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

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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.

classpath build jar maven-2
4 Answers

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>
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