Java的反射

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最新推荐文章于 2025-03-26 20:25:07 发布

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文章标签：反射

                    版权声明：版权声明：本文为博主原创文章，转载请注明出处。                        https://blog.youkuaiyun.com/lililuni/article/details/83449088                    </div>
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                                        <p>&nbsp;&nbsp;学到spring框架的时候，发现反射思想很重要，故特此写下此文，以加深理解。</p>

文章目录

1：反射概述
2：Class对象特点
3：反射的使用
4 : 反射的其他用处
- 1:通过反射运行配置文件内容
- 2:通过反射越过泛型检查

1：反射概述

JAVA反射机制是在运行状态中，对于任意一个类，都能够知道这个类的所有属性和方法；对于任意一个对象，都能够调用它的任意一个方法和属性；这种动态获取的信息以及动态调用对象的方法的功能称为java语言的反射机制。
实际上，我们创建的每一个类也都是对象，即类本身是java.lang.Class类的实例对象。这个实例对象称之为类对象，也就是Class对象。那么，Class对象又是什么对象呢？

2：Class对象特点

下图是Class类的api（图片来自于Java基础之—反射（非常重要））

在这里插入图片描述
从图中可以得出以下几点：

Class 类的实例对象表示正在运行的 Java 应用程序中的类和接口。也就是jvm中有很多的实例，每个类都有唯一的Class对象。
Class 类没有公共构造方法。Class 对象是在加载类时由 Java 虚拟机自动构造的。也就是说我们不需要创建，JVM已经帮我们创建了。
Class 对象用于提供类本身的信息，比如有几种构造方法，有多少属性，有哪些普通方法

3：反射的使用

假设我们现在有一个Hero类

package pojo;
public class Hero {
	public String name; //姓名
    public float hp; //血量
    public float armor; //护甲
    public int moveSpeed; //移动速度
}

 
 1
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5
6
7

1:获取类对象

获取类对象有3种方式

Class.forName（）（常用）
Hero.class
new Hero().getClass()

在一个JVM中，一种类，只会有一个类对象存在。所以以上三种方式取出来的类对象，都是一样。（此处准确是在ClassLoader下,只有一个类对象）

示例：

package pojo;
public class ObjectTest {
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    String className <span class="token operator">=</span> <span class="token string">"pogo.Hero"</span><span class="token punctuation">;</span>
	<span class="token keyword">try</span> <span class="token punctuation">{</span>
    	<span class="token comment">//获取类对象的第一种方式</span>
        Class <span class="token class-name">pClass1</span> <span class="token operator">=</span> Class<span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span>className<span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//获取类对象的第二种方式</span>
        Class <span class="token class-name">pClass2</span> <span class="token operator">=</span> Hero<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">;</span>
        <span class="token comment">//获取类对象的第三种方式</span>
        Class <span class="token class-name">pClass3</span> <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Hero</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>pClass1<span class="token operator">==</span>pClass2<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//输出true</span>
        System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>pClass1<span class="token operator">==</span>pClass3<span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//输出true</span>
    <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">ClassNotFoundException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">// TODO Auto-generated catch block</span>
        e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

}
}

三种方式中，常用第一种，第二种需要导入类的包，依赖太强，不导包就抛编译错误。第三种对象都有了还要反射干什么。一般都第一种，一个字符串可以传入也可写在配置文件中等多种方法。

2 利用反射机制创建对象

基本步骤
与传统的通过new 来获取对象的方式不同反射机制，反射会先拿到Hero的“类对象”,然后通过类对象获取“构造器对象”再通过构造器对象创建一个对象，具体步骤：

1.获取类对象 Class class = Class.forName("pojo.Hero");
2.获取构造器对象 Constructor con = clazz.getConstructor(形参.class);
3 获取对象 Hero hero =con.newInstance(实参);

上面是最简单的获取方法，当Hero的构造方法不是无参构造方法时，获取构造器对象略有不同，见下面测试：

构造方法不同时，获取构造器对象的方法

示例：

Hero类添加6种构造方法

//---------------构造方法-------------------
	//（默认的构造方法）
	Hero(String str){
		System.out.println("(默认)的构造方法 s = " + str);
	}
<span class="token comment">//无参构造方法</span>
<span class="token keyword">public</span> <span class="token function">Hero</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"调用了公有、无参构造方法执行了。。。"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">//有一个参数的构造方法</span>
<span class="token keyword">public</span> <span class="token function">Hero</span><span class="token punctuation">(</span><span class="token keyword">char</span> name<span class="token punctuation">)</span><span class="token punctuation">{</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"姓名："</span> <span class="token operator">+</span> name<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">//有多个参数的构造方法</span>
<span class="token keyword">public</span> <span class="token function">Hero</span><span class="token punctuation">(</span>String name <span class="token punctuation">,</span><span class="token keyword">float</span> hp<span class="token punctuation">)</span><span class="token punctuation">{</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"姓名："</span><span class="token operator">+</span>name<span class="token operator">+</span><span class="token string">"血量："</span><span class="token operator">+</span> hp<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">//受保护的构造方法</span>
<span class="token keyword">protected</span> <span class="token function">Hero</span><span class="token punctuation">(</span><span class="token keyword">boolean</span> n<span class="token punctuation">)</span><span class="token punctuation">{</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"受保护的构造方法 n = "</span> <span class="token operator">+</span> n<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token comment">//私有构造方法</span>
<span class="token keyword">private</span> <span class="token function">Hero</span><span class="token punctuation">(</span><span class="token keyword">float</span> hp<span class="token punctuation">)</span><span class="token punctuation">{</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"私有的构造方法   血量："</span><span class="token operator">+</span> hp<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

通过反射机制获取对象

package test;
public class ConstructorTest {
<span class="token comment">/*
 * 通过Class对象可以获取某个类中的：构造方法、成员变量、成员方法；并访问成员；
 * 
 * 1.获取构造方法：
 * 		1).批量的方法：
 * 			public Constructor[] getConstructors()：所有"公有的"构造方法
            public Constructor[] getDeclaredConstructors()：获取所有的构造方法(包括私有、受保护、默认、公有)
     
 * 		2).获取单个的方法，并调用：
 * 			public Constructor getConstructor(Class... parameterTypes):获取单个的"公有的"构造方法：
 * 			public Constructor getDeclaredConstructor(Class... parameterTypes):获取"某个构造方法"可以是私有的，或受保护、默认、公有；
 * 		
 * 2.创建对象
 * 		Constructor对象调用newInstance(Object... initargs)
 */</span>

 
<span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token keyword">throws</span> Exception <span class="token punctuation">{</span>
	<span class="token comment">//1.加载Class对象</span>
	Class <span class="token class-name">clazz</span> <span class="token operator">=</span> Class<span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span><span class="token string">"pojo.Hero"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	
	
	<span class="token comment">//2.获取所有公有构造方法</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"**********************所有公有构造方法*********************************"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	Constructor<span class="token punctuation">[</span><span class="token punctuation">]</span> conArray <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getConstructors</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">for</span><span class="token punctuation">(</span>Constructor c <span class="token operator">:</span> conArray<span class="token punctuation">)</span><span class="token punctuation">{</span>
		System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	
	
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"************所有的构造方法(包括：私有、受保护、默认、公有)***************"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	conArray <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getDeclaredConstructors</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token keyword">for</span><span class="token punctuation">(</span>Constructor c <span class="token operator">:</span> conArray<span class="token punctuation">)</span><span class="token punctuation">{</span>
		System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>c<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"*****************获取公有、无参的构造方法*******************************"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	Constructor con <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getConstructor</span><span class="token punctuation">(</span>null<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">//1&gt;、因为是无参的构造方法所以类型是一个null,不写也可以：这里需要的是一个参数的类型，切记是类型</span>
	<span class="token comment">//2&gt;、返回的是描述这个无参构造函数的类对象。</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"con = "</span> <span class="token operator">+</span> con<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">//调用构造方法</span>
	Object obj <span class="token operator">=</span> con<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	
	
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">"******************获取私有构造方法，并调用*******************************"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	con <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getDeclaredConstructor</span><span class="token punctuation">(</span><span class="token keyword">float</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>con<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token comment">//调用构造方法</span>
	con<span class="token punctuation">.</span><span class="token function">setAccessible</span><span class="token punctuation">(</span><span class="token boolean">true</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//暴力访问(忽略掉访问修饰符)</span>
	obj <span class="token operator">=</span> con<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token number">100</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

输出：

**********************所有公有构造方法*********************************
public pojo.Hero(java.lang.String,float)
public pojo.Hero(char)
public pojo.Hero()
************所有的构造方法(包括：私有、受保护、默认、公有)***************
private pojo.Hero(float)
protected pojo.Hero(boolean)
public pojo.Hero(java.lang.String,float)
public pojo.Hero(char)
public pojo.Hero()
pojo.Hero(java.lang.String)
*****************获取公有、无参的构造方法*******************************
con = public pojo.Hero()
调用了公有、无参构造方法执行了。。。
******************获取私有构造方法，并调用*******************************
private pojo.Hero(float)
私有的构造方法   血量：100.0

总结：

1.获取构造器对象方法：
1).批量的方法：
public Constructor[] getConstructors()：所有"公有的"构造方法
public Constructor[] getDeclaredConstructors()：获取所有的构造方法(包括私有、受保护、默认、公有)
2).获取单个的方法:
public Constructor getConstructor(Class… parameterTypes): 获取单个的"公有的"构造方法
public Constructor getDeclaredConstructor(Class…parameterTypes):获取"某个构造方法"可以是私有的，或受保护、默认、公有；

3: 获取成员变量并使用

基本步骤

1.获取HeroPlus类的对象 new方法/第2章中的方法 h
2. 获取属性 Field f1 = h.getDeclaredField("属性名")
3. 修改属性 f1.set(h，实参)，注意这里的h是对象，不是类对象

示例：

新增HeroPlus类

package pojo;
public class HeroPlus {
	public String name;
    public float hp;
    public int damage;
    public int id;
<span class="token keyword">public</span> String <span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">setName</span><span class="token punctuation">(</span>String name<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">this</span><span class="token punctuation">.</span>name <span class="token operator">=</span> name<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token function">HeroPlus</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
     
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token function">HeroPlus</span><span class="token punctuation">(</span>String string<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    name <span class="token operator">=</span>string<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> String <span class="token function">toString</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token string">"Hero [name="</span> <span class="token operator">+</span> name <span class="token operator">+</span> <span class="token string">"]"</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">boolean</span> <span class="token function">isDead</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">// TODO Auto-generated method stub</span>
    <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">attackHero</span><span class="token punctuation">(</span>HeroPlus h2<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>name<span class="token operator">+</span> <span class="token string">" 正在攻击 "</span> <span class="token operator">+</span> h2<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

}

获取属性并修改

package test;
public class ParaTest {
	 public static void main(String[] args) {
         HeroPlus h =new HeroPlus();
         //使用传统方式修改name的值为garen
         h.name = "garen";
     <span class="token keyword">try</span> <span class="token punctuation">{</span>
         <span class="token comment">//获取类HeroPlus的名字叫做name的字段</span>
         Field f1<span class="token operator">=</span> h<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getDeclaredField</span><span class="token punctuation">(</span><span class="token string">"name"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
         <span class="token comment">//修改这个字段的值</span>
         f1<span class="token punctuation">.</span><span class="token function">set</span><span class="token punctuation">(</span>h<span class="token punctuation">,</span> <span class="token string">"teemo"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
         <span class="token comment">//打印被修改后的值</span>
         System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>h<span class="token punctuation">.</span>name<span class="token punctuation">)</span><span class="token punctuation">;</span>
          
     <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
         <span class="token comment">// TODO Auto-generated catch block</span>
         e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
     <span class="token punctuation">}</span>

}
}

补充：
getField和getDeclaredField的区别
getField 只能获取public的，包括从父类继承来的字段。
getDeclaredField 可以获取本类所有的字段，包括private的，但是不能获取继承来的字段。 (注：这里只能获取到private的字段，但并不能访问该private字段的值,除非加上setAccessible(true))

4: 获取成员方法并使用

获取HeroPlus类的对象 h
获取成员方法：
public Method getMethod(String name ，Class<?>… parameterTypes):获取"公有方法"；（包含了父类的方法也包含Object类）
public Method getDeclaredMethods(String name ，Class<?>… parameterTypes) :获取成员方法，包括私有的(不包括继承的)
参数解释：
  name : 方法名；
  Class … : 形参的Class类型对象
调用方法
Method --> public Object invoke(Object obj,Object… args):
参数说明：
  obj : 要调用方法的对象；
  args:调用方式时所传递的实参；

示例：

package test;
public class MethodTest {
	public static void main(String[] args) {
 HeroPlus h <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">HeroPlus</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
 
 <span class="token keyword">try</span> <span class="token punctuation">{</span>
     <span class="token comment">// 获取这个名字叫做setName，参数类型是String的方法</span>
     Method m <span class="token operator">=</span> h<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">getMethod</span><span class="token punctuation">(</span><span class="token string">"setName"</span><span class="token punctuation">,</span> String<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
     <span class="token comment">// 对h对象，调用这个方法</span>
     m<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>h<span class="token punctuation">,</span> <span class="token string">"盖伦"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
     <span class="token comment">// 使用传统的方式，调用getName方法</span>
     System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>h<span class="token punctuation">.</span><span class="token function">getName</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

 <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
     <span class="token comment">// TODO Auto-generated catch block</span>
     e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
 <span class="token punctuation">}</span>

}
}

5: 获取main方法并使用

示例：

HeroPlus 新增main方法

public static void main(String[] args) {
		System.out.println("执行main方法");
	}

 
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通过下面步骤获取main方法¹

package test;
public class MainTest {
	public static void main(String[] args) {
		try {
			//1、获取HeroPlus对象的字节码
			Class clazz = Class.forName("pojo.HeroPlus");
		<span class="token comment">//2、获取main方法,第一个参数：方法名称，第二个参数：方法形参的类型，</span>
		 Method methodMain <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getMethod</span><span class="token punctuation">(</span><span class="token string">"main"</span><span class="token punctuation">,</span> String<span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
		<span class="token comment">//3、调用main方法</span>
		<span class="token comment">// methodMain.invoke(null, new String[]{"a","b","c"});</span>
		<span class="token comment">//第一个参数，对象类型，因为方法是static静态的，所以为null可以，第二个参数是String数组，这里要注意在jdk1.4时是数组，jdk1.5之后是可变参数</span>
		<span class="token comment">//这里拆的时候将  new String[]{"a","b","c"} 拆成3个对象。所以需要将它强转。</span>
		 methodMain<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>null<span class="token punctuation">,</span> <span class="token punctuation">(</span>Object<span class="token punctuation">)</span><span class="token keyword">new</span> <span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span><span class="token punctuation">{</span><span class="token string">"a"</span><span class="token punctuation">,</span><span class="token string">"b"</span><span class="token punctuation">,</span><span class="token string">"c"</span><span class="token punctuation">}</span><span class="token punctuation">)</span><span class="token punctuation">;</span><span class="token comment">//方式一</span>
		<span class="token comment">// methodMain.invoke(null, new Object[]{new String[]{"a","b","c"}});//方式二</span>
		
	<span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">Exception</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
		e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
	
	
<span class="token punctuation">}</span>

}

4 : 反射的其他用处

反射非常强大，但是学习了之后，会不知道该如何使用，反而觉得还不如直接调用方法来的直接和方便²。

通常来说，需要在学习了Spring 的依赖注入，反转控制之后，才会对反射有更好的理解，但是刚学到这里的同学，不一定接触了Spring，所以在这里举两个例子，来演示一下反射的一种实际运用。

1:通过反射运行配置文件内容

首先准备两个业务类

package service;
public class Service1 {
    public void doService1(){
        System.out.println("业务方法1");
    }
}

package service;
public class Service2 {
	public void doService2(){
        System.out.println("业务方法2");
    }
}

当需要从第一个业务方法切换到第二个业务方法的时候，使用非反射方式，必须修改代码，并且重新编译运行，才可以达到效果

package service;
public class CommonTest {
	  public static void main(String[] args) {
		  //new Service1().doService1();
		  //必须重新修改代码
	        new Service2().doService2();
	    }
}

 
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使用反射方式则方便很多

首先准备一个配置文件，就叫做spring.txt吧, 放在src目录下。
里面存放的是类的名称，和要调用的方法名。首先准备一个配置文件，就叫做spring.txt吧, 放在src目录下。里面存放的是类的名称，和要调用的方法名。
在测试类Test中，首先取出类名称和方法名，然后通过反射去调用这个方法。
当需要从调用第一个业务方法，切换到调用第二个业务方法的时候，不需要修改一行代码，也不需要重新编译，只需要修改配置文件spring.txt，再运行即可。

示例：
spring.txt内容

class=reflection.Service1
method=doService1

 
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测试类

package service;
public class ReflectTest {
	@SuppressWarnings({ "rawtypes", "unchecked" })
    public static void main(String[] args) throws Exception {
    <span class="token comment">//从spring.txt中获取类名称和方法名称</span>
    File springConfigFile <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">File</span><span class="token punctuation">(</span><span class="token string">"H:\\eclpise-workspace\\reflect-demo\\src\\spring.txt"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    Properties springConfig<span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Properties</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    springConfig<span class="token punctuation">.</span><span class="token function">load</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">FileInputStream</span><span class="token punctuation">(</span>springConfigFile<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    String className <span class="token operator">=</span> <span class="token punctuation">(</span>String<span class="token punctuation">)</span> springConfig<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token string">"class"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    String methodName <span class="token operator">=</span> <span class="token punctuation">(</span>String<span class="token punctuation">)</span> springConfig<span class="token punctuation">.</span><span class="token function">get</span><span class="token punctuation">(</span><span class="token string">"method"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
     
    <span class="token comment">//根据类名称获取类对象</span>
    Class <span class="token class-name">clazz</span> <span class="token operator">=</span> Class<span class="token punctuation">.</span><span class="token function">forName</span><span class="token punctuation">(</span>className<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//根据方法名称，获取方法对象</span>
    Method m <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getMethod</span><span class="token punctuation">(</span>methodName<span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//获取构造器</span>
    Constructor c <span class="token operator">=</span> clazz<span class="token punctuation">.</span><span class="token function">getConstructor</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//根据构造器，实例化出对象</span>
    Object service <span class="token operator">=</span> c<span class="token punctuation">.</span><span class="token function">newInstance</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token comment">//调用对象的指定方法</span>
    m<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>service<span class="token punctuation">)</span><span class="token punctuation">;</span>
     
<span class="token punctuation">}</span>

}

2:通过反射越过泛型检查

泛型是在编译期间起作用的。在编译后的.class文件中是没有泛型的。所有比如T或者E类型啊，本质都是通过Object处理的。所以可以通过使用反射来越过泛型。

示例：

package test;
public class GenericityTest {
	public static void main(String[] args) throws Exception{
ArrayList<span class="token generics function"><span class="token punctuation">&lt;</span>String<span class="token punctuation">&gt;</span></span> list <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList</span><span class="token operator">&lt;</span><span class="token operator">&gt;</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"this"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
list<span class="token punctuation">.</span><span class="token function">add</span><span class="token punctuation">(</span><span class="token string">"is"</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//	strList.add(5);报错</span>

<span class="token comment">/********** 越过泛型检查    **************/</span>

<span class="token comment">//获取ArrayList的Class对象，反向的调用add()方法，添加数据</span>
Class <span class="token class-name">listClass</span> <span class="token operator">=</span> list<span class="token punctuation">.</span><span class="token function">getClass</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> 
<span class="token comment">//获取add()方法</span>
Method m <span class="token operator">=</span> listClass<span class="token punctuation">.</span><span class="token function">getMethod</span><span class="token punctuation">(</span><span class="token string">"add"</span><span class="token punctuation">,</span> Object<span class="token punctuation">.</span><span class="token keyword">class</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token comment">//调用add()方法</span>
m<span class="token punctuation">.</span><span class="token function">invoke</span><span class="token punctuation">(</span>list<span class="token punctuation">,</span> <span class="token number">5</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

<span class="token comment">//遍历集合</span>
<span class="token keyword">for</span><span class="token punctuation">(</span>Object obj <span class="token operator">:</span> list<span class="token punctuation">)</span><span class="token punctuation">{</span>
	System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>obj<span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
<span class="token punctuation">}</span>

}

反射部分先记录到这。
接下来计划认真理解下spring的依赖注入和反转控制！

GitHub源码：java反射

Java基础之—反射（非常重要） ↩︎
有什么用
后记：
本文部分内容引用自csdn的敬业的小码哥和How2jJava，如有侵权，请联系我 ↩︎

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