4 Overview of Java Web Start Technology

Java Web Start is an application-deployment technology that enables your users to launch full-featured applications with a single click from any web browser. Users can download and launch applications without going through complicated installation procedures.

With Java Web Start, your users launch applications by clicking a web page link. If the application is not present on their computer, Java Web Start automatically downloads all necessary files. It then caches the files on the computer so that the application is always ready to be relaunched anytime the user wants—either from an icon on the desktop or from the browser link. No matter which method is used to launch the application, the most current version of the application is always presented.

The technology underlying Java Web Start is the Java Network Launching Protocol & API (JNLP). This technology was developed through the Java Community Process (JCP). Java Web Start is the reference implementation (RI) for the JNLP specification. The JNLP technology defines, among other things, the JNLP file, which is a standard file format that describes how to launch an application. The JNLP specification is available at JSR 56: Java Network Launching Protocol and API.

Java Web Start allows you to launch Java-technology-based applications directly from the Web. An application can be launched in three different ways:

• From a web browser by clicking a link

• From desktop icons or the Start Menu

• From the Java Cache Viewer

Regardless of which way is used, Java Web Start will connect back to the web server each time an application is launched, to check whether an updated version of the application is available.

Applications can be deployed from any standard web server. Java Web Start leverages existing internet technology, such as the HTTP protocol and web servers, so existing infrastructure for deploying HTML-based content can be reused to deploy Java Technology-based applications using Java Web Start.

To deploy your application to client machines, ensure that all files that contain your application are accessible through a web server. This typically requires copying one or more JAR files and a JNLP file into the web server's directories. Enabling the website to support Java Web Start is similar to deploying HTML-based content. In addition, to use Java Web Start, the web server must be configured to support the application/x-java-jnlp-file MIME type.

Step 1   Configure the web server to use the Java Web Start MIME type.

Many web servers come with the Java Web Start MIME type configured by default. If your web server does not, configure it so that all files with the .jnlp file extension are set to the application/x-java-jnlp-file MIME type.

Most web browsers use the MIME type returned with the contents from the web server to determine how to handle the particular content. The server must return the application/x-java-jnlp-file MIME type for JNLP files in order for Java Web Start to be invoked.

Each web server has a specific way in which to add MIME types. For example, for the Apache web server, you add the following line to the .mime.types configuration file:

application/x-java-jnlp-file JNLP

Check the documentation for the specifics of your web server.

Step 2   Create a JNLP file for the application.

The easiest way to create this file is to modify an existing JNLP file with your requirements. A simple JNLP file is shown in the following example:

<?xml version="1.0" encoding="utf-8"?>
<jnlp spec="9.0+"
  codebase="http://example.com/demos/helloworld" 
  href="HelloWorld.jnlp">
  <information>
    <title>HelloWorld</title>
    <description>HelloWorld demo application</description>
  </information>

  <resources>
    <j2se version="9"/>
    <jar href="HelloWorld.jar" size="47013" download="eager" />
  </resources>

  <application-desc main-class="HelloWorld"/>
</jnlp>

The syntax and format for the JNLP file is described in JNLP File Syntax.

Step 3   Make the application accessible on the web server.

Ensure that your application's JAR files and the JNLP file are accessible at the URLs listed in the JNLP file.

Step 4   Create the web page that launches the application.

Create the web page and include one of the following options for starting a Java Web Start application:

• Use a link to the JNLP file, as shown in the following examples:

  <a href="/some/path/HelloWorld.jnlp">Launch HelloWorld demo</a>
  
  <a href="https://docs.oracle.com/javase/tutorialJWS/samples/deployment/dynamictree_webstartJWSProject/dynamictree_webstart.jnlp">Launch DynamicTree demo</a>
  
  <a href="http://docs.oracle.com/javase/tutorialJWS/samples/deployment/dynamictree_webstartJWSProject/dynamictree_webstart.jnlp">Launch DynamicTree demo</a>
  

• Use JavaScript, as shown in the following example:

  <script src="https://www.java.com/js/deployJava.js"></script>
  <script>
      var url = "https://docs.oracle.com/javase/tutorialJWS/samples/deployment/dynamictree_webstartJWSProject/dynamictree_webstart.jnlp";
      deployJava.createWebStartLaunchButton(url, '1.6.0');
  </script>
  

• Use a link with the jnlp: schema, as shown in the following example:

  <a href="jnlp:https://docs.oracle.com/javase/tutorialJWS/samples/deployment/dynamictree_webstartJWSProject/dynamictree_webstart.jnlp">Launch DynamicTree demo</a>
  

• Copy the JavaScript code from the HTML file generated by the Java Packager tool.

  If you are using the Java Packager tool, see Create the Web Page.

Java Web Start includes a protocol handler to handle the custom URI schemes jnlp: and jnlps:. Use these schemes as a direct way to start Java Web Start applications.

The protocol handler is automatically installed on Windows and macOS systems. It must be manually installed on Linux systems.

• Installing the Protocol Handler for Chrome

• Installing the Protocol Handler in Firefox