Java 反射机制

package com.example.java.javakuangstudy.Reflection;
​
/**
 * 反射练习-->得到class类的几种方式
 */
public class Text01 {
    public static void main(String[] args) throws ClassNotFoundException {
        Person person = new Student();
        System.out.println("我这个人是：" + person.name);
​
        //TODO 方式一：通过对象获取
        Class<? extends Person> c1 = person.getClass();
        System.out.println(c1.hashCode());
​
        //TODO 方式二：
        Class<?> c2 = Class.forName("com.example.java.javakuangstudy.Reflection.Student");
        System.out.println(c2.hashCode());
​
        //TODO 方式三：通过类名.class获取
        Class<Student> c3 = Student.class;
        System.out.println(c3.hashCode());
​
        //TODO 方式四：基本内置类型的包装类都有一个type属性
        Class<Integer> c4 = Integer.TYPE;
        System.out.println(c4);
​
        //TODO 获取父类类型
        Class<?> c5 = c1.getSuperclass();
        System.out.println(c5);
    }
}
​
class Person {
    public String name;
​
    public Person(String name) {
        this.name = name;
    }
​
    public Person() {
    }
​
    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                '}';
    }
}
​
class Student extends Person {
    public Student() {
        this.name = "学生";
    }
}
​
class Teacher extends Person {
    public Teacher() {
        this.name = "老师";
    }
}

package com.example.java.javakuangstudy.Reflection;
​
/**
 * 测试类什么时候会初始化
 */
public class Text02 {
    static {
        System.out.println("Main被加载");
    }
    public static void main(String[] args) throws ClassNotFoundException {
        //TODO 通过new 主动引用
        //Son son = new Son();
​
        //TODO 通过反射也会产生主动引用
        //Class.forName("com.example.java.javakuangstudy.Reflection.Son");
​
        //TODO 不会产生类的引用的方法
        //TODO 1、子类调用父类的静态方法
        //int b = Son.b;
        //System.out.println(b);
​
        // TODO 2、通过数组
        //Son[] arr = new Son[5];  //只打印--- Main被加载  （父类子类都不加载）
​
        //TODO 3、调用常量
        System.out.println(Son.M);
    }
​
}
​
class Father {
    static int b = 2;
​
    static {
        System.out.println("父类被加载");
    }
}
​
class Son extends Father {
    static {
        System.out.println("子类被加载");
        m = 300;
    }
​
    static int m = 100;
​
    static final int M = 1;
}

package com.example.java.javakuangstudy.Reflection;
​
import java.lang.reflect.Constructor;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
​
/**
 * 动态的创建对象 通过反射
 */
@SuppressWarnings("all")
public class Text03 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
        //TODO 获得class对象
        Class c1 = Class.forName("com.example.java.javakuangstudy.Reflection.User");
​
        //TODO 构造一个对象
        User user = (User) c1.newInstance();  //本质是调用了类的无参构造
        System.out.println(user);
​
        //TODO 通过构造器创建对象
        Constructor constructor = c1.getDeclaredConstructor(String.class, int.class, int.class);
        User user1 = (User) constructor.newInstance("liulianglin", 24, 251314);
        System.out.println(user1);
​
        //TODO 通过反射调用普通方法
        User user2 = (User) c1.newInstance();
        //TODO 通过反射获取一个方法
        Method setName = c1.getDeclaredMethod("setName", String.class);
        //TODO invoke激活的意思 （对象，"方法的值"）
        setName.invoke(user2, "刘亮林");
        System.out.println(user2.getName());
​
        //TODO 通过反射操作方法
        System.out.println("----------------------");
        User user3 = (User) c1.newInstance();
        Field name = c1.getDeclaredField("name");
        //TODO 不能直接操作私有属性，我们需要关闭程序的安全检测，将setAccessible(true) 可提高效率
        name.setAccessible(true);
        name.set(user3, "liulianglin");
        System.out.println(user3.getName());
    }
}

package com.example.java.javakuangstudy.Reflection;
​
import java.lang.annotation.*;
import java.lang.reflect.Field;
​
/**
 * 练习反射操作注解
 */
@SuppressWarnings("all")
public class Text04 {
    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
        Class c1 = Class.forName("com.example.java.javakuangstudy.Reflection.Student2");
        //TODO 通过反射获取全部注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
            System.out.println(annotation); //@com.example.java.javakuangstudy.Reflection.Tablekuang(value=db_student)
        }
        //TODO 获得注解的value值
        Tablekuang tablekuang = (Tablekuang) c1.getAnnotation(Tablekuang.class);
        String value = tablekuang.value();
        System.out.println(value);   //db_student
​
        //TODO 获得类指定的注解
        Field name = c1.getDeclaredField("id");
        Fieldkuang annotation = name.getAnnotation(Fieldkuang.class);
        System.out.println(annotation.columnName());   //db_id
        System.out.println(annotation.type());         //int
        System.out.println(annotation.length());       //18
    }
}
​
@Tablekuang("db_student")
class Student2 {
​
    @Fieldkuang(columnName = "db_id", type = "int", length = 18)
    private int id;
    @Fieldkuang(columnName = "db_age", type = "int", length = 18)
    private int age;
    @Fieldkuang(columnName = "db_name", type = "varchar", length = 18)
    private String name;
​
    public Student2() {
    }
​
    public Student2(int id, int age, String name) {
        this.id = id;
        this.age = age;
        this.name = name;
    }
​
    public int getId() {
        return id;
    }
​
    public void setId(int id) {
        this.id = id;
    }
​
    public int getAge() {
        return age;
    }
​
    public void setAge(int age) {
        this.age = age;
    }
​
    public String getName() {
        return name;
    }
​
    public void setName(String name) {
        this.name = name;
    }
​
    @Override
    public String toString() {
        return "Student2{" +
                "id=" + id +
                ", age=" + age +
                ", name='" + name + '\'' +
                '}';
    }
}
​
//TODO 类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Tablekuang {
    String value();
}
​
//TODO 属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Fieldkuang {
    String columnName();
​
    String type();
​
    int length();
}