【JavaSE】反射机制 && Class类 && 反射具体操作详解

Class<?> clazz = Person.class; // 具体获取方式看后面

public static void main(String[] args){
    // 1. 使用Class.forName()，注意要指定包名，并且最好捕捉异常
    Class<?> c1 = null;
    try {
        c1 = Class.forName("Reflection.Student");
    } catch(ClassNotFoundException e) {
        e.printStackTrace();
    }

    // 2. 使用指定类的class静态变量
    Class<?> c2 = Student.class;

    // 3. 使用对象实例的getClass()方法
    Student st = new Student();
    Class<?> c3 = st.getClass();

    // 一个类在JVM中只会有一个Class实例，因此c1、c2、c3都指向同一个Class实例
    System.out.println(c1 == c2); // true
    System.out.println(c2 == c3); // true
    System.out.println(c3 == c1); // true
}

public static void main(String[] args) {
    try {
        // 1. 先获取Class对象
        Class c = Class.forName("Reflection.Student");

        // 2. 然后调用newInstance()方法创建对象，注意要强转成Student类型
        Student p = (Student)c.newInstance();
        System.out.println(p);

    } catch (ClassNotFoundException e) {
        throw new RuntimeException(e);
    } catch (InstantiationException e) {
        throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}

public static void main(String[] args) {
    try {
        // 1. 获取Class对象
        Class<?> cs = Class.forName("Reflection.Student");

        // 2. 获取无参的构造方法来创建对象
        Constructor<?> non_arg_constructor = cs.getDeclaredConstructor();
        Student s1 = (Student)non_arg_constructor.newInstance();
        System.out.println(s1);

        // 3. 获取带参的构造方法来创建对象，因为带参构造方法是私有的，所以要设置权限为true
        Constructor<?> arg_constructor = cs.getDeclaredConstructor(String.class, int.class);
        arg_constructor.setAccessible(true);
        Student s2 = (Student)arg_constructor.newInstance("liren", 18);
        System.out.println(s2);
        
    } catch (ClassNotFoundException e) {
        throw new RuntimeException(e);
    } catch (NoSuchMethodException e) {
        throw new RuntimeException(e);
    } catch (InvocationTargetException e) {
        throw new RuntimeException(e);
    } catch (InstantiationException e) {
        throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}

// 运行结果：
Student()
Student{name='bit', age=18}
Student(String,name)
Student{name='liren', age=18}

public static void main(String[] args) {
    // 使用反射操作字段
    try {
        Class<?> cs = Class.forName("Reflection.Student");

        // 1. 输出类名，打印字段信息
        System.out.println("ClassName: " + cs.getName());
        for(Field e : cs.getDeclaredFields()) {
            System.out.println(e.getName() + " " + e.getType().getName() + " " + Modifier.toString(e.getModifiers()));
        }

        // 2.1 获取私有属性name
        Field namefield = cs.getDeclaredField("name");
        namefield.setAccessible(true); // 对于私有属性，需要设置Accessible为true

        // 2.2 获取该类对象，并设置name属性的值，然后用namefield.get()获取这个私有值
        Student st = (Student)cs.newInstance();
        namefield.set(st, "liren");
        System.out.println("反射私有属性修改了name：" + namefield.get(st));

    } catch (ClassNotFoundException | NoSuchFieldException e) {
        throw new RuntimeException(e);
    } catch (InstantiationException e) {
        throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    }
}

// 运行结果：
ClassName: Reflection.Student
name java.lang.String private
age int public
Student()
反射私有属性修改了name：liren

public static void main(String[] args) {
    try {
        Class<Student> cs = Student.class;

        // 获取带参的私有方法，然后设置权限，最后使用invoke()方法进行调用
        Method func_method = cs.getDeclaredMethod("function", String.class);
        func_method.setAccessible(true);
        func_method.invoke((Student)cs.newInstance(), "我是给私有的function函数传的参数");

        // 还可以输出方法的指定信息
        System.out.println("方法名：" + func_method.getName());
        System.out.println("修饰符为：" + Modifier.toString(func_method.getModifiers()));

    } catch (NoSuchMethodException e) {
        throw new RuntimeException(e);
    } catch (InvocationTargetException e) {
        throw new RuntimeException(e);
    } catch (IllegalAccessException e) {
        throw new RuntimeException(e);
    } catch (InstantiationException e) {
        throw new RuntimeException(e);
    }
}

// 运行结果：
Student()
我是给私有的function函数传的参数
方法名：function
修饰符为：private

class Student {
    // 私有属性name
    private String name = "bit";
    // 公有属性age
    public int age = 18;

    // 不带参数的构造⽅法
    public Student() {
        System.out.println("Student()");
    }

    private Student(String name, int age) {
        this.name = name;
        this.age = age;
        System.out.println("Student(String,name)");
    }

    private void eat() {
        System.out.println("i am eat");
    }

    void sleep() {
        System.out.println("i am pig");
    }

    private void function(String str) {
        System.out.println(str);
    }

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