How a maven dependecy can be defined without version tag?

Question

I have a pom which works without defining dependency version in pom works fine and another one without dependency version which does not work.

The one which works:

<project>
    <parent>
        <artifactId>artifact-parent</artifactId>
        <groupId>group-parent</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <dependencies>
        <dependency>
            <groupId>group-a</groupId>
            <artifactId>artifact-a</artifactId>
        </dependency>
        <dependency>
            <groupId>group-a</groupId>
            <artifactId>artifact-b</artifactId>
        </dependency>
    </dependencies>
</project>

This one which does not work:

<project>
    <dependencies>
        <dependency>
            <groupId>group-a</groupId>
            <artifactId>artifact-a</artifactId>
        </dependency>
        <dependency>
            <groupId>group-a</groupId>
            <artifactId>artifact-b</artifactId>
        </dependency>
    </dependencies>
</project>

The only thing differ in these two seems to be:

<parent>
    <artifactId>artifact-parent</artifactId>
    <groupId>group-parent</groupId>
    <version>1.0-SNAPSHOT</version>
</parent>

Second one does not work it seems fine to me but my question is why the first one works ?

Quoted from maven pom reference :

This trinity represents the coordinate of a specific project in time, demarcating it as a dependency of this project.

So my question is how the first one is working?

Answer 1

The main thing to notice here is:

<parent>
    <artifactId>artifact-parent</artifactId>
    <groupId>group-parent</groupId>
    <version>1.0-SNAPSHOT</version>
</parent>

Version of the dependency looks like to be defined in the parent pom. This can be something like this:

<project>
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>group-a</groupId>
                <artifactId>artifact-a</artifactId>
                <version>1.0</version>
            </dependency>
            <dependency>
                <groupId>group-a</groupId>
                <artifactId>artifact-b</artifactId>
                <version>1.0</version>
            </dependency>
        </dependencies>
    </dependencyManagement>
</project>

Again quoting from the doc :

This is because the minimal set of information for matching a dependency reference against a dependencyManagement section is actually {groupId, artifactId, type, classifier}.

Here we did not need to define the {type, classifier} as it is same as default value which is as follows :

<type>jar</type>
<classifier><!-- no value --></classifier>

If this value differ form the default, you need to define it explicitly in both parent pom and child pom.