diff --git a/src/main/org/apache/tools/ant/taskdefs/optional/javacc/JavaCC.java b/src/main/org/apache/tools/ant/taskdefs/optional/javacc/JavaCC.java
index 4921a98ec..20e2530ed 100644
--- a/src/main/org/apache/tools/ant/taskdefs/optional/javacc/JavaCC.java
+++ b/src/main/org/apache/tools/ant/taskdefs/optional/javacc/JavaCC.java
@@ -228,8 +228,9 @@ public class JavaCC extends Task {
         if (target == null || !target.isFile()) {
             throw new BuildException("Invalid target: " + target);
         }
-        final File javaFile = new File(
-            target.toString().substring(0, target.toString().indexOf(".jj")) + ".java");
+
+        // determine if the generated java file is up-to-date
+        final File javaFile = getOutputJavaFile(outputDirectory, target);
         if (javaFile.exists() && target.lastModified() < javaFile.lastModified()) {
             project.log("Target is already built - skipping (" + target + ")");
             return;
@@ -264,4 +265,36 @@ public class JavaCC extends Task {
             throw new BuildException("Failed to launch JavaCC: " + e);
         }
     }
+
+    /**
+     * Determines the output Java file to be generated by the given grammar
+     * file.
+     * 
+     */
+    private File getOutputJavaFile(File outputdir, File srcfile)
+    {
+        String path = srcfile.getPath();
+
+        // Extract file's base-name
+        int startBasename = path.lastIndexOf(File.separator);
+        if ( startBasename != -1 ) {
+            path = path.substring(startBasename+1);
+        }
+
+        // Replace the file's extension with '.java'
+        int startExtn = path.lastIndexOf('.');
+        if (startExtn != -1) {
+            path = path.substring(0, startExtn) + ".java";
+        }
+        else {
+            path += ".java";
+        }
+
+        // Change the directory
+        if (outputdir != null) {
+            path = outputdir + File.separator + path;
+        }
+
+        return new File(path);
+    }
 }