One interesting challenge I encountered is the need or ability to make an Java application extensible by providing additional classes and configuation. Ideally extension should happen by dropping a properly crafted JAR file into a specified location and restard the server. Along the line I learned about Java's classpath. This is what is to be shared here.

Act one: onto the classpath

When you start off with Java, you would expect, that you simply can set the classpath varible either using an environment variable or the java -cp parameter. Then you learn the hard way, that java -jar and java -cp are mutually exclusive. After a short flirt with fatJAR, you end up with a directory structure like this:

The secreingredient to make this work is the manifest file inside the myApp.jar. It needs to be told to put all jar files in libs onto the classpath too. In maven, it looks like this:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <version>${maven.jar.plugin.version}</version>
    <configuration>
        <archive>
            <manifest>
                <mainClass>com.hcl.domino.keep.Launch</mainClass>
            </manifest>
            <manifestEntries>
                <Class-Path>.</Class-Path>
                <Class-Path>libs/*</Class-Path>
            </manifestEntries>
        </archive>
    </configuration>
</plugin>

Now, that all JARS are successfully availble on the classspath, we can try to retrieve them.