JNLP : Java Glossary

JNLP (Java Network Launching Protocol) for installing the JVM (Java Virtual Machine) and Java apps. Sun has written a specific implementation of it called Java Web Start.

The Java Web Start entry will explain how to set up the *.jnlp to javaws.exe association in your browser and desktop.

JNLP Versions

New in version 1.5, Java version 1.5 or later:

• splash image type.
• shortcut, associations, related-content
• js2e java-vm-args
• os, arch, and platform
• added ampersand (&) modifier to the version-range argument to allow compound versions.

• update and its args check and policy.
• association now has description and icon.
• java element to eventually replace j2se.

Sample JNLP 1.5

Sample JNLP 1.6

Here is an example using the most recent JNLP 1.6 specification.

The ordering of the sections matters. You must do them in the order jnlp, information, security, update, resources, application-desc, applet-desc, component-desc, installer-desc.

Within the resources sections, order is again important: java, j2se, jar, nativelib, extension, property, package.

You nest <java inside <resources. However, you can nest another <resources inside the <java to describe the resources to use for that JVM only.

Icons

Off/line

JVM Versions

Every time Sun releases a new JRE (Java Runtime Environment), you need to add it to all your JNLP files if you want your clients to be able to access your web start apps with it.

Multiple JNLP files

1. When you have an installer. It needs its own separate JNLP file.
2. When some jars are signed with one certificate and others with another. You need an JNLP file for each different certificate and its jars.

<!-- chaining off an auxiliary jnlp file from the main one -->
<extension name="Installer" href="esperinstaller.jnlp"/>

JNLP Tokens

JNLP Validation

You can check that your JNLP file is correctly formed using an XSD (XML Scheme Definition) Schema originally from Vampqh. You must copy the JNLP 1.0 XSD schema posted below into the current directory as file jnlp1.xsd or jnlp6.xsd then run the Java validation posted below with:

rem Validate a JNLP file with CMP ValidateJNLP utility
java.exe ValidateJNLP jnlp6.xsd C:\mydir\myapp.jnlp

The XSD Schemas

This is an XSD schema to validate JNLP 1.0 :

This is an XSD schema to validate JNLP 1.5. It was mechanically generated by Stylus Studio from the Sun JNLP DTD (Document Type Definition). I modified it slightly to allow both :

The Validator Program

The above validator is not user friendly. You can get a good idea what it is looking for by reading the XSD file. You would be surprised how many errors you will find.

Unfortunately, Vampqh’s fancier validator, Vanessa, is no longer available.

DTD for JNLP

<!ELEMENT update>
should be:
<!ELEMENT update EMPTY>
and
<!ELEMENT shortcut (desktop? menu?)>
should be:
<!ELEMENT shortcut (desktop?, menu?)>

Snooping

One way to learn JNLP tricks is to snoop on the JNLP files of other people’s applications.

rem jnlpview.bat viewing a JNLP file.
javaws.exe -viewer
rem then right click on the installed JWS app and click show JNLP file.

Gotchas

• If you make any changes to your JNLP file, you must increment the <jnlp version= or else your clients will not bother to use the new JNLP file.
• There does not appear to be a way to request the 32-bit windows version of the JVM. If you only have 32-bit JNI (Java Native Interface), often it will attempt to use the 64-bit version and fail.
• There is no naming convention to keep track of whether a Windows DLL (Dynamic Link Library) is 32-bit or 64-bit. You have to keep them in separate directories/jars.
• The one tool you do have is the arch parameter which you set to x86 for 32-bit and amd64 for 64-bit.

Learning More