ant available什么任务?

<taskdef resource="net/sf/antcontrib/antcontrib.properties" classpath="${ant-contrib.jar}"/>

<available classname="org.apache.tools.ant.DynamicAttribute" property="ant-1.6.2"/>
<fail unless="ant-1.6.2" message="AppFuse requires Ant 1.6.2 for faster unit testing"/>

这几个任务是什么意思啊