单例模式

package com.m01.program.maven_1;
public class Singleton2 {
    private Singleton2(){}
    private static Singleton2 s=null;
    public static  Singleton2 getSingleton2(){
        if(s==null){
            synchronized (Singleton2.class) {
                if(s==null){
                    s=new Singleton2();
                }
            }
        }
        return s;
    }
}

package com.m01.program.maven_1;
public class SingletonTest1 {
    private SingletonTest1(){}
    private static  SingletonTest1 s=new SingletonTest1();
    public static SingletonTest1 getSingletonTest1(){
        return s;
    }
}

package com.m01.program.maven_1;

public class Singleton3 {

    private static class getSingle{
         private static Singleton3  s=new Singleton3();
    }
    private Singleton3(){}
    public static Singleton3 getSingleton3(){
        return getSingle.s;
    }
}

package com.m01.program.maven_1;

public enum Singleton5 {
    INSTANCE;
    public void doWhat(){
        System.out.println("OOXX");
    }
}

package com.m01.program.maven_1;
public class Singleton3 {
     static {
          s=new Singleton3();
    }
    private Singleton3(){}
    private static Singleton3 s;
    public static Singleton3 getSingleton3(){
        return s;
    }
}