Java foundation building 35-reflection (the bottom layer of the framework, must be mastered)

Table of contents

1. Reflection mechanism 

1. A demand elicits reflection

2. Schematic diagram of reflection

3. Reflection related classes

4. Reflective call optimization

Two, Class class

1. Class class analysis

2. Common methods of Class

3. Various ways to obtain class objects (six types)

4. Which types have Class objects

3. Class loading

1. Dynamic loading and static loading

2. Class loading process

3. Five stages of class loading

4. Reflection to obtain the structural information of the class

1. Get class structure information

2. Create objects through reflection

3. Manipulating attributes through reflection

4. Manipulating methods via reflection

5. Case exercises

1) Operational attributes

2) Create a file using reflection

---

1. Reflection mechanism 

1. A demand elicits reflection

re.properties

classfullpath=com.feiyang.basic17_reflect.Cat
--method=hi
method=run

package com.feiyang.basic17_reflect;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Cat {
    private String name="招财猫";
    public void hi(){
        System.out.println("hi " + name);
    }

    public void run(){
        System.out.println("run " + name);
    }
}

package com.feiyang.basic17_reflect;

import java.io.FileInputStream;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Properties;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflectionQuestion {

    public static void main(String[] args) throws IOException, ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
        //需求：根据配置文件指定信息，创建Cat对象并调用方法

        //传统实现方式：
        /*Cat c = new Cat();
        c.hi();*/

        //明白反射

        //1.读去配置文件
        Properties properties = new Properties();
        properties.load(new FileInputStream("src\\re.properties"));
        String classfullpath = properties.get("classfullpath").toString();
        String method = properties.get("method").toString();
        System.out.println("classfullpath=" + classfullpath);
        System.out.println("method=" + method);

        //创建对象，传统方法行不通 =》 反射机制
        Class aClass = Class.forName(classfullpath); //获取Class对象
        Object o = aClass.newInstance();  //实例化对象

        Method method1 = aClass.getMethod(method);  //通过Class对象获取方法（方法对象）
        method1.invoke(o);  //调用方法

    }
}

Results of the:

classfullpath=com.feiyang.basic17_reflect.Cat

method=run

run lucky cat

2. Schematic diagram of reflection

Schematic diagram of the reflection mechanism:

3. Reflection related classes

package com.feiyang.basic17_reflect;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.Properties;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Reflection01 {
    public static void main(String[] args) throws Exception {
        //1.读去配置文件
        Properties properties = new Properties();
        properties.load(new FileInputStream("src\\re.properties"));
        String classfullpath = properties.get("classfullpath").toString();
        String method = properties.get("method").toString();

        Class c = Class.forName(classfullpath); //获取Class对象
        Object o = c.newInstance();  //实例化对象


        //获取构造器（无参）
        Constructor constructor = c.getConstructor();
        System.out.println(constructor);

        //获取构造器（有参）
        Constructor constructor1 = c.getConstructor(String.class, int.class);
        System.out.println(constructor1);

        //获取属性
        Field age = c.getField("age");
        System.out.println(age.get(o));

        //获取方法
        Method method1 = c.getMethod(method);
        method1.invoke(o);  //调用方法

    }
}

Results of the:

public com.feiyang.basic17_reflect.Cat()

public com.feiyang.basic17_reflect.Cat(java.lang.String,int)

10

run lucky cat

4. Reflective call optimization

Code example:

re.properties

classfullpath=com.feiyang.basic17_reflect.Cat
method=hi

package com.feiyang.basic17_reflect;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Cat {
    private String name="招财猫";
    public int age = 10;

    public Cat(){

    }

    public Cat(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public void hi(){
        //System.out.println("hi " + name);
    }

    public void run(){
        //System.out.println("run " + name);
    }
}

package com.feiyang.basic17_reflect;

import java.io.FileInputStream;
import java.lang.reflect.Method;
import java.util.Properties;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Reflection02 {
    public static void main(String[] args) throws Exception {
        m1();
        m2();
        m3();

    }

    //传统方法调用
    public static void m1(){
        Cat cat = new Cat();
        long l = System.currentTimeMillis();
        for (int i = 0; i < 900000000; i++) {
            cat.hi();
        }
        long l1 = System.currentTimeMillis();
        System.out.println(l1-l);

    }


    //反射方式调用
    public static void m2() throws Exception {
        //1.读去配置文件
        Properties properties = new Properties();
        properties.load(new FileInputStream("src\\re.properties"));
        String classfullpath = properties.get("classfullpath").toString();
        String method = properties.get("method").toString();

        Class c = Class.forName(classfullpath); //获取Class对象
        Object o = c.newInstance();  //实例化对象

        //获取方法
        Method method1 = c.getMethod(method);
        long l = System.currentTimeMillis();
        for (int i = 0; i < 900000000; i++) {
            method1.invoke(o);  //调用方法
        }
        long l1 = System.currentTimeMillis();
        System.out.println(l1-l);
    }

    //反射调用优化（禁止安全检查）

    public static void m3() throws Exception {
        //1.读去配置文件
        Properties properties = new Properties();
        properties.load(new FileInputStream("src\\re.properties"));
        String classfullpath = properties.get("classfullpath").toString();
        String method = properties.get("method").toString();

        Class c = Class.forName(classfullpath); //获取Class对象
        Object o = c.newInstance();  //实例化对象

        //获取方法
        Method method1 = c.getMethod(method);
        method1.setAccessible(false);
        long l = System.currentTimeMillis();
        for (int i = 0; i < 900000000; i++) {
            method1.invoke(o);  //调用方法
        }
        long l1 = System.currentTimeMillis();
        System.out.println(l1-l);
    }

}

Relection02 execution result:

0

1954

1838

Two, Class class

1. Class class analysis

First look at the class diagram of the Class class:

It can be seen that the Class class is the same as other classes, and also inherits the Object class, but its usage method and completed functions are somewhat special, that's all (so, don't think it is very mysterious)

Example:

package com.feiyang.basic17_reflect;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:  Class类分析
 */
public class Class01 {
    public static void main(String[] args) throws ClassNotFoundException {

        //传统方法
        /**
         * debug发现：
         *
         * ClassLoader类的方法，加载
         * public Class<?> loadClass(String name) throws ClassNotFoundException {
         *         return loadClass(name, false);
         *     }
         */
        Cat cat = new Cat();

        //第2点：Class类对象不是new出来的，而是系统创建的
        /**
         * debug发现：
         *
         * 同样是ClassLoader类的方法，加载
         * public Class<?> loadClass(String name) throws ClassNotFoundException {
         *         return loadClass(name, false);
         *     }
         */
        Class c = Class.forName("com.feiyang.basic17_reflect.Cat");


        //第3点：对于某个类的Class类对象，在内存中只有一份，因为类只加载一次
        //此处要把上面的new 注释掉，否则，debug不到ClassLoader

    }
}

2. Common methods of Class

package com.feiyang.basic17_reflect;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Car {
    public String brand = "奔驰";
    public double price = 30.0;
    public String color = "灰色";

}

package com.feiyang.basic17_reflect;

import java.lang.reflect.Field;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Class02 {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchFieldException {
        //<?> 表示类型不确定
        //1 获取Car类对应的Class对象
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.Car");

        //2 输出cls
        System.out.println(cls);//显示cls对象，是哪个类的Class对象，class com.feiyang.basic17_reflect.Car
        System.out.println(cls.getClass());//显示cls运行类型，class java.lang.Class

        //3 得到包名
        System.out.println(cls.getPackage().getName());//com.feiyang.basic17_reflect

        //4 得到全类名
        System.out.println(cls.getName());//com.feiyang.basic17_reflect.Car

        //5 通过cls创建对象实例
        Car car = (Car)cls.newInstance();
        System.out.println(car);//com.feiyang.basic17_reflect.Car@610455d6

        //6 通过反射获取属性
        Field brand = cls.getField("brand");
        System.out.println(brand.get(car));//奔驰

        //7 通过反射给属性赋值
        brand.set(car,"宝马");
        System.out.println(brand.get(car));//宝马

        //8 得到所有属性
        Field[] fields = cls.getFields();
        for (Field f : fields) {
            System.out.println(f.getName());
        }
        /**
         * brand
         * price
         * color
         */

        for (Field f : fields) {
            System.out.println(f.get(car));
        }
        /**
         * 宝马
         * 30.0
         * 灰色
         */


    }
}

3. Various ways to obtain class objects (six types)

package com.feiyang.basic17_reflect;

import com.sun.scenario.effect.impl.sw.sse.SSEBlend_SRC_OUTPeer;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class GetClass_ {
    public static void main(String[] args) throws ClassNotFoundException {
        //1. Class.forName  通过读取配置文件
        String classPath = "com.feiyang.basic17_reflect.Car";
        Class<?> cls1 = Class.forName(classPath);
        System.out.println(cls1);  

        //2. 类名.class   应用场景：用于参数传递
        Class<Car> cls2 = Car.class;
        System.out.println(cls2);

        //3. 对象.getClass()  应用场景：有对象实例
        Car car = new Car();
        Class cls3 = car.getClass();
        System.out.println(cls3);

        //4. 通过类加载器来获取到类的class对象
        ClassLoader classLoader = car.getClass().getClassLoader();
        Class cls4 = classLoader.loadClass(classPath);
        System.out.println(cls4);

        //其实，上面的cls1、cls2、cls3、cls4
        System.out.println(cls1);
        System.out.println(cls2);
        System.out.println(cls3);
        System.out.println(cls4);//class com.feiyang.basic17_reflect.Car

        //5. 基本数据类型通过 .class获取Class对象
        Class<Integer> integerClass = int.class;
        Class<Character> characterClass = char.class;
        Class<Boolean> booleanClass = boolean.class;
        System.out.println(integerClass);//int

        //6. 基本数据类型对应的包装类可以通过.TYPE获取Class对象
        Class<Integer> type = Integer.TYPE;
        Class<Character> type1 = Character.TYPE;
        System.out.println(type);//int
        System.out.println(type1);//char

        System.out.println(integerClass.hashCode() == type.hashCode());//结果为true


    }
}

4. Which types have Class objects

package com.feiyang.basic17_reflect;

import java.io.Serializable;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class AllTypeClass {
    public static void main(String[] args) {
        Class<String> cls1 = String.class;
        Class<Serializable> cls2 = Serializable.class;
        Class<Integer[]> cls3 = Integer[].class;
        Class<float[][]> cls4 = float[][].class;
        Class<Deprecated> cls5 = Deprecated.class;
        Class<Thread.State> cls6 = Thread.State.class;
        Class<Long> cls7 = long.class;
        Class<Void> cls8 = void.class;
        Class<Class> cls9 = Class.class;

        System.out.println(cls1);
        System.out.println(cls2);
        System.out.println(cls3);
        System.out.println(cls4);
        System.out.println(cls5);
        System.out.println(cls6);
        System.out.println(cls7);
        System.out.println(cls8);
        System.out.println(cls9);


    }
}

printout:

class java.lang.String

interface java.io.Serializable

class [Ljava.lang.Integer;

class [[F

interface java.lang.Deprecated

class java.lang.Thread$State

long

void

class java.lang.Class

3. Class loading

1. Dynamic loading and static loading

package com.feiyang.basic17_reflect;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Scanner;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class classLoad_ {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
        Scanner scanner = new Scanner(System.in);
        System.out.println("请输入您要查询的序号： ");
        String next = scanner.next();
        switch (next){
            case "1":
                Car car = new Car();//静态加载，编译期就会加载Car类，找不到会报错
                System.out.println(car.brand);
                break;
            case "2":
                Class<?> cls = Class.forName("com.feiyang.basic17_reflect.Person");//动态加载，运行时加载，只有进入该条件时才会加载，编译期即使没有Person类也不会报错
                Object o = cls.newInstance();
                Method say = cls.getMethod("say");
                say.invoke(o);
                break;
            default:
                System.out.println("输入有误！");
                break;
        }
    }
}

class Person{
    public void say(){
        System.out.println("hi");
    }
}

2. Class loading process

3. Five stages of class loading

1) load

2) connect

3) Initialize

4. Reflection to obtain the structural information of the class

1. Get class structure information

package com.feiyang.basic17_reflect;

import org.junit.jupiter.api.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflectionUtils {

    public static void main(String[] args) {

    }

    @Test
    public void api_03() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        Method[] declaredMethods = cls.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println("获取本类中所有的方法：" + declaredMethod.getName()
            + "该方法的修饰符值：" + declaredMethod.getModifiers()
            + "该方法的返回值类型：" + declaredMethod.getReturnType());

            //输出当前方法的形参类型数组
            Class<?>[] parameterTypes = declaredMethod.getParameterTypes();
            for (Class<?> parameterType : parameterTypes) {
                System.out.println("该方法的参数类型：" + parameterType.getName());
            }
        }
    }

    @Test
    public void api_02() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        Field[] declaredFields = cls.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println("获取本类中所有的属性：" +declaredField.getName()
            + "该属性的修饰符值：" + declaredField.getModifiers()
            + "该属性的类型：" + declaredField.getType());
        }
    }

    @Test
    public void api_01() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        System.out.println("获取全类名：" + cls.getName());

        System.out.println("==");
        System.out.println("获取简单类名：" +cls.getSimpleName());

        System.out.println("==");
        Field[] fields = cls.getFields();
        for (Field field : fields) {
            System.out.println("获取所有public修饰的属性，包括本类以及父类：" +field.getName());
        }

        System.out.println("==");
        Field[] declaredFields = cls.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println("获取本类中所有的属性：" +declaredField.getName());
        }

        System.out.println("==");
        Method[] methods = cls.getMethods();
        for (Method method : methods) {
            System.out.println("获取所有public修饰的方法，包括本类以及父类：" + method.getName());
        }

        System.out.println("==");
        Method[] declaredMethods = cls.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println("获取本类中所有的方法：" + declaredMethod.getName());
        }

        System.out.println("==");
        Constructor<?>[] constructors = cls.getConstructors();
        for (Constructor<?> constructor : constructors) {
            System.out.println("获取所有public修饰的构造器，包括本类：" + constructor.getName());
        }

        System.out.println("==");
        Constructor<?>[] declaredConstructors = cls.getDeclaredConstructors();
        for (Constructor<?> declaredConstructor : declaredConstructors) {
            System.out.println("获取本类中所有的构造器：" + declaredConstructor.getName());
        }

        System.out.println("==");
        Package aPackage = cls.getPackage();
        System.out.println("获取包信息：" + aPackage);

        System.out.println("==");
        Class<?> superclass = cls.getSuperclass();
        System.out.println("获取父类Class对象信息：" + superclass.getName());

        System.out.println("==");
        Class<?>[] interfaces = cls.getInterfaces();
        for (Class<?> anInterface : interfaces) {
            System.out.println("获取接口信息：" + anInterface.getName());
        }

        System.out.println("==");
        Annotation[] annotations = cls.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println("获取注解信息：" + annotation);
        }

    }
}

interface IA{

}

@Deprecated
class People implements IA{
    public String name;
    protected int age;
    String address;
    private double sal;

    public People() {
    }

    public People(String name, int age, String address, double sal) {
        this.name = name;
        this.age = age;
        this.address = address;
        this.sal = sal;
    }

    public void m1(String a,int b){
        System.out.println("m1");
    }

    protected String m2(){
        System.out.println("m2");
        return null;
    }

    void m3(){
        System.out.println("m3");
    }

    private void m4(){
        System.out.println("m4");
    }
}

api_01() execution result:

Get the full class name: com.feiyang.basic17_reflect.People

==

Get simple class name: People

==

Get all public modified attributes, including this class and parent class: name

==

Get all attributes in this class: name

Get all the attributes in this class: age

Get all attributes in this class: address

Get all attributes in this class: sal

==

Get all public modified methods, including this class and parent class: m1

Get all public modified methods, including this class and parent class: wait

Get all public modified methods, including this class and parent class: wait

Get all public modified methods, including this class and parent class: wait

Get all public modified methods, including this class and parent class: equals

Get all public modified methods, including this class and parent class: toString

Get all public modified methods, including this class and parent class: hashCode

Get all public modified methods, including this class and parent class: getClass

Get all public modified methods, including this class and parent class: notify

Get all public modified methods, including this class and parent class: notifyAll

==

Get all the methods in this class: m1

Get all the methods in this class: m2

Get all the methods in this class: m4

Get all the methods in this class: m3

==

Get all public modified constructors: com.feiyang.basic17_reflect.People

Get all public modified constructors: com.feiyang.basic17_reflect.People

==

Get all constructors in this class: com.feiyang.basic17_reflect.People

Get all constructors in this class: com.feiyang.basic17_reflect.People

==

Get package information: package com.feiyang.basic17_reflect

==

Obtain parent class Class object information: java.lang.Object

==

Get interface information: com.feiyang.basic17_reflect.IA

==

Get annotation information: @java.lang.Deprecated()

package com.feiyang.basic17_reflect;

import org.junit.jupiter.api.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflectionUtils {

    public static void main(String[] args) {

    }

    @Test
    public void api_03() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        Method[] declaredMethods = cls.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println("获取本类中所有的方法：" + declaredMethod.getName()
            + "该方法的修饰符值：" + declaredMethod.getModifiers()
            + "该方法的返回值类型：" + declaredMethod.getReturnType());

            //输出当前方法的形参类型数组
            Class<?>[] parameterTypes = declaredMethod.getParameterTypes();
            for (Class<?> parameterType : parameterTypes) {
                System.out.println("该方法的参数类型：" + parameterType.getName());
            }
        }
    }

    @Test
    public void api_02() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        Field[] declaredFields = cls.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println("获取本类中所有的属性：" +declaredField.getName()
            + "该属性的修饰符值：" + declaredField.getModifiers()
            + "该属性的类型：" + declaredField.getType());
        }
    }

    @Test
    public void api_01() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        System.out.println("获取全类名：" + cls.getName());

        System.out.println("==");
        System.out.println("获取简单类名：" +cls.getSimpleName());

        System.out.println("==");
        Field[] fields = cls.getFields();
        for (Field field : fields) {
            System.out.println("获取所有public修饰的属性，包括本类以及父类：" +field.getName());
        }

        System.out.println("==");
        Field[] declaredFields = cls.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println("获取本类中所有的属性：" +declaredField.getName());
        }

        System.out.println("==");
        Method[] methods = cls.getMethods();
        for (Method method : methods) {
            System.out.println("获取所有public修饰的方法，包括本类以及父类：" + method.getName());
        }

        System.out.println("==");
        Method[] declaredMethods = cls.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println("获取本类中所有的方法：" + declaredMethod.getName());
        }

        System.out.println("==");
        Constructor<?>[] constructors = cls.getConstructors();
        for (Constructor<?> constructor : constructors) {
            System.out.println("获取所有public修饰的构造器，包括本类：" + constructor.getName());
        }

        System.out.println("==");
        Constructor<?>[] declaredConstructors = cls.getDeclaredConstructors();
        for (Constructor<?> declaredConstructor : declaredConstructors) {
            System.out.println("获取本类中所有的构造器：" + declaredConstructor.getName());
        }

        System.out.println("==");
        Package aPackage = cls.getPackage();
        System.out.println("获取包信息：" + aPackage);

        System.out.println("==");
        Class<?> superclass = cls.getSuperclass();
        System.out.println("获取父类Class对象信息：" + superclass.getName());

        System.out.println("==");
        Class<?>[] interfaces = cls.getInterfaces();
        for (Class<?> anInterface : interfaces) {
            System.out.println("获取接口信息：" + anInterface.getName());
        }

        System.out.println("==");
        Annotation[] annotations = cls.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println("获取注解信息：" + annotation);
        }

    }
}

interface IA{

}

@Deprecated
class People implements IA{
    public String name;
    protected int age;
    String address;
    private double sal;

    public People() {
    }

    public People(String name, int age, String address, double sal) {
        this.name = name;
        this.age = age;
        this.address = address;
        this.sal = sal;
    }

    public void m1(String a,int b){
        System.out.println("m1");
    }

    protected String m2(){
        System.out.println("m2");
        return null;
    }

    void m3(){
        System.out.println("m3");
    }

    private void m4(){
        System.out.println("m4");
    }
}

api_02() execution result:

Get all attributes in this class: name Modifier value of this attribute: 1 Type of this attribute: class java.lang.String

Get all attributes in this class: age Modifier value of this attribute: 4 Type of this attribute: int

Get all attributes in this class: address Modifier value of this attribute: 0 Type of this attribute: class java.lang.String

Get all attributes in this class: sal Modifier value of this attribute: 2 Type of this attribute: double

package com.feiyang.basic17_reflect;

import org.junit.jupiter.api.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflectionUtils {

    public static void main(String[] args) {

    }

    @Test
    public void api_03() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        Method[] declaredMethods = cls.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println("获取本类中所有的方法：" + declaredMethod.getName()
            + "该方法的修饰符值：" + declaredMethod.getModifiers()
            + "该方法的返回值类型：" + declaredMethod.getReturnType());

            //输出当前方法的形参类型数组
            Class<?>[] parameterTypes = declaredMethod.getParameterTypes();
            for (Class<?> parameterType : parameterTypes) {
                System.out.println("该方法的参数类型：" + parameterType.getName());
            }
        }
    }

    @Test
    public void api_02() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        Field[] declaredFields = cls.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println("获取本类中所有的属性：" +declaredField.getName()
            + "该属性的修饰符值：" + declaredField.getModifiers()
            + "该属性的类型：" + declaredField.getType());
        }
    }

    @Test
    public void api_01() throws ClassNotFoundException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
        System.out.println("获取全类名：" + cls.getName());

        System.out.println("==");
        System.out.println("获取简单类名：" +cls.getSimpleName());

        System.out.println("==");
        Field[] fields = cls.getFields();
        for (Field field : fields) {
            System.out.println("获取所有public修饰的属性，包括本类以及父类：" +field.getName());
        }

        System.out.println("==");
        Field[] declaredFields = cls.getDeclaredFields();
        for (Field declaredField : declaredFields) {
            System.out.println("获取本类中所有的属性：" +declaredField.getName());
        }

        System.out.println("==");
        Method[] methods = cls.getMethods();
        for (Method method : methods) {
            System.out.println("获取所有public修饰的方法，包括本类以及父类：" + method.getName());
        }

        System.out.println("==");
        Method[] declaredMethods = cls.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println("获取本类中所有的方法：" + declaredMethod.getName());
        }

        System.out.println("==");
        Constructor<?>[] constructors = cls.getConstructors();
        for (Constructor<?> constructor : constructors) {
            System.out.println("获取所有public修饰的构造器，包括本类：" + constructor.getName());
        }

        System.out.println("==");
        Constructor<?>[] declaredConstructors = cls.getDeclaredConstructors();
        for (Constructor<?> declaredConstructor : declaredConstructors) {
            System.out.println("获取本类中所有的构造器：" + declaredConstructor.getName());
        }

        System.out.println("==");
        Package aPackage = cls.getPackage();
        System.out.println("获取包信息：" + aPackage);

        System.out.println("==");
        Class<?> superclass = cls.getSuperclass();
        System.out.println("获取父类Class对象信息：" + superclass.getName());

        System.out.println("==");
        Class<?>[] interfaces = cls.getInterfaces();
        for (Class<?> anInterface : interfaces) {
            System.out.println("获取接口信息：" + anInterface.getName());
        }

        System.out.println("==");
        Annotation[] annotations = cls.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println("获取注解信息：" + annotation);
        }

    }
}

interface IA{

}

@Deprecated
class People implements IA{
    public String name;
    protected int age;
    String address;
    private double sal;

    public People() {
    }

    public People(String name, int age, String address, double sal) {
        this.name = name;
        this.age = age;
        this.address = address;
        this.sal = sal;
    }

    public void m1(String a,int b){
        System.out.println("m1");
    }

    protected String m2(){
        System.out.println("m2");
        return null;
    }

    void m3(){
        System.out.println("m3");
    }

    private void m4(){
        System.out.println("m4");
    }
}

api_03() execution result:

Get all the methods in this class: m1 The modifier value of this method: 1 The return value type of this method: void

The parameter type of this method: java.lang.String

The parameter type of this method: int

Get all the methods in this class: m2 The modifier value of this method: 4 The return value type of this method: class java.lang.String

Get all the methods in this class: m4 Modifier value of this method: 2 Return value type of this method: void

Get all the methods in this class: m3 Modifier value of this method: 0 Return value type of this method: void

package com.feiyang.basic17_reflect;

    import org.junit.jupiter.api.Test;

    import java.lang.annotation.Annotation;
    import java.lang.reflect.Constructor;
    import java.lang.reflect.Field;
    import java.lang.reflect.Method;

    /**
     * @author：飞扬
     * @公众hao：程序员飞扬
     * @description:
     */
    public class ReflectionUtils {

        public static void main(String[] args) {

        }

        @Test
        public void api_04() throws ClassNotFoundException {
            Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
            Constructor<?>[] declaredConstructors = cls.getDeclaredConstructors();
            for (Constructor<?> declaredConstructor : declaredConstructors) {
                System.out.println("获取本类中所有的构造器：" + declaredConstructor.getName()
                + "该构造器的修饰符值" + declaredConstructor.getModifiers());

                Class<?>[] parameterTypes = declaredConstructor.getParameterTypes();
                for (Class<?> parameterType : parameterTypes) {
                    System.out.println("该构造器的参数类型" + parameterType.getName());
                }
            }
        }

        @Test
        public void api_03() throws ClassNotFoundException {
            Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
            Method[] declaredMethods = cls.getDeclaredMethods();
            for (Method declaredMethod : declaredMethods) {
                System.out.println("获取本类中所有的方法：" + declaredMethod.getName()
                + "该方法的修饰符值：" + declaredMethod.getModifiers()
                + "该方法的返回值类型：" + declaredMethod.getReturnType());

                //输出当前方法的形参类型数组
                Class<?>[] parameterTypes = declaredMethod.getParameterTypes();
                for (Class<?> parameterType : parameterTypes) {
                    System.out.println("该方法的参数类型：" + parameterType.getName());
                }
            }
        }

        @Test
        public void api_02() throws ClassNotFoundException {
            Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
            Field[] declaredFields = cls.getDeclaredFields();
            for (Field declaredField : declaredFields) {
                System.out.println("获取本类中所有的属性：" +declaredField.getName()
                + "该属性的修饰符值：" + declaredField.getModifiers()
                + "该属性的类型：" + declaredField.getType());
            }
        }

        @Test
        public void api_01() throws ClassNotFoundException {
            Class<?> cls = Class.forName("com.feiyang.basic17_reflect.People");
            System.out.println("获取全类名：" + cls.getName());

            System.out.println("==");
            System.out.println("获取简单类名：" +cls.getSimpleName());

            System.out.println("==");
            Field[] fields = cls.getFields();
            for (Field field : fields) {
                System.out.println("获取所有public修饰的属性，包括本类以及父类：" +field.getName());
            }

            System.out.println("==");
            Field[] declaredFields = cls.getDeclaredFields();
            for (Field declaredField : declaredFields) {
                System.out.println("获取本类中所有的属性：" +declaredField.getName());
            }

            System.out.println("==");
            Method[] methods = cls.getMethods();
            for (Method method : methods) {
                System.out.println("获取所有public修饰的方法，包括本类以及父类：" + method.getName());
            }

            System.out.println("==");
            Method[] declaredMethods = cls.getDeclaredMethods();
            for (Method declaredMethod : declaredMethods) {
                System.out.println("获取本类中所有的方法：" + declaredMethod.getName());
            }

            System.out.println("==");
            Constructor<?>[] constructors = cls.getConstructors();
            for (Constructor<?> constructor : constructors) {
                System.out.println("获取所有public修饰的构造器，包括本类：" + constructor.getName());
            }

            System.out.println("==");
            Constructor<?>[] declaredConstructors = cls.getDeclaredConstructors();
            for (Constructor<?> declaredConstructor : declaredConstructors) {
                System.out.println("获取本类中所有的构造器：" + declaredConstructor.getName());
            }

            System.out.println("==");
            Package aPackage = cls.getPackage();
            System.out.println("获取包信息：" + aPackage);

            System.out.println("==");
            Class<?> superclass = cls.getSuperclass();
            System.out.println("获取父类Class对象信息：" + superclass.getName());

            System.out.println("==");
            Class<?>[] interfaces = cls.getInterfaces();
            for (Class<?> anInterface : interfaces) {
                System.out.println("获取接口信息：" + anInterface.getName());
            }

            System.out.println("==");
            Annotation[] annotations = cls.getAnnotations();
            for (Annotation annotation : annotations) {
                System.out.println("获取注解信息：" + annotation);
            }

        }
    }

    interface IA{

    }

    @Deprecated
    class People implements IA{
        public String name;
        protected int age;
        String address;
        private double sal;

        public People() {
        }

        public People(String name, int age, String address, double sal) {
            this.name = name;
            this.age = age;
            this.address = address;
            this.sal = sal;
        }

        public void m1(String a,int b){
            System.out.println("m1");
        }

        protected String m2(){
            System.out.println("m2");
            return null;
        }

        void m3(){
            System.out.println("m3");
        }

        private void m4(){
            System.out.println("m4");
        }
    }

api_04() execution result:

Get all constructors in this class: com.feiyang.basic17_reflect.People The modifier value of this constructor is 1

Get all constructors in this class: com.feiyang.basic17_reflect.People The modifier value of this constructor is 1

The parameter type of the constructor is java.lang.String

The parameter type of the constructor is int

The parameter type of the constructor is java.lang.String

The parameter type of the constructor is double

2. Create objects through reflection

package com.feiyang.basic17_reflect;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflecCreateInstance {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchMethodException, InvocationTargetException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.User");

        //调用类中public修饰的无参构造器
        Object o = cls.newInstance();
        System.out.println(o);

        //调用类中指定构造器（公有的）
        Constructor<?> declaredConstructor = cls.getConstructor(String.class);
        Object o1 = declaredConstructor.newInstance("feiyang");
        System.out.println(o1);

        //调用类中指定构造器（含私有的）
        Constructor<?> declaredConstructor1 = cls.getDeclaredConstructor(int.class, String.class);
        declaredConstructor1.setAccessible(true);//设置私有属性可访问
        Object feiyang = declaredConstructor1.newInstance(18, "feiyang");
        System.out.println(feiyang);

    }
}
class User{
    private int age;
    private String name;

    public User() {
    }

    public User(String name) {
        this.name = name;
    }

    private User(int age, String name) {
        this.age = age;
        this.name = name;
    }

    @Override
    public String toString() {
        return "User{" +
                "age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}

User{age=0, name='null'}

User{age=0, name='feiyang'}

User{age=18, name='feiyang'}

3. Manipulating attributes through reflection

package com.feiyang.basic17_reflect;

import java.lang.reflect.Field;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflectAccessProperty {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, InstantiationException, IllegalAccessException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.Student");
        Object o = cls.newInstance();
        System.out.println(o.getClass());

        //使用反射来操作属性（公有属性）
        Field name = cls.getField("name");
        name.set(o,"张三丰");
        System.out.println(o);
        System.out.println(name.get(o));

        //使用反射来操作属性（私有属性）
        Field age = cls.getDeclaredField("age");
        age.setAccessible(true);
        age.set(o,100);
        System.out.println(o);
        System.out.println(age.get(o));

    }
}

class Student {
    public String name;

    private static int age;

    public Student() {
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                '}';
    }
}

Results of the:

class com.feiyang.basic17_reflect.Student

Student{name='Zhang Sanfeng'}

Zhang Sanfeng

Student{name='Zhang Sanfeng'}

100

4. Manipulating methods via reflection

package com.feiyang.basic17_reflect;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class ReflectAccessMethod {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InstantiationException, IllegalAccessException, InvocationTargetException {
        // 得到Boss类对象的Class对象
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.Boss");
        Object o = cls.newInstance();

        //得到hi方法对象(公有方法)
        Method hi = cls.getMethod("hi", String.class);
        hi.invoke(o, "我是xxx");

        //得到say方法（私有方法）
        Method say = cls.getDeclaredMethod("say", int.class, String.class, char.class);
        say.setAccessible(true);
        System.out.println(say.invoke(o, 100, "张三丰", '男'));

        //在反射中，方法如果有返回值统一返回Object,但是他的运行时类型和方法定义的返回类型一致
        Object retVal = say.invoke(o,25, "周芷若", '女');
        System.out.println(retVal.getClass());
    }
}

class Boss{
    public int age;
    private static String name;

    public Boss() {
    }

    private static String say(int n,String s,char c){
        return n + " " + s + " " + c;
    }

    public void hi(String s){
        System.out.println("hi " + s);
    }
}

hi i'm xxx

100 Zhang Mitutoyo

class java.lang.String

5. Case exercises

1) Operational attributes

package com.feiyang.basic17_reflect;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Homework01 {
    public static void main(String[] args) throws ClassNotFoundException, InstantiationException, IllegalAccessException, NoSuchFieldException, NoSuchMethodException, InvocationTargetException {
        Class<?> cls = Class.forName("com.feiyang.basic17_reflect.PrivateTest");
        Object o = cls.newInstance();

        Field name = cls.getDeclaredField("name");

        name.setAccessible(true);
        name.set(o,"tom");
        
        Method getName = cls.getMethod("getName");
        System.out.println(getName.invoke(o));

    }
}

class PrivateTest{
    private String name = "hellokitty";
    public String getName(){
        return name;
    }
}

printout:

tom

2) Create a file using reflection

package com.feiyang.basic17_reflect;

import java.io.File;
import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

/**
 * @author：飞扬
 * @公众hao：程序员飞扬
 * @description:
 */
public class Homework02 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
        Class<?> cls = Class.forName("java.io.File");

        //得到所有的构造器
        Constructor<?>[] constructors = cls.getDeclaredConstructors();
        for (Constructor<?> constructor : constructors) {
            System.out.println("File构造器=" + constructor);
        }

        //指定得到public java.io.File(java.lang.String)
        Constructor<?> declaredConstructor = cls.getDeclaredConstructor(String.class);
        String filePath = "d:\\mynew.txt";
        Object file = declaredConstructor.newInstance(filePath);

        //得到createNewFile()方法对象
        Method createNewFile = cls.getDeclaredMethod("createNewFile");
        createNewFile.invoke(file);//创建文件，调用的是createNewFile()
        
        System.out.println("File的运行类型=" + file.getClass());
        System.out.println("创建文件成功=" + filePath);


    }
}

Results of the:

File constructor = public java.io.File(java.lang.String, java.lang.String)

File constructor = public java.io.File(java.lang.String)

File构造器=private java.io.File(java.lang.String,java.io.File)

File constructor = public java.io.File(java.io.File, java.lang.String)

File constructor = public java.io.File(java.net.URI)

File constructor = private java.io.File(java.lang.String, int)

File's running type = class java.io.File

File created successfully = d:\mynew.txt