FaqDebugMojo

Can I debug Maven builds (mojo code) from the IDE?

Yes. The following steps were tested with a NB 7.0 development build but ought to apply to earlier releases as well.

I made a "Java Application" and added

<build>
    <plugins>
        <plugin>
            <groupId>org.codehaus.mojo.webstart</groupId>
            <artifactId>webstart-maven-plugin</artifactId>
            <version>1.0-beta-1</version>
        </plugin>
    </plugins>
</build>

In the properties dialog of the demo project, I added a custom action to run a sample mojo while debugging the build, so that my nbactions.xml looks like this:

<?xml version="1.0" encoding="UTF-8"?>
<actions>
    <action>
        <actionName>CUSTOM-debug-mojo</actionName>
        <displayName>Debug Mojo</displayName>
        <goals>
            <goal>-e</goal>
            <goal>package</goal>
            <goal>webstart:jnlp-inline</goal>
        </goals>
        <properties>
            <jpda.listen>maven</jpda.listen>
        </properties>
    </action>
</actions>

(You can also run the same goals without creating a custom action, if you don't mind retyping the above information each time.) As expected (given that I have configured nothing related to JNLP in this project), it fails with a stack trace:

java.lang.NullPointerException
        at org.codehaus.mojo.webstart.AbstractJnlpMojo.execute(AbstractJnlpMojo.java:214)

If I check out https://svn.codehaus.org/mojo/tags/webstart-1.0-beta-1/webstart-maven-plugin and open it in the IDE, and set a breakpoint on that line, then when running the custom action the debugger stops at that line and shows me that jnlp = null.