单例（一）

从最简单的单例开始：

package com.garinzhang.designpattern;



public class SimpleSingleton {

    private static SimpleSingleton instance = null;

    private SimpleSingleton() {}

    public static SimpleSingleton getInstance() {

        if(instance == null) {

            instance = new SimpleSingleton();

        }

        return instance;

    }

    public void printOut() {

        System.out.println("instance created!");

    }

}

以上例子有两个问题：

1. 通过JAVA里的反射可以产生构造函数为private的类的实例，看如下代码(这种方式可以让所有单例失效)：

package com.garinzhang.designpattern;



import java.lang.reflect.Constructor;

import java.lang.reflect.InvocationTargetException;

import java.util.ArrayList;

import java.util.List;



public class GetSimpleSingletonInstanceByReflection {

    static List<SimpleSingleton> list = new ArrayList<SimpleSingleton>();

    

    @SuppressWarnings("unchecked")

    public static SimpleSingleton getInstance() {

        Constructor<SimpleSingleton>[] constructors = null;

        try {

            constructors = (Constructor<SimpleSingleton>[]) Class.forName("com.garinzhang.designpattern.SimpleSingleton").getDeclaredConstructors();

        } catch(ClassNotFoundException e) {

            e.printStackTrace();

        }

        Constructor<SimpleSingleton> con = constructors[0];

        con.setAccessible(true);

        SimpleSingleton ss = null;

        try {

            ss = con.newInstance();

        } catch(IllegalArgumentException e) {

            e.printStackTrace();

        } catch(InvocationTargetException e) {

            e.printStackTrace();

        } catch(IllegalAccessException e) {

            e.printStackTrace();

        } catch(InstantiationException e) {

            e.printStackTrace();

        }

        return ss;

    }

    public static void main(String[] args) {

        SimpleSingleton ss = getInstance();

        System.out.println(ss instanceof SimpleSingleton);

        ss.printOut();

    }

}

 2. 不是线程安全的。线程安全的定义：如果你的代码所在的进程中有多个线程在同时运行，而这些线程可能会同时运行这段代码。如果每次运行结果和单线程运行的结果是一样的，而且其他的变量的值也和预期的是一样的，就是线程安全的；

package com.garinzhang.designpattern;



public class ThreadSafeSimpleSingleton {

    private static ThreadSafeSimpleSingleton instance = null;

    private ThreadSafeSimpleSingleton() {}

    public synchronized static ThreadSafeSimpleSingleton getInstance() {

        if(instance == null) {

            instance = new ThreadSafeSimpleSingleton();

        }

        return instance;

    }

}

GOF里的单例：

饿汉式单例类、懒汉式单例类、登记式单例类（http://www.cnblogs.com/whgw/archive/2011/10/05/2199535.html）