Building an executable jar with Ant

An executable jar file has a defined default class, allowing the default class to be invoked without explicitly identifying it via the command line. On a supporting OS (such as Mac OS/X or Microsoft Windows), this allows the class to be invoked by double-clicking the file.
I know that making a jar file executable requires an entry in the manifest file. I can usually remember that the attribute in question is Main-Class (although I'm usually fuzzy about the proper capitalization). The problem is remembering the correct command line options to insert my own MANIFEST.MF file into a jar file.
All in all, it is much simpler to just use ant to handle it for me. Let's start with the example from my Ant Hello World Revisited. The new ant build file follows, with the new lines shown in bold. Note that I've also designated the "jar" task as the default task.

$ cat hello.xml
<project default="jar">
  <target name="compile">
    <javac srcdir="." />
  </target>
  <target name="jar" depends="compile">
    <jar destfile="hello.jar"
         basedir="."
         includes="**/*.class">
      <manifest>
        <attribute name="Main-Class" value="hello" />
      </manifest>
    </jar>
  </target>
</project>

$ cat hello.java
public class hello {
    public static void main( String[] args )
    {
        System.out.println( "Hello World" );
    }
}

Now we simply build our jar file:

$ rm *.jar
$ ant -f hello.xml
Buildfile: hello.xml

compile:

jar:
      [jar] Building jar: /Tutorial/Ant/Jar/hello.jar

BUILD SUCCESSFUL
Total time: 2 seconds

And execute the default class by using the -jar command line option to java:

$ java -jar hello.jar
Hello World