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Deploying Java Web Start Applications

This section describes the basics of deploying an application using Java Web Start. Deploying an application involves the following steps:

1. Setting up the Web Server
2. Creating the JNLP File
3. Placing the Application on the Web Server
4. Creating the Web Page

This section uses the Notepad example application to demonstrate Java Web Start technology. You can find all the source files for the Notepad application example in the \demo\plugin\jfc\Notepad directory within the JDK installation directory.

The key to running an application with Java Web Start is the Java Network Launching Protocol, or JNLP, file. The JNLP file is an XML file that contains elements and attributes that tell Java Web Start how to run the application.

An Example JNLP File

Following is the JNLP file for the Notepad demo:

<?xml version="1.0" encoding="utf-8"?>

<jnlp spec="1.0"
      codebase="http://javaweb.eng/~mh156787/tutorial/jar/jws/" 
      href="Notepad.jnlp">
   <information>
      <title>Notepad Demo</title>
      <vendor>Sun Microsystems, Inc.</vendor>
   </information>
   <resources>     
   	<jar href="Notepad.jar"/>   
	<j2se version="1.3+"
	      href="http://java.sun.com/products/autodl/j2se"/>
   </resources>
   <application-desc main-class="Notepad"/>
</jnlp>

If you click this link to the Notepad.jnlp file, you open and run the Notepad application.

JNLP File Contents

The following table describes the elements and attributes in the sample JNLP file.

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Note : This table does not include all possible contents of the JNLP file. For more information, see the Java Network Launching Protocol & API Specification (JSR-56) .

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JNLP File Contents

Element	Contents	Description
jnlp	spec codebase href	The spec attribute must be 1.0 or higher. The default is 1.0+. This attribute can typically be omitted. The codebase attribute specifies the base location for all relative URLs specified in href attributes in the JNLP file. The href specifies the URL of the JNLP file itself.
information	title vendor	The title element specifies the title of the application. The vendor element specifies the provider of the application.
resources	jar j2se	The jar element contains the attribute href that specifies the JAR file that is part of the application's class path. The JAR file is loaded using a ClassLoader object. The JAR file typically contains classes that for the particular application, but can also contain other resources, such as icons and configuration files, that are available through the getResource mechanism. The j2se element contains the attribute version that specifies the Java platform on which to run the application. The + symbol following the version signifies that the application can run on the specified version of the Java platform or a later version. The href attribute of the j2se element points to the URL from which the specified version of the Java platform can be automatically downloaded.
application-desc	@main-class	The application-desc element indicates that the JNLP file is to launch an application. The main-class attribute specifies the entry point for the application; that is, the class that contains the public static void main(String[] args) method where execution begins. You can omit the main-class attribute if the first JAR file specified in the JNLP file contains a manifest file containing the main-class header.

Encoding JNLP Files

Java Web Start supports encoding of JNLP files in any character encoding supported by the Java platform. For more information on character encoding in Java, see the Supported Encodings Guide . To encode a JNLP file, specify an encoding in the XML prolog of that file. For example, the following line indicates that the JNLP file is encoded in UTF-16.

<?xml version="1.0" encoding="utf-16"?>

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Note : The XML prolog itself must be UTF-8-encoded.

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Once you've completed to preceding steps, you are ready to write a Web page that gives users access to your application. Adding a link to your application in a Web page for users with Java Web Start already installed is simple; however, you must also design your Web page for users that might not have Java Web Start installed.

Adding the Basic Link to the JNLP File

In order to enable your users to launch the application from a Web page, you must include a link to the application's JNLP file from that Web page. To add this link, you use the standard HTML link syntax, with the href attribute specifying the location of the JNLP file:

<a href="Notepad.jnlp">Launch Notepad Application</a>

Assuming Java Web Start is installed on the client computer, when the user clicks this link, Java Web Start executes the application based on the instructions in the JNLP file.

Adding the Link when Java Web Start is not Installed

For users who might not have Java Web Start installed, you must write scripts in your Web page to:

1. Detect which browser the user has.
2. Detect whether Java Web Start is installed.
3. If Java Web Start is not installed, either auto-install it, or direct the user to a download page.

For more information and sample scripts to use for these steps, see the Java Web Start Guide .

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