Ivy fails to resolve a dependency, unable to find cause

While using ivy:retrieve, it fails to resolve the dependency that should be downloaded. The output looks like this:

Buildfile: C:\Users\Simon\workspace\apollo\build.xml
init:
resolve:

BUILD FAILED
C:\Users\Simon\workspace\apollo\build.xml:42: Problem: failed to create task or type antlib:org.apache.ivy.ant:retrieve
Cause: The name is undefined.
Action: Check the spelling.
Action: Check that any custom tasks/types have been declared.
Action: Check that any <presetdef>/<macrodef> declarations have taken place.
No types or tasks have been defined in this namespace yet

This appears to be an antlib declaration. 
Action: Check that the implementing library exists in one of:
        -C:\Users\Simon\eclipse\plugins\org.apache.ant_1.8.2.v20120109-1030\lib
        -C:\Users\Simon\.ant\lib
        -a directory added on the command line with the -lib argument


Total time: 348 milliseconds

The relevant section of the build.xml looks like so:

  <target name="resolve" depends="init">
    <ivy:retrieve pattern="${lib}/[artifact]-[revision].[ext]" sync="true" />
  </target>

Here is also a list of what it should be downloading (from the build.xml)

  <target name="doc" depends="build">
    <javadoc sourcepath="${src}" classpathref="libraries" access="private" destdir="${doc}" windowtitle="Apollo">
      <doclet name="org.jboss.apiviz.APIviz" pathref="libraries">
        <param name="-sourceclasspath" value="${bin}" />
        <param name="-author" />
        <param name="-version" />
        <param name="-use" />
        <param name="-nopackagediagram" />
      </doclet>
      <doctitle><![CDATA[<h1>Apollo</h1>]]></doctitle>
      <link href="http://download.oracle.com/javase/6/docs/api/" />
      <link href="http://docs.jboss.org/netty/3.2/api/" />
      <link href="http://guava-libraries.googlecode.com/svn/trunk/javadoc/" />
      <link href="http://www.junit.org/apidocs/" />
      <link href="http://commons.apache.org/compress/apidocs/" />
      <link href="http://jruby.org/apidocs/" />
    </javadoc>
  </target>

ANT cannot find the ivy jar. Needs to be downloaded, extracted, and the ivy-x.y.z.jar placed into one of the following locations:

• $ANT_HOME/lib
• $HOME/.ant/lib

Enabling ivy

Ivy is packaged as an antlib, so to enable it you need to do the following

1) Declare the ivy namespace at the top of the build file

<project ..... xmlns:ivy="antlib:org.apache.ivy.ant">

2) Include the ivy jar in one of the ant library directories

Your error message indictates some of the possible locations for antlibs:

This appears to be an antlib declaration. 
Action: Check that the implementing library exists in one of:
        -C:\Users\Simon\eclipse\plugins\org.apache.ant_1.8.2.v20120109-1030\lib
        -C:\Users\Simon\.ant\lib
        -a directory added on the command line with the -lib argument

Note:

The beauty of an antlib is that you don't need to perform the taskdef (It's optional if you want to place the ivy jar in a non-standard location)

How to bootstrap a build

Even though ivy is an ANT sub-project, for some inexplicable reason ivy is not packaged with ANT....

I normally include the following target in my build files to setup a new environment:

<target name="bootstrap" description="Used to install the ivy task jar">
    <mkdir dir="${user.home}/.ant/lib"/>
    <get dest="${user.home}/.ant/lib/ivy.jar" src="http://search.maven.org/remotecontent?filepath=org/apache/ivy/ivy/2.2.0/ivy-2.2.0.jar"/>
</target>

It downloads the ivy jar from Maven Central.

Since all other ANT tasks can subsequently be downloaded using ivy, few people object to this little piece of ugliness at the top of the build file.

If you can't put the ivy libs in the classpath for ant you will need to define it yourself:

<path id="ivy.lib.path">
    <fileset dir="path/to/dir/with/ivy/jar" includes="*.jar"/>
</path>
<taskdef resource="org/apache/ivy/ant/antlib.xml"
         uri="antlib:org.apache.ivy.ant" classpathref="ivy.lib.path"/>

This bit is missing from the getting started tutorial, but listed here: http://ant.apache.org/ivy/history/2.2.0/ant.html

When you Run Ant task, in the classpath ensure that ivy.jar is there. In the eclipse -> Run As-> Ant Build -> Edit configuration -> Classpath tab. Even though Eclipse would have the ivy.jar in the ANT Home, for some reason it's not called.