一、什么是反射

二、如何使用反射

1.第一步：加载类，获取类的字节码（获取类的Class对象）

public class reflectTest {public static void main(String[] args) throws Exception {Class c1 =  Student.class; //通过类名得到学生类的Class对象System.out.println ( c1.getName () );//全类名  study.reflect.StudentSystem.out.println ( c1.getSimpleName () );//简名： StudentClass c2 = Class.forName ( "study.reflect.Student" );//必须填入类的全类名System.out.println (c1==c2);//true   拿到的是同一个对象Student s= new Student ();Class  c3 = s.getClass ();System.out.println (c2==c3);//true}
}

2.获取类的构造器

import org.junit.Test;import java.lang.reflect.Constructor;public class Test2Constructor {@Testpublic void test1(){//1.反射第一步必须先得到这个类的Class对象Class c1=Cat.class;//2.获取类的全部构造器
//        Constructor[] constructors = c1.getConstructors ();//只能拿public修饰的构造器Constructor[] constructors = c1.getDeclaredConstructors ();//获取所有声明的构造器，不管是什么修饰符的//3.遍历数组中的每个构造器对象for (Constructor constructor : constructors) {System.out.println (constructor.getName ()+"--->"+constructor.getParameterCount ());//获得构造器的参数数量}}@Testpublic void testGetConstructor() throws Exception {//1.反射第一步必须先得到这个类的Class对象Class c=Cat.class;//2.获取某个构造器：无参构造器Constructor constructor = c.getConstructor ();//只能拿public修饰的构造器  须填入参数类型Constructor declaredConstructor = c.getDeclaredConstructor ();//可以拿到所有权限修饰的构造器System.out.println (constructor.getName ()+"--->"+constructor.getParameterCount ());System.out.println (declaredConstructor.getName ()+"--->"+declaredConstructor.getParameterCount ());//3.获取有参数的构造器Constructor constructor1 = c.getConstructor ( String.class, int.class );Constructor constructor2 = c.getDeclaredConstructor ( String.class, int.class );System.out.println (constructor1.getName ()+"--->"+constructor1.getParameterCount ());System.out.println (constructor2.getName ()+"--->"+constructor2.getParameterCount ());}
}

3.用构造器可以通过反射初始化对象

import org.junit.Test;import java.lang.reflect.Constructor;public class Test2Constructor {@Testpublic void test1(){//1.反射第一步必须先得到这个类的Class对象Class c1=Cat.class;//2.获取类的全部构造器
//        Constructor[] constructors = c1.getConstructors ();//只能拿public修饰的构造器Constructor[] constructors = c1.getDeclaredConstructors ();//获取所有声明的构造器，不管是什么修饰符的//3.遍历数组中的每个构造器对象for (Constructor constructor : constructors) {System.out.println (constructor.getName ()+"--->"+constructor.getParameterCount ());//获得构造器的参数数量}}@Testpublic void testGetConstructor() throws Exception {//1.反射第一步必须先得到这个类的Class对象Class c=Cat.class;//2.获取某个构造器：无参构造器
//        Constructor constructor = c.getConstructor ();//只能拿public修饰的构造器  须填入参数类型Constructor declaredConstructor = c.getDeclaredConstructor ();//可以拿到所有权限修饰的构造器
//        System.out.println (constructor.getName ()+"--->"+constructor.getParameterCount ());System.out.println (declaredConstructor.getName ()+"--->"+declaredConstructor.getParameterCount ());//3.获取有参数的构造器
//        Constructor constructor1 = c.getConstructor ( String.class, int.class );Constructor constructor2 = c.getDeclaredConstructor ( String.class, int.class );
//        System.out.println (constructor1.getName ()+"--->"+constructor1.getParameterCount ());System.out.println (constructor2.getName ()+"--->"+constructor2.getParameterCount ());declaredConstructor.setAccessible ( true );//禁止检查访问权限Cat cat = (Cat) declaredConstructor.newInstance ();System.out.println ( cat );constructor2.setAccessible ( true );Cat cat1 = (Cat) constructor2.newInstance ( "咪咪", 2 );System.out.println ( cat1 );}
}

4.获取类的成员变量

import org.junit.Test;import java.lang.reflect.Field;public class Test3Constructor {@Testpublic void testGetFields() throws Exception {//1.反射第一步，必须先拿到类的Class对象Class c1=Cat.class;//2.获取类的全部成员变量Field[] fields = c1.getDeclaredFields ();//3.遍历数组for (Field field : fields) {System.out.println ( field.getName ()+"---->"+field.getType () );}//4.定位某个成员变量Field fname = c1.getDeclaredField ( "name" );System.out.println (fname.getName ()+"--->"+ fname.getType () );//5.复制Cat cat=new Cat ();fname.setAccessible ( true );fname.set ( cat,"加菲猫" );System.out.println ( cat );//6.取值String name = (String) fname.get ( cat );System.out.println ( name );}
}

5.获取类的成员方法，并执行方法

import org.junit.Test;import java.lang.reflect.Method;public class TestMethod {@Testpublic void getMehtod() throws Exception {//1.反射第一步：先得到class对象Class c=Cat.class;//2.获取类的全部成员方法Method[] methods = c.getMethods ();//只能获取public修饰的方法for (Method method : methods) {System.out.println ( method.getName ()+"---->"+method.getParameterCount ()+"---->"+method.getReturnType () );}System.out.println ("-----------------------------------------");Method[] declaredMethods = c.getDeclaredMethods ();//获取所有权限的方法for (Method declaredMethod : declaredMethods) {System.out.println ( declaredMethod.getName ()+"---->"+declaredMethod.getParameterCount ()+"---->"+declaredMethod.getReturnType ());//返回返回值类型}System.out.println ("-----------------------------------------");//3.获取某个方法对象Method run = c.getDeclaredMethod ( "run" );System.out.println (run.getName ()+"---->"+run.getParameterCount ()+"---->"+run.getReturnType ());//run---->0---->voidCat cat = new Cat ();run.setAccessible ( true );run.invoke (cat);//必须传入触发方法的对象}
}

三、反射的作用，应用场景

1.反射的作用

2.使用反射做一个简易版的框架

框架类

import java.io.FileOutputStream;
import java.io.PrintStream;
import java.lang.reflect.Field;public class ObjectFrame {//保存任意对象的字段和其数据保存带文件中public static void saveObject(Object obj) throws Exception {PrintStream ps = new PrintStream (new FileOutputStream ( "\\xuexiaaa\\src\\666.txt" ,true),true );//obj 是任意对象，到底要有多少个字段要保存？Class c = obj.getClass ();String cName = c.getSimpleName ();ps.println ("--------------------"+c+"----------------------");//2.从这个类中提取它的全部成员变量Field[] fields = c.getDeclaredFields ();//3.遍历每个成员变量for (Field field : fields) {//4.拿到成员变量的名字field.setAccessible ( true );String name = field.getName ();//5.拿到这个成员变量在对象中的数据String value =  field.get ( obj )+"";ps.println (name+"="+value);}ps.close ();}
}

测试类

public class TestFrame {public static void main(String[] args) throws Exception {Student s1 = new Student ( "吴彦祖", 18, "男" );Teacher t1 = new Teacher ( "八八", 99.9 );//需求把任意对象的字段名和其对应的信息ObjectFrame.saveObject ( s1 );ObjectFrame.saveObject ( t1 );}
}