单例模式

另类饿汉单例模式


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

懒汉模式 （懒汉，线程安全）
public class Singleton{
　　private static Singleton instance;
　　private Singleton(){}
　　public static synchronized Singleton getInstance(){
　　　　if(instance==null){
　　　　　　instance=new Singleton();
　　　　}
　　　　return instance;
　　}

}

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;  
    }  
}  

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