一个例子让你了解Java反射机制

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 getDemo5();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;}@Overridepublic void walk(int m) {// TODO Auto-generated method stubSystem.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

反射机制调用静态方法：

Class c;c = Class.forName("class name");Method m = c.getMethod("method name", new Class[] {int.class, int.class, int.class,int.class});m.invoke(c, new Object[] {1,2, 3, 4});