Single jar post.

Author: jacobsimpson

_posts/2015-12-20-single-jar.md

@@ -0,0 +1,105 @@
+---
+layout: post
+title:  "Unified Jar"
+date:   2015-12-20 09:23:07
+categories: java jar
+---
+
+Sometimes I have a little program built in some JVM language. Usually it's a
+simple command line utility, maybe something that tests database connectivity
+via a JDBC connection, a simple JMS utility or a utility analogous to ping but
+for UDP.
+
+The point is, I want to make it easy to set it up and move it from machine to
+machine.  I just want to run it from the command line and have it work.
+
+But, it's got several code files, maybe a resource, and I used something for a
+network library, and a command line parsing library...
+
+What I would like is a consolidated jar, one jar that has what is needed, it is
+configured so it can be run with `java -jar Util.jar`, and it will work.
+
+Obviously, the build system should be responsible for producing the new
+consolidated artifact.
+
+For the simplest version, where there are only code files, no jars, it's pretty
+straight forward:
+
+{% highlight xml %}
+<target name="dist" depends="compile"
+        description="generate the distribution" >
+    <!-- Create the distribution directory -->
+    <mkdir dir="${dist}"/>
+
+    <jar jarfile="${dist}/Util.jar" basedir="${build}">
+        <manifest>
+            <attribute name="Main-Class" value="Main"/>
+        </manifest>
+    </jar>
+</target>
+{% endhighlight %}
+
+Next, say there I used some libraries, something for parsing command line parameters.
+
+There are two basic approaches to this. One is to create a jar containing jars.
+A custom class loader that knows how to read jars from inside jars is required
+to make this work. There are a couple available tools that do this. Eclipse can
+do this through the GUI or [One Jar](http://one-jar.sourceforge.net/).
+
+This repo covers (but does not necessarily advocate) a second approach.
+Unjarring the libraries and creating a new jar with all the classes from the
+compile and the libraries in a single jar.
+
+Adding this line:
+
+{% highlight xml %}
+<zipfileset src="lib/commons-cli-1.3.1.jar" includes="**/*.class"/>
+{% endhighlight %}
+
+will unzip the library jar, extract the .class files and include them the new
+jar that is being created. In context it looks like this:
+
+{% highlight xml %}
+<target name="dist" depends="compile">
+   <mkdir dir="${dist}"/>
+
+   <jar jarfile="${dist}/util.jar" basedir="${classes}">
+        <manifest>
+            <attribute name="Main-Class" value="Main"/>
+        </manifest>
+        <zipfileset src="lib/commons-cli-1.3.1.jar" includes="**/*.class"/>
+    </jar>
+</target>
+{% endhighlight %}
+
+Now there is a single jar that contains all the dependencies necessary to run
+the program.
+
+There is an additional evolution possible. On Unix style systems it is possible
+to create a single consolidated file that can be set executable.
+
+By taking a simple bash script:
+
+{% highlight bash %}
+#! /bin/bash
+
+exec /usr/bin/java -jar $0 "$@"
+{% endhighlight %}
+
+and munging that on the beginning of a jar file, the result is a file that will
+execute on its own, as long as there is a /usr/bin/java executable installed.
+
+By adding these lines to the Ant task, a concatenated file with the script and
+jar will be created.
+
+{% highlight xml %}
+<concat destfile="${build}/util" binary="yes">
+    <filelist dir="src/main/bash" files="header.sh"/>
+    <filelist dir="${dist}" files="util.jar"/>
+</concat>
+<chmod file="${build}/util" perm="ugo+x"/>
+{% endhighlight %}
+
+A sample project is available
+[here](https://github.com/jacobsimpson/consolidated_jar/tree/ant-build).
+