设计模式 之 单例模式

单例主要有两种，一种懒汉，一种饿汉，区别在于懒汉在getInstance的时候，才会去初始化单例，而饿汉在类加载之后就立即进行初始化。可见懒汉节省资源，而饿汉的在于天生的线程安全

1.懒汉式单例

package com.test.pattern;

class Singleton {
    private static Singleton singleton;
    private Singleton(){
        System.out.println("hello I'm singleton "+this.hashCode());
    }
    public static Singleton getSingleton() {
        if(singleton == null)
            singleton = new Singleton();
        return singleton;
    }
}
public class SingletonTest {
    public static void main(String[] args) {
        Singleton.getSingleton();
        Singleton.getSingleton();
        Singleton.getSingleton();
    }
}

这种单例模式是线程不安全的(但如果将getSingleton方法声明为synchronized即可使之线程安全):

package com.test.pattern;

/**
 * @author wangx
 * @Date: 2016年8月8日 
 * @func: 懒汉式单例多线程不安全
 * @Copyright: 2016 wangx. All rights reserved.
 */
public class SingletonMultithread {
    public static void main(String[] args) {
        new Thread(new Task()).start();
        new Thread(new Task()).start();
    }
}

class Task implements Runnable {

    @Override
    public void run() {
        Singleton.getSingleton();
        Singleton.getSingleton();
    }
}

运行结果:

hello I'm singleton 1025866005
hello I'm singleton 1449742585

2.饿汉式单例

package com.test.pattern;

/**
 * @author wangx
 * @Date: 2016年8月9日 
 * @func: 饿汉式单例
 * @Copyright: 2016 wangx. All rights reserved.
 */
class Singleton2 {
    private static Singleton2 singleton=new Singleton2();
    private Singleton2() {
        System.out.println("I'm singleton "+this.hashCode());
    }
    public static Singleton2 getInstance() {
        return singleton;
    }
}
public class SingletonTest2 {
    public static void main(String[] args) {
        new Thread(new Task2()).start();
        new Thread(new Task2()).start();
    }
}

class Task2 implements Runnable{
    @Override
    public void run() {
        Singleton2.getInstance();
    }
}

终：通过反射机制破坏单例

package com.test.reflection;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

class HelloWorld {
    public void sayHello() {
        System.out.println("hello world");
    }
}
class Singleton {
    private Singleton(){}
    private static Singleton singleton;
    public Singleton getSingleton() {
        if(singleton == null)
            singleton = new Singleton();
        return singleton;
    }
    public void sayHello() {
        System.out.println("hello I'm singleton "+this.hashCode());
    }
}
public class ReflectionTest {
    public static void main(String[] args) throws NoSuchMethodException, SecurityException, ClassNotFoundException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, InstantiationException {
        Constructor constructor = Class.forName("com.test.reflection.Singleton").getDeclaredConstructor();
        constructor.setAccessible(true);
        Method sayHello = Class.forName("com.test.reflection.Singleton").getDeclaredMethod("sayHello", null);
        sayHello.invoke(constructor.newInstance(),null);
        sayHello.invoke(constructor.newInstance(),null);
    }
}