build.xml

<project name="hello" basedir=".">
    <property name="src" location="src"/>
    <property name="build" location="build"/>
    <property name="classes" location="${build}/classes"/>
    <property name="jar" location="${build}/jar"/>
    <target name="setup">
        <tstamp/>
        <mkdir dir="${classes}"/>
        <mkdir dir="${jar}"/>
    </target>
    <target name="build" depends="setup" description="compile the source">
        <javac srcdir="${src}" destdir="${classes}" includeantruntime="false"/>
    </target>
    <target name="jar" depends="build">
        <jar destfile="${jar}/Hello.jar" basedir="${classes}">
            <manifest>
                <attribute name="Main-Class" value="br.ufes.hello.Hello"/>
            </manifest>
        </jar>
    </target>
    <target name="run" depends="jar">
        <java jar="${jar}/Hello.jar" fork="true"/>
    </target>
    <target name="clean">
        <delete dir="${build}"/>
    </target>
</project>

Hello.java

package br.ufes.hello;

public class Hello {

    public static void main(String[] args) {
        System.out.println("Hello, world!");
    }

}