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());
}
}