add an optional attribute to import

note this is differnet from a failonerror attribute in
that it only checks for the existance of the
imported file, other errors in the imported
file would cause the build to fail


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@275513 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/import.html

@@ -83,6 +83,15 @@ imported files.<br>
       </td>
       <td valign="top" align="center">Yes</td>
     </tr>
+    <tr>
+      <td valign="top">optional<br>
+      </td>
+      <td valign="top">
+        if true, do not issue stop the build if the file does not exist,
+        default is false.<br>
+      </td>
+      <td valign="top" align="center">No</td>
+    </tr>
   </tbody>
 </table>
 <h3><br>

src/main/org/apache/tools/ant/taskdefs/ImportTask.java

@@ -64,8 +64,6 @@ import java.io.IOException;
 import java.util.Vector;
 
 /**
- * <i>EXPERIMENTAL:</i> This task is experimental and may be under continual
- * change till Ant1.6 ships; it may even be omitted from the product.
  * <p>
  * Task to import another build file into the current project.
  * <p>
@@ -96,7 +94,18 @@ import java.util.Vector;
  */
 public class ImportTask extends Task {
     private String file;
+    private boolean optional;
 
+    /**
+     * sets the optional attribute
+     *
+     * @param optional if true ignore files that are not present,
+     *                 default is false
+     */
+    public void setOptional(boolean optional) {
+        this.optional = true;
+    }
+    
     /**
      * the name of the file to import. How relative paths are resolved is still
      * in flux: use absolute paths for safety.
@@ -151,9 +160,15 @@ public class ImportTask extends Task {
         }
 
         if (!importedFile.exists()) {
-                throw new BuildException(
-                    "Cannot find " + file + " imported from "
-                    + buildFile.getAbsolutePath());
+            String message =
+                "Cannot find " + file + " imported from "
+                + buildFile.getAbsolutePath();
+            if (optional) {
+                getProject().log(message, Project.MSG_VERBOSE);
+                return;
+            } else {
+                throw new BuildException(message);
+            }
         }
 
         importedFile = new File(getPath(importedFile));