In this example I will use the Ivy library. It is kind of a silly example since Ivy itself is a tool so no jar files needs to exist in the project directory.
Download and place the external jar file in your project directory. I placed the file ivy-2.1.0-rc2.jar in a directory called lib.
Use a zip-program (for example 7Zip) and look in your jar file to see where the file antlib.xml is. For Ivy this is org/apache/ivy/ant/antlib.xml.
The next step is the make up a namespace. I say makeup, because you can use whatever you like. However it is good to use the form antlib:java package. This way it will work when you one day remove the external library from your project and add in the ant-library directory on your local computer (then you wont need the taskdef). For Ivy the namespace is antlib:org.apache.ivy.ant. For more information see http://ant.apache.org/manual/CoreTypes/antlib.html.
Next is to write the build.xml file:
<project name="ivyhgtest" xmlns:ivy="antlib:org.apache.ivy.ant"> <target name="resolve" description="--> retreive dependencies with ivy"> <path id="ivy.classpath"> <fileset dir="lib/" includes="ivy*.jar" /> </path> <taskdef resource="org/apache/ivy/ant/antlib.xml" classpathref="ivy.classpath" uri="antlib:org.apache.ivy.ant" /> <ivy:retrieve /> </target> </project>
The content of xmlns:ivy="antlib:org.apache.ivy.ant" must match the uri="antlib:org.apache.ivy.ant". The XML namespace xmlns:ivy is arbitrary but it must match the task namespace <ivy:retrieve />.Choose a good namespace that maches your external feature (only one word).
Note how I used a wildcard to reference the ivy-2.1.0-rc2.jar. It can be good if a new version of the jar is published.
I can now access all the tasks defined in the antlib.xml file with the ivy namespace.