java - Maven and dependent modules

Question

Colleagues have been touting the wonders of maven and its magical dependency stuff but I'm finding that it fails at what I would consider the obvious use.

Suppose I have a root folder with a master POM.

Then underneath I have some projects, call them A and B

B requires A and so the POM in the B folder has the appropriate dependency entry in it

Now, back in the root folder, in a profile, I specify that I want to build B.

When I perform the usual mvn clean install, I get a failure because A was not built.

My friends tell me I have to specify both A and B in that main profile in the root.

But isn't the whole point of dependency management that maven sees B, goes to the B POM file where it sees the dependency on A and so it should go build A automatically.

Answer 1

With the master POM:

~/scratch/pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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>scratch</groupId>
    <artifactId>scratch</artifactId>
    <packaging>pom</packaging>
    <version>1.0-SNAPSHOT</version>

    <modules>
        <module>nipple</module>
        <module>cabbage</module>
    </modules>
</project>

And the module POMs:

~/scratch/nipple/pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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">

    <parent>
        <artifactId>scratch</artifactId>
        <groupId>scratch</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <modelVersion>4.0.0</modelVersion>

    <groupId>scratch</groupId>
    <artifactId>nipple</artifactId>
    <version>1.0-SNAPSHOT</version>

</project>

~/scratch/cabbage/pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<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">

    <parent>
        <artifactId>scratch</artifactId>
        <groupId>scratch</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <modelVersion>4.0.0</modelVersion>

    <groupId>scratch</groupId>
    <artifactId>cabbage</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>scratch</groupId>
            <artifactId>nipple</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
    </dependencies>

</project>

I can issue mvn package in the root directory after clearing out my local repository and end up with all the modules built. (Into empty-ish JARs, but built.)

Maven seems to look for dependencies either in the repository, or in the build in progress. It will not automatically traverse your project structure when you're only building a single module, because it's not required that you even have the parent project on your computer, much less one directory above the current module. (The parent-child relationship isn't even bijective.)

A reason why this is so might be because a directory layout where the location of modules would be predictable is in no way mandatory. It's even somewhat common and acceptable that the layout for the above example be like this:

projects
|
+--scratch
|  |
|  +--scratch-parent
|  |  |
|  |  +--pom.xml [The POM of scratch:scratch:1.0-SNAPSHOT]
|  |
|  +--nipple
|  |  |
|  |  +--pom.xml [The POM of scratch:mod1:1.0-SNAPSHOT]
|  |
|  +--cabbage
|  |  |
|  |  +--pom.xml [The POM of scratch:mod2:1.0-SNAPSHOT]

In this case, the <modules> section of the parent POM would be:

<modules>
    <module>../nipple</module>
    <module>../cabbage</module>
</modules>

Notice that there is nothing saying which artifact ID is in which module. It just serves to tell Maven that these are filesystem locations where to search for other artifacts related to this build.