java反射机制

1、什么是反射机制？

java反射机制是指在运行状态中，对于任意一个类，都能知道这个类的所有属性和方法;对于任意一个对象，都能调用它的任意一个方法;

这种动态获取信息和动态调用方法的功能称为java反射机制。

2、反射机制的几种方法

User代码：

package com.reflect;

public class User {
    private String username;
    
    private String password;
    
    private Integer age;
    
    public Integer add(Integer i , Integer j){
        return i + j;
    }
    

    public User(){}
    
    public User(String username,String password,Integer age){
        this.username = username;
        this.password = password;
        this.age = age;
    }
    
    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }
    
    
}

ReflectMethod方法：

package com.reflect;

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

public class ReflectMethod {

    // 能过反射获取一个对象的方法和属性
    public static void reflectTest(String className)
            throws ClassNotFoundException {
        // 获取Class对象
        Class<?> c = Class.forName(className);

        // 属性
        Field[] f = c.getDeclaredFields();
        for (int i = 0; i < f.length; i++) {
            System.out.println("属性：" + f[i].getName());
        }

        // 方法
        Method[] m = c.getDeclaredMethods();
        for (int i = 0; i < m.length; i++) {
            System.out.println("方法：" + m[i].getName());
        }
    }

    // 通过反射调用方法
    public static void useMethod(String className)
            throws ClassNotFoundException, InstantiationException,
            IllegalAccessException, NoSuchMethodException, SecurityException,
            IllegalArgumentException, InvocationTargetException {
        // 获取Class对象
        Class<?> c = Class.forName(className);

        // 通过Class获取对象,用newInstance()方法
        Object obj = c.newInstance();

        // 能过反射调用方法,传入参数是方法名和这个方法所需要的参数的Class对象的数组，如果add()里需要三个Integer类型的参数，
        // new Class[]数组就要有3个参数
        Method addMethod = c.getMethod("add", new Class[] { Integer.class,
                Integer.class });

        // 调用目标方法，传入参数
        Object result = addMethod.invoke(obj, new Object[] { 1, 2 });
        System.out.println(result);
    }

    // 通过构造方法，获取对象，第一种方法
    public static Object noneParamObj1(String className)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException {
        Class<?> c = Class.forName(className);
        return c.newInstance();
    }

    // 通过构造方法，获取对象，第二种方法
    public static Object noneParamObj2(String className)
            throws InstantiationException, IllegalAccessException,
            ClassNotFoundException, IllegalArgumentException,
            InvocationTargetException, NoSuchMethodException, SecurityException {

        Class<?> c = Class.forName(className);
        Constructor<?> con = c.getConstructor(new Class[] {});
        Object obj = con.newInstance(new Object[] {});
        return obj;
    }

    // 通过反射克隆对象
    public static Object cloneInstance(String className)
            throws ClassNotFoundException, NoSuchMethodException,
            SecurityException, InstantiationException, IllegalAccessException,
            IllegalArgumentException, InvocationTargetException {
        Class<?> c = Class.forName(className);

        // 获取构造方法
        Constructor<?> con = c.getConstructor(new Class[] {});
        // 通过构造方法获取一个新的对象
        Object obj = con.newInstance(new Object[] {});
        // 获取属性数组
        Field[] fields = c.getDeclaredFields();

        for (int i = 0; i < fields.length; i++) {
            Field field = fields[i];
            // 获取属性名字
            String fieldName = field.getName();
            // 首字母大字
            String firstLetter = fieldName.substring(0, 1).toUpperCase();
             
            //获取set和get方法名字
            String getMethodName = "get" + firstLetter + fieldName.substring(1);
            String setMethodName = "set" + firstLetter + fieldName.substring(1);
            
            
            //获取set和get方法
            Method setMethod = c.getMethod(setMethodName, new Class[]{field.getType()});
            Method getMethod = c.getMethod(getMethodName, new Class[]{});
            
            // 调用原对象的getXXX()方法 
           Object value = getMethod.invoke(obj, new Object[] {});
            // 调用拷贝对象的setXXX()方法 
           setMethod.invoke(obj, new Object[] { value });
        }
        User user = (User)obj;
        user.setPassword("123");
        System.out.println(user.getPassword());
        return obj;
    }
}

Main代码

package com.reflect;

import java.lang.reflect.InvocationTargetException;

public class Main {
    public static void main(String[] args) { 
        try {
            ReflectMethod.cloneInstance("com.reflect.User");
        } catch (ClassNotFoundException | NoSuchMethodException
                | SecurityException | InstantiationException
                | IllegalAccessException | IllegalArgumentException
                | InvocationTargetException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}