单例兑现方式

单例实现方式
1：//效率低下，线程安全
public class SingleTon1 {
public final static SingleTon1 singleTonDemo=new SingleTon1();
private SingleTon1(){

}
public static SingleTon1 getInstance(){
return singleTonDemo;
}
}

2：//线程不安全
public class SingleTon21 {
public static SingleTon21 singleTonDemo=null;
private SingleTon21(){

}
public static SingleTon21 getSingleton(){
if(singleTonDemo==null){
singleTonDemo=new SingleTon21();
}
return singleTonDemo;
}
}

3://线程安全
public class SingleTon22 {
private static SingleTon22 singleTon21Demo=null;
private SingleTon22(){

}
// 线程安全，可以同时多线程，但是效率也不是很高
public static SingleTon22 getSingleTon21(){
if(singleTon21Demo==null){
synchronized (SingleTon22.class) {
if(singleTon21Demo==null){
singleTon21Demo=new SingleTon22();
}
}
}
return singleTon21Demo;
}
// 线程安全，但是效率不高，永远只有一个线程能访问,同时返回一个对象
public static synchronized SingleTon22 getSingleTon22(){
if(singleTon21Demo==null){
singleTon21Demo=new SingleTon22();
}
return singleTon21Demo;
}
}

4:private SingleTonBobLee(){

}
static class SingleTonHolder{
static SingleTonBobLee singleTonDemo=new SingleTonBobLee();
}
public static SingleTonBobLee getSingleTonBobLee(){
return SingleTonHolder.singleTonDemo;
}