Problem: My browser shows the JNLP file for my application as plain text.

Most likely, your Web server is not aware of the proper MIME type for JNLP files. See the Setting up the Web server section for more information.

Furthermore, if you are using a proxy server, ensure that the update versions of the files are returned, by updating the time stamp of the resources on the Web server such that the proxies will update their caches.

Problem: When I click on a link to a JNLP file with Internet Explorer and Java Web Start launches, I get the message "Could not load file/URL specified: C:\Documents and Settings\...\application[1].jnlp".

This problem only occurs with Internet Explorer. It is usually caused by a no-cache directive from the Web or proxy server, which causes Internet Explorer to not write the JNLP file to the local disk.

This frequently happens when you upgrade a Tomcat-based server, as later versions set the no-cache directive by default for any resource that is within a security-constraint in the web.xml file. Try removing the relevant URI from the security constraint.

This problem can also occur because of a full cache, or if the cache is turned off in Internet Explorer.

The most likely cause is that your browser and Java Web Start have different proxy settings. To modify Java Web Start proxy settings:

1. Open the Java Application Cache Viewer.
2. From the Edit menu, select Preferences. The Java Control Panel opens.
3. Click Network Settings. The Network Settings dialog box opens.
4. Select Use proxy server, enter the proxy server name, and port number (typically 8080).
5. Click OK.

Problem: When I try to launch my JNLP file, I get the following error:

MissingFieldException[ The following required field is missing from the 
launch file: (|||)]
    at com.sun.javaws.jnl.XMLFormat.parse(Unknown Source)
    at com.sun.javaws.jnl.LaunchDescFactory.buildDescriptor(Unknown Source)
    at com.sun.javaws.jnl.LaunchDescFactory.buildDescriptor(Unknown Source)
    at com.sun.javaws.jnl.LaunchDescFactory.buildDescriptor(Unknown Source)
    at com.sun.javaws.Main.launchApp(Unknown Source)
    at com.sun.javaws.Main.continueInSecureThread(Unknown Source)
    at com.sun.javaws.Main.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

Often this error occurs when your XML is malformed. You can stare at the code until you figure it out but it is easier to run an XML syntax checker over the file. (NetBeans IDE and jEdit both provide XML syntax checkers.)

However, this error can occur in a other situations and the above was caused by the following line in an otherwise well-formed XML file:

<description kind="short">Demonstrates choosing the drop location in the target <code>TransferHandler</code></description>

The error was caused by the illegal embedded code tags.