Java One-Jar Example Using Ant and Eclipse

One-Jar is an amazingly clever and simple tool that let's you package an entire Java application containing external jar files all into one Jar. But just like anything, it's simple and easy to use only AFTER you figure out how to get it working. Having just worked through using One-jar, I thought I'd lay out the steps needed to successfully use One-jar in Eclipse using Ant.

Step 1: Get One-jar. Go to the One-jar download page on Source-Forge and download the "one-jar-ant-task-0.97.jar" file.

Step 2: Integrate this jar into Eclipse so that when running Ant, Ant has access to the one-jar Ant Task. If you don't do this, you may get and error like:

/eclipse/helloworkspace/HelloOneJar/build.xml:54: Problem: failed to create task or type one-jar
Cause: The name is undefined.
Action: Check the spelling.
Action: Check that any custom tasks/types have been declared.
Action: Check that any / declarations have taken place.

Move the jar to your Eclipse plugins folder. For me, that was in /eclipse/plugins. You can really put this anywhere you want, but it made sense for me to put it here as Eclipse stores a lot of jars it needs here.

In Eclipse, open preferences, drill down to Ant-->Runtime, highlight Ant Home Entries, and click "Add external JARs...". Find the one-jar JAR, and add it here.

Step 3: Create Ant build file. For demonstration and testing purposes, I created a tiny Java Project called HelloOneJar which contains a Hello class. All the Hello class does is log two messages using log4j. Here is a screenshot of the project structure and the Hello class:

build.properties

project.name=hello-onejar
lib.dir=lib
src.dir=src
build.dir=build
dist.dir=dist

build.xml

<?xml version="1.0"?>
<project name="hello-onejar" default="onejar" basedir=".">
 
    <taskdef name="one-jar" classname="com.simontuffs.onejar.ant.OneJarTask" onerror="report" />
 
    <property file="build.properties"/>
 
    <tstamp>
       <format property="timestamp" pattern="yyyy-MM-dd HH:mm:ss" />
    </tstamp>
 
    <path id="classpath">
     <fileset dir="${lib.dir}" />
    </path>

    <target name="clean">
        <echo>Cleaning the ${build.dir}</echo>
        <delete dir="${build.dir}"/>
        <delete dir="${dist.dir}"/>
    </target>

    <target name="init" depends="clean">
        <echo>Creating the build directory</echo>
        <mkdir dir="${build.dir}"/>
        <mkdir dir="${dist.dir}"/>
    </target>
 
    <target name="compile" depends="init">
        <echo>Compile the source files</echo>
        <javac srcdir="${src.dir}" destdir="${build.dir}" debug="on">
            <classpath refid="classpath"/>
        </javac>
    </target>

 <target name="mainjar" depends="compile">
       <jar jarfile="${dist.dir}/main.jar">
         <manifest>
            <attribute name="Built-By" value="${user.name}"/>
             <attribute name="Build-Date" value="${timestamp}"/>                 
             <attribute name="Main-Class" value="com.hello.Hello"/> 
           </manifest>
          <fileset dir="${build.dir}">
              <include name="**/*.class"/>
          </fileset>
        </jar>
 </target>

    <target name="onejar" depends="mainjar">
     
        <!-- Construct the One-JAR file -->   
  <one-jar destfile="${dist.dir}/${project.name}.jar">
         <main jar="${dist.dir}/main.jar">
         </main>
         <lib>
             <fileset dir="${lib.dir}" />
         </lib>
  </one-jar>
    </target>
 
</project>

The follwing parts are very important!

<taskdef name="one-jar" classname="com.simontuffs.onejar.ant.OneJarTask" onerror="report" />
without that line, the one-jar task is undefined. Remember to tell Ant where this class is in step 2!

<attribute name="Main-Class" value="com.hello.Hello"/>
without this in the manifest you may get an error like this when you run your jar:

Exception in thread "main" java.lang.Exception: hello-onejar.jar main class was not found (fix: add main/main.jar with a Main-Class manifest attribute, or specify -Done-jar.main.class=), or use One-Jar-Main-Class in the manifest
at com.simontuffs.onejar.Boot.run(Boot.java:327)
at com.simontuffs.onejar.Boot.main(Boot.java:168)

Step 4: Run the Ant build. In this example, I run the "onejar" task that I defined. This build file first creates a jar called main.jar that contains my application code. This jar has it's own manifest and could be ran separately. The onejar target wraps main.jar in hello-one-jar.jar and packages all the jar(s) in the lib folder inside. Both jars are created in the ./dist folder.

Step 5: Inspect the built jar. This is how I did this on my Mac. First I created a folder: /HelloOneJar and moved the hello-onejar.jar into it. Then, in terminal:

cd /HelloOneJar
jar -xvf hello-onejar.jar

Now the jar is unpacked and we can see what's in it. You can do the same for main.jar and verify that the manifest file was created correctly.

Step 6: Run the jar and make sure it all works. This is how I did this on my Mac. First I created a folder: /HelloOneJar and moved the hello-onejar.jar into it. Then, in terminal:

cd /HelloOneJar/
java -jar hello-onejar.jar

The following output appeared in Terminal:

0 [main] INFO com.hello.Hello - Hello.
1 [main] INFO com.hello.Hello - Bye Bye.

Exactly what I expected!

Piece of Cake!!