Spring Boot war Packaging

Spring boot recommends creating an executable jar with an embedded container(tomcat or jetty) during build time and using this executable jar as a standalone process at runtime. It is common however to deploy applications to an external container instead and Spring boot provides packaging the applications as a war specifically for this kind of a need.

My focus here is not to repeat the already detailed Spring Boot instructions on creating the war artifact, but on testing the created file to see if it would reliably work on a standalone container. I recently had an issue when creating a war from a Spring Boot project and deploying it on Jetty and this is essentially a learning from that experience.

The best way to test if the war will work reliably will be to simply use the jetty-maven and/or the tomcat maven plugin, with the following entries to the pom.xml file:

<plugin>
 <groupId>org.apache.tomcat.maven</groupId>
 <artifactId>tomcat7-maven-plugin</artifactId>
 <version>2.2</version>
</plugin>
<plugin>
 <groupId>org.eclipse.jetty</groupId>
 <artifactId>jetty-maven-plugin</artifactId>
 <version>9.2.3.v20140905</version>
</plugin>

With the plugins in place, starting up the war with the tomcat plugin:

mvn tomcat7:run

and with the jetty plugin:

mvn jetty:run

If there any issues with the way the war has been created, it should come out at start-up time with these containers. For eg, if I were to leave in the embedded tomcat dependencies:

<dependency>
 <groupId>org.springframework.boot</groupId>
 <artifactId>spring-boot-starter-tomcat</artifactId>
</dependency> 

then when starting up the maven tomcat plugin, an error along these lines will show up:

java.lang.ClassCastException: org.springframework.web.SpringServletContainerInitializer cannot be cast to javax.servlet.ServletContainerInitializer

an indication of a servlet jar being packaged with the war file, fixed by specifying the scope as provided in the maven dependencies:

<dependency>
 <groupId>org.springframework.boot</groupId>
 <artifactId>spring-boot-starter-tomcat</artifactId>
 <scope>provided</scope>
</dependency> 

This gave me an error when started up using the jetty runtime, and the fix again is to mark the underlying tomcat dependencies as provided, replace above with the following:

<dependency>
 <groupId>org.springframework</groupId>
 <artifactId>spring-websocket</artifactId>
</dependency>
<dependency>
 <groupId>org.apache.tomcat.embed</groupId>
 <artifactId>tomcat-embed-websocket</artifactId>
 <scope>provided</scope>
</dependency>
<dependency>
 <groupId>org.springframework</groupId>
 <artifactId>spring-messaging</artifactId>
</dependency>