单例模式常见的几种形式

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
}