Java反射（Reflection）

Bileton

已于 2024-11-01 15:42:31 修改

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分类专栏： Java 文章标签： java 反射 reflect

于 2024-11-01 12:34:16 首次发布

本文链接：https://blog.youkuaiyun.com/weixin_74305514/article/details/143424350

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Java 反射（Reflection）是一种在运行时分析和操作 Java 程序的能力，可以动态地获取类的结构信息、调用方法、访问字段等。

reflect

反射是由 java.lang.reflect 包支持的，主要类包括 Class、Method、Field 和 Constructor 等。

示例类

public class Student {

    // 属性
    private String name;
    private int age;
    public static String school = "High School";

    // 构造方法
    public Student() {
        this.name = "Unknown";
        this.age = 0;
    }
    
    private Student(String name) {
        this.name = name;
        this.age = 18; // 默认年龄
    }

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

    // 公有方法
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

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

    public void introduce() {
        System.out.println("Hi, my name is " + name + ", and I am " + age + " years old.");
    }

    // 私有方法
    private void printSecret() {
        System.out.println("This is a private method.");
    }

    // 静态方法
    public static String getSchool() {
        return school;
    }

    public static void setSchool(String schoolName) {
        school = schoolName;
    }
}

获取Class对象

通过对象获取
Class<?> clazz = object.getClass();

    public static void main(String[] args) {
        Student stu = new Student("Bileton",21);
        Class stuClass = stu.getClass();
        System.out.println(stuClass.getName()); // Student
    }

通过类获取
Class<?> clazz = ClassName.class;

    public static void main(String[] args) {
        Class stuClass = Student.class;
        System.out.println(stuClass.getName()); // Student
    }

通过全类名获取类
Class<?> clazz = Class.forName("com.example.SomeClass");

    public static void main(String[] args) throws ClassNotFoundException {
        Class strClass = Class.forName("java.lang.String");
        System.out.println(strClass.getName()); // java.lang.String
    }

获取构造方法

获取公有构造方法对象的数组
Constructor<?>[] getConstructors()

    public static void main(String[] args) {
        Class stuClass = Student.class;
        Constructor[] constructors = stuClass.getConstructors();
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }
    }

获取所有构造方法对象的数组
Constructor<?>[] getDeclaredConstructors()

    public static void main(String[] args) {
        Class stuClass = Student.class;
        Constructor[] constructors = stuClass.getDeclaredConstructors();
        for (Constructor constructor : constructors) {
            System.out.println(constructor);
        }
    }

获取单个公有构造方法对象
Constructor<?> getConstructor()

    public static void main(String[] args) throws NoSuchMethodException {
        Class stuClass = Student.class;
        Constructor constructor1 = stuClass.getConstructor();
        Constructor constructor2 = stuClass.getConstructor(String.class,int.class);
        System.out.println(constructor1);
        System.out.println(constructor2);
    }

获取单个构造方法对象

    public static void main(String[] args) throws NoSuchMethodException {
        Class stuClass = Student.class;
        Constructor constructor = stuClass.getDeclaredConstructor(String.class);
        System.out.println(constructor);
    }

调用公有构造方法

    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
        Class stuClass = Student.class;
        Constructor constructor = stuClass.getConstructor(String.class,int.class);
        Student stu = (Student) constructor.newInstance("Bileton",21);
        System.out.println(stu.getName()+" "+stu.getAge()); // Bileton 21
    }

调用私有构造方法
使用setAccessible(true)突破 Java 的访问控制限制，使得私有成员（字段、方法或构造方法）可以被访问。

    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException {
        Class stuClass = Student.class;
        Constructor constructor = stuClass.getDeclaredConstructor(String.class);
        constructor.setAccessible(true);
        Student stu = (Student) constructor.newInstance("Bileton");
        System.out.println(stu.getName()+" "+stu.getAge()); // Bileton 18
    }

获取成员变量

获取公有成员变量对象的数组
Field[] getFields()

    public static void main(String[] args) {
        Class stuClass = Student.class;
        Field[] fields = stuClass.getFields(); 
        for (Field field : fields) {
            System.out.println(field.getName()); // school
        }
    }

获取所有成员变量对象的数组
Field[] getDeclaredFields()

    public static void main(String[] args) {
        Class stuClass = Student.class;
        Field[] fields = stuClass.getDeclaredFields();
        for (Field field : fields) {
            System.out.println(field);
        }
    }

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获取单个公有成员变量对象
Field[] getField(String name)

    public static void main(String[] args) throws NoSuchFieldException {
        Class stuClass = Student.class;
        Field fields = stuClass.getField("school");
        System.out.println(fields);
        // public static java.lang.String Student.school
    }

获取单个成员变量对象
Field[] getDeclaredField(String name)

    public static void main(String[] args) throws NoSuchFieldException {
        Class stuClass = Student.class;
        Field fields = stuClass.getDeclaredField("name");
        System.out.println(fields);
        // private java.lang.String Student.name
    }

获取和访问字段

    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException, NoSuchFieldException {
        Class stuClass = Student.class;
        Constructor constructor = stuClass.getConstructor(String.class,int.class);
        Student Stu = (Student) constructor.newInstance("Bileton",21);
        Field name_field = stuClass.getDeclaredField("name");
        Field age_field = stuClass.getDeclaredField("age");
        name_field.setAccessible(true);
        age_field.setAccessible(true);
        String name = (String) name_field.get(Stu);
        int age = (int) age_field.get(Stu);
        System.out.println(name+" "+age); // Bileton 21
        name_field.set(Stu,"bileton");
        age_field.set(Stu,18);
        name = (String) name_field.get(Stu);
        age = (int) age_field.get(Stu);
        System.out.println(name+" "+age); // bileton 18
    }

获取成员方法

获取公有成员方法对象的数组，包括继承的
Method[] getMethods()

    public static void main(String[] args) throws NoSuchFieldException {
        Class stuClass = Student.class;
        Method[] methods = stuClass.getMethods();
        for (Method method : methods) {
            System.out.println(method);
        }
    }

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获取所有成员方法对象的数组，不包括继承的
Method[] getDeclaredMethods()

	     public static void main(String[] args) throws NoSuchFieldException {
	        Class stuClass = Student.class;
	        Method[] methods = stuClass.getDeclaredMethods();
	        for (Method method : methods) {
	            System.out.println(method);
	        }
	    }

在这里插入图片描述

获取单个公有成员方法对象
Method getMethod(String name,Class<?>…parameterTypes)

    public static void main(String[] args) throws NoSuchMethodException {
        Class stuClass = Student.class;
        Method method = stuClass.getMethod("introduce");
        System.out.println(method);
    }

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获取单个成员方法对象
Method getDeclaredMethod(String name,Class<?>…parameterTypes)

    public static void main(String[] args) throws NoSuchMethodException {
        Class stuClass = Student.class;
        Method method = stuClass.getDeclaredMethod("printSecret");
        System.out.println(method);
    }

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调用成员方法

    public static void main(String[] args) throws NoSuchMethodException, InvocationTargetException, InstantiationException, IllegalAccessException, NoSuchFieldException {
        Class stuClass = Student.class;
        Constructor constructor = stuClass.getConstructor(String.class,int.class);
        Student Stu = (Student) constructor.newInstance("Bileton",21);
        Method printSecret = stuClass.getDeclaredMethod("printSecret");
        printSecret.setAccessible(true);
        printSecret.invoke(Stu); // This is a private method.
    }

其他方法

getModifiers()
获取成员（类、方法、构造器或字段）的修饰符。
getParameters()
获取方法的参数信息。
这个方法返回一个 Parameter[] 数组，每个 Parameter 对象代表方法的一个参数。
setAccessible(true)
临时取消权限校验
getName()
获取各种对象的名称
getType()
获取字段或参数的类型
get()
获取字段的值
set()
修改字段的值
invoke()
调用方法