Java reflection and code implementation

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Get an instance of the Class class

public class Test02 {
    
    
    public static void main(String[] args) throws ClassNotFoundException {
    
    
        Person person =new Student();
        System.out.println("这个人是："+person.name);
        //方式一：通过对象获取
        Class c1=person.getClass();
        //方式二：forname获取
        Class c2=Class.forName("Student");
        System.out.println(c2.hashCode());
        //方式三：通过.class获得
        Class c3 = Student.class;
        System.out.println(c3.hashCode());
        //方式四：基本内置类型的包装类都有一个Type属性
        Class c4=Integer.TYPE;
        System.out.println(c4);
        //获得父类类型
        Class c5=c1.getSuperclass();
        System.out.println(c5);
    }
}
class Person{
    
    
   public String name;
    public Person(){
    
    
    }

    public Person(String name) {
    
    
        this.name = name;
    }

    @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";

    }
}

Which classes can have Class objects?

class : Outer class, member (member inner class, static inner class), local inner class, anonymous inner class.
interface: interface
[] : array
enum : enum
annotation : annotation @interface
primitive type : basic data type
void

public class Test03 {
    
    
    public static void main(String[] args) {
    
    
        Class c1=Object.class;//类
        Class c2=Comparable.class;//接口
        Class c3=String[].class;//一维数组
        Class c4=int[][].class;//二位数组
        Class c5=Override.class;//注解
        Class c6= ElementType.class;//枚举
        Class c7=Integer.class;//基本数据类型
        Class c8=void.class;//void
        Class c9=Class.class;//Class
        //快速换行复制ctrl+D
        //多行选择复制alt
        System.out.println(c1);
        System.out.println(c2);
        System.out.println(c3);
        System.out.println(c4);
        System.out.println(c5);
        System.out.println(c6);
        System.out.println(c7);
        System.out.println(c8);
        System.out.println(c9);
        //只要元素类型与维度(一维数组跟多维数组)一样，就是同一个Class.
        int[] a=new int[10];
        int[] b=new int[100];
        System.out.println(a.getClass().hashCode());
        System.out.println(b.getClass().hashCode());
    }
}

Java memory analysis

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Class loader

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public static void main(String[] args) throws ClassNotFoundException {
    
    
        //获取系统类的加载器
        ClassLoader systemClassLoader=ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader);
        //获取系统类加载器的父类加载器-->扩展类加载器
        ClassLoader parent =systemClassLoader.getParent();
        System.out.println(parent);
        //获取扩展类加载器的父类加载器-->根加载器（c/c++）
        ClassLoader parent1 = parent.getParent();
        System.out.println(parent1);
        //测试当前类是那个加载器加载的
        ClassLoader classLoader=Class.forName("Test04").getClassLoader();
        System.out.println(classLoader);
        //测试JDK内置的类是那个加载器加载的
        classLoader=Class.forName("java.lang.Object").getClassLoader();
        System.out.println(classLoader);
        //如何获得系统类加载器可以加载的路径
        System.out.println(System.getProperty("java.class.path"));

    }

Get the complete structure of the runtime class

Field、Method、Constructor、Superclass、Interface、Annotation

public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException, NoSuchMethodException {
    
    
        Class c1=Class.forName("User");
        User user = new User();
        c1=user.getClass();
        //获得类的名字
        System.out.println(c1.getName());//获取包名+类名
        System.out.println(c1.getSimpleName());//获得类名
        //获得类的属性
        Field[] fields=c1.getFields();//只能找到public属性
        fields=c1.getDeclaredFields();//找到全部的属性
        for(Field field:fields){
    
    
            System.out.println(field);
        }
        //获得指定属性的值
        Field name =c1.getDeclaredField("name");
        System.out.println(name);
        System.out.println("***********************");
        //获得类的方法
        Method[] methods = c1.getMethods();//获得本类及其父类的全部public方法
        for(Method method:methods){
    
    
            System.out.println("正常的："+method);
        }
        methods=c1.getDeclaredMethods();//获得本类的所有方法
        for(Method method:methods){
    
    
            System.out.println("getDeclaredMethods："+method);
        }
        //获得指定方法
        //重载
        Method getName=c1.getMethod("getName",null);
        Method setName=c1.getMethod("setName",String.class);
        //Method setName=c1.getMethod("getName",String.class);
        System.out.println(getName);
        System.out.println(setName);

        //获得指定的构造器
        System.out.println("***********************");
        Constructor[] constructors = c1.getConstructors();
        for (Constructor constructor : constructors) {
    
    
            System.out.println(constructor);
        }
        Constructor[] declaredConstructors = c1.getDeclaredConstructors();
        for (Constructor declaredConstructor : declaredConstructors) {
    
    
            System.out.println("#"+declaredConstructor);

        }
        //获得指定的构造器
        Constructor declaredConstructor=c1.getDeclaredConstructor(String.class,int.class,int.class);
        System.out.println("指定:"+declaredConstructor);

    }

What can be done with the Class object?

 public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, NoSuchMethodException, InvocationTargetException, NoSuchFieldException {
    
    
        //获得class对象
        Class c1=Class.forName("User");
        //构造一个对象
        User user  = (User) c1.newInstance();//本质是调用的无参构造器
        System.out.println(user);

        //通过构造器创建对象
        Constructor declaredConstrcutor=c1.getDeclaredConstructor(String.class,int.class,int.class);

        User user2=(User) declaredConstrcutor.newInstance("张苏杭",001,18);
        System.out.println(user2);
        //通过反射调用普通方法
        User user3=(User)c1.newInstance();//通过反射创建一个对象
        //通过反射获取一个方法
        Method setName=c1.getDeclaredMethod("setName", String.class);
        setName.invoke(user3,"苏杭");//激活invoke(对象，"方法的值")
        System.out.println(user3.getName());
        //通过反射操作属性
        User user4=(User)c1.newInstance();
        Field name=c1.getDeclaredField("name");
        //不能直接操作私有属性，我们需要关闭程序的安全检测，属性或者方法的name.setAccessible(true)
        name.setAccessible(true);//
        name.set(user4,"书行");
        System.out.println(user4.getName());

    }

Reflection operation notes

Requirements:
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Code:

import java.lang.annotation.*;
import java.lang.reflect.Field;
//练习反射操作注解
public class Test07 {
    
    

    public static void main(String[] args) throws ClassNotFoundException, NoSuchFieldException {
    
    
        Class c1=Class.forName("Student2");
        //通过反射获取注解
        Annotation[] annotations = c1.getAnnotations();
        for (Annotation annotation : annotations) {
    
    
            System.out.println(annotation);
        }
        //获得注解的value的值。
        Tablesu annotation1=(Tablesu)c1.getAnnotation(Tablesu.class);
        String value = annotation1.value();
        System.out.println(value);
        //获得类指定的注解
        Field f=c1.getDeclaredField("name");
        Fieldsu annotation=f.getAnnotation(Fieldsu.class);
        System.out.println(annotation.columnName());
        System.out.println(annotation.type());
        System.out.println(annotation.length());
    }

}
@Tablesu("db_student")
class Student2{
    
    
    @Fieldsu(columnName= "db_id",type = "int",length = 10)
    private int id;
    @Fieldsu(columnName= "db_age",type = "int",length = 10)
    private int age;
    @Fieldsu(columnName= "db_name",type = "varchar",length = 10)
    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 + '\'' +
                '}';
    }
}
//类名的注解
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@interface Tablesu{
    
    
    String value();
}
//属性的注解
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
@interface Fieldsu{
    
    
    String columnName();
    String type();
    int length();
}