java design pattern - singleton pattern

2个原则：
There should be only one instance allowed for a class
We should allow global point of access to that single instance

1. 使用反射,reflection 

package Singleton;

import java.lang.reflect.Constructor;


//using Reflection we can set the private constructor to become accessible
//at runtime

public class SingletonDemo2{
    //early initialization
    private static SingletonDemo2 instance = new SingletonDemo2();

    private SingletonDemo2(){
        System.out.println("Creating .... ");
        if(instance !=null){
            throw new RuntimeException("Can't create instance,please use get instance");
        }
    }

    public static SingletonDemo2 getInstance(){
        return instance;
    }


    public static void main(String[] args) throws Exception{
        SingletonDemo2 s1 = SingletonDemo2.getInstance();
        SingletonDemo2 s2 = SingletonDemo2.getInstance();

        print("s1", s1);
        print("s2", s2);

        Class clazz= Class.forName("Singleton.SingletonDemo2");
        Constructor<SingletonDemo2> ctor = clazz.getDeclaredConstructor();
        ctor.setAccessible(true);
        SingletonDemo2 s3 = ctor.newInstance();

        print("s3", s3);
    }

    static void print(String name, SingletonDemo2 obj){
        String str;
        str=String.format("Object: %s, Hahscode: %d", name,obj.hashCode());
        System.out.println(str);
    }

}

2.  如果clone 对象的话，则throws Exception

package Singleton;

import java.lang.reflect.Constructor;

//not allow clone
public class SingletonDemo3 implements Cloneable{
    //early initialization
    private static SingletonDemo3 instance = new SingletonDemo3();

    private SingletonDemo3(){
        System.out.println("Creating .... ");
        if(instance !=null){
            throw new RuntimeException("Can't create instance,please use get instance");
        }
    }

    public static SingletonDemo3 getInstance(){
        return instance;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException{
        if(instance != null) throw new CloneNotSupportedException();
        return super.clone();
    }

    public static void main(String[] args) throws Exception{
        SingletonDemo3 s1 = SingletonDemo3.getInstance();
        SingletonDemo3 s2 = SingletonDemo3.getInstance();

        print("s1", s1);
        print("s2", s2);

        SingletonDemo3 s3= (SingletonDemo3)s2.clone();
        print("s3", s3);
    }

    static void print(String name, SingletonDemo3 obj){
        String str;
        str=String.format("Object: %s, Hahscode: %d", name,obj.hashCode());
        System.out.println(str);
    }

}

3.  允许序列化

package Singleton;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.lang.reflect.Constructor;

//not allow clone
public class SingletonDemo4 implements Serializable{
    //early initialization
    private static SingletonDemo4 instance = new SingletonDemo4();

    private SingletonDemo4(){
        System.out.println("Creating .... ");
        if(instance !=null){
            throw new RuntimeException("Can't create instance,please use get instance");
        }
    }

    private Object readResolve(){
        System.out.println("applying read resolve");
        return instance;
    }

    public static SingletonDemo4 getInstance(){
        return instance;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException{
        if(instance != null) throw new CloneNotSupportedException();
        return super.clone();
    }

    public static void main(String[] args) throws Exception{
        SingletonDemo4 s1 = SingletonDemo4.getInstance();
        SingletonDemo4 s2 = SingletonDemo4.getInstance();

        print("s1", s1);
        print("s2", s2);

        ObjectOutputStream oos= new ObjectOutputStream(new FileOutputStream("s2.txt"));
        oos.writeObject(s2);

        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("s2.txt"));
        SingletonDemo4 s3= (SingletonDemo4)ois.readObject();
        print("s3", s3);
    }

    static void print(String name, SingletonDemo4 obj){
        String str;
        str=String.format("Object: %s, Hahscode: %d", name,obj.hashCode());
        System.out.println(str);
    }

}

 
4. 如果允许多线程

package Singleton;

import java.lang.reflect.Constructor;

public class SingletonDemo implements Cloneable{
    //lazy initialization
    private static SingletonDemo instance =null;

    private SingletonDemo(){
        System.out.println("Creating .... ");
    }

    //in multithreaded environment
    public static synchronized SingletonDemo getInstance(){
    //public static SingletonDemo getInstance(){
        if(instance == null){
            instance = new SingletonDemo();
        }
        return instance;
    }

    @Override
    protected Object clone() throws CloneNotSupportedException{
        //if(instance != null) throw new CloneNotSupportedException();
        return super.clone();
    }

    private Object readResolve(){
        System.out.println("applying read resolve....");
        return instance;
    }

    public static void main(String[] args) throws Exception{
        SingletonDemo s1 = SingletonDemo.getInstance();
        SingletonDemo s2 = SingletonDemo.getInstance();

        print("s1", s1);
        print("s2", s2);

        SingletonDemo s3 = (SingletonDemo)s2.clone();

        print("s3", s3);
    }

    static void print(String name, SingletonDemo obj){
        String str;
        str=String.format("Object: %s, Hahscode: %d", name,obj.hashCode());
        System.out.println(str);
    }

}