if (instance == null) {  
	        instance = new Singleton();  
	    }  
	    return instance;  
    }  
} 

3.饿汉

利用了classloder的机制来保证初始化instance时只有一个线程

Singleton类被装载instance就会被实例化 ,所以线程是安全的.

public class Singleton {  
    private static Singleton instance = new Singleton();  
    private Singleton (){}
    
    public static Singleton getInstance() {  
    	return instance;  
    }  
} 

4.饿汉变种

同3

public class Singleton {  
    private Singleton instance = null;  
    static {  
    	instance = new Singleton();  
    }  
    private Singleton (){} 
    
    public static Singleton getInstance() {  
    	return this.instance;  
    }  
} 

5.静态内部类

• 与3 4的区别是 可以lazy loading懒加载
  调用getInstance方法时，才会显示装载SingletonHolder类，从而实例化instance

public class Singleton {  
    private static class SingletonHolder {  
    	private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}
    
    public static final Singleton getInstance() {  
    	return SingletonHolder.INSTANCE;  
    }  
} 

6.枚举

不仅能避免多线程同步问题，而且还能防止反序列化重新创建新的对象

public enum Singleton {  
    INSTANCE;  
    public void whateverMethod() {  
    }  
}  

7.双重校验锁

public class Singleton {
    private volatile static Singleton singleton;  
    private Singleton (){}
    
    public static Singleton getSingleton() {
	    if (singleton == null) {
	        synchronized (Singleton.class) {  
		        if (singleton == null) {  
		            singleton = new Singleton();  
		        }
	        }
	    }
	    return singleton;
    }  
}