Maven - how can I add an arbitrary classpath entry to a jar?

I have an unusual situation where I need to add an arbitrary classpath entry (that points to a jar file) into the manifest of an executable jar. (This is for a Swing desktop application.)

The maven-jar-plugin generates the "Class-Path" entry for the jar manifest using the maven dependencies, and there doesn't appear to be any way of adding arbitrary entries.

I also looked at hard-coding the arbitrary classpath entry into the batch file that starts the application, using the "-classpath" parameter, but I can't figure out how to get Maven to filter the classpath into a batch file.

I found that there is an easy solution for this problem. You can add a <Class-Path> element to <manifestEntries> element, and set <addClassPath>true</addClassPath> to <manifest> element. So value of <Class-Path> element is added to class-path automatically. Example:

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-jar-plugin</artifactId>
  <configuration>
    <archive>
      <manifest>
        <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
        <addClasspath>true</addClasspath>
        <mainClass>your.main.Class</mainClass>
      </manifest>
      <manifestEntries>
        <Class-Path>../conf/</Class-Path>
      </manifestEntries>
    </archive>
  </configuration>
</plugin>

Update: Here's how to filter a classpath into a custom manifest.

The maven-dependency-plugin's build-classpath goal can be configured to output the classpath to a file in the properties format (i.e. classpath=[classpath]). You then configure the filters element to use the generated classpath file, and configure the resources directory to be filtered.

For example:

<build>
  <plugins>
    <plugin>
      <artifactId>maven-dependency-plugin</artifactId>
      <version>2.1</version>
      <executions>
        <execution>
          <phase>generate-resources</phase>
          <goals>
            <goal>build-classpath</goal>
          </goals>
        </execution>
      </executions>
      <configuration>
        <outputFilterFile>true</outputFilterFile>
        <outputFile>${project.build.directory}/classpath.properties</outputFile>
      </configuration>
    </plugin>
    <plugin>
      <artifactId>maven-jar-plugin</artifactId>
      <configuration>
        <archive>
          <manifestFile>
            ${project.build.outputDirectory}/META-INF/MANIFEST.MF
          </manifestFile>
        </archive>
      </configuration>
    </plugin>
  </plugins>
  <filters>
    <filter>${project.build.directory}/classpath.properties</filter>
  </filters>
  <resources>
    <resource>
      <directory>src/main/resources</directory>
      <filtering>true</filtering>
    </resource>
  </resources>
</build>

Then specify the following in src/main/resources/META-INF/Manifest.MF:

Bundle-Version: 4.0.0
...
Classpath: ${classpath};[specify additional entries here]

Note: there is a bug with this processing using the standard window path separator (\), the generate path is stripped of separators (note it works fine on Linux). You can get the classpath to be generated correctly for Windows by specifying <fileSeparator>\\\\</fileSeparator> in the build-classpath goal's configuration.

---

You can customise the manifest in the jar-plugin's configuration. To do so you'd add something like this to your pom.

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-jar-plugin</artifactId>
  ...
  <configuration>
    <archive>
      <index>true</index>
      <manifest>
        <addClasspath>true</addClasspath>
      </manifest>
      <manifestEntries>
        <mode>development</mode>
        <url>${pom.url}</url>
        <key>value</key>
      </manifestEntries>
    </archive>
  </configuration>
  ...
</plugin>

The full archiver specification provides quite a few options. See the examples page for options on configuring the classpath.

If none of these work for you, you can define your own Manifest, set up properties containing the required entries and use a filter to populate the manifest with those properties

Try to do it like they do in this bug, i.e. merge entries using manifestEntries/Class-Path element

https://issues.apache.org/jira/browse/MJAR-41

I was able to get a slightly modified version of Rich Seller's approach working, avoiding the Error assembling JAR: Unable to read manifest file (line too long) issue that was mentioned in the comments.

I wanted to get all dependencies copied via the dependency-maven-plugin referenced in the .jar file's Manifest Class-Path. I could not use the <addClasspath>true</addClasspath> option of the Maven Jar Plugin as that put too much in my Jar Classpath (I'm only copying a selection of dependencies over).

Here's how I got this to work.

First I use the Maven Dependency Plugin to do the copying and at the same time build a classpath variable. Using the <outputProperty> I put this in a property rather than a file:

  <plugin>
    <artifactId>maven-dependency-plugin</artifactId>
    <version>3.2.0</version>
    <executions>
      <execution>
        <phase>generate-resources</phase>
        <goals>
          <goal>copy-dependencies</goal>
          <goal>build-classpath</goal>
        </goals>
        <configuration>
          <outputDirectory>${project.build.directory}/lib</outputDirectory>              

          <!-- These properties are for build-classpath. It creates a classpath for the copied
               dependencies and puts it in the ${distro.classpath} property. The jar Class-Path
               uses spaces as separators. Unfortunately <pathSeparator> configuration property
               does not work with a space as value, so the pathSeparator is set to a character
               here and this is then replaced later using the regex-property plugin. -->
          <prefix>lib</prefix>
          <outputProperty>distro.classpath</outputProperty>
          <pathSeparator>:</pathSeparator>
        </configuration>
      </execution>
    </executions>
  </plugin>

The syntax of the Jar Manifest Class-Path uses a space as separators. While the dependency plugin has a <pathSeparator> property, this one unfortunatly ignores the value if it is a space. So I just hardcode that one to some value and then use the build-helper-maven-plugin to replace it to that space I need:

  <plugin>
    <groupId>org.codehaus.mojo</groupId>
    <artifactId>build-helper-maven-plugin</artifactId>
    <version>3.2.0</version>
    <executions>
      <execution>
        <phase>process-resources</phase>
        <goals>
          <goal>regex-property</goal>
        </goals>
        <configuration>
          <!-- Here the value of property for the jar the Class-Path is replaced to have a space
               as separator. Unfortunately <replacement> does not work if a single space if specified
               so this uses the surrounding .jar and lib to provide some content. -->
          <name>distro.classpath.replaced</name>
          <value>${distro.classpath}</value>
          <regex>[.]jar[:]lib</regex>
          <replacement>.jar lib</replacement>
        </configuration>
      </execution>
    </executions>
  </plugin>

Here, also the <replacement> value doesn't work if it's just a space, so I'm surrounding it with the text that exists around it.

Finally I can use the Maven Jar Plugin to pick up the property that was replaced with the space as separator. Because I pass the value of the classpath here in the maven definition (rather than picking it up from a filtered file) the line length constraints of the Manifest file will automatically be handled, and no 'line too long' problems appear:

  <plugin>
    <artifactId>maven-jar-plugin</artifactId>
    <version>3.2.0</version>
    <configuration>
      <archive>
        <manifest>
          <mainClass>org.acme.Main</mainClass>
        </manifest>
        <manifestEntries>
          <!-- Include the computed classpath with all copied dependencies in the jar here -->
          <Class-Path>${distro.classpath.replaced}</Class-Path>
        </manifestEntries>
      </archive>
    </configuration>
  </plugin>