Why order of Maven dependencies matter?

Question

I thought that the order of Maven dependencies doesn't matter before and regard this as a pro of it. And this is my old pom.xml's dependencies:

<dependencies>      <dependency>         <groupId>org.glassfish.jersey.containers</groupId>         <artifactId>jersey-container-servlet</artifactId>         <version>2.19</version>     </dependency>      <dependency>         <groupId>org.springframework</groupId>         <artifactId>spring-web</artifactId>         <version>4.1.7.RELEASE</version>     </dependency>      <dependency>         <groupId>org.glassfish.jersey.ext</groupId>         <artifactId>jersey-spring3</artifactId>         <version>2.19</version>     </dependency>      <dependency>         <groupId>org.glassfish.jersey.media</groupId>         <artifactId>jersey-media-moxy</artifactId>         <version>2.19</version>     </dependency>  </dependencies> 

It works well, and today I wanna move spring dependency to the bottom so that those jersey related can be together. However then I can no longer make it working, my Jetty complains:

[ERROR] Failed to execute goal org.eclipse.jetty:jetty-maven-plugin:9.3.0.M1:run (default-cli) on project mtest: Execution default-cli of goal org.eclipse.jetty:jetty-maven-plugin:9.3.0.M1:run failed: A required class was missing while executing org.eclipse.jetty:jetty-maven-plugin:9.3.0.M1:run: org/apache/commons/logging/LogFactory 

That is really confusing, so do I have to concern about dependencies order? How do I know the correct order?

Answer 1

The order of dependencies does matter because of how Maven resolves transitive dependencies, starting with version 2.0.9. Excerpt from the documentation:

(...) this determines what version of a dependency will be used when multiple versions of an artifact are encountered. (...) You can always guarantee a version by declaring it explicitly in your project's POM. (...) since Maven 2.0.9 it's the order in the declaration that counts: the first declaration wins.