Spring #Autowired working without context:annotation-config [duplicate] - spring

Question

This question already has answers here:
Difference between <context:annotation-config> and <context:component-scan>
(15 answers)
Closed 3 years ago.
I've tested the behavior of auto wiring in case the context:annotation-config element is missing in the application context xml file. To my surprise it worked just the same.
So here is my question:
How come an AutowiredAnnotationBeanPostProcessor is registered in the ApplicationContext even though the context:annotation-config element is missing from the application context configuration file, or what else mechanism makes this configuration work?
I'm using Spring version 3.0.6.RELEASE
This is the project pom file:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven- v4_0_0.xsd"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<modelVersion>4.0.0</modelVersion>
<groupId>org.springframework.samples.spring</groupId>
<artifactId>spring-utility</artifactId>
<version>1.0.0.CI-SNAPSHOT</version>
<packaging>jar</packaging>
<name>Spring Utility</name>
<url>http://www.springframework.org</url>
<description>
<![CDATA[
This project is a minimal jar utility with Spring configuration.
]]>
</description>
<properties>
<maven.test.failure.ignore>true</maven.test.failure.ignore>
<spring.framework.version>3.0.6.RELEASE</spring.framework.version>
</properties>
<dependencies>
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.7</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-test</artifactId>
<version>${spring.framework.version}</version>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context</artifactId>
<version>${spring.framework.version}</version>
</dependency>
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.14</version>
</dependency>
</dependencies>
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-compiler-plugin</artifactId>
<configuration>
<source>1.5</source>
<target>1.5</target>
</configuration>
</plugin>
</plugins>
</build>
</project>
This is the application context configuration file, with the context:annotation-config element commented out:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">
<description>Example configuration to get you started.</description>
<!-- context:annotation-config/ -->
<context:component-scan base-package="com.foo.ch04" />
</beans>
A MessageProvider, that will be used as collaborator by a dependent bean MessageRenderer
package com.foo.ch04.helloworld;
import org.springframework.stereotype.Service;
#Service("messageProvider")
public class HelloWorldMessageProvider implements MessageProvider {
public String getMessage() {
return "Hello, World!";
}
}
The MessageRenderer, whose dependency messageProvider gets auto-injected:
package com.foo.ch04.helloworld;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
#Service("messageRenderer")
public class StandardOutMessageRenderer implements MessageRenderer {
private MessageProvider messageProvider;
public void render() {
if (messageProvider == null) {
throw new RuntimeException(
"You must set the property messageProvider before rendering message.");
}
System.out.println(messageProvider.getMessage());
}
#Autowired
public void setMessageProvider(MessageProvider provider) {
messageProvider = provider;
}
public MessageProvider getMessageProvider() {
return messageProvider;
}
}
The test application loading the application context and testing the messageRenderer:
package com.foo.ch04.helloworld;
import org.springframework.context.support.GenericXmlApplicationContext;
public class DeclareSpringComponents {
public static void main(String[] args) {
GenericXmlApplicationContext context = new GenericXmlApplicationContext();
context.load("classpath:META-INF/spring/app-context-annotation.xml");
context.refresh();
MessageRenderer renderer = context.getBean("messageRenderer",
MessageRenderer.class);
renderer.render();
}
}
Even though the is missing in the application context configuration file, the message "Hello, World!" is written to stdout when the application is run.

Answer 1

The use of <context:component-scan /> implies annotation based configuration and as such specifying <context:annotation-config/> is redundant.

Answer 2

'context:annotation-config' This annotation-config tag is used to process the auto wired beans declared in the application context XML file. If the auto wired bean could be discovered using the scope of 'context:component-scan' tag then no need to use 'context:annotation-config' tag