1.在运行时判断任意一个对象所属的类
2.在运行时构造任意一个类的对象
3.在运行时判断任意一个类所具有的成员变量和方法(通过反射甚至可以调用private方法)
4.在运行时调用任意一个对象的方法(注意:前提都是在运行时,而不是在编译时)
位于java。lang。reflect包中
--Class类:代表一个类
--Filed类:代表类的成员变量
--Method类:代表类的方法
--Constructor类:代表类的构造方法
--Array类:提供了动态创建数组,以及访问数组的元素的静态方法。该类中的所有方法都是静态方法
Java允许我们从多种途径为一个类class生成对应的Class对象。
--运用 getClass():Object类中的方法,每个类都拥有此方法
String str="abc";
Class cl=str.getClass();
--运用 Class。getSuperclass():Class类中的方法,返回该Class的父类的Class
--运用 Class。forName()静态方法:
--运用 ,Class:类名.class
--运用primitive wrapper classes的TYPE语法: 基本类型包装类的TYPE,如:Integer.TYPE
注意:TYPE的使用,只适合原生(基本)数据类型
package domain;
public interface Fruit {
public abstract void eat();
}
package domain;
public class Apple implements Fruit{
@Override
public void eat() {
System.out.println("Alan Turing died eating an poisoned apple");
}
}
package domain;
public class Banana implements Fruit{
@Override
public void eat() {
System.out.println("banaba is very nice");
}
}
package reflect;
import domain.Fruit;
public class FruitReflection {
public void printer(String classPath) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
Class<?> fruit = Class.forName(classPath);
Fruit ck = (Fruit) fruit.newInstance();
ck.eat();
}
public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException {
new FruitReflection().printer("domain.Banana");
new FruitReflection().printer("domain.Apple");
}
}
运行结果:
banaba is very nice
Alan Turing died eating an poisoned apple
二、通过反射调用接口、构造函数及父类
package domain;
public interface China {
public static final String name = "Alex";
public static final int age = 23;
public void sayChina();
public void sayHello(String name, int age);
}
package domain;
public class ChinaImp implements China {
private String name;
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public ChinaImp() {
}
public ChinaImp(String name) {
this.name = name;
}
@Override
public void sayChina() {
System.out.println("Say hello China");
}
@Override
public void sayHello(String name, int age) {
System.out.println(name + " " + age);
}
}
package reflect;
import java.lang.reflect.Constructor;
public class ChinaReflection {
public static void main(String args[]) {
Class c = null;
try {
c = Class.forName("domain.ChinaImp"); //domain is the package of ChinaImp.java
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
Class interfaces[] = c.getInterfaces();
Class superClass = c.getSuperclass();
Constructor[] cons = c.getConstructors();
for (int i = 0; i < interfaces.length; i++) {
System.out.println("interface -- >" + interfaces[i].getName());
}
System.out.println("SuperClass -- >" + superClass.getName());
for (int i = 0; i < cons.length; i++){
System.out.println("constructor name -- >" + cons[i]);
}
}
}
运行结果:
interface -- >domain.China
SuperClass -- >java.lang.Object
constructor name -- >public domain.ChinaImp(java.lang.String)
constructor name -- >public domain.ChinaImp()
三、通过反射获得定义的字段、方法、方法参数、方法修饰符、返回类型、以及特定类的方法的调用。
package reflect;
import domain.ChinaImp;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;
public class ChinaReflection_2 {
public static void main(String[] args) throws NoSuchMethodException, IllegalAccessException, InstantiationException, InvocationTargetException {
Class ci = null;
try {
ci = Class.forName("domain.ChinaImp");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
Field[] fields = ci.getFields();
Field[] declaredFields = ci.getDeclaredFields();
Method methods[] = ci.getMethods();
Class<?>[] para = null;
for (int i = 0; i < fields.length; i++) {
System.out.println("all fields -- > " + fields[i].getName());
}
System.out.println("****************************************");
for (int i = 0; i < declaredFields.length; i++) {
System.out.println("declared fields -- > " + declaredFields[i].getName());
}
System.out.println("****************************************");
for (int i = 0; i < methods.length; i++) {
System.out.println("method name -- > " + methods[i].getName());
para = methods[i].getParameterTypes();
for (int j = 0; j < para.length; j++) {
System.out.println("paramater -- > " + para[j].getName());
}
int modifier = methods[i].getModifiers();
System.out.println("Modifiers -- > " + Modifier.toString(modifier));
Class<?> returnType = methods[i].getReturnType();
System.out.println("return type -- > " + returnType.getName());
System.out.println(" --------------------------------------- ");
}
System.out.println("invoke method");
Method invokeMethods = ci.getMethod("sayChina");
invokeMethods.invoke(ci.newInstance());
invokeMethods = ci.getMethod("sayHello", String.class, int.class);
invokeMethods.invoke(ci.newInstance(), "David Beckham", 37); //函数调用会隐含地把调用该函数的对象传到函数的第一个函数,
//所以此处需要把该函数的对象传到invoke中去
System.out.println("invoke setter and getter");
ChinaImp obj = (ChinaImp) ci.newInstance();
setter(ci, "Name", obj);
getter(ci, "Name", obj);
}
private static void setter(Class ci, String name, ChinaImp chinaImp) throws InstantiationException {
Method method = null;
try {
method = ci.getMethod("set" + name, String.class);
} catch (NoSuchMethodException e) {
e.printStackTrace();
}
try {
System.out.println("call setter method");
method.invoke(chinaImp, "Robin Van Persie");
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
public static void getter(Class clazz, String attr, ChinaImp chinaImp) {
Method method = null;
try {
method = clazz.getMethod("get" + attr);
} catch (NoSuchMethodException e) {
e.printStackTrace();
}
try {
System.out.println("getter value : " + method.invoke(chinaImp));
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}
}
四、通过反射构造出的对象去调用相应类的方法package domain;
public class Person {
private String name;
private int age;
public Person() {
}
public Person(int age) {
this.age = age;
}
public Person(String name) {
this.name = name;
}
public Person(String name, int age) {
this.age = age;
this.name = name;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
@Override
public String toString() {
return "[" + this.getName() + " : " + this.getAge() + "]";
}
}
package reflect;
import domain.Person;
public class PersonReflection {
public static void main(String[] args) {
Class person1 = null;
try {
person1 = Class.forName("domain.Person");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
Person person2 = null;
try {
person2 = (Person) person1.newInstance();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
person2.setName("David Beckham");
person2.setAge(37);
System.out.println(person2);
}
}
运行结果:[David Beckham : 37]
package reflect;
import domain.Person;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
public class PersonReflection_2 {
public static void main(String args[]) {
Class per = null;
try {
per = Class.forName("domain.Person");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
Constructor<?> cons[] = per.getConstructors();
Person person1 = null;
Person person2 = null;
Person person3 = null;
Person person4 = null;
try {
person1 = (Person) cons[0].newInstance();
person2 = (Person) cons[3].newInstance(37);
person3 = (Person) cons[2].newInstance("David Beckham");
person4 = (Person) cons[1].newInstance("David Beckham", 37);
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
System.out.println(person1);
System.out.println(person2);
System.out.println(person3);
System.out.println(person4);
}
}
运行结果:[null : 0]
[null : 37]
[David Beckham : 0]
[David Beckham : 37]
package domain;
public class ClassLoader_model {
}
package reflect;
import domain.ClassLoader_model;
public class ClassLoaderReflection {
public static void main(String args[]){
ClassLoader_model classLoader_model = new ClassLoader_model();
System.out.println("ClassLoader Name : " + classLoader_model.getClass().getClassLoader().getClass().getName());
}
}
运行结果:ClassLoader Name : sun.misc.Launcher$AppClassLoader
其实在java中有三种类类加载器。
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。
package domain;
public class RVP {
public String name;
int age;
String height;
}
package reflect;
import domain.RVP;
import java.lang.reflect.Field;
public class RVPReflection {
public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, IllegalAccessException, InstantiationException {
Class<?> cl = Class.forName("domain.RVP");
Field field = cl.getDeclaredField("name");
RVP rvp = (RVP) cl.newInstance();
field.setAccessible(true);
field.set(rvp, "Van Persie");
System.out.println(field.get(rvp));
}
}
Van Persie