Java反射

1. 定义

JAVA反射（放射）机制：“程序运行时，允许改变程序结构或变量类型，这种语言称为动态语言”。从这个观点看，Perl，Python，Ruby是动态语言，C++，Java，C#不是动态

语言。但是JAVA有着一个非常突出的动态相关机制：Reflection，用在Java身上指的是我们可以于运行时加载、探知、使用编译期间完全未知的classes。换句话说，Java程序

可以加载一个运行时才得知名称的class，获悉其完整构造（但不包括methods定义），并生成其对象实体、或对其fields设值、或唤起其methods。其中，有一个Class类型，

它是实现反射的起源，针对任何您想探勘的类，唯有先为它产生一个Class 对象，接下来才能经由后者唤起为数十多个的Reflection APIs。而关于jvm如何加载类的过程，见文

章：Java类加载机制深度解析。

2. 实例应用

案例接口

public interface China {public void sayHello(String name, Integer age);}

案例类

public class Person implements China{private int age;private String name;public int getAge() {return age;}public final void setAge(Integer age) {this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}@Overridepublic String toString() {return "Person [age=" + age + ", name=" + name + "]";}public Person(int age, String name) {super();this.age = age;this.name = name;}public Person(int age){super();this.age = age;}public Person(String name){super();this.name = name;}public Person() {super();}@Overridepublic void sayHello(String name, Integer age) {System.out.println("hello "+ name + " : " + age);}}

测试类

public class ReflectTest {/** * 通过Class.forName和ClassLoader来加载类 * @throws ClassNotFoundException */@Testpublic void test1() throws ClassNotFoundException{Class<?> demo1 = null;Class<?> demo2 = null;Class<?> demo3 = null;Class<?> demo4 = null;demo1 = Class.forName("cn.test.reflect.Demo");demo2 = new Demo().getClass();demo3 = Demo.class;demo4 = Demo.class.getClassLoader().loadClass("cn.test.reflect.Demo");System.out.println(demo1.getName());System.out.println(demo2.getName());System.out.println(demo3.getName());System.out.println(demo4.getName());}/** * 通过Class.forName("").newInstance()生成新对象，调用的是无参构造函数 * @throws ClassNotFoundException * @throws InstantiationException * @throws IllegalAccessException */@Testpublic void test2() throws ClassNotFoundException, InstantiationException, IllegalAccessException{Class<?> demo = Class.forName("cn.test.reflect.Person");Person person = (Person) demo.newInstance();person.setAge(12);person.setName("张三");System.out.println(person);}/** * 打印出该类所有的信息：属性，方法，构造函数 * @throws ClassNotFoundException */@Testpublic void test3() throws ClassNotFoundException{Class<?> demo = Class.forName("cn.test.reflect.Person");System.out.println("类属性");Field[] fields = demo.getDeclaredFields();for(Field field : fields){int mo = field.getModifiers();System.out.print(Modifier.toString(mo)+" ");System.out.print(field.getType().getName()+" ");System.out.print(field.getName());System.out.println();}System.out.println("所有方法");Method[] methods = demo.getDeclaredMethods();for(Method method : methods){Class<?>[] parms = method.getParameterTypes();int mo = method.getModifiers();System.out.print(Modifier.toString(mo)+" ");System.out.print(method.getName()+"(");int i=1;for(Class<?> parm : parms){System.out.print(parm.getName() + " arg"+i++);if(i<=parms.length){System.out.print(",");}}System.out.print(")");System.out.println();}Constructor<?>[] constructors = demo.getConstructors();System.out.print("构造方法：");for(Constructor<?> constructor : constructors){System.out.println();Class<?>[] parms = constructor.getParameterTypes();int mo = constructor.getModifiers();System.out.print(Modifier.toString(mo)+" ");System.out.print(constructor.getName()+"(");int i=1;for(Class<?> parm : parms){System.out.print(parm.getName() + " arg"+i++);if(i<=parms.length){System.out.print(",");}}System.out.print("){}");}}/** * 获取接口 * @throws ClassNotFoundException  */@Testpublic void test4() throws ClassNotFoundException{Class<?> demo = Class.forName("cn.test.reflect.Person");Class<?>[] intes = demo.getInterfaces();for(Class<?> interf : intes){System.out.println(interf.getName());}}/** * 获取父类 * @throws ClassNotFoundException */@Testpublic void test5() throws ClassNotFoundException{Class<?> demo = Class.forName("cn.test.reflect.Person");Class<?> superClass = demo.getSuperclass();System.out.println(superClass.getName());}/** * 反射调用类的中任意方法，需要知道方法名 * @throws ClassNotFoundException  * @throws IllegalAccessException  * @throws InstantiationException  * @throws SecurityException  * @throws NoSuchMethodException  * @throws InvocationTargetException  * @throws IllegalArgumentException  */@Testpublic void test6() throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException{Class<?> demo = Class.forName("cn.test.reflect.Person");Person person = (Person) demo.newInstance();Method sayHelloMethod = demo.getMethod("sayHello", String.class, Integer.class);sayHelloMethod.invoke(person, "张三", 20);Method setAgeMethod = demo.getMethod("setAge", Integer.class);setAgeMethod.invoke(person, 2);Method getAgeMethod = demo.getMethod("getAge");System.out.println(getAgeMethod.invoke(person));Method setNameMethod = demo.getMethod("setName", String.class);setNameMethod.invoke(person, "李四");Method getNameMethod = demo.getMethod("getName");System.out.println(getNameMethod.invoke(person));Method toStringMethod = demo.getMethod("toString");System.out.println(toStringMethod.invoke(person));}/** * 反射设置属性值 * @throws ClassNotFoundException * @throws InstantiationException * @throws IllegalAccessException * @throws NoSuchMethodException * @throws SecurityException * @throws IllegalArgumentException * @throws InvocationTargetException * @throws NoSuchFieldException  */@Testpublic void test7() throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, SecurityException, IllegalArgumentException, InvocationTargetException, NoSuchFieldException{Class<?> demo = Class.forName("cn.test.reflect.Person");Person person = (Person) demo.newInstance();Field ageField = demo.getDeclaredField("age");ageField.setAccessible(true);ageField.set(person, 10);Field nameField = demo.getDeclaredField("name");nameField.setAccessible(true);nameField.set(person, "王武");System.out.println(person);}}