You should add your junit.jar into the classpath definition in your ant file.
There are many way to do it, one example is:
<junit printsummary="yes" haltonfailure="yes">
<classpath>
<path refid="your.classpath.refid" />
<fileset dir="${junit.dir}">
<include name="**/junit.jar" />
</fileset>
</classpath>
...
</junit>
See Ant Manual for details on setting up your classpath.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…