Java反射

认识反射、获取类

反射是什么？

反射学什么？

反射第一步：加载类，获取字节码：Class对象

Student

public class Student {
    private String name;
    private int age;
    private char sex;
    private double height;
    private String hobby;

    public Student() {
    }

    public Student(String name, int age, char sex, double height, String hobby) {
        this.name = name;
        this.age = age;
        this.sex = sex;
        this.height = height;
        this.hobby = hobby;
    }

    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 char getSex() {
        return sex;
    }

    public void setSex(char sex) {
        this.sex = sex;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    public String getHobby() {
        return hobby;
    }

    public void setHobby(String hobby) {
        this.hobby = hobby;
    }
}

**Test1Class **

/**
 * 目标：获取Class对象。
 */
public class Test1Class {
    public static void main(String[] args) throws Exception {
        Class c1 = Student.class;
        System.out.println(c1.getName()); // 全类名(包名+类名)
        System.out.println(c1.getSimpleName()); // 简名：Student

        Class c2 = Class.forName("com.wuxuan.Junit_reflect_annotation_proxy_app.d2_reflect.Student");//填全类名
        System.out.println(c1 == c2);

        Student s = new Student();
        Class c3 = s.getClass();
        System.out.println(c3 == c2);
    }
}

总结

获取类的构造器

Cat

public class Cat {
    public static int a;
    public static final String COUNTRY = "中国";
    private String name;
    private int age;

    public Cat(){
        System.out.println("无参数构造器执行了~~");
    }

    private Cat(String name, int age) {
        System.out.println("有参数构造器执行了~~");
        this.name = name;
        this.age = age;
    }

    private void run(){
        System.out.println("🐱跑的贼快~~");
    }

    public void eat(){
        System.out.println("🐱爱吃猫粮~");
    }

    private String eat(String name){
        return "🐱最爱吃:" + name;
    }

    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;
    }

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

**Test2Constructor **

/**
 * 目标：掌握获取类的构造器，并对其进行操作。
 */
public class Test2Constructor {
    @Test
    public void testGetConstructors(){
        // 1、反射第一步：必须先得到这个类的Class对象
        Class c = Cat.class;
        // 2、获取类的全部构造器
        // Constructor[] constructors = c.getConstructors();
        Constructor[] constructors = c.getDeclaredConstructors();
        // 3、遍历数组中的每个构造器对象
        for (Constructor constructor : constructors) {
            System.out.println(constructor.getName() + "--->"
            + constructor.getParameterCount());
        }
    }

    @Test
    public void testGetConstructor() throws Exception {
        // 1、反射第一步：必须先得到这个类的Class对象
        Class c = Cat.class;
        // 2、获取类的某个构造器：无参数构造器
        Constructor constructor1 = c.getDeclaredConstructor();
        System.out.println(constructor1.getName() + "--->"
                + constructor1.getParameterCount());
        constructor1.setAccessible(true); // 禁止检查访问权限
        Cat cat = (Cat) constructor1.newInstance();
        System.out.println(cat);

        AtomicInteger a;


        // 3、获取有参数构造器
        Constructor constructor2 =
                c.getDeclaredConstructor(String.class, int.class);
        System.out.println(constructor2.getName() + "--->"
                + constructor2.getParameterCount());
        constructor2.setAccessible(true); // 禁止检查访问权限
        Cat cat2 = (Cat) constructor2.newInstance("叮当猫", 3);
        System.out.println(cat2);

    }
}

获取类的成员变量

Cat

public class Cat {
    public static int a;
    public static final String COUNTRY = "中国";
    private String name;
    private int age;

    public Cat(){
        System.out.println("无参数构造器执行了~~");
    }

    private Cat(String name, int age) {
        System.out.println("有参数构造器执行了~~");
        this.name = name;
        this.age = age;
    }

    private void run(){
        System.out.println("🐱跑的贼快~~");
    }

    public void eat(){
        System.out.println("🐱爱吃猫粮~");
    }

    private String eat(String name){
        return "🐱最爱吃:" + name;
    }

    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;
    }

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

**Test3Field **

/**
 * 目标：掌握获取类的成员变量，并对其进行操作。
 */
public class Test3Field {
    @Test
    public void testGetFields() throws Exception {
        // 1、反射第一步：必须是先得到类的Class对象
        Class c = Cat.class;
        // 2、获取类的全部成员变量。
        Field[] fields = c.getDeclaredFields();
        // 3、遍历这个成员变量数组
        for (Field field : fields) {
            System.out.println(field.getName() +  "---> "+ field.getType());
        }
        // 4、定位某个成员变量
        Field fName = c.getDeclaredField("name");
        System.out.println(fName.getName() + "--->" + fName.getType());

        Field fAge = c.getDeclaredField("age");
        System.out.println(fAge.getName() + "--->" + fAge.getType());

        // 赋值
        Cat cat = new Cat();
        fName.setAccessible(true); // 禁止访问控制权限
        fName.set(cat, "卡菲猫");
        System.out.println(cat);

        // 取值
        String name = (String) fName.get(cat);
        System.out.println(name);
    }
}

获取类的成员方法

Cat

public class Cat {
    public static int a;
    public static final String COUNTRY = "中国";
    private String name;
    private int age;

    public Cat(){
        System.out.println("无参数构造器执行了~~");
    }

    private Cat(String name, int age) {
        System.out.println("有参数构造器执行了~~");
        this.name = name;
        this.age = age;
    }

    private void run(){
        System.out.println("🐱跑的贼快~~");
    }

    public void eat(){
        System.out.println("🐱爱吃猫粮~");
    }

    private String eat(String name){
        return "🐱最爱吃:" + name;
    }

    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;
    }

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

Test4Method

/**
 * 目标：掌握获取类的成员方法，并对其进行操作。
 */
public class Test4Method {
    @Test
    public void testGetMethods() throws Exception {
        //  1、反射第一步：先得到Class对象。
        Class c = Cat.class;
        // 2、获取类的全部成员方法。
        Method[] methods = c.getDeclaredMethods();
        // 3、遍历这个数组中的每个方法对象
        for (Method method : methods) {
            System.out.println(method.getName() + "--->"
                    + method.getParameterCount() + "---->"
                    + method.getReturnType());
        }
        //  4、获取某个方法对象
        Method run = c.getDeclaredMethod("run"); // 拿run方法，无参数的
        System.out.println(run.getName() + "--->"
                + run.getParameterCount() + "---->"
                + run.getReturnType());

        Method eat = c.getDeclaredMethod("eat", String.class);
        System.out.println(eat.getName() + "--->"
                + eat.getParameterCount() + "---->"
                + eat.getReturnType());

        Cat cat = new Cat();
        run.setAccessible(true); // 禁止检查访问权限
        Object rs = run.invoke(cat); // 调用无参数的run方法，用cat对象触发调用的。
        System.out.println(rs);

        eat.setAccessible(true); // 禁止检查访问权限
        String rs2 = (String) eat.invoke(cat, "鱼儿");
        System.out.println(rs2);
    }
}

作用、应用场景

Student

public class Student {
    private String name;
    private int age;
    private char sex;
    private double height;
    private String hobby;

    public Student() {
    }

    public Student(String name, int age, char sex, double height, String hobby) {
        this.name = name;
        this.age = age;
        this.sex = sex;
        this.height = height;
        this.hobby = hobby;
    }

    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 char getSex() {
        return sex;
    }

    public void setSex(char sex) {
        this.sex = sex;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    public String getHobby() {
        return hobby;
    }

    public void setHobby(String hobby) {
        this.hobby = hobby;
    }
}

**Teacher **

public class Teacher {
    private String name;
    private double salary;

    public Teacher() {
    }

    public Teacher(String name, double salary) {
        this.name = name;
        this.salary = salary;
    }

    public String getName() {
        return name;
    }

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

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}

**ObjectFrame **

public class ObjectFrame {
    // 目标：保存任意对象的字段和其数据到文件中去
    public static void saveObject(Object obj) throws Exception {
	    PrintStream ps = new PrintStream(new FileOutputStream("D:\\Code\\JDBC\\Basic\\src\\main\\java\\com\\wuxuan\\Junit_reflect_annotation_proxy_app\\d2_reflect\\data.txt", true));
        // obj是任意对象，到底有多少个字段要保存。
        Class c = obj.getClass();
        String cName = c.getSimpleName();
        ps.println("---------------" + cName + "------------------------");
        // 2、从这个类中提取它的全部成员变量
        Field[] fields = c.getDeclaredFields();
        // 3、遍历每个成员变量。
        for (Field field : fields) {
            // 4、拿到成员变量的名字
            String name = field.getName();
            // 5、拿到这个成员变量在对象中的数据。
            field.setAccessible(true); // 禁止检查访问控制
            String value = field.get(obj) + "";
            ps.println(name + "=" + value);
        }
        ps.close();
    }
}

**Test5Frame **

/**
 * 目标：使用反射技术：设计一个保存对象的简易版框架。
 */
public class Test5Frame {
    @Test
    public void save() throws Exception {
        Student s1 = new Student("黑马吴彦祖", 45, '男', 185.3, "蓝球，冰球，阅读");
        Teacher t1 = new Teacher("播妞", 999.9);

        // 需求：把任意对象的字段名和其对应的值等信息，保存到文件中去。
        ObjectFrame.saveObject(s1);
        ObjectFrame.saveObject(t1);
    }
}