JAVA反射

一、反射机制概念

二、反射应用场景

1、源代码阶段

1）用法：Class clazz=Class.forName("类的全路径")；

2）作用：多用于配置文件，将类名定义在配置文件中，读取文件内容，加载程序类；

2、Class类对象阶段

1）用法：类名.class，通过类名的属性class获取；

2）作用：多用于参数的传递；

3、运行时阶段

1）用法：对象.getClass()；getClass()方法在object类中定义着；

2）作用：多用于对象获取字节码的方式

注意：同一个字节码文件（*.class）在一次程序运行中，只会被加载一次，不论通过哪一种方式获取的Class对象都是同一个。

三、Class对象功能

1、获取成员变量

public Field getField(String name)                 //获取类中public的成员变量

public Field[] getFields()                         // 获取类中所有的public的成员变量

public Field getDeclaredField(String name)         //可以获取类中private的成员变量

public Field[] getDeclaredFields()                 //获取类中所有成员信息

2、获取构造方法

package com.dgs.entity;
 
public class User {
 
    private String username ;
    private String password ;
 
    public String age ;
    public String sex ;
 
    public User() {
    }
 
    public User(String username, String password, String age, String sex) {
        this.username = username;
        this.password = password;
        this.age = age;
        this.sex = sex;
    }
 
    private User(String username, String password) {
        this.username = username;
        this.password = password;
    }
 
    private User(String username, String password, String 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 String getAge() {
        return age;
    }
 
    public void setAge(String age) {
        this.age = age;
    }
 
    public String getSex() {
        return sex;
    }
 
    public void setSex(String sex) {
        this.sex = sex;
    }
}

package com.dgs.main;
import com.dgs.entity.User;
 
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
 
public class TestMain {
    public static void main(String[] args) throws Exception {
        Class userClass = User.class;
        Constructor constructor1 = userClass.getConstructor();
        System.out.println(constructor1);
        Constructor constructor2 = userClass.getConstructor(String.class, String.class, String.class, String.class);
        System.out.println(constructor2);
 
        Constructor[] constructors = userClass.getConstructors();
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }
 
        Constructor declaredConstructor1 = userClass.getDeclaredConstructor(String.class, String.class);
        System.out.println(declaredConstructor1);
 
        Constructor[] declaredConstructors = userClass.getDeclaredConstructors();
        for (Constructor declaredConstructor : declaredConstructors) {
            System.out.println(declaredConstructor);
        }
 
    }
}

3、获取成员方法

package com.dgs.main;
import com.dgs.entity.User;
 
import java.lang.reflect.Method;
 
public class TestMain {
    public static void main(String[] args) throws Exception {
        User user = new User();
        Class userClass = user.getClass();
 
        Method getSex = userClass.getMethod("getSex");
        System.out.println(getSex);
 
        Method[] methods = userClass.getMethods();
        for (Method method : methods) {
            System.out.println(method);
        }
 
        Method setSex = userClass.getDeclaredMethod("setSex",String.class);
        System.out.println(setSex);
 
        Method[] declaredMethods = userClass.getDeclaredMethods();
        for (Method declaredMethod : declaredMethods) {
            System.out.println(declaredMethod);
        }
    }
}

4、具体使用

package com.dgs.main;
import com.dgs.entity.User;
 
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
 
public class TestMain {
    public static void main(String[] args) throws Exception {
        User user = new User();
        Class userClass = user.getClass();
 
        Constructor constructor = userClass.getConstructor(String.class, String.class, String.class, String.class);
 
        Object userObject = constructor.newInstance("dgs", "1234", "22", "男");
        System.out.println(userObject);
 
        Method setSex = userClass.getMethod("setSex", String.class);
        setSex.invoke(userObject, "女");
        System.out.println(userObject);
 
        Field username = userClass.getDeclaredField("username");
        username.setAccessible(true);
        username.set(userObject,"dgj");
        System.out.println(userObject);
    }
}

Field成员变量

操作：

        ①设置值 void set(Object obj,Object value)

        ②获取值 get(Object obj)

        ③忽略访问权限修饰符的安全检查setAccessible(true)

Constructor：构造方法

创建对象：

        ①T newInstance(Object... initargets)

        ②：如果使用空参数构造方法创建对象，操作可以简化：Class对象的newInstance

Method：方法对象

执行方法：Object invoke(Object obj,Object... args)