Java Essentials Java annotations easy to get started

Annotations are simply configuration, which is a special configuration, and can be replaced with annotations. It then uses reflection to get the information of the annotations.

How to define an annotation

You create a new annotation definition in the IDE, which is structured like this:

package com.nicchagil.exercise.springbootexercise.annotation;public @interface MyFirstAnnotation {}

Then there are about 4 of the above structure plus some configuration, of course, this configuration is added in the form of annotations =_=!

Where is this annotation used

Where this annotation will be applied can be configured as follows:

Keep at what time

When to keep:

Annotations are reflected in the document

@Documented

Whether the subclass inherits annotations from the parent class

@Inherited

Get the information of annotations with reflection

Let's define an annotation first:

package com.nicchagil.exercise.springbootexercise.annotation;import java.lang.annotation.Documented;import java.lang.annotation.ElementType;import java.lang.annotation.Retention;import java.lang.annotation.RetentionPolicy;import java.lang.annotation.Target;@Target(ElementType.METHOD)@Retention(RetentionPolicy.RUNTIME)@Documentedpublic @interface PojoPostProcessing {        public Class targetClass();        public String[] whiteProperties();}

Use annotations, such as here I set to a method:

    @PojoPostProcessing(targetClass=User.class, whiteProperties={"name"})    public List<User> selectXxx(String id) {        ......    }

Reflection gets the information for the annotation:

    @Test    public void test() throws NoSuchMethodException, SecurityException, ClassNotFoundException {        Class clazz = Class.forName("com.nicchagil.exercise.springbootexercise.service.UserService");                Method method = clazz.getMethod("selectXxx", String.class);                boolean isAnnotationPresent = method.isAnnotationPresent(PojoPostProcessing.class);        if (!isAnnotationPresent) {            return;        }                PojoPostProcessing dpp = (PojoPostProcessing)method.getAnnotation(PojoPostProcessing.class);        this.logger.info("dpp : {}", dpp);    }

Log:

dpp : @com.nicchagil.exercise.springbootexercise.annotation.PojoPostProcessing(targetClass=class com.nicchagil.exercise.springbootexercise.mapper.entity.User, whiteProperties=[name])

Java Essentials Java annotations easy to get started