How does maven decide when to use the target folder for classpath

Question

I have a question regarding how maven calculates the classpath during building. Specifically, what controls when the "target/classes" is used and when the "jar" from a repository (local/remote) is used.

I have a project on version 1.0.0-SNAPSHOT where the artifacts have NOT been installed/deployed so there is no "jar" in some repository (remote or local) to resolve them. I want to run "generate-sources" WITHOUT installing locally (no 'mvn install' run).

The structure looks like this:

parent-prj
parent-prj/sub-prj
parent-prj/gen-src-prj <--- This depends on 'sub-prj'

When I run "mvn -am -pl parent-prj/gen-src-prj generate-sources" in order to just generate some java files, it does not work:

[ERROR] Failed to execute goal on project gen-src-prj: Could
 not resolve dependencies for project 
mygrp:gen-src-prj:jar:1.0.0-SNAPSHOT: 
Could not find artifact 
mygrp:sub-prj:jar:1.0.0-SNAPSHOT -> [Help 1]

Using debug output and adding "dependency:build-classpath" I can confirm that maven ignores the presence of "sub-prj" in the reactor and looks for a "jar" somewhere which it can't find. Yet the project is printed in the reactor summary:

[INFO] Reactor Summary:
[INFO] 
[INFO] parent-prj ..................................... SUCCESS [  0.625 s]
[INFO] sub-prj ........................................ SUCCESS [  0.018 s]
[INFO] gen-src-prj .................................... FAILURE [  0.040 s]

The interesting thing I noticed is that running the compile goal works fine! This uses sub-prj/target/classes (as shown by dependency:build-classpath) and has no trouble generating the sources and even compiling them: "mvn -am -pl parent-prj/gen-src-prj compile"

So here are the points I want to understand:

1. Why does the compile goal work but the generate-sources doesn't work?
2. At what point does maven decide to use the output folder of previous projects on the reactor classpath instead of looking for a jar?
3. Is there a way for generate-sources to run directly as I want it EVEN WITHOUT having its dependencies resolved?

Regarding (3) my generation tool is a utility invoked by:

    <plugin>
        <groupId>org.codehaus.mojo</groupId>
        <artifactId>exec-maven-plugin</artifactId>
        <version>1.6.0</version>
        <executions>
            <execution>
                <phase>generate-sources</phase>
                <goals>
                    <goal>java</goal>
                </goals>
            </execution>
        </executions>

The tool reads some XML in src/main/resources and generates Java files and does NOT need anything in its class-path (so there is no need for maven to resolve it).

Also note that I would be interested to understand (1) and (2) even if a solution for (3) is provided.

---

EDIT: Per comment request, adding full example

parent-prj/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>mygrp</groupId>
    <artifactId>parent-prj</artifactId>
    <version>1.0.0-SNAPSHOT</version>

    <packaging>pom</packaging>

    <modules>
        <module>sub-prj</module>
    <module>gen-src-prj</module>
    </modules>

    <build>
        <pluginManagement>
            <plugins>
                <plugin>
                    <groupId>org.apache.maven.plugins</groupId>
                    <artifactId>maven-compiler-plugin</artifactId>
                    <version>3.3</version>
                    <configuration>
                        <source>1.8</source>
                        <target>1.9</target>
                        <encoding>UTF-8</encoding>
                    </configuration>
                </plugin>
            </plugins>
        </pluginManagement>
    </build>
</project>

parent-prj/sub-prj/pom.xml

<?xml version="1.0"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
    xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>mygrp</groupId>
        <artifactId>parent-prj</artifactId>
        <version>1.0.0-SNAPSHOT</version>
    </parent>

    <artifactId>sub-prj</artifactId>
</project>

parent-prj/gen-src-prj/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>

    <parent>
        <groupId>mygrp</groupId>
        <artifactId>parent-prj</artifactId>
        <version>1.0.0-SNAPSHOT</version>
    </parent>

    <artifactId>gen-src-prj</artifactId>

    <dependencies>
        <dependency>
            <groupId>mygrp</groupId>
            <artifactId>sub-prj</artifactId>
            <version>${project.version}</version>
        </dependency>
    </dependencies>

    <build>
        <plugins>
            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>exec-maven-plugin</artifactId>
                <version>1.6.0</version>
                <executions>
                    <execution>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>java</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <includeProjectDependencies>false</includeProjectDependencies>
                    <includePluginDependencies>true</includePluginDependencies>
                    <mainClass>uk.co.real_logic.sbe.SbeTool</mainClass>
                    <systemProperties>
                        <systemProperty>
                            <key>sbe.output.dir</key>
                            <value>${project.build.directory}/generated-sources/java</value>
                        </systemProperty>
                        <systemProperty>
                            <key>sbe.validation.warnings.fatal</key>
                            <value>true</value>
                        </systemProperty>
                    </systemProperties>
                    <arguments>
                        <argument>${project.build.resources[0].directory}/Examples.xml</argument>
                    </arguments>
                    <workingDirectory>${project.build.directory}/generated-sources/java</workingDirectory>
                </configuration>
                <dependencies>
                    <dependency>
                        <groupId>uk.co.real-logic</groupId>
                        <artifactId>sbe-tool</artifactId>
                        <version>1.7.10</version>
                    </dependency>
                </dependencies>
            </plugin>

            <plugin>
                <groupId>org.codehaus.mojo</groupId>
                <artifactId>build-helper-maven-plugin</artifactId>
                <version>3.0.0</version>
                <executions>
                    <execution>
                        <id>add-source</id>
                        <phase>generate-sources</phase>
                        <goals>
                            <goal>add-source</goal>
                        </goals>
                        <configuration>
                            <sources>
                                <source>target/generated-sources/java/</source>
                            </sources>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>
</project>

---

EDIT: Armed with the knowledge from the answers I have come up with this workaround that allows one to achieve the desired behaviour. I list the dependencies in a profile that is active by default, then use another profile to run generate-sources with no dependencies active, like follows:

parent-prj/gen-src-prj/pom.xml

<profiles>
    <profile>
        <id>default</id>
        <activation>
            <activeByDefault>true</activeByDefault>
        </activation>
        <dependencies>
            <dependency>
                <groupId>mygrp</groupId>
                <artifactId>sub-prj</artifactId>
                <version>${project.version}</version>
            </dependency>
        </dependencies>
    </profile>

    <profile>
        <id>excludeDependency</id>
        <dependencies>
        </dependencies>
    </profile>
</profiles>

To generate sources with above, use: mvn -PexcludeDependency generate-sources

Answer 1

Maven can reference only output generated in current Session (during currently executing shell command). It uses the most "mature" place to look for the "output":

• If compile is run - the classes end up in the target/classes dir, thus other modules can reference that
• If package is run - then target/*.jar is created and this jar file ends up in the classpath instead
• If install is run - then jar file ends up in the local repository - which is what ends up on the classpath

So there are 3 factors that impede your task:

• maven-exec-plugin requires dependency resolution (as pointed out by @mondaka)
• Your module1 references module2
• generate-sources is run before the compilation. Thus module2 is not yet prepared to be used as a dependency.

So if you want to do it your way - you'll have to run at least compile phase each time you use anything from the Default Lifecycle. Or you could write your own plugin that doesn't require dependency resolution.

Answer 2

This problem is related to an open maven bug:

https://issues.apache.org/jira/browse/MNG-3283

The issue says: "The problem only occurs when a plugin binds itself to the generate-sources phase and has @requiresDependencyResolution".

I have checked that exec-maven-plugin Mojo have indeed requiresDependencyResolution = ResolutionScope.TEST. You can see that on https://github.com/mojohaus/exec-maven-plugin/blob/master/src/main/java/org/codehaus/mojo/exec/ExecJavaMojo.java

Then, your only option is to use compile or process-classes phases. This is a Major open bug from 2007...