Certain CDNs seem to require User-Agent headers, and return HTTP 403 (forbidden) without them. As such, the following wro4j directives both fail in wro4j-maven-plugin v1.7.7:

<group name="core"> <js>http://cdn.datatables.net/1.10.5/js/jquery.dataTables.min.js</js&gt; <js>http://code.highcharts.com/highcharts.js</js&gt; </group>

The error:

[ERROR] Cannot ignore missing resource: ro.isdc.wro.model.resource.Resource@b326f69[JS,http://cdn.datatables.net/1.10.5/js/jquery.dataTables.js,false] [ERROR] Exception while merging resources: Server returned HTTP response code: 403 for URL: http://cdn.datatables.net/1.10.5/js/jquery.dataTables.js [ERROR] Exception occured while processing: ro.isdc.wro.WroRuntimeException: Exception while merging resources: Server returned HTTP response code: 403 for URL: http://cdn.datatables.net/1.10.5/js/jquery.dataTables.js, class: ro.isdc.wro.WroRuntimeException,caused by: java.io.IOException ro.isdc.wro.WroRuntimeException: Exception while merging resources: Server returned HTTP response code: 403 for URL: http://cdn.datatables.net/1.10.5/js/jquery.dataTables.js at ro.isdc.wro.model.group.processor.GroupsProcessor.process(GroupsProcessor.java:86)