单例几种常见的形式

原文地址：http://forestqqqq.iteye.com/blog/1896723

一，饿汉式单例

//饿汉式单例1
public class Singleton1 {
    private static final Singleton1 instance = new Singleton1();
    private Singleton1(){}
    public static Singleton1 getInstance(){
        return instance;
    }
}

//饿汉式单例2
class Singleton{
    private static class Single{
        static final Singleton instance = new Singleton();
    }
    private Singleton(){}
    public static Singleton getInstance(){
        return Single.instance;
    }
}

 

二，懒汉式单例

//懒汉式单例
public class Singleton2 {
    private static Singleton2 instance = null;
    private Singleton2(){}
    public static synchronized Singleton2 getInstance(){
        if(instance == null){
            instance = new Singleton2();
        }
        return instance;
    }
}

 

三，DCL双重锁检测式单例

//DCL双重锁检测式单例
public class Singleton3 {
    private volatile static Singleton3 instance = null;
    private Singleton3(){}
    public static Singleton3 getInstance(){
        if(instance == null){
            synchronized(Singleton3.class){
                if(instance == null){
                    instance = new Singleton3();
                }
            }
        }
        return instance;
    }
//参考文章：http://www.ibm.com/developerworks/cn/java/j-dcl.html
}

 

 

四，登记式单例

import java.lang.reflect.Constructor;
import java.util.HashMap;
import java.util.Map;

//登记式单例
public class Singleton4 {
    private  static final Map<String,Singleton4> instances =
            new HashMap<String,Singleton4>();
    static{
        Singleton4 instance = new Singleton4();
        instances.put(instance.getClass().getName(), instance);
    }
    protected Singleton4(){}
    public static synchronized Singleton4 getInstance(String name){
        if(name == null){
            name = Singleton4.class.getName();
        }
        if(instances.get(name) == null){
            try {
                Constructor con = Class.forName(name).getDeclaredConstructor();
                con.setAccessible(true);
                instances.put(name, (Singleton4)con.newInstance());
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return instances.get(name);
    }
//参考文章：http://www.cnblogs.com/whgw/archive/2011/10/05/2199535.html
}