java单例模式

1.

1.懒汉式  不安全 用到时候在加载
public class Singleton (){
    private static Singleton instance;
    private Singleton (){}  //构造方法私有化 防止 new Singleton
    public static Singleton getInstance(){
        if(instance == null){
            instance = new Singleton ();
        }
    return instance;
    }
}
2.懒汉式 线程安全  
public class Singleton (){
    private static Singleton instance;
    
    private Singleton (){};
    public synchronized Singleton getInstance(){
    if( instance == null){
        instance = new Singleton
    }
    return instance;
    }        
}
3.饿汉  类初始化即实例 instance
public class Singleton {  
    private static Singleton instance = new Singleton();  
    private Singleton (){}  
    public static Singleton getInstance() {  
    return instance;  
    }  
}  
4.静态内部类 调用时才加载(意义上应该属于懒汉式)
public class Singleton {  
    private static class SingletonHolder {  
    private static final Singleton INSTANCE = new Singleton();  
    }  
    private Singleton (){}  
    public static final Singleton getInstance() {  
    return SingletonHolder.INSTANCE;  
    }  
}  
5.枚举
public enum Singleton(){
    INSTANCE;
    public void whateverMethod() {  
    }  
}
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
6.双重校验锁
public class Singleton {  
    private volatile static Singleton instance;  
    private Singleton (){}  
    public static Singleton getInstance() {  
    if (instance== null) {  
        synchronized (Singleton.class) {  
        if (instance== null) {  
            instance= new Singleton();  
        }  
        }  
    }  
    return instance;  
    }  
}