谢谢大家的回答。经过进一步调查，我弄清楚了如何使其工作。我必须完全重新编写build.xml。这里是：

<property file="build.properties" />

<path id="cobertura.classpath">
    <fileset dir="${cobertura.dir}">
        <include name="**/*.jar" />
    </fileset>
</path>

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

<target name="init">
    <mkdir dir="${classes.dir}" />
    <mkdir dir="${instrumented.dir}" />
    <mkdir dir="${reports.xml.dir}" />
    <mkdir dir="${reports.html.dir}" />
    <mkdir dir="${coverage.xml.dir}" />
    <mkdir dir="${coverage.summaryxml.dir}" />
    <mkdir dir="${coverage.html.dir}" />
</target>

<target name="compile" depends="init">
    <javac srcdir="${src.dir}" destdir="${classes.dir}" debug="yes" includeantruntime="false">
        <classpath refid="cobertura.classpath" />
    </javac>
    <javac srcdir="${test.src.dir}" destdir="${classes.dir}" debug="yes" includeantruntime="false">
        <classpath refid="cobertura.classpath" />
    </javac>
</target>

<target name="instrument" depends="init,compile">
    <!--
            Remove the coverage data file and any old instrumentation.
    -->
    <delete file="cobertura.ser"/>
    <delete dir="${instrumented.dir}" />

    <!--
            Instrument the application classes, writing the
            instrumented classes into ${build.instrumented.dir}.
    -->
    <cobertura-instrument todir="${instrumented.dir}">
        <!--
                The following line causes instrument to ignore any
                source line containing a reference to log4j, for the
                purposes of coverage reporting.
        -->
        <ignore regex="org.apache.log4j.*" />

        <fileset dir="${classes.dir}">
            <!--
                    Instrument all the application classes, but
                    don't instrument the test classes.
            -->
            <include name="**/*.class" />
            <exclude name="**/*Test.class" />
        </fileset>
    </cobertura-instrument>
</target>

<target name="test" depends="init,compile">
    <junit fork="yes" dir="${basedir}" failureProperty="test.failed">
        <!--
                Note the classpath order: instrumented classes are before the
                original (uninstrumented) classes.  This is important.
        -->
        <classpath location="${instrumented.dir}" />
        <classpath location="${classes.dir}" />

        <!--
                The instrumented classes reference classes used by the
                Cobertura runtime, so Cobertura and its dependencies
                must be on your classpath.
        -->
        <classpath refid="cobertura.classpath" />

        <formatter type="xml" />
        <test name="${testcase}" todir="${reports.xml.dir}" if="testcase" />
        <batchtest todir="${reports.xml.dir}" unless="testcase">
            <fileset dir="${test.src.dir}">
                <include name="**/*Test.java" />
            </fileset>
        </batchtest>
    </junit>

    <!-- JUnit Report in HTML -->
    <junitreport todir="${reports.xml.dir}">
        <fileset dir="${reports.xml.dir}">
            <include name="TEST-*.xml" />
        </fileset>
        <report format="frames" todir="${reports.html.dir}" />
    </junitreport>

</target>

<target name="coverage-check">
    <cobertura-check branchrate="34" totallinerate="100" />
</target>

<target name="coverage-report">
    <!--
            Generate an XML file containing the coverage data using
            the "srcdir" attribute.
    -->
    <cobertura-report srcdir="${src.dir}" destdir="${coverage.xml.dir}" format="xml" />
</target>

<target name="summary-coverage-report">
    <!--
            Generate an summary XML file containing the coverage data using
            the "srcdir" attribute.
    -->
    <cobertura-report srcdir="${src.dir}" destdir="${coverage.summaryxml.dir}" format="summaryXml" />
</target>

<target name="alternate-coverage-report">
    <!--
            Generate a series of HTML files containing the coverage
            data in a user-readable form using nested source filesets.
    -->
    <cobertura-report destdir="${coverage.html.dir}">
        <fileset dir="${src.dir}">
            <include name="**/*.java"/>
        </fileset>
    </cobertura-report>
</target>

<target name="clean" description="Remove all files created by the build/test process.">
    <delete dir="${classes.dir}" />
    <delete dir="${instrumented.dir}" />
    <delete dir="${reports.dir}" />
    <delete file="cobertura.log" />
    <delete file="cobertura.ser" />
</target>

<target name="coverage" depends="compile,instrument,test,coverage-report,summary-coverage-report,alternate-coverage-report" description="Compile, instrument ourself, run the tests and generate JUnit and coverage reports."/>

属性定义在开头引用的build.properties文件中