Ant has built-in property ${ant.java.version}, you can directly use it to get Java or JVM version, the following example prints out Java version.

Build.xml
<target name="print-version"> 
   <echo>Java/JVM version: ${ant.java.version}</echo> 
   <echo>Java/JVM detail version: ${java.version}</echo> 
</target>


The output getting from example:

[echo] Java/JVM version: 1.5
[echo] Java/JVM detail version: 1.5.0_08

How I can Check Java/JVM version in Ant?

Ant has task condition which sets a property if a certain condition holds true, we compare current Java version to 1.5/1.6, if false then fail the build.

Filename: Build.xml
<fail message="Unsupported Java version: ${ant.java.version}. 
  Make sure that the Java version is 1.5 or greater.">
  <condition>
    <not>
      <or>
        <equals arg1="${ant.java.version}" arg2="1.5"/>
        <equals arg1="${ant.java.version}" arg2="1.6"/>
      </or>
    </not>
  </condition>
</fail>

Example 2 to check version in Ant:
We compare current Java version to 1.5/1.6 and set result back to property of condition, this property can be used later on.

<project basedir="." default="check-java-version">
  <target name="get-java-version">
    <condition property="java.version">
      <not>
        <or>
          <equals arg1="${ant.java.version}" arg2="1.5"/>
          <equals arg1="${ant.java.version}" arg2="1.6"/>
        </or>
      </not>
    </condition>
  </target>
  <target name="check-java-version" depends="get-java-version" unless="java.version">
    <fail message="Unsupported Java version: ${ant.java.version}. 
    Make sure that the Java version is 1.5 or greater."/>
  </target>
</project>

here is no “greater than” or “less than” logic, but certainly we can use Ant and, or, not combine multiple conditions to archive the needs.