[零基础学JAVA]Java SE应用部分-36.反射机制与工厂设计模式

本季目标

Java的反射机制
工厂模式综合讲解

1、什么叫反射
Java.lang.reflect包下
正常情况下我们可以通过类实例化一个对象，那么通过反射实际上就可以通过一个对象得到此类完整的包.类名称。

package org.michael;    
class Person{    
        private String name;    
        private int age;    
        public void setName(String name){    
                this.name = name;    
        }    
        public void setAge(int age){    
                this.age = age;    
        }    
        public String getName(){    
                return this.name;    
        }    
        public int getAge(){    
                return this.age;    
        }    
}    
public class Demo01{    
        public static void main(String args[]){    
                Person p = new Person();    
                //假设现在不知道p是那个类的对象，则可以通过反射机制找到    
                Class c = null;    
                c = p.getClass();    
                System.out.println(c.getName());    
        }    
}

看下效果：

除了可以找到对象所在的包.类名称，实际上也可以把所有的方法名称列出来。

package        org.michael;    
import java.lang.reflect.*;    
class Person{    
        private String name;    
        private int age;    
        public void setName(String name){    
                this.name = name;    
        }    
        public void setAge(int age){    
                this.age = age;    
        }    
        public String getName(){    
                return this.name;    
        }    
        public int getAge(){    
                return this.age;    
        }    
}    
public class Demo02{    
        public static void main(String args[]){    
                Person p = new Person();    
                //假设现在不知道p是那个类的对象，则可以通过反射机制找到    
                Class c = null;    
                c = p.getClass();    
                Method m[] = c.getMethods();    
                for(int i=0;i<m.length;i++){    
                System.out.println(m[i]);    
                }    
        }    
}

2、研究Class类
Class类的构造方法被私有化了，外部无法直接看见，所以其内部必然有一个方法可以取得Class 实例。
public static Class<?> forName(String className) throws ClassNotFoundException
此方法可以返回Class类的实例，此方法接收一个完整的包.类名称。
通过newInstance方法，可以将传入的完整的字符串（包.类名称）实例化。

package        org.michael;    
import java.lang.reflect.*;    
class Person{    
        private String name;    
        private int age;    
        public void setName(String name){    
                this.name = name;    
        }    
        public void setAge(int age){    
                this.age = age;    
        }    
        public String getName(){    
                return this.name;    
        }    
        public int getAge(){    
                return this.age;    
        }    
}    
public class Demo03{    
        public static void main(String args[]){    
                Person p = null;    
                Class c = null;    
                try{    
                        c = Class.forName("org.michael.Person");    
                }catch (Exception e){}    
                try{    
                        p = (Person)c.newInstance();    
                }catch (Exception e){}    
                //上面两行代码也可以使用下面一行代码取代哈~    
                // p = (Person)Class.forName("org.michael.Person").newInstance();    
                p.setName("Michael");    
                p.setAge(30);    
                System.out.println(p.getName()+"--->"+p.getAge());    
        }    
}

如果要使用以上的代码去实例化一个对象，则必须有一个前途条件：在对象所在的类中必须有一个无参构造方法，如果没有此无参构造，则肯定会出现错误。

package        org.michael;    
import java.lang.reflect.*;    
class Person{    
        private String name;    
        private int age;    
        public Person(String name,int age){    
                this.name = name;    
                this.age = age;    
        }    
        public void setName(String name){    
                this.name = name;    
        }    
        public void setAge(int age){    
                this.age = age;    
        }    
        public String getName(){    
                return this.name;    
        }    
        public int getAge(){    
                return this.age;    
        }    
}    
public class Demo04{    
        public static void main(String args[]){    
                Person p = null;    
                Class c = null;    
                try{    
                        c = Class.forName("org.michael.Person");    
                        p = (Person)c.newInstance();    
                }catch (Exception e){    
                        System.out.println(e);    
                }    
                System.out.println(p.getName()+"--->"+p.getAge());    
        }    
}

在此时如果想继续通过此操作为对象进行实例化，则可以通过构造方法类（Constructor）完成。

package        org.michael;    
import java.lang.reflect.*;    
class Person{    
        private String name;    
        private int age;    
        public Person(String name,int age){    
                this.name = name;    
                this.age = age;    
        }    
        public void setName(String name){    
                this.name = name;    
        }    
        public void setAge(int age){    
                this.age = age;    
        }    
        public String getName(){    
                return this.name;    
        }    
        public int getAge(){    
                return this.age;    
        }    
}    
public class Demo05{    
        public static void main(String args[]){    
                Person p = null;    
                Class c = null;    
                try{    
                        c = Class.forName("org.michael.Person");    
                        Constructor[] cs = c.getConstructors();    
                        Object obj[] = new Object[]{"Michael",30};    
                        //一个类中会有多个构造方法，所以此时返回一个数组    
                        p = (Person)cs[0].newInstance(obj);    
                }catch (Exception e){    
                        System.out.println(e);    
                }    
                System.out.println(p.getName()+"--->"+p.getAge());    
        }    
}

反射机制实际上是我们所有框架的一个基础，那么现在就利用反射机制完成一个高可扩展性的工厂设计。

回顾：工厂设计

interface Fruit{    
        public void grow();    
        public void eat();    
}    
class Apple implements Fruit{    
        public void grow(){    
                System.out.println("苹果在生长...");    
        }    
        public void eat(){    
                System.out.println("吃苹果...");    
        }    
}    
class Orange implements Fruit{    
        public void grow(){    
                System.out.println("橘子在生长...");    
        }    
        public void eat(){    
                System.out.println("吃橘子...");    
        }    
}    
class Factory{    
        public static Fruit getFruit(int i){    
                Fruit f = null;    
                if (i==1){    
                        f = new Apple();    
                }    
                if (i==2){    
                        f = new Orange();    
                }    
                return f;    
        }    
}    
public class Demo06{    
        public static void main(String args[]){    
                Fruit f = Factory.getFruit(1);    
                f.grow();    
        }    
}

客户端只与工厂和直接的接口有关了，而与其他的无关，但是有个问题，如果现在要扩展了子类，则工厂也必须同时进行修改。那么有没有一种方法，可以让子类扩充之后不去修改工厂呢？肯定是有的，通过Class.forName 完成。

interface Fruit{    
        public void grow();    
        public void eat();    
}    
class Apple implements Fruit{    
        public void grow(){    
                System.out.println("苹果在生长...");    
        }    
        public void eat(){    
                System.out.println("吃苹果...");    
        }    
}    
class Orange implements Fruit{    
        public void grow(){    
                System.out.println("橘子在生长...");    
        }    
        public void eat(){    
                System.out.println("吃橘子...");    
        }    
}    
class Banana implements Fruit{    
        public void grow(){    
                System.out.println("香蕉在生长...");    
        }    
        public void eat(){    
                System.out.println("吃香蕉...");    
        }    
}    
class Factory{    
        public static Fruit getFruit(String className){    
                Fruit f = null;    
                try{    
                        f = (Fruit)Class.forName(className).newInstance();    
                }catch (Exception e){}    
                return f;    
        }    
}    
public class Demo07{    
        public static void main(String args[]){    
                Fruit f = Factory.getFruit("Banana");    
                f.grow();    
        }    
}

但是此程序依然有一个缺点，现在的输入的包.类名称实际上长度非常的短，如果包.类名称的长度过长了，则在使用的时候就比较麻烦了。所以最好可以找个代号进行替代。
使用Hashtable的子类 —— Properties完成。

import java.util.*;    
import java.io.*;    
interface Fruit{    
        public void grow();    
        public void eat();    
}    
class Apple implements Fruit{    
        public void grow(){    
                System.out.println("苹果在生长...");    
        }    
        public void eat(){    
                System.out.println("吃苹果...");    
        }    
}    
class Orange implements Fruit{    
        public void grow(){    
                System.out.println("橘子在生长...");    
        }    
        public void eat(){    
                System.out.println("吃橘子...");    
        }    
}    
class Banana implements Fruit{    
        public void grow(){    
                System.out.println("香蕉在生长...");    
        }    
        public void eat(){    
                System.out.println("吃香蕉...");    
        }    
}    
class Factory{    
        public static Fruit getFruit(String className){    
                Fruit f = null;    
                try{    
                        f = (Fruit)Class.forName(className).newInstance();    
                }catch (Exception e){}    
                return f;    
        }    
}    
class InputData{    
        private BufferedReader buf = null;    
        public InputData(){    
                this.buf = new BufferedReader(new InputStreamReader(System.in));    
        }    
        public String getString(){    
                String str = null;    
                try{    
                        str = this.buf.readLine();    
                }catch (Exception e){}    
                return str;    
        }    
}    
public class Demo08{    
        public static void main(String args[]){    
                Properties p = new Properties();    
                p.setProperty("a","Apple");    
                p.setProperty("o","Orange");    
                p.setProperty("b","Banana");    
                System.out.println(p);    
                System.out.print("请选择所需要的类型：");    
                String str = new InputData().getString();    
                //进一步扩展，现在可以由用户自己输入要使用的类型    
                Fruit f = Factory.getFruit(p.getProperty(str));    
                f.grow();    
        }    
}

如果现在再增加子类呢？
属性文件肯定不够了。
所以此时为了达到好的效果，则最好可以将属性保存起来，之后通过修改保存的文件达到属性的扩充。

import java.util.*;    
import java.io.*;    
interface Fruit{    
        public void grow();    
        public void eat();    
}    
class Apple implements Fruit{    
        public void grow(){    
                System.out.println("苹果在生长...");    
        }    
        public void eat(){    
                System.out.println("吃苹果...");    
        }    
}    
class Orange implements Fruit{    
        public void grow(){    
                System.out.println("橘子在生长...");    
        }    
        public void eat(){    
                System.out.println("吃橘子...");    
        }    
}    
class Banana implements Fruit{    
        public void grow(){    
                System.out.println("香蕉在生长...");    
        }    
        public void eat(){    
                System.out.println("吃香蕉...");    
        }    
}    
class Factory{    
        public static Fruit getFruit(String className){    
                Fruit f = null;    
                try{    
                        f = (Fruit)Class.forName(className).newInstance();    
                }catch (Exception e){}    
                return f;    
        }    
}    
class PropertyOperate{    
        private Properties pro = null;    
        public PropertyOperate(){    
                this.pro = new Properties();    
                this.load();    
        }    
        //设置一个返回方法    
        public Properties getPro(){    
                return this.pro;    
        }    
        //从文件中读出属性，如果文件不存在，则创建一个默认的    
        private void save(){    
                pro.setProperty("a","Apple");    
                pro.setProperty("o","Orange");    
                //保存在文件之中    
                try{    
                        pro.storeToXML(new FileOutputStream(new File("e:\\fruit.xml")),"FRUIT FACTORY");    
                }catch (Exception e){}    
        }    
        private void load(){    
                File f = new File("e:\\fruit.xml");    
                if(f.exists()){    
                        //文件存在则可以读取    
                        try{    
                                pro.loadFromXML(new FileInputStream(f));    
                        }catch (Exception e){}    
                }else{    
                        //进行创建    
                        this.save();    
                }    
        }    
}    
class InputData{    
        private BufferedReader buf = null;    
        public InputData(){    
                this.buf = new BufferedReader(new InputStreamReader(System.in));    
        }    
        public String getString(){    
                String str = null;    
                try{    
                        str = this.buf.readLine();    
                }catch(Exception e){}    
                return str;    
        }    
}    
public class Demo09{    
        public static void main(String args[]){    
                Properties p = new PropertyOperate().getPro();    
                System.out.println(p);    
                System.out.print("请选择所需要的类型：");    
                String str = new InputData().getString();    
                //进一步扩展，现在可以由用户自己输入要使用的类型    
                Fruit f = Factory.getFruit(p.getProperty(str));    
                f.grow();    
        }    
}

如果此时要想新增加可以操作的子类，则就需要配置fruit.xml文件即可。
此种代码是典型的配置与程序相分离，程序直接有配置文件有关。某一个部分的修改不影响其他程序。—— 思想必须建立起来。

总结
对象的产生到底有多少种方法了：
· 直接用new关键字产生：直接，但是代码间会产生严重的耦合性
· 可以通过工厂传递引用：直接，但是必须考虑到代码以后的可维护性
· 通过对象克隆可以完成
· 通过Class.forName()进行反射加载完成。

####################Michael分割线######################