Java学习之路---高级编程（反射，反射获取构造方法并创建对象）

反射 获取反射对象：

三种获取Class类型的对象

三种方式获取反射对象：

对象类：

package JavaAPI.FanShe.Test1.ch01;

import java.util.Objects;

public class Student {
    private String name;
    private int age;

    public Student() {
    }

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }

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

测试类：

package JavaAPI.FanShe.Test1.ch01;

public class Test {
    public static void main(String[] args) throws ClassNotFoundException {
        
        Class c1 = Student.class;
        System.out.println(c1);

        Class c2 = Student.class;
        System.out.println(c2);             //说明编译时只会生成一次字节码文件
        System.out.println("--------");
        
        Student s=new Student();
        Class c3 = s.getClass();
        System.out.println(c3);
        System.out.println("--------");
        
        Class c4 = Class.forName("JavaAPI.FanShe.Test1.ch01.Student");
        System.out.println(c4);
    }
}

反射获取构造方法并使用： 获取多个构造方法：

有两种常用的方法可以获取多个构造方法：

1.getConstructors(Class)：获取所有的公共的构造方法，返回一个Constructor集合 2.getDeclaredConstructors(Class)：获取所有的构造方法，返回一个Constructor集合

不同包下的对象：
学生类：

package JavaAPI.FanShe.Test1.ch01;

import java.util.Objects;

public class Student {
    private String name;
    private int age;

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
    public void show1(){

    }
    private void show2(){

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

测试类：

package JavaAPI.FanShe.Test1.ch02;

import JavaAPI.FanShe.Test1.ch01.Student;

import java.lang.reflect.Constructor;

public class Test {
    public static void main(String[] args) {
        //获取反射对象
        Class c = Student.class;
        

        Constructor[] cs1 = c.getConstructors();
        for (Object o:cs1)
            System.out.println(o);
        System.out.println("---------------");
        
        Constructor[] cs2 = c.getDeclaredConstructors();
        for (Object o:cs2)
            System.out.println(o);
    }
}

获取单个构造方法并创建对象：

两种方法获取单个对象

getConstructor(Class) ：获取单个公共的对象 getDeclaredConstructor(Class) ：获取单个所有可指定类型的对象 使用newInstance(参数)：可以使用获取到的构造方法创建对象

示例：
学生类：

package JavaAPI.FanShe.Test1.ch01;

import java.util.Objects;

public class Student {
    private String name;
    private int age;

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

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Student student = (Student) o;
        return age == student.age && Objects.equals(name, student.name);
    }

    @Override
    public int hashCode() {
        return Objects.hash(name, age);
    }
    public void show1(){

    }
    private void show2(){

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

测试类：

package JavaAPI.FanShe.Test1.ch02;

import JavaAPI.FanShe.Test1.ch01.Student;

import java.lang.reflect.Constructor;


public class Test2 {
    public static void main(String[] args) throws Exception {
    
        
         
        Class c = Class.forName("JavaAPI.FanShe.Test1.ch01.Student");
        
        
         
        Constructor[] con = c.getDeclaredConstructors();
        for (Object o:con)
        System.out.println(o);
        
        
         
        Constructor c1 = c.getConstructor();
        System.out.println("获取到的公共无参构造方法为："+c1);
        
        
         
        Object s1 = c1.newInstance();
        System.out.println("创建的对象为："+s1);
        System.out.println("---------");

        
        Constructor c2 = c.getConstructor(String.class, int.class);
        System.out.println("获取到的公共带参构造方法为："+c2);


        
        Student s2 = (Student)c2.newInstance("小明",15);
        System.out.println("创建的对象为："+s2);
        System.out.println("---------");

        Constructor c3 = c.getDeclaredConstructor(String.class);
        System.out.println("获取到的私有带参构造方法为："+c3);
		Object s3 = c3.newInstance("小王");
        System.out.println("创建的对象为："+s3);
    }
}

可以看到，最后一种方式报错，是因为私有构造方法不能创建对象，但是在反射中，可以使用暴力反射来构造对象

setAccessible(true)，暴力反射使虚拟机取消访问检查

示例：

Constructor c3 = c.getDeclaredConstructor(String.class);
c3.setAccessible(true);   //暴力反射
System.out.println("获取到的私有带参构造方法为："+c3);
Object s3 = c3.newInstance("小王",15);
System.out.println("创建的对象为："+s3);

小结：