JNLP File Syntax

This chapter covers the following topics:

Introduction
Example
JNLP Elements

Introduction

The format used in this release is that specified in JSR-00056: Java Network Launching Protocol and API. 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, 6.0, 6.0.10, 6.0.18, 7.0, or a wildcard such as 1.0+. It denotes the minimum version of the JNLP Specification that this jnlp file can work with.	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 systems 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, 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
	install	The optional install attribute can be used in a shortcut element to describe the application's preference for being considered "installed". If the value is "true" the application prefers to be considered installed. The default value of the install attribute is "false". On Windows, this determines if the application appears in the Add and Remove Programs panel.	7.0
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. If this element is included, either the offline-allowed element must also be included, or the href attribute must be set for the jnlp element.	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 eagerly, lazily, or for progress indication.	1.0 (6.0.18)
	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
application-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
	progress-class	The name of the class containing the implementation of DownloadServiceListener, that may be used to indicate download progress.	6.0.18
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
	progress-class	The name of the class containing the implementation of DownloadServiceListener, that may be used to indicate download progress.	6.0.18
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
	progress-class	The name of the class containing the implementation of DownloadServiceListener, that may be used to indicate download progress.	6.0.18
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"/> 
    <shortcut online="false" install="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 omitted. 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

os attribute: This attribute contains a list of operating system names for this element. Read the discussion of the resources element later for a full discussion of the os attribute.

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:

one-line: If a reference to the application is going to appear on one row in a list or a table, this description will be used.
short: If a reference to the application is going to be displayed in a situation where there is room for a paragraph, this description is used.
tooltip: If a reference to the application is going to appear in a tooltip, this description is used.

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, JPEG, ICO, or PNG format. The icons are used to represents the application

during launch when Java Web Start presents the application to the user;
in the Java Application Cache Viewer;
in desktop shortcuts.

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 attribute can have one of the following values:

default: The specified image is used in the Java Application Cache Viewer. It is also used if an icon element with a particular kind attribute has not been specified. Not specifying the kind attribute is the same as specifying it with the default value.
splash: The specified image is used as the splash image on the second and subsequent launches of the application. During the initial launch of the application, before resources are downloaded, the default splash image is used.
shortcut: The specified image is used in desktop shortcuts.

A JNLP file can contain multiple icon elements that differ by their kind attribute. This enables you to specify different icon images for your 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 its sub-elements provide hints that the JNLP Client may or may not use. The shortcut element can contain the optional online and install attributes, 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. If the association element is included, either the offline-allowed element must also be included, or the href attribute must be set for the jnlp element to ensure that the application can be located and run.

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:

description element: A short description of the related content.
icon element: The icon can be used by the JNLP Client to identify the related content to the user.

`security` Element

Each sandbox application is 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 has full access to the client machine and local network. All JAR files must be signed. The user is prompted to accept the certificate and agree to run the application.

`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 running the application.

For example:

`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 os attribute contains a list of operating system names for a resource. For example, you could use multiple resources definitions, with different os attributes, to supply a native library for multiple operating systems.

The os attribute contains a list of operating system names separated by spaces. At runtime, the os values are compared with the beginning of the os.name system property to find a match. For example, an os attribute value of "Windows" matches both "Windows Vista" and "Windows 7" operating systems.

If you wish to list an operating system whose name contains a space, use a backslash to indicate that the space is part of the operating system name. The following example will specifically match "Windows Vista" and "Windows 7":

    <resources os="Windows\ Vista Windows\ 7">
      <jar href="hello.jar"/>
    </resources>

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 JSR-00056: Java Network Launching Protocol and API 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"/>
    </resources>

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, lazily, or contains a custom progress implementation. For example:

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

A jar denoted with download="progress" will be downloaded eagerly, and may be used to indicate progress while downloading other resources.

The j2se element specifies what Java 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 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"/

<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 (unix platforms only) */
 -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, as well as properties beginning with either "javaws." or "jnlp.", are considered "secure" and will be passed to the VM in this way:

    sun.java2d.noddraw,
    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,
    sun.awt.disableMixing,
    sun.lang.ClassLoader.allowArraySyntax

For an untrusted application, system properties set in the JNLP file will only be set by Java Web Start if they are considered secure.

For information about properties that affect the behavior of Java Web Start applications, see Properties That Affect the Behavior of Rich Internet Applications.

`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>

The (optional) progress-class attribute can be used to indicate that the class of this name implements the javax.jnlp.DownloadServiceListener interface. This class may be loaded first and used to indicate the progress of other resources being downloaded and verified.

`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.

JNLP File Syntax

Introduction

Example

JNLP Elements

jnlp Element

information Element

security Element

update Element

resources Element

application-desc Element

applet-desc Element

component-desc Element

installer-desc Element