Implementing a Custom @Autowired-like Annotation in Spring

The tutorial shows a step‑by‑step method for building a custom annotation that behaves like Spring's @Autowired , allowing developers to quickly experiment with dependency injection without writing complex code.

Six source files are required:

Custom annotation class package com.aqin.custom.annotation; import java.lang.annotation.*; @Target({ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE}) @Retention(RetentionPolicy.RUNTIME) @Documented public @interface AqinAutowired { boolean required() default true; }

BeanPostProcessor MyAnnotationBeanPostProcessor that extends AutowiredAnnotationBeanPostProcessor and replaces the default autowired type with AqinAutowired.class (the crucial line is this.autowiredAnnotationTypes.add(AqinAutowired.class); ).

Controller MyAnnotationController using the new annotation: package com.aqin.custom.annotation; public class MyAnnotationController { @AqinAutowired private MyAnnotationService myAnnotationService; public void testMyAnnotation() { myAnnotationService.test(); } }

Service MyAnnotationService that simply prints a message: package com.aqin.custom.annotation; public class MyAnnotationService { public void test() { System.out.println("(*≧ω≦)～"); } }

XML configuration populateBean.xml that registers the processor, controller and service beans: <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.springframework.org/schema/beans https://www.springframework.org/schema/beans/spring-beans.xsd"> <bean id="myAnnotationBeanPostProcessor" class="com.aqin.custom.annotation.MyAnnotationBeanPostProcessor"/> <bean id="myAnnotationController" class="com.aqin.custom.annotation.MyAnnotationController"/> <bean id="myAnnotationService" class="com.aqin.custom.annotation.MyAnnotationService"/> </beans>

Test class Test that loads the XML and invokes the controller method: package com.aqin.custom.annotation; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class Test { public static void main(String[] args) { ApplicationContext ctx = new ClassPathXmlApplicationContext("populateBean.xml"); MyAnnotationController bean = ctx.getBean(MyAnnotationController.class); bean.testMyAnnotation(); } }

Running the Test class prints the expected output, confirming that the custom annotation is correctly processed by the BeanPostProcessor and the dependency is injected.

The article also analyses the internal Spring mechanisms: during bean definition merging, applyMergedBeanDefinitionPostProcessors() iterates over MergedBeanDefinitionPostProcessor implementations, invoking postProcessMergedBeanDefinition() on both the original AutowiredAnnotationBeanPostProcessor and the custom MyAnnotationBeanPostProcessor . The custom processor registers AqinAutowired in the autowiredAnnotationTypes set, so fields annotated with it are treated identically to those annotated with @Autowired .

Finally, the author invites readers to discuss the approach, provides additional resources, and includes promotional links and QR codes, but the core instructional content remains a practical guide for extending Spring's dependency injection.