JNLP File Syntax


This chapter covers the following topics:

Introduction

The format used in this release is that specified in the Java Network Launching Protocol & API Specification (JSR-56) version 6.0. This document describes the most commonly used elements of a JNLP file. For a complete description of the format, refer to the specification.

JNLP File Contents

Element

Attributes

Description

Since

Required

jnlp


This is the main xml element for a jnlp file. Everything is contained within the jnlp element.

1.0

yes


spec

The spec attribute can be 1.0, 1.5.0, or 6.0 or can use the wildcards such as 1.0+. It denoted the minimum version of the JNLP Specification that this jnlp file can work woth.

1.0



codebase

The codebase attribute specifies the base location for all relative URLs specified in href attributes in the JNLP file.

1.0



href

The href specifies the URL of the JNLP file itself.

1.0



version

The version of the application being launched, as well as the version of the JNLP file itself.

1.0


information


The information element contains other elements that describe the application and it's source.

1.0

yes


os

Specifies the operating system for which this information element should be considered.

1.5.0



arch

Specifies the architecture for which this information element should be considered

1.5.0



platform

Specifies the platform for which this information element should be considered.

1.5.0



locale

Specifies the locale for which this information element should be considered.

1.5.0


title


The title element specifies the title of the application.

1.0

yes

vendor


The vendor element specifies the provider of the application.

1.0

yes

homepage


The homepage of the application.

1.0



href

A URL pointing to where more information on this application can be found.

1.0

yes

description


A short statement describing the application.

1.0



kind

An indicator as to what type of description this is, legal values are one-line, short, and tooltip.

1.0


icon


Describes an icon that can be used to identify the application to the user.

1.0



href

A URL pointing to the icon file, may be in one of the following formats: gif, jpg, png, ico.

1.0

yes


kind

Indicates the suggested use of the icon, can be: default, selected, disabled, rollover, splash, or shortcut.

1.0



width

Can be used to indicate the resolution of the image.

1.0



height

Can be used to indicate the resolution of the image.

1.0



depth

Can be used to indicate the resolution of the image.

1.0


offline-allowed


Indicates that this application can operate when the client system is disconnected from the network.

1.0


shortcut


The shortcut element can be used to indicate an application's preferences for desktop integration.

1.5.0



online

Can be used to describe the application's preference for creating a shortcut to run online or offline.

1.5.0


desktop


Can be used to indicate an application's preference for putting a shortcut on the users desktop.

1.5.0


menu


Can be used to indicate an application's preference for putting a menu item in the users start menus.

1.5.0



sub-menu

Can be used to indicate an application's preference for where to place the menu item.

1.5.0


association


Can be used to hint to the JNLP client that it wishes to be registered with the operating system as the primary handler of certain extensions and a certain mime-type.

1.5.0



extensions

Contains a list of file extensions (separated by spaces) that the application requests it be registered to handle.

1.5.0



mime-type

Contains the mime-type that the application requests it be registered to handle.

1.5.0


related-content


Describes an additional piece of related content that may be integrated with the application.

1.5.0



href

A URL pointing to the related content.

1.5.0

yes






update


The update element is used to indicate the preferences for how application updates should be handled by the JNLP Client.

6.0



check

Indicates the preference for when the JNLP Client should check for updates. It can be always, timeout, or background..

6.0



policy

Indicates the preference for how the JNLP Client should handle an application update when it is known an update is available before the application is launched. It can be always, prompt-update, or prompt-run.

6.0





1.0


security


This element can be used to request enhanced permissions.

1.0


all-permissions


Requests that the application be run with all permissions.

1.0


j2ee-application-client-permissions


Requests that the application be run with a permission set that meets the security specifications of the J2EE Application Client environment.

1.0





1.0


resources


Describes all the resources that are needed for an application.

1.0

yes


os

Specifies the operating system for which the resources element should be considered.

1.0



arch

Specifies the architecture for which the resources element should be considered.

1.0



locale

Specifies that the locales for which the resources element should be considered.



java (or j2se)


Specifies what version(s) of Java to run the application with.

6.0 (java)



version

Describes an ordered list of version ranges to use.

1.0

yes


href

The URL denoting the supplier of this version of java, and where it may be downloaded from.

1.0



java-vm-args

Indicates an additional set of standard and non-standard virtual machine arguments that the application would prefer the JNLP Client to use when launching Java.

1.0



initial-heap-size

Indicates the initial size of the Java heap.

1.0



max-heap-size

Indicates the maximum size of the Java heap.

1.0


jar


Specifies a JAR file that is part of the application's classpath.

1.0

yes


href

The URL of the jar file.

1.0

yes


version

The requested version of the jar file. Requires using the version-based download protocol

1.0



main

Indicates if this jar contains the class containing the main method of the application.

1.0



download

Can be used to indicate this jar may be downloaded lazily, or when needed.

1.0



size

Indicates the downloadable size of the jar file in bytes.

1.0



part

Can be used to group resources together so they will be downloaded at the same time.

1.0


nativelib


Specifies a JAR file that contains native libraries in it's root directory.

1.0



href

The URL of the jar file.

1.0

yes


version

The requested version of the jar file. Requires using the version-based download protocol

1.0



download

Can be used to indicate this jar may be downloaded lazily.

1.0



size

Indicates the downloadable size of the jar file in bytes.

1.0



part

Can be used to group resources together so they will be downloaded at the same time.

1.0


extension


Contains pointer to an additional component-desc or installer-desc to be used with this application.

1.0



href

The URL to the additional extension jnlp file.

1.0

yes


version

The version of the additional extension jnlp file.

1.0



name

The name of the additional extension jnlp file

1.0


ext-download


Can be used in an extension element to denote the parts contained in a component-extension.

1.0



ext-part

Describes the name of a part that can be expected to be found in the extension.

1.0

yes


download

Can be used to indicate this extension may be downloaded eagerly or lazily.

1.0



part

Denotes the name of a part in this jnlp file to include the extension in.

1.0


package


Can be used to indicate to the JNLP Client which packages are implemented in which JAR files.

1.0



name

Package name contained in the jar files of the given part.

1.0

yes


part

Part name containing the jar files that include the given package name.

1.0

yes


recursive

Can be used to indicated that all package names beginning with the given name, can be found in the given part.

1.0


property


Defines a system property that will be available through the System.getProperty and System.getProperties methods.

1.0



name

Name of the system property.

1.0

yes


value

Value it will be set to.

1.0

yes




1.0




Note: A jnlp file must contain one of application-desc, applet-desc, component-desc, or installer-desc

1.0

yes

applicaation-desc


Denotes this is the jnlp file for an application.

1.0



main-class

The name of the class containing the public static void main(String[]) method of the application.

1.0

yes

argument


Each argument contains (in order) an additional argument to be passed to main.

1.0


applet-desc


Denotes this is the jnlp file for an applet.

1.0



main-class

This is the name of the main Applet class.

1.0

yes


documentbase

The document base for the Applet as a URL.

1.0



name

Name of the Applet.

1.0

yes


width

The width of the applet in pixels.

1.0

yes


height

The height of the applet in pixels

1.0

yes

param


A set of parameters that can be passed into the applet.

1.0



name

The name of this parameter

1.0

yes


value

The value of this parameter

1.0

yes

component-desc


Denotes this is the jnlp file for a component extension.

1.0


installer-desc


Denotes this is the jnlp file for an installed extension.

1.0



main-class

The name of the class containing the public static void main(String[]) method of the installer.

1.0

yes

The JNLP file is an XML document. The following shows a complete example of a JNLP file.

Example

<?xml version="1.0" encoding="utf-8"?>
<!-- JNLP File for SwingSet2 Demo Application -->
<jnlp
  spec="6.0+"
  codebase="http://my_company.com/jaws/apps"
  href="swingset2.jnlp">
  <information>
    <title>SwingSet2 Demo Application</title>
    <vendor>Sun Microsystems, Inc.</vendor>
    <homepage href="docs/help.html"/>
    <description>SwingSet2 Demo Application</description>
    <description kind="short">A demo of the capabilities 
    of the Swing Graphical User Interface.</description>
    <icon href="images/swingset2.jpg"/>
    <icon kind="splash" href="images/splash.gif"/>
    <offline-allowed/> 
    <association>
      <mime-type="application-x/swingset2-file"/>
      <extensions="swingset2"/> 
    </association>
    <shortcut online="false">
      <desktop/>
      <menu submenu="My Corporation Apps"/>
    </shortcut>
  </information>
  <information os="linux">   
    <title> SwingSet2 Demo on Linux </title>
    <homepage href="docs/linuxhelp.html">
  </information>
  <security>
      <all-permissions/>
  </security>
  <resources>
    <j2se version="1.6+" java-vm-args="-esa -Xnoclassgc"/>
    <jar href="lib/SwingSet2.jar"/>
  </resources>
  <application-desc main-class="SwingSet2"/>
</jnlp>

The example shows the basic outline of the document. The root element is jnlp, which has four subelements: information, security, resources, and application-desc. In addition, Java Web Start also supports launching applets by using the applet-desc element. The elements are described in more detail below.

JNLP Elements

jnlp Element

spec attribute: This attribute must be 1.0 or higher to work with this release. The default value is "1.0+". Thus, it can typically be omited. Note that this version supports both spec version 1.0, version 1.5, and version 6.0, whereas previous versions support only 1.0 and 1.5.  A jnlp file specifying spec="6.0+" will work with this version, but not previous versions of Java Web Start.

codebase attribute: All relative URLs specified in href attributes in the JNLP file are using this URL as a base.

href attribute: This is a URL pointing to the location of the JNLP file itself.

information Element

title element: The name of the application.

vendor element: The name of the vendor of the application.

homepage element: Contains a single attribute, href, which is a URL locating the home page for the Application. It is used by the Java Application Cache Viewer to point the user to a Web page where more information about the application can be found.

description element: A short statement about the application. Description elements are optional. The kind attribute defines how the description should be used. It can have one of the following values:

Only one description element of each kind can be specified. A description element without a kind is used as a default value. Thus, if Java Web Start needs a description of kind short, and it is not specified in the JNLP file, then the text from the description without an attribute is used.

All descriptions contain plain text. No formatting, such as with HTML tags, is supported.

icon element: Contains an HTTP URL to an image file in either GIF or JPEG format. The icons are used to represents the application

A 64x64 icon is shown during download; in the Java Application Cache Viewer and in desktop shortcuts a 32x32 icon is used. Java Web Start automatically resizes an icon to the appropriate size.

Optional width and height attributes can be used to indicate the size of the images.

The optional kind="splash" attribute may be used in an icon element to indicate that the image is to be used as a "splash" screen during the launch of an application. If the JNLP file does not contain an icon element with kind="splash" attribute, Java Web Start will construct a splash screen using other items from the information Element.

If the JNLP file does not contain any icon images, the splash image will consist of the application's title and vendor, as taken from the JNLP file.

The first time an application is launched following the addition or modification of the icon element in the JNLP file, the old splash image will still be displayed. The new splash image will appear on the second and subsequent launches of the application.

offline-allowed element: The optional offline-allowed element indicates if the application can be launched offline.

If offline-allowed is specified, then the application can be launched offline by the Java Application Cache Viewer, and shortcuts can be created which launch the application offline.

If an application is launched offline, it will not check for updates and the API call BasicService.isOffline() will return true.

The offline-allowed element also controls how Java Web Start checks for an update to an application. If the element is not specified—i.e., the application is required to be online to run—Java Web Start will always check for an updated version before launching the application. And if an update is found, the new application will be downloaded and launched. Thus, it is guaranteed that the user always runs the latest version of the application. The application, however, must be run online.

If offline-allowed is specified, Java Web Start will also check to see if an update is available. However, if the application is already downloaded the check will timeout after a few seconds, in which case the cached application will be launched instead. Given a reasonably fast server connection, the latest version of the application will usually be run, but it is not guaranteed. The application, however, can be run offline.

shortcut element: The optional shortcut element can be used to indicate an application's preferences for desktop integration. The shortcut element and it's sub-elements provide hints that the JNLP Client may or may not use. The shortcut element can contain the optional online attribute, and the two optional sub-elements, desktop and menu.

association element: The optional association element is a hint to the JNLP client that it wishes to be registered with the operating system as the primary handler of certain extensions and a certain mime-type. The association element must have the extensions and mime-type attributes.

related-content element: The optional related-content element describes an additional piece of related content, such as a readme file, help pages, or links to registration pages, as a hint to a JNLP Client. The application is asking that this content be included in its desktop integration. The related-content element has a mandatory href and title attribute. It can contain any of the following two sub-elements:

security Element

Each application is, by default, run in a restricted execution environment, similar to the Applet sandbox.  The security element can be used to request unrestricted access.

If the all-permissions element is specified, the application will have full access to the client machine and local network. If an application requests full access, then all JAR files must be signed. The user will be prompted to accept the certificate the first time the application is launched.

update Element

The update element is used to indicate the preferences for how application updates should be handled by Java Web Start.

The update element can contain the following two optional attributes:

check attribute: The check attribute indicates the preference for when the JNLP Client should check for updates, and can have one of the three values: "always", "timeout", and "background"

A value of "always" means to always check for updates before launching the application.

A value of "timeout" (default) means to check for updates until timeout before launching the application. If the update check is not completed before the timeout, the application is launched, and the update check will continue in the background.

A value of "background" means to launch the application while checking for updates in the background.

policy attribute: The policy attribute indicates the preference for how the JNLP Client should handle an application update when it is known an update is available before the application is launched, and can have one of the following three values: "always", "prompt-update", and "prompt-run"

A value of "always" (default) means to always download updates without any prompt.

A value of "prompt-update" means to ask the user if he/she wants to download and run the updated version, or launch the cached version.

A value of "prompt-run" means to ask the user if he/she wants to download and run the updated version, or cancel and abort runing the application.

For example:

<update check="always" policy="prompt-update">

resources Element

The resources element is used to specify all the resources, such as Java class files, native libraries, and system properties, that are part of the application.  A resource definition can be restricted to a specific operating system, architecture, or locale using the os, arch, and locale attributes.

The resources element has six different possible subelements: jar, nativelib, j2se, property, package, and extension. The package and extension elements are not discussed in this developer's guide. See the Java Network Launching Protocol & API Specification (JSR-56) version 6.0 for details.

A jar element specifies a JAR file that is part of the application's classpath.  For example:

  <jar href="myjar.jar"/>

The jar file will be loaded into the JVM using a ClassLoader object.  The jar file will typically contain Java classes that contain the code for the particular application, but can also contain other resources, such as icons and configuration files, that are available through the getResource mechanism.

A nativelib element specifies a JAR file that contains native libraries.  For example:

    <nativelib href="lib/windows/corelib.jar"/>

The JNLP client must ensure that each file entry in the root directory of the JAR file (i.e., /) can be loaded into the running process using the System.loadLibrary method.  Each entry must contain a platform-dependent shared library with the correct naming convention, e.g., *.dll on Windows or lib*.so on Solaris/Linux.  The application is responsible for doing the actual call to System.loadLibrary.

Native libraries would typically be included in a resources element that is geared toward a particular operating system and architecture.  For example:

    <resources os="SunOS" arch="sparc">
        <nativelib href="lib/solaris/corelibs.jar"/>
    </resource>

By default, jar and nativelib resources will be downloaded eagerly, i.e., they are downloaded and available locally to the JVM running the application before the application is launched.  The jar and nativelib elements also allow a resource to be specified as lazy.  This means the resource does not have to be downloaded onto the client system before the application is launched.

The download attribute is used to control whether a resource is downloaded eagerly or lazily.  For example:

    <jar href="sound.jar" download="lazy"/>
    <nativelib href="native-sound.jar" download="eager"/>

The j2se element specifies what Java(TM) Platform Standard Edition Runtime Environment (JRE) versions an application is supported on, as well as standard parameters to the Java Virtual Machine.  If several JREs are specified, this indicates a prioritized list of the supported JREs, with the most preferred version first.  For example:

       <j2se version="1.3" initial-heap-size="64m" max-heap-size="128m"/>
       <j2se version="1.4.2+" href="http://java.sun.com/products/autodl/j2se" java-vm-args="-esa -Xnoclassgc"/>

The version attribute refers, by default, to a platform version (specification version) of the Java(TM) Platform Standard Edition. Currently defined platform versions are 1.2, 1.3, 1.4, 1.5 and 1.6. (A platform version will not normally contain a micro version number; e.g., 1.4.2.)

Exact product versions (implementation versions) may also be specified. by including the href attribute. For example, 1.3.1_07, 1.4.2, or 1.5.0-beta2 by Sun Microsystems, Inc. For example,

<j2se version="1.4.2" href="http://java.sun.com/products/autodl/j2se"/

or

<j2se version="1.4.2_04" href="http://java.sun.com/products/autodl/j2se"/>

If a platform version is specified (i.e., no href attribute is provided), Java Web Start will not consider an installed non-FCS (i.e., milestone) JRE as a match. E.g., a request of the form

<j2se version="1.4+"/>

would not consider an installed 1.4.1-ea or 1.4.2-beta JRE as a match for the request. Starting with 1.3.0, a JRE from Sun Microsystems, Inc., is by convention a non-FCS (milestone) JRE if there is a dash (-) in the version string.

The java-vm-args attribute of the j2se element specifies a preferred set of virtual machine arguments to use when launching java.

<j2se version="1.4+" java-vm-args="-ea -Xincgc"/>

The following java-vm-args are supported by this version: 

 -d32,							        /* use a 32-bit data model if available */
 -client,                      				        /* to select the client VM */
 -server,						        /* to select the server VM */
 -verbose,                    				        /* enable verbose output */
 -version,         		        		        /* print product version and exit */
 -showversion,					                /* print product version and continue */       					      
 -help,                        					/* print this help message */
 -X,                           					/* print help on non-standard options */
 -ea,                          					/* enable assertions */
 -enableassertions,            					/* enable assertions */
 -da,                          					/* disable assertions */
 -disableassertions,           					/* disable assertions */
 -esa,                         					/* enable system assertions */
 -enablesystemassertions,      					/* enable system assertions */
 -dsa,                         					/* disable system assertione */
 -disablesystemassertions,     					/* disable system assertione */
 -Xmixed,                      					/* mixed mode execution (default) */
 -Xint,                        					/* interpreted mode execution only */
 -Xnoclassgc,                  					/* disable class garbage collection */
 -Xincgc,                      					/* enable incremental garbage collection */
 -Xbatch,                      					/* disable background compilation */
 -Xprof,                       					/* output cpu profiling data */
 -Xdebug,                      					/* enable remote debugging */
 -Xfuture,                     					/* enable strictest checks, anticipating future default */
 -Xrs,                         					/* reduce use of OS signals by Java/VM (see documentation) */
 -XX:+ForceTimeHighResolution, 					/* use high resolution timer */
 -XX:-ForceTimeHighResolution, 					/* use low resolution (default) */

Plus any argument starting with one of the following: 

-ea,                          /* enable assertions for classes */
-enableassertions,            /* enable assertions for classes */
-da,                          /* disable assertions for classes */
-disableassertions,           /* disable assertions for classes */
-verbose,                     /* enable verbose output */
-Xms,                         /* set initial Java heap size */
-Xmx,                         /* set maximum Java heap size */
-Xss,                         /* set java thread stack size */
-XX:NewRatio,                 /* set Ratio of new/old gen sizes */
-XX:NewSize,                  /* set initial size of new generation */
-XX:MaxNewSize,               /* set max size of new generation */
-XX:PermSize,                 /* set initial size of permanent gen */
-XX:MaxPermSize,              /* set max size of permanent gen */
-XX:MaxHeapFreeRatio,         /* heap free percentage (default 70) */
-XX:MinHeapFreeRatio,         /* heap free percentage (default 40) */
-XX:UseSerialGC,              /* use serial garbage collection */
-XX:ThreadStackSize,          /* thread stack size (in KB) */
-XX:MaxInlineSize,            /* set max num of bytecodes to inline */
-XX:ReservedCodeCacheSize,    /* Reserved code cache size (bytes) */
-XX:MaxDirectMemorySize,

The property element defines a system property that will be available through the System.getProperty and System.setProperties methods. It has two required attributes: name and value. For example:

<property name="key" value="overwritten"/>

Properties set in the jnlp file will normally be set by Java Web Start after the VM is started but before the application is invoked. Some properties are considered "secure" properties and can be passed as -Dkey=value arguments on the java invocation command line.

The following properties are considered "secure" and will be passed to the VM in this way: 

    sun.java2d.noddraw,
    javax.swing.defaultlf,
    javaws.cfg.jauthenticator,
    swing.useSystemFontSettings,
    swing.metalTheme,
    http.agent,
    http.keepAlive,
    sun.awt.noerasebackground,
    sun.java2d.opengl,
    sun.java2d.d3d,
    java.awt.syncLWRequests,
    java.awt.Window.locationByPlatform,
    sun.awt.erasebackgroundonresize,
    swing.noxp,
    swing.boldMetal,
    awt.useSystemAAFontSettings,
    sun.java2d.dpiaware,

For an untrusted application, system properties set in the JNLP file will only be set by Java Web Start if they are considered secure, or if the property name begins with "jnlp." or "javaws.".

application-desc Element

The application element indicates that the JNLP file is launching an application (as opposed to an applet). The application element has an optional attribute, main-class, which can be used to specify the name of the application's main class, i.e., the class that contains the public static void main(String argv[]) method where execution must begin.

The main-class attribute can be omitted if the first JAR file specified in the JNLP file contains a manifest file containing the main class.

Arguments can be specified to the application by including one or more nested argument elements. For example:

  <application-desc main-class="Main">
    <argument>arg1</argument>
    <argument>arg2</argument>
  </application-desc>

applet-desc Element

Java Web Start has support for launching Java applets. This support provides easy migration of existing code to Java Web Start.

An applet is launched using the applet-desc element instead of the application-desc element. For example:

  <applet-desc
      documentBase="http://..."
      name="TimePilot"
      main-class="TimePilot.TimePilotApp"
      width="527"
      height="428">
    <param name="key1" value="value1"/>
    <param name="key2" value="value2"/>
  </applet-desc>

The JAR files that make up the applet are described using the resources element as for applications. The documentBase must be provided explicitly since a JNLP file is not embedded in an HTML page. The rest of the attributes correspond to the respective HTML applet tag elements.

The main-class attribute is used instead of the code attribute.  The main-class attribute is assigned the name of the Applet class (without the .class extension).  This attribute can be omitted if the Applet class can be found from the Main-Class manifest entry in the main JAR file.

Note: Applets must be packaged in JAR files in order to work with Java Web Start.

component-desc Element

The component-desc element denotes that this jnlp file is not an application or an applet but an extension that can be used as a resource in an application, applet or another extension.

A component extension is typically used to factor out a set of resources that are shared between multiple applications or that have separate security needs.

installer-desc Element

The installer-desc element denotes that this jnlp file is an installer extension that defines an application that will be run only once, the first time this extension jnlp file is used in an application, applet or another extension.

An installer extension is typically used to install platform specific native code that requires a more complicated setup than simply loading a native library into the VM.