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javamavendependenciespom.xml

Why version number in maven dependency is skipped at times?

I am fairly new to maven's capabilities.. I have seen that in pom.xml where dependencies are put, at times, only groupID and artifact id are mentioned and version is skipped. why is this? For example the below dependency is from springsource website http://spring.io/guides/gs/authenticating-ldap/

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-security</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.security</groupId>
        <artifactId>spring-security-ldap</artifactId>
        <version>3.2.4.RELEASE</version>
    </dependency>
    <dependency>
        <groupId>org.apache.directory.server</groupId>
        <artifactId>apacheds-server-jndi</artifactId>
        <version>1.5.5</version>
    </dependency>
</dependencies>

But elsewhere in stackoverflow it was also mentioned that version is not optional. I would be glad if someone could explain this.

Solution

  • Yes, version is not optional.

    Consider a multimodule application which has 10 modules, say module1, module2.. module10.Assume that all of these 10 projects use the spring-boot-starter-web. In case these 10 modules are interdependent, you might want to use the same version of the spring-boot-starter-web in each of these 10.

    Now just imagine how complicated it would be if you were to maintain the same version number in all of these 10 pom files and then update all of them when you want to use a newer version of spring-boot-starter-web. Wouldn't it be better if this information can be managed centrally?

    Maven has got something known a <dependencyManagement/> tag to get around this and centralize dependency information.

    For the example which you have provided, below set of links will help you understand how the version number is resolved even though it's not present in the pom which you are looking at.

    Look at the parent tag of the pom you are looking at (https://github.com/spring-guides/gs-authenticating-ldap/blob/master/complete/pom.xml)

    Now lets go to that parent and see if the versions are specified in the dependencyManagement section of that pom(https://github.com/spring-projects/spring-boot/blob/master/spring-boot-starters/spring-boot-starter-parent/pom.xml). No it's not defined there as well. Now lets look at the parent of parent. https://github.com/spring-projects/spring-boot/blob/master/spring-boot-dependencies/pom.xml. Oh yea, we have got the version numbers there.

    Similar to dependencyManagement, plugins can be managed in the pluginManagement section of the pom.

    Hope that explains it.

    Refer : dependencyManagement, pluginManagement