Why maven-war-plugin behaves differently than the standard maven dependency resolving mechanism

Question

Is there a reason maven-war-plugin behaves differently than the standard maven dependency resolving mechanism? I have a war maven project which has a dependency:

    <dependency>
        <groupId>GroupA</groupId>
        <artifactId>DependencyA</artifactId>
        <exclusions>
            <exclusion>
                <groupId>GroupB</groupId>
                <artifactId>DependencyB</artifactId>
            </exclusion>
        </exclusions>
    </dependency>

The above dependency is correctly excluded and I can verify on the dependency hierarchy view. When I build the war with maven it includes the DependencyB in WEB-INF/lib directory, so I have to explicitly define to exclude it using the maven-war-plugin as so:

        <plugin>
            <artifactId>maven-war-plugin</artifactId>
            <configuration>
                <warSourceDirectory>src/main/webapp</warSourceDirectory>
                <packagingExcludes>
                    WEB-INF/lib/dependencyB.jar,
                </packagingExcludes>
            </configuration>
        </plugin>

I have to do the above cause conflicts between versions arise on my project. Why is this happening? Is there another way to achieve this?

UPDATE

So I created a test case just to showcase you what I mean, if I was not clear enough before. And the corresponding pom's are as below:

ArtifactA:

<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>GroupA</groupId>
<artifactId>ArtifactA</artifactId>
<packaging>war</packaging>
<version>0.0.1-SNAPSHOT</version>
<name>ArtifactA Maven Webapp</name>
<url>http://maven.apache.org</url>
<dependencies>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>3.8.1</version>
        <scope>test</scope>
    </dependency>

    <dependency>
        <groupId>GroupB</groupId>
        <artifactId>ArtifactB</artifactId>
        <type>war</type>
        <version>0.0.1-SNAPSHOT</version>
        <exclusions>
            <exclusion>
                <groupId>GroupC</groupId>
                <artifactId>ArtifactC</artifactId>
            </exclusion>
        </exclusions>
    </dependency>
</dependencies>


<build>
    <finalName>ArtifactA</finalName>
</build>

ArtifactB

<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/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>GroupB</groupId>
<artifactId>ArtifactB</artifactId>
<packaging>war</packaging>
<version>0.0.1-SNAPSHOT</version>
<name>ArtifactB Maven Webapp</name>
<url>http://maven.apache.org</url>
<dependencies>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>3.8.1</version>
        <scope>test</scope>
    </dependency>

    <dependency>
        <groupId>GroupC</groupId>
        <artifactId>ArtifactC</artifactId>
        <version>0.0.1-SNAPSHOT</version>
    </dependency>
</dependencies>
<build>
    <finalName>ArtifactB</finalName>
</build>

ArtifactC

<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>GroupC</groupId>
<artifactId>ArtifactC</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>

<name>ArtifatcC</name>
<url>http://maven.apache.org</url>

<properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
</properties>

<dependencies>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>3.8.1</version>
        <scope>test</scope>
    </dependency>
</dependencies>

In the dependency Hierarchy in Eclipse, I do not see the dependency ArtifactC. enter image description here

And when building the ArtifactA and unzipping the .war I get: enter image description here

As you can see the transitive dependency is included in the .war in the WEB-INF/lib. I hope this is more clear now.

Some other details to add:

I'm calling maven from eclipse
maven-war-plugin version 2.1.1, also tested 2.5
Maven version 3.0.4 embedded in Eclipse

Is transitive DependencyB being included by a different dependency (not A) by chance? You might need to exclude it in more than one place. — user944849
Please add you full pom. Which Maven version? Which version maven-war-plugin do you use? How did you call maven? From command line? — khmarbaise

Jörn Horstmann Jörn Horstmann · Accepted Answer · 2014-12-04T17:39:38

The problem is not so much that the war plugin handles dependency exclusion differently, but that it handles dependencies of <type>war</type> specially. It creates a war overlay out of these, meaning it takes the whole contents of the war and puts the projects web resources on top. The war dependency really has no transitive dependencies, it already includes all jar files.

If using war overlays is really your intention then using packagingExcludes is the correct solution. If you instead want to depend on the classes in artifact B then you should modify the pom so it creates a separate artifact (using the attachClasses configuration) and depend on this classes artifact. Exclusion of transitive dependencies should then work as with any other jar dependency.

Why maven-war-plugin behaves differently than the standard maven dependency resolving mechanism

2 Answers