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Table of Contents

1. Overview

The tutorial illustrates how to Bootstrap a Web Application with Spring and also discusses how to make the jump from XML to Java without having to completely migrate the entire XML configuration.

Further reading:

Bootstrap a Simple Application using Spring Boot

This is how you start understanding Spring Boot.

Read more

Configure a Spring Boot Web Application

Some of the more useful configs for a Spring Boot application.

Read more

Migrating from Spring to Spring Boot

See how to properly migrate from a Spring to Spring Boot.

Read more

2. The Maven pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" 
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="
      http://maven.apache.org/POM/4.0.0 
      http://maven.apache.org/xsd/maven-4.0.0.xsd">
   <modelVersion>4.0.0</modelVersion>
   <groupId>org</groupId>
   <artifactId>rest</artifactId>
   <version>0.0.1-SNAPSHOT</version>
   <packaging>war</packaging>

   <dependencies>

      <dependency>
         <groupId>org.springframework</groupId>
         <artifactId>spring-webmvc</artifactId>
         <version>${spring.version}</version>
         <exclusions>
            <exclusion>
               <artifactId>commons-logging</artifactId>
               <groupId>commons-logging</groupId>
            </exclusion>
         </exclusions>
      </dependency>
      
   </dependencies>

   <build>
      <finalName>rest</finalName>

      <plugins>
         <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <version>3.1</version>
            <configuration>
               <source>1.6</source>
               <target>1.6</target>
               <encoding>UTF-8</encoding>
            </configuration>
         </plugin>
      </plugins>
   </build>

   <properties>
      <spring.version>4.0.5.RELEASE</spring.version>
   </properties>

</project>

2.1. The cglib Dependency Before Spring 3.2

You may wonder why cglib is a dependency – it turns out there is a valid reason to include it – the entire configuration cannot function without it. If removed, Spring will throw:

Caused by: java.lang.IllegalStateException: CGLIB is required to process @Configuration classes. Either add CGLIB to the classpath or remove the following @Configuration bean definitions

The reason this happens is explained by the way Spring deals with @Configuration classes. These classes are effectively beans, and because of this they need to be aware of the Context, and respect scope and other bean semantics. This is achieved by dynamically creating a cglib proxy with this awareness for each @Configuration class, hence the cglib dependency.

Also, because of this, there are a few restrictions for Configuration annotated classes:

  • Configuration classes should not be final
  • They should have a constructor with no arguments

2.2. The cglib Dependency in Spring 3.2 and Beyond

Starting with Spring 3.2, it is no longer necessary to add cglib as an explicit dependency. This is because Spring is in now inlining cglib – which will ensure that all class based proxying functionality will work out of the box with Spring 3.2.

The new cglib code is placed under the Spring package: org.springframework.cglib (replacing the original net.sf.cglib). The reason for the package change is to avoid conflicts with any cglib versions already existing on the classpath.

Also, the new cglib 3.0 is now used, upgraded from the older 2.2 dependency (see this JIRA issue for more details).

Finally, now that Spring 4.0 is out in the wild, changes like this one (removing the cglib dependency) are to be expected with Java 8 just around the corner – you can watch this Spring Jira to keep track of the Spring support, and the Java 8 Resources page to keep tabs on the that.

3. The Java-based Web Configuration

@Configuration
@ImportResource({
  "classpath*:/rest_config.xml"
})
@ComponentScan( basePackages = "org.rest" )
@PropertySource({
  "classpath:rest.properties", 
  "classpath:web.properties" 
})
public class AppConfig{

   @Bean
   public static PropertySourcesPlaceholderConfigurer
     properties() {
 
      return new PropertySourcesPlaceholderConfigurer();
   }
}

First, the @Configuration annotation – this is the main artifact used by the Java-based Spring configuration; it is itself meta-annotated with @Component, which makes the annotated classes standard beans and as such, also candidates for component-scanning. The main purpose of @Configuration classes is to be sources of bean definitions for the Spring IoC Container. For a more detailed description, see the official docs.

Then, @ImportResource is used to import the existing XML based Spring configuration. This may be the configuration which is still being migrated from XML to Java, or simply the legacy configuration that you wish to keep. Either way, importing it into the Container is essential for a successful migration, allowing small steps without to much risk. The equivalent XML annotation that is replaced is:

<import resource=”classpath*:/rest_config.xml” />

Moving on to @ComponentScan – this configures the component scanning directive, effectively replacing the XML:

<context:component-scan base-package="org.rest" />

As of Spring 3.1, the @Configuration are excluded from classpath scanning by default – see this JIRA issue. Before Spring 3.1 though, these classes should have been excluded explicitly:

excludeFilters = { @ComponentScan.Filter( Configuration.class ) }

The @Configuration classes should not be auto-discovered because they are already specified and used by the Container – allowing them to be rediscovered and introduced into the Spring context will result in the following error:

Caused by: org.springframework.context.annotation.ConflictingBeanDefinitionException: Annotation-specified bean name ‘webConfig’ for bean class [org.rest.spring.AppConfig] conflicts with existing, non-compatible bean definition of same name and class [org.rest.spring.AppConfig]

And finally, using the @Bean annotation to configure the properties support – PropertySourcesPlaceholderConfigurer is initialized in a @Bean annotated method, indicating it will produce a Spring bean managed by the Container. This new configuration has replaced the following XML:

<context:property-placeholder
location="classpath:persistence.properties, classpath:web.properties"
ignore-unresolvable="true"/>

For a more in-depth discussion on why it was necessary to manually register the PropertySourcesPlaceholderConfigurer bean, see the Properties with Spring Tutorial.

3.1. The web.xml

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="
    http://java.sun.com/xml/ns/javaee"
    xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
       http://java.sun.com/xml/ns/javaee 
       http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    id="rest" version="3.0">

   <context-param>
      <param-name>contextClass</param-name>
      <param-value>
         org.springframework.web.context.support.AnnotationConfigWebApplicationContext
      </param-value>
   </context-param>
   <context-param>
      <param-name>contextConfigLocation</param-name>
      <param-value>org.rest.spring.root</param-value>
   </context-param>
   <listener>
      <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
   </listener>

   <servlet>
      <servlet-name>rest</servlet-name>
      <servlet-class>
         org.springframework.web.servlet.DispatcherServlet
      </servlet-class>
      <init-param>
         <param-name>contextClass</param-name>
         <param-value>
            org.springframework.web.context.support.AnnotationConfigWebApplicationContext
         </param-value>
      </init-param>
      <init-param>
         <param-name>contextConfigLocation</param-name>
         <param-value>org.rest.spring.rest</param-value>
      </init-param>
      <load-on-startup>1</load-on-startup>
   </servlet>
   <servlet-mapping>
      <servlet-name>rest</servlet-name>
      <url-pattern>/api/*</url-pattern>
   </servlet-mapping>

   <welcome-file-list>
      <welcome-file />
   </welcome-file-list>

</web-app>

First, the root context is defined and configured to use AnnotationConfigWebApplicationContext instead of the default XmlWebApplicationContext. The newer AnnotationConfigWebApplicationContext accepts @Configuration annotated classes as input for the Container configuration and is needed in order to set up the Java-based context.

Unlike XmlWebApplicationContext, it assumes no default configuration class locations, so the “contextConfigLocation” init-param for the Servlet must be set. This will point to the java package where the @Configuration classes are located; the fully qualified name(s) of the classes are also supported.

Next, the DispatcherServlet is configured to use the same kind of context, with the only difference that it’s loading configuration classes out of a different package.

Other than this, the web.xml doesn’t really change from an XML to a Java based configuration.

4. Conclusion

The presented approach allows for a smooth migration of the Spring configuration from XML to Java, mixing the old and the new. This is important for older projects, which may have a lot of XML based configuration that cannot be migrated all at once.

This way, in a migration, the XML beans can be ported in small increments.

In the next article on REST with Spring, I cover setting up MVC in the project, configuration of the HTTP status codes, payload marshaling, and content negotiation.

As always, the code presented in this article is available over on Github. This is a Maven based project, so it should be easy to import and run as it is.

Go deeper into Spring Security with the course:

>> LEARN SPRING SECURITY

Sort by:   newest | oldest | most voted
Chris Beams
Guest

Hi Eugen,

Note that as of Spring 3.1 RC2, use of [email protected](Configuration.class) is no longer necessary. See https://jira.springsource.org/browse/SPR-8808.

Would you mind updating your examples to depend on RC2 and eliminating this bit from the @ComponentScan definition?

Thanks for a great series of posts!

Eugen Paraschiv
Guest

Thank you for letting me know (as well as for the actual work). The update is now included in the article.

Amol Khanolkar
Guest

Can you tell me some advantages of using java based configurations over XML based and when to choose which one?

Torben Vesterager
Guest

Simple – you have one file for configuration and functionality – instead of jumping between 2

Eugen Paraschiv
Guest

Type safety is a big one, and ease of use is a close second.

Stephane
Guest

And you can use a debugger if needed.

Eugen Paraschiv
Guest

I updated the article – thanks.

Stephane
Guest

It’d be nice to also include a showcase on how to replace the web.xml file by a class implementing the WebApplicationInitializer interface.

Eugen Paraschiv
Guest

I am planning to discuss that in an upcoming article.

Stephane
Guest

I could in fact do it, and it wasn’t that hard. The only thing that I could not cut is how to move the elements of the log4j config into a Java config. Google was dry on this one. Maybe your next article would shed some light on this 🙂 Thanks again.

Stephane
Guest

It’s funny, my comment was automatically closed by a tag.

Anuj Patel
Guest

Thanks ! It really helps to clear concepts and will help while doing actual Implementation

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