diff --git a/src/etc/testcases/taskdefs/jar.xml b/src/etc/testcases/taskdefs/jar.xml
index 9b94dea98..46ac4f5b6 100644
--- a/src/etc/testcases/taskdefs/jar.xml
+++ b/src/etc/testcases/taskdefs/jar.xml
@@ -189,4 +189,15 @@
     </jar>
   </target>
 
+  <!-- bug 17780 -->
+  <target name="testUpdateIfOnlyManifestHasChanged"
+          depends="test4">
+    <jar destfile="${tmp.jar}" update="true">
+      <manifest>
+        <attribute name="Foo" value="bar"/>
+      </manifest>
+    </jar>
+    <mkdir dir="${tmp.dir}"/>
+    <unzip src="${tmp.jar}" dest="${tmp.dir}"/>
+  </target>
 </project>
diff --git a/src/testcases/org/apache/tools/ant/taskdefs/JarTest.java b/src/testcases/org/apache/tools/ant/taskdefs/JarTest.java
index 492146399..b4c9fafc7 100644
--- a/src/testcases/org/apache/tools/ant/taskdefs/JarTest.java
+++ b/src/testcases/org/apache/tools/ant/taskdefs/JarTest.java
@@ -67,6 +67,7 @@ import org.apache.tools.ant.BuildFileTest;
 public class JarTest extends BuildFileTest {
 
     private static String tempJar = "tmp.jar";
+    private static String tempDir = "jartmp/";
     private Reader r1, r2;
 
     public JarTest(String name) {
@@ -170,9 +171,9 @@ public class JarTest extends BuildFileTest {
         executeTarget("testManifestStaysIntact");
 
         r1 = new FileReader(getProject()
-                            .resolveFile("jartmp/manifest"));
+                            .resolveFile(tempDir + "manifest"));
         r2 = new FileReader(getProject()
-                            .resolveFile("jartmp/META-INF/MANIFEST.MF"));
+                            .resolveFile(tempDir + "META-INF/MANIFEST.MF"));
         Manifest mf1 = new Manifest(r1);
         Manifest mf2 = new Manifest(r2);
         assertEquals(mf1, mf2);
@@ -219,4 +220,10 @@ public class JarTest extends BuildFileTest {
         executeTarget("testCreateWithEmptyFileset");
         executeTarget("testCreateWithEmptyFileset");
     }
+
+    public void testUpdateIfOnlyManifestHasChanged() {
+        executeTarget("testUpdateIfOnlyManifestHasChanged");
+        File jarXml = getProject().resolveFile(tempDir + "jar.xml");
+        assertTrue(jarXml.exists());
+    }
 }