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JarClassLoader

What is it?

This is just a copy of JDotSoft JarClassLoader, currently there are no functional modifications, the copyright is theirs. For more information visit their web site.

I found the tool because my usual favourite tool One-JAR for creating JARs with included nested dependencies (I do not like shaded JARs) failed in a situation where I wanted to use signed third-party JARs in connection with my unsigned own one. Specifically, this was about the BouncyCastle crypto libraries. I tried JarClassLoader, integrated it into my Maven build and - it worked! Thank you, JDotSoft. :-)

Why am I re-publishing it?

I spoke to the creators and asked them to publish it as a little JAR on Maven Central, but they refused to do so due to maintenance effort. They said, as it only consists of one class every user can just copy the source code into her project instead of adding a dependency. I do disagree on that approach and think that duplication is the enemy of maintainability. Because I saw that the licence is GPL2, I decided to take the initiative and publish it myself.

This is what OSS is all about, right? Take the source code, modify it (or not) and re-publish it under the conditions of the original licence. This is what I did here.

In order to do that I need to create signed JARs. I also wanted a separate Javadoc JAR, so I had to modify the Javadoc a little bit in order to make the checker pass because it threw errors (build with JDK 8). Content-wise, there are no changes.

Why group ID de.scrum-master instead of com.jdotsoft? Because I do not want to claim another company's domain name and rather publish under a domain name which is really mine. I did keep their package name com.jdotsoft.jarloader, though, because other users who until now have used the identical class and want to migrate to using the Maven dependency do not need to change anything in their source code, the import stays the same.

How to use with Maven

The description Using Maven to build executable JAR on the original web site is valid only if you have the JarClassLoader copied into your project. When using it as an external Maven artifact, we additionally need to make sure that

  • its class files get unpacked and inlined into our own one-jar artifact and
  • that the JAR itself does not get copied into our own one-jar artifact because it cannot be used from there anyway (hen vs. egg problem).

So we have something like this:

<properties>
  <java.source-target.version>1.8</java.source-target.version>
  <jar-class-loader.version>1.0</jar-class-loader.version>
  <one-jar-main-class>de.scrum_master.smime.OneJarLauncher</one-jar-main-class>
</properties>

<dependencies>
  <!-- JarClassLoader tool for creating executeable uber JAR -->
  <dependency>
    <groupId>de.scrum-master</groupId>
    <artifactId>jar-class-loader</artifactId>
    <version>${jar-class-loader.version}</version>
  </dependency>
</dependencies>

<plugins>

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.7.0</version>
    <configuration>
      <source>${java.source-target.version}</source>
      <target>${java.source-target.version}</target>
    </configuration>
  </plugin>

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-dependency-plugin</artifactId>
    <version>3.0.2</version>
    <executions>
      <execution>
        <id>copy</id>
        <phase>prepare-package</phase>
        <goals>
          <goal>copy-dependencies</goal>
        </goals>
        <configuration>
          <!-- Exclude JarClassLoader artifact and anything else you do not need -->
          <excludeArtifactIds>jar-class-loader,junit</excludeArtifactIds>
          <outputDirectory>${project.build.directory}/classes</outputDirectory>
        </configuration>
      </execution>
      <execution>
        <id>unpack-jar-class-loader</id>
        <phase>prepare-package</phase>
        <goals>
          <goal>unpack</goal>
        </goals>
        <configuration>
          <outputDirectory>${project.build.directory}/classes</outputDirectory>
          <!-- Make sure to unpack classes from JarClassLoader artifact -->
          <artifact>de.scrum-master:jar-class-loader:${jar-class-loader.version}</artifact>
          <includes>**/*.class</includes>
        </configuration>
      </execution>
    </executions>
  </plugin>

  <plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-jar-plugin</artifactId>
    <version>3.0.2</version>
    <configuration>
      <archive>
        <manifest>
          <mainClass>${one-jar-main-class}</mainClass>
          <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
        </manifest>
      </archive>
    </configuration>
  </plugin>

</plugins>

The difference to the original are just

  • the additional jar-class-loader in <excludeArtifactIds> and
  • the additional execution <id>unpack-jar-class-loader</id>.