Annotations (c) parsing the annotations (the annotations on analysis method, analysis based on the annotation)

Notes Analysis: annotation get property values

Using reflection techniques : java.lang.reflect.AnnotatedElement Interface
   Interface reflected implementation classes:
      the AccessibleObject, Class, the Constructor, Field,, Method,, the Package
   abstract methods:
       Boolean isAnnotationPresent (Class annotationClass)
      determines the specified type (Constructor, Class, Method ...) on whether the annotation containing the specified
parameters:
      Class annotationClas: Class transmitting specified annotation file object; want to judge if there is no annotation Book, passed Book.class
return value:
      Boolean: containing the specified annotation, returns true; not specified annotation return to false
       T getAnnotation (Class annotationClass)
      Gets the specified type (Constructor, Class, Method ...) annotation
parameters:
      Class annotationClass: Specifies the file transfer Class annotations; want to obtain Book annotations, passing Book.class
return value:
      T: returns is specified annotation
      annotation does not exist, return null

Meta-annotation of use:

package com.ccc.demo08Annotation;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Target({ElementType.METHOD,ElementType.TYPE})//声明注解的使用位置:成员方法上,类/接口上
@Retention(RetentionPolicy.RUNTIME)//声明注解的声明周期(.java,.class,运行后的内存中)
public @interface Book {
    //书名
    public abstract String value();
    //价格，默认值为 100
    public abstract double price() default 100;
    //多位作者
    public abstract String[] authors();
}

Analytical element analysis method on :( annotation, the annotation on annotation parser class)

package com.ccc.demo08Annotation;

import com.itheima.demo05Annotation.MyAnnotation01;
import com.itheima.demo05Annotation.MyAnnotation03;
import org.junit.Test;

import java.lang.annotation.Annotation;
import java.lang.reflect.Method;
import java.util.Arrays;

@Book(value = "三国演义",price = 88,authors = "罗贯中")
public class ParseAnnotation {
    @Book(value = "水浒传",authors = "施耐庵")
    @MyAnnotation03("aaa")
    public void method(){}

Notes on the analytical method for
analysis:

1. Gets the current class (ParseAnnotation) of the class file object
2. Get all members of the methods in the current class, returns an array of Method
3. Through the array, each acquiring a Method Object
4. Use Method isAnnotationPresent object determination, whether there is a specified method Book annotations
5. If the method specified Book annotations, using the specified method for obtaining Book Notes on getAnnotation
6. Use annotation name attribute name () Gets annotation property values

    @Test
    public void getAnnotationMethod() throws ClassNotFoundException {
        
        //1.获取当前类ParseAnnotation的class文件对象
        Class clazz = Class.forName("com.itheima.demo08Annotation.ParseAnnotation");
        
        //2.获取当前类中的所有成员方法,返回一个Method数组
        //Method[] methods = clazz.getMethods();
        Method[] methods = clazz.getDeclaredMethods();
        
        //3.遍历数组,获取每一个Method对象
        for (Method m : methods) {
            //System.out.println(m.getName()+"-->"+m.isAnnotationPresent(Book.class));
            
            //4.使用Method对象中的方法isAnnotationPresent判断,方法上是否有指定的Book注解
            if(m.isAnnotationPresent(Book.class)){
                
                //5.如果方法指定的Book注解,使用getAnnotation获取方法上指定的Book注解
                Book book = m.getAnnotation(Book.class);
                
                //6.使用注解名.属性名()获取注解的属性值
                System.out.println(book.value());
                System.out.println(book.price());
                System.out.println(Arrays.toString(book.authors()));
            }
        }
    }

Analytical notes on class
analysis:

1. Gets the current class (ParseAnnotation) of the class file object
2. Use isAnnotationPresent determines whether the class has a specified annotation Book
3. If there Book annotations, use getAnnotation method to get to the Book Notes
4. Use annotation name attribute name (), get property values ​​annotation

    @Test
    public void  getAnnotationClass(){
        
        //1.获取当前类ParseAnnotation的class文件对象
        Class clazz = ParseAnnotation.class;
        
        //2.使用isAnnotationPresent判断类上是否有指定的Book注解
        boolean b = clazz.isAnnotationPresent(Book.class);
        //System.out.println(b);
        
        //3.如果有Book注解,使用getAnnotation方法获取到Book注解
        if(b){
            //Annotation是所有接口的父接口，而Annotation没有book类型的属性，
           								 //所以得向下转型为book类型
			Book book  = (Book)clazz.getAnnotation(Book.class);
            
            //4.使用注解名.属性名(),获取注解的属性值
            String value = book.value();
            System.out.println(value);
            double price = book.price();
            System.out.println(price);
            String[] authors = book.authors();
            System.out.println(Arrays.toString(authors));
        }
    }
}