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
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;
}
@Override
public 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();
}
@Override
public void sayHello(String name, Integer age) {
System.out.println("hello "+ name + " : " + age);
}
}
public class ReflectTest {
/**
* 通过Class.forName和ClassLoader来加载类
* @throws ClassNotFoundException
*/
@Test
public 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
*/
@Test
public 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
*/
@Test
public 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
*/
@Test
public 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
*/
@Test
public 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
*/
@Test
public 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
*/
@Test
public 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);
}
}