diff --git a/README.md b/README.md
index 60d1b2ce..65af8726 100644
--- a/README.md
+++ b/README.md
@@ -46,9 +46,9 @@ Building with Apache Ant
 
   * If you are using OpenJDK version older than 9:
 
-    Comment the `sourcever9` line in the file `build.properties`.
-    When it is commented, `module-info.java` and other files
-    requiring Java 9 won't be built. `xz.jar` won't be a modular JAR.
+    To build with OpenJDK 8, use `-Djava8only=true` on the `ant`
+    command line. Then the files requiring Java >= 9 won't be built,
+    and `xz.jar` won't be a modular JAR.
 
   * If you are using OpenJDK version older than 8:
 
diff --git a/build.properties b/build.properties
index fc24e3b9..4f6c63f3 100644
--- a/build.properties
+++ b/build.properties
@@ -14,14 +14,12 @@ debug = true
 #
 # sourcever9 does the same as sourcever but for files that require Java 9 or
 # later. The resulting classes are packaged as multi-release JAR, including
-# module-info.java. If sourcever9 is commented out, these files won't be
-# built but the package will still work.
+# module-info.java.
 #
-# If you need to build this on OpenJDK 8:
-#   - Set sourcever = 8
-#   - Comment the sourcever9 line below to disable building Java 9 files.
+# To build with OpenJDK 8, use -Djava8only=true on the ant command line.
 sourcever = 8
 sourcever9 = 9
+java8only = false
 
 # If true, javadoc is called with en_US locale to force the headings and
 # such strings from javadoc to be in English, independent of the system
diff --git a/build.xml b/build.xml
index 9635ff12..bce9977a 100644
--- a/build.xml
+++ b/build.xml
@@ -55,18 +55,13 @@
             <compilerarg compiler="modern" line="-implicit:none"/>
         </javac>
 
-        <!-- Build Java 9 files only if sourcever9 is set: -->
-        <mkdir dir="${classes9_dir}"/>
-        <condition property="exclude_java9" value="**">
-            <not><isset property="sourcever9"/></not>
-        </condition>
-
-        <javac srcdir="." sourcepath="${src9_dir}:${src_dir}"
+        <mkdir unless:true="${java8only}" dir="${classes9_dir}"/>
+        <javac unless:true="${java8only}"
+               srcdir="." sourcepath="${src9_dir}:${src_dir}"
                destdir="${classes9_dir}"
                includeAntRuntime="false" debug="${debug}"
                release="${sourcever9}"
-               includesfile="fileset-src9.txt"
-               excludes="${exclude_java9}">
+               includesfile="fileset-src9.txt">
             <compilerarg compiler="modern" line="-Xlint"/>
             <compilerarg compiler="modern" line="-implicit:none"/>
         </javac>
@@ -79,14 +74,14 @@
         <jar destfile="${jar_dir}/xz.jar">
             <fileset dir="${classes_dir}" includes="org/tukaani/xz/**"/>
             <zipfileset prefix="META-INF/versions/9/" dir="${classes9_dir}"
-                        if:set="sourcever9"/>
+                        unless:true="${java8only}"/>
             <manifest>
                 <attribute name="Implementation-Title" value="${title}"/>
                 <attribute name="Implementation-Version" value="${version}"/>
                 <attribute name="Implementation-URL" value="${homepage}"/>
                 <attribute name="Sealed" value="true"/>
                 <attribute name="Multi-Release" value="true"
-                           if:set="sourcever9"/>
+                           unless:true="${java8only}"/>
                 <!-- Attributes for OSGi bundles: -->
                 <attribute name="Bundle-ManifestVersion" value="2"/>
                 <attribute name="Bundle-SymbolicName" value="org.tukaani.xz"/>