How to Configure Cobertura with testNG in Ant

Here is my build xml. After I run the target "coverage-test", in command line it displays that the test is passed. Then "cobertura.reports" target is run. But coverage tool report displays no test coverage. What should i do to solve the problem. Can any one explain how to properly configure testNG and Cobertura, in Ant target ?

 <project basedir=".">

    <property name="src.dir" value="${basedir}/src"/>
    <property name="lib.dir" value="${basedir}/lib"/>
    <property name="build" value="${basedir}/build"/>
    <property name="tng.conf.dir" value="${src.dir}/conf"/>
    <property name="cobertura.dir" value="${lib.dir}/cobertura" />
    <property name="coverage-output" value="${basedir}/coverage-output" />
    <property name="reports" value="${basedir}/reports" />


    <taskdef name="testng" classname="org.testng.TestNGAntTask">
        <classpath>
            <pathelement location="lib/testng-6.8.jar"/>
        </classpath>
    </taskdef>

    <path id="class_path">
        <pathelement path="${build}"/>
        <fileset dir="${lib.dir}">
            <include name="**/*.jar"/>
        </fileset>
    </path>

    <path id="cobertura.classpath">
        <fileset dir="${cobertura.dir}">
            <include name="cobertura-1.9.4.1.jar" />
            <include name="log4j-1.2.17.jar"/>
            <include name="asm-3.1.jar"/>
            <include name="asm-tree-3.0.jar"/>
            <include name="jakarta-oro.jar"/>
            <include name="lib/**/*.jar"/></fileset>
    </path>

    <taskdef classpathref="cobertura.classpath" resource="tasks.properties" />


    <!--<cobertura-instrument todir="${instrumented.dir}">-->
        <!--<ignore regex="org.apache.log4j.*" />-->
        <!--<fileset dir="${build}">-->
            <!--<include name="**/*.class" />-->
            <!--<exclude name="**/*Test.class" />-->
        <!--</fileset>-->
    <!--</cobertura-instrument>-->

    <target name="clean" description="Clear previous build class files">
        <delete dir="${build}" quiet="true"></delete>
        <delete file="cobertura.ser" />
        <delete dir="${coverage-output}" quiet="true"></delete>
        <delete dir="${reports}" quiet="true"></delete>

        <!--<delete dir="${coverage-output}/" />-->
    </target>


    <target name="compile" description="compile java class">
        <mkdir dir="${build}"/>
        <mkdir dir="${coverage-output}"/>
        <mkdir dir="${reports}"/>
        <javac srcdir="${src.dir}" destdir="${build}" debug="yes">
            <classpath refid="class_path"></classpath>
        </javac>

        <copy todir="${build}">
            <fileset dir="${src.dir}">
                <include name="**/*.xml"/>
                <include name="**/*.properties"/>
            </fileset>
        </copy>
    </target>

    <!--<target name="ng-test" description="Check very specific test ng target only">-->
        <!--<testng classpathref="class_path" groups="test-fl-one">-->
            <!--<classfileset dir="${build}" includes="**/*Test.class"></classfileset>-->
        <!--</testng>-->
    <!--</target>-->
    <!---->
    <!--<target name="test-by-xml" description="Check by Testng xml" depends="cobertura" >-->
        <!--<testng classpathref="class_path" outputdir="${build}" haltonfailure="false">-->
            <!--<classpath location="${build}"></classpath>-->
            <!--<xmlfileset dir="${tng.conf.dir}" includes="testng.xml"/>-->
        <!--</testng>-->
    <!--</target>-->

    <target name="cobertura" depends="compile">

        <cobertura-instrument todir="${coverage-output}">

            <fileset dir="${build}">
                <include name="**/*.class" />
                <!--<exclude name="test/*.class"/>-->
            </fileset>
        </cobertura-instrument>
    </target>

    <target name="coverage-test" depends="cobertura">
        <testng classpathref="class_path" groups="test-fl-one">
            <classfileset dir="${coverage-output}" includes="test/*.class" />
        </testng>
    </target>

    <target name="cobertura.reports">
        <cobertura-report format="html" srcdir="${build}" destdir="${reports}" />
    </target>

</project>