BasicService.showDocument("file://...jnlp"); will throw the following exception:
Exception in thread "AWT-EventQueue-0" java.security.AccessControlException: access denied (java.io.FilePermission F:\tests\javaws\generic\netbeans\notepad.jnlp read)
        at java.security.AccessControlContext.checkPermission(Unknown Source)
        at java.security.AccessController.checkPermission(Unknown Source)
        at java.lang.SecurityManager.checkPermission(Unknown Source)
        at java.lang.SecurityManager.checkRead(Unknown Source)
        at java.io.File.isDirectory(Unknown Source)
        at sun.net.www.protocol.file.FileURLConnection.connect(Unknown Source)
        at com.sun.deploy.net.BasicHttpRequest.doRequest(BasicHttpRequest.java:166)
        at com.sun.deploy.net.BasicHttpRequest.doRequest(BasicHttpRequest.java:94)
        at com.sun.deploy.net.BasicHttpRequest.doHeadRequest(BasicHttpRequest.java:64)
        at com.sun.deploy.net.DownloadEngine.isJnlpURL(DownloadEngine.java:624)
        at com.sun.jnlp.BasicServiceImpl.showDocument(BasicServiceImpl.java:69)

note - the code in JNLPCLassLoader has specifically given read permission to :
F:\tests\javaws\generic\netbeans\Notepad.jnlp
so problem may be limited to nfs mounted files jnlp file on windows