一个例子让你了解Java反射机制

水深无声 2022-09-17 12:19 147阅读 0赞

**本文来自：blog.csdn.net/ljphhj**

**JAVA反射机制：**

**通俗地说,反射机制就是可以把一个类,类的成员(函数,属性),当成一个对象来操作,希望读者能理解,也就是说,类,类的成员,我们在运行的时候还可以动态地去操作他们.**

**理论的东东太多也没用，下面我们看看实践 Demo ～**

**Demo:**

package cn.lee.demo;
    
    import java.lang.reflect.Constructor;
    import java.lang.reflect.Field;
    import java.lang.reflect.InvocationTargetException;
    import java.lang.reflect.Method;
    import java.lang.reflect.Modifier;
    import java.lang.reflect.TypeVariable;
    
    public class Main {
    	/**
    	 * 为了看清楚Java反射部分代码，所有异常我都最后抛出来给虚拟机处理！
    	 * @param args
    	 * @throws ClassNotFoundException
    	 * @throws InstantiationException
    	 * @throws IllegalAccessException
    	 * @throws InvocationTargetException 
    	 * @throws IllegalArgumentException 
    	 * @throws NoSuchFieldException 
    	 * @throws SecurityException 
    	 * @throws NoSuchMethodException 
    	 */
    	public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, SecurityException, NoSuchFieldException, NoSuchMethodException {
    		// TODO Auto-generated method stub
    		
    		//Demo1.  通过Java反射机制得到类的包名和类名
    		Demo1();
    		System.out.println("===============================================");
    		
    		//Demo2.  验证所有的类都是Class类的实例对象
    		Demo2();
    		System.out.println("===============================================");
    		
    		//Demo3.  通过Java反射机制，用Class 创建类对象[这也就是反射存在的意义所在]，无参构造
    		Demo3();
    		System.out.println("===============================================");
    		
    		//Demo4:  通过Java反射机制得到一个类的构造函数，并实现构造带参实例对象
    		Demo4();
    		System.out.println("===============================================");
    		
    		//Demo5:  通过Java反射机制操作成员变量, set 和 get
    		Demo5();
    		System.out.println("===============================================");
    		
    		//Demo6: 通过Java反射机制得到类的一些属性： 继承的接口，父类，函数信息，成员信息，类型等
    		Demo6();
    		System.out.println("===============================================");
    		
    		//Demo7: 通过Java反射机制调用类中方法
    		Demo7();
    		System.out.println("===============================================");
    		
    		//Demo8: 通过Java反射机制获得类加载器
    		Demo8();
    		System.out.println("===============================================");
    		
    	}
    	
    	/**
    	 * Demo1: 通过Java反射机制得到类的包名和类名
    	 */
    	public static void Demo1()
    	{
    		Person person = new Person();
    		System.out.println("Demo1: 包名: " + person.getClass().getPackage().getName() + "，" 
    				+ "完整类名: " + person.getClass().getName());
    	}
    	
    	/**
    	 * Demo2: 验证所有的类都是Class类的实例对象
    	 * @throws ClassNotFoundException 
    	 */
    	public static void Demo2() throws ClassNotFoundException
    	{
    		//定义两个类型都未知的Class , 设置初值为null, 看看如何给它们赋值成Person类
    		Class<?> class1 = null;
            Class<?> class2 = null;
            
            //写法1, 可能抛出 ClassNotFoundException [多用这个写法]
            class1 = Class.forName("cn.lee.demo.Person");
            System.out.println("Demo2:(写法1) 包名: " + class1.getPackage().getName() + "，" 
    				+ "完整类名: " + class1.getName());
            
            //写法2
            class2 = Person.class;
            System.out.println("Demo2:(写法2) 包名: " + class2.getPackage().getName() + "，" 
    				+ "完整类名: " + class2.getName());
    	}
    	
    	/**
    	 * Demo3: 通过Java反射机制，用Class 创建类对象[这也就是反射存在的意义所在]
    	 * @throws ClassNotFoundException 
    	 * @throws IllegalAccessException 
    	 * @throws InstantiationException 
    	 */
    	public static void Demo3() throws ClassNotFoundException, InstantiationException, IllegalAccessException
    	{
    		Class<?> class1 = null;
    		class1 = Class.forName("cn.lee.demo.Person");
    		//由于这里不能带参数，所以你要实例化的这个类Person，一定要有无参构造函数哈～
    		Person person = (Person) class1.newInstance();
    		person.setAge(20);
    		person.setName("LeeFeng");
    		System.out.println("Demo3: " + person.getName() + " : " + person.getAge());
    	}
    	
    	/**
    	 * Demo4: 通过Java反射机制得到一个类的构造函数，并实现创建带参实例对象
    	 * @throws ClassNotFoundException 
    	 * @throws InvocationTargetException 
    	 * @throws IllegalAccessException 
    	 * @throws InstantiationException 
    	 * @throws IllegalArgumentException 
    	 */
    	public static void Demo4() throws ClassNotFoundException, IllegalArgumentException, InstantiationException, IllegalAccessException, InvocationTargetException
    	{
    		Class<?> class1 = null;
    		Person person1 = null;
    		Person person2 = null;
    		
    		class1 = Class.forName("cn.lee.demo.Person");
    		//得到一系列构造函数集合
    		Constructor<?>[] constructors = class1.getConstructors();
    		
    		person1 = (Person) constructors[0].newInstance();
    		person1.setAge(30);
    		person1.setName("leeFeng");
    		
    		person2 = (Person) constructors[1].newInstance(20,"leeFeng");
    		
    		System.out.println("Demo4: " + person1.getName() + " : " + person1.getAge()
    				+ "  ,   " + person2.getName() + " : " + person2.getAge()
    				);
    		
    	}
    	
    	/**
    	 * Demo5: 通过Java反射机制操作成员变量, set 和 get
    	 * 
    	 * @throws IllegalAccessException 
    	 * @throws IllegalArgumentException 
    	 * @throws NoSuchFieldException 
    	 * @throws SecurityException 
    	 * @throws InstantiationException 
    	 * @throws ClassNotFoundException 
    	 */
    	public static void Demo5() throws IllegalArgumentException, IllegalAccessException, SecurityException, NoSuchFieldException, InstantiationException, ClassNotFoundException
    	{
    		Class<?> class1 = null;
    		class1 = Class.forName("cn.lee.demo.Person");
    		Object obj = class1.newInstance();
    		
    		Field personNameField = class1.getDeclaredField("name");
    		personNameField.setAccessible(true);
    		personNameField.set(obj, "胖虎先森");
    		
    		
    		System.out.println("Demo5: 修改属性之后得到属性变量的值：" + personNameField.get(obj));
    		
    	}
    	
    
    	/**
    	 * Demo6: 通过Java反射机制得到类的一些属性： 继承的接口，父类，函数信息，成员信息，类型等
    	 * @throws ClassNotFoundException 
    	 */
    	public static void Demo6() throws ClassNotFoundException
    	{
    		Class<?> class1 = null;
    		class1 = Class.forName("cn.lee.demo.SuperMan");
    		
    		//取得父类名称
    		Class<?>  superClass = class1.getSuperclass();
    		System.out.println("Demo6:  SuperMan类的父类名: " + superClass.getName());
    		
    		System.out.println("===============================================");
    		
    		
    		Field[] fields = class1.getDeclaredFields();
    		for (int i = 0; i < fields.length; i++) {
    			System.out.println("类中的成员: " + fields[i]);
    		}
    		System.out.println("===============================================");
    		
    		
    		//取得类方法
    		Method[] methods = class1.getDeclaredMethods();
    		for (int i = 0; i < methods.length; i++) {
    			System.out.println("Demo6,取得SuperMan类的方法：");
    			System.out.println("函数名：" + methods[i].getName());
    			System.out.println("函数返回类型：" + methods[i].getReturnType());
    			System.out.println("函数访问修饰符：" + Modifier.toString(methods[i].getModifiers()));
    			System.out.println("函数代码写法： " + methods[i]);
    		}
    		
    		System.out.println("===============================================");
    		
    		//取得类实现的接口,因为接口类也属于Class,所以得到接口中的方法也是一样的方法得到哈
    		Class<?> interfaces[] = class1.getInterfaces();
    		for (int i = 0; i < interfaces.length; i++) {
    			System.out.println("实现的接口类名: " + interfaces[i].getName() );
    		}
    		
    	}
    	
    	/**
    	 * Demo7: 通过Java反射机制调用类方法
    	 * @throws ClassNotFoundException 
    	 * @throws NoSuchMethodException 
    	 * @throws SecurityException 
    	 * @throws InvocationTargetException 
    	 * @throws IllegalAccessException 
    	 * @throws IllegalArgumentException 
    	 * @throws InstantiationException 
    	 */
    	public static void Demo7() throws ClassNotFoundException, SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException, InstantiationException
    	{
    		Class<?> class1 = null;
    		class1 = Class.forName("cn.lee.demo.SuperMan");
    		
    		System.out.println("Demo7: \n调用无参方法fly()：");
    		Method method = class1.getMethod("fly");
    		method.invoke(class1.newInstance());
    		
    		System.out.println("调用有参方法walk(int m)：");
    		method = class1.getMethod("walk",int.class);
    		method.invoke(class1.newInstance(),100);
    	}
    	
    	/**
    	 * Demo8: 通过Java反射机制得到类加载器信息
    	 * 
    	 * 在java中有三种类类加载器。[这段资料网上截取]
    
    		1）Bootstrap ClassLoader 此加载器采用c++编写，一般开发中很少见。
    
    		2）Extension ClassLoader 用来进行扩展类的加载，一般对应的是jre\lib\ext目录中的类
    
    		3）AppClassLoader 加载classpath指定的类，是最常用的加载器。同时也是java中默认的加载器。
    	 * 
    	 * @throws ClassNotFoundException 
    	 */
    	public static void Demo8() throws ClassNotFoundException
    	{
    		Class<?> class1 = null;
    		class1 = Class.forName("cn.lee.demo.SuperMan");
    		String nameString = class1.getClassLoader().getClass().getName();
    		
    		System.out.println("Demo8: 类加载器类名: " + nameString);
    	}
    	
    	
    	
    }
    /**
     * 
     * @author xiaoyaomeng
     *
     */
    class  Person{
    	private int age;
    	private String name;
    	public Person(){
    		
    	}
    	public Person(int age, String name){
    		this.age = age;
    		this.name = name;
    	}
    
    	public int getAge() {
    		return age;
    	}
    	public void setAge(int age) {
    		this.age = age;
    	}
    	public String getName() {
    		return name;
    	}
    	public void setName(String name) {
    		this.name = name;
    	}
    }
    
    class SuperMan extends Person implements ActionInterface
    {
    	private boolean BlueBriefs;
    	
    	public void fly()
    	{
    		System.out.println("超人会飞耶～～");
    	}
    	
    	public boolean isBlueBriefs() {
    		return BlueBriefs;
    	}
    	public void setBlueBriefs(boolean blueBriefs) {
    		BlueBriefs = blueBriefs;
    	}
    
    	@Override
    	public void walk(int m) {
    		// TODO Auto-generated method stub
    		System.out.println("超人会走耶～～走了" + m + "米就走不动了！");
    	}
    }
    interface ActionInterface{
    	public void walk(int m);
    }

**个人觉得使用反射机制的一些地方：**

**1.工厂模式：Factory类中用反射的话，添加了一个新的类之后，就不需要再修改工厂类Factory了  
2.数据库JDBC中通过Class.forName(Driver).来获得数据库连接驱动  
3.分析类文件：毕竟能得到类中的方法等等  
4.访问一些不能访问的变量或属性：破解别人代码**

**Demo下载地址：[http://download.csdn.net/detail/u011133213/6420969][http_download.csdn.net_detail_u011133213_6420969]**

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[http_download.csdn.net_detail_u011133213_6420969]: http://download.csdn.net/detail/u011133213/6420969
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