Java单例模式 多种实现方式

package SinglePag;
/*
 * 构造函数私有化，结合锁+静态的概念 实现单例模式
 */
public class SingleDemo {
    //防止类的实例化
    private SingleDemo(){
        
    }
    //静态 实例 属性
    private static SingleDemo singleD;
    //获取对象实例 （单例的方式 ）,锁住当前类（加锁）
    
    public synchronized static SingleDemo getInstance(){
        if(singleD == null){
            singleD = new SingleDemo();
        }
        return singleD;
    }
}

package SinglePag;

public class SingleWinform {
    public static void main(String[] args) {
        SingleDemo singleDemo = SingleDemo.getInstance();
        System.out.println(singleDemo.hashCode());
        SingleDemo singleDemo2 = SingleDemo.getInstance();
        System.out.println(singleDemo2.hashCode());
    }
}

package SinglePag;

public class SingleInnerStatic {
    //静态内置类
    private static class InnerStatic{
        //静态私有 实例属性
        private static SingleInnerStatic singleInstance;
        static{
            singleInstance = new SingleInnerStatic();
        }
    }
    
    //私有化构造函数，防止实例化
    private SingleInnerStatic(){
        
    }
    
    public static SingleInnerStatic getInstance(){
        return InnerStatic.singleInstance;
    }
}

package SinglePag;

public class SingleStaticWinform {
   public static void main(String[] args) {
       SingleInnerStatic singleInnerStatic =SingleInnerStatic.getInstance();
       System.out.println(singleInnerStatic.hashCode());
       
       SingleInnerStatic singleInnerStatic2 = SingleInnerStatic.getInstance();
       System.out.println(singleInnerStatic2.hashCode());
   }
}