Java 如何安全的发布对象

package com.rumenz.task.single;


//线程安全
//饿汉模式
//静态代码块初始化
public class SingletonExample {
  private SingletonExample(){
    //初始化操作
  }
  private static SingletonExample singletonExample=null;

  static {
    singletonExample=new SingletonExample();
  }

  public static SingletonExample getInstance(){
    return singletonExample;
  }
}

//或者
package com.rumenz.task.single;
//线程安全
//饿汉模式
//静态代码块初始化
public class SingletonExample {
  private SingletonExample(){

    //初始化操作

  }
  private static SingletonExample singletonExample=new SingletonExample();



  public static SingletonExample getInstance(){
    return singletonExample;
  }
}

package com.rumenz.task.single;

//线程安全
//懒汉模式
public class SingletonExample1 {

  private SingletonExample1() {
    //初始化操作
  }
  // 1、memory = allocate() 分配对象的内存空间
  // 2、ctorInstance() 初始化对象
  // 3、instance = memory 设置instance指向刚分配的内存
  // 单例对象 volatile + 双重检测机制 -> 禁止指令重排
  private volatile static SingletonExample1 singletonExample1=null;
  //静态工厂方法
  public static SingletonExample1 getInstance(){
    if(singletonExample1==null){ //双重检测
      synchronized(SingletonExample1.class){ //同步锁
        if(singletonExample1==null){
          singletonExample1=new SingletonExample1();
        }
      }
    }
    return singletonExample1;
  }
}

package com.rumenz.task.single;

public class SingletonExample3 {
  //私有构造函数
  private SingletonExample3(){
    //初始化操作
  }

  private static SingletonExample3 singletonExample3=null;
  //静态的工厂方法
  public static synchronized SingletonExample3 getSingletonExample3(){
     if(singletonExample3==null){
       singletonExample3=new SingletonExample3();
     }
     return singletonExample3;
  }
}

package com.rumenz.task.single;

public class SingletonExample4 {

  //私有构造函数
  private SingletonExample4(){
    //初始化
  }
  public static SingletonExample4 getSingletonExample4(){
    return Singleton.INSTANCE.getSingleton();
  }
  private enum Singleton{
    INSTANCE;
    private SingletonExample4 singleton;
    Singleton(){
      singleton=new SingletonExample4();
    }
    public SingletonExample4 getSingleton(){
      return singleton;
    }

  }
}