In order for an application to be launched from a web page via JNLP, the page
must include a link to the JNLP file. E.g., to be able to launch application
app.jnlp on a web site http://www.MySite.com, the page
needs to include the following link:
<a href=http://www.MySite.com/app.jnlp>Launch the application</a>
It may be the case, however, that Java Web Start is not installed on the user's computer. Thus the page needs to include logic (scripts) to take account of this. In fact, the page should include logic for the following:
Detect if Java Web Start is installed
The scripts, and the HTML for the auto-install page, are discussed below.
Here is the first script that should be run on a web page for launching an application via JNLP:
<SCRIPT LANGUAGE="JavaScript">
var javawsInstalled = 0; var javaws142Installed=0;
var javaws150Installed=0;
isIE = "false";
if (navigator.mimeTypes && navigator.mimeTypes.length) {
x = navigator.mimeTypes['application/x-java-jnlp-file'];
if (x) { javawsInstalled = 1;
javaws142Installed=1; javaws150Installed=1;
}
}
else {
isIE = "true";
}
</SCRIPT>
This script looks at the navigator.mimeTypes object and the navigator.mimeTypes.length
var to decide if the browser is Netscape or IE. If length is 0,
it is assumed the browser is IE, as with IE the navigator.mimeTypes
array is defined but always empty. If length is non-zero, then the browser is
assumed to be Netscape and the JNLP MIME type is checked to see if it exists
on Netscape. If so, javawsInstalled, javaws142Installed, and javaws150Installed are all
set to 1. With Netscape it is not possible to determine which particular version
of Java Web Start is installed, so all four variables are set to 1.
The above JavaScript should be followed by a VBScript that sets variables related to Internet Explorer browers:
<SCRIPT LANGUAGE="VBScript">
on error resume next
If isIE = "true" Then
If Not(IsObject(CreateObject("JavaWebStart.isInstalled"))) Then
javawsInstalled = 0
Else
javawsInstalled = 1
End If
If Not(IsObject(CreateObject("JavaWebStart.isInstalled.1.4.2.0"))) Then
javaws142Installed = 0
Else
javaws142Installed = 1
End If
If Not(IsObject(CreateObject("JavaWebStart.isInstalled.1.5.0.0"))) Then
javaws150Installed = 0
Else
javaws150Installed = 1
End If
End If
</SCRIPT>
This VBScript is executed if the variable isIE from the preceeding
JavaScript is "true"; i.e., if the end-user's browser is Internet Explorer.
This script instantiates the isInstalled COM object in JavaWebStart.dll,
and this object determines four things:
After the above two scripts have been executed, the variables javawsInstalled, javaws142Installed, and javawsInstalled150
will be set to either 1 or 0, as follows:
javawsInstalledjavaws142Installedjavaws150InstalledInternet Explorer 1 if any version of Java Web Start is installed; 0 otherwise. 1 if Java Web Start 1.4.2 is installed; 0 otherwise. 1 if Java Web Start 1.5.0 is installed; 0 otherwise. Netscape Navigator 1 if any version of Java Web Start is installed; 0 otherwise. 1 if any version of Java Web Start is installed; 0 otherwise. 1 if any version of Java Web Start is installed; 0 otherwise.
An additional JavaScript can be used to decide whether to:
The following JavaScript handles these scenarios:
<script language="JavaScript"> /* Note that the logic below always launches the JNLP application
*if the browser is Gecko based. This is because it is not possible
*to detect MIME type application/x-java-jnlp-file on Gecko-based browsers.
*/ if (javawsInstalled || (navigator.userAgent.indexOf("Gecko") !=-1)) { document.write("<a href=http://www.MySite.com/app.jnlp>Launch the application</a>"); } else { document.write("Click "); document.write("<a href=http://java.sun.com/PluginBrowserCheck? pass=http://www.MySite.com/download.html& fail=http://java.sun.com/j2se/1.5.0/download.html>here</a> "); document.write("to download and install JRE 5.0 and the application."); } </SCRIPT>
Notes:
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If javawsInstalled is 1, indicating that Java Web Start is already available
on the client, then the script provides a link to the application's jnlp
file. If Java Web Start is not installed on the client, the script instead provides a
link to the PluginBrowserCheck program on the java.sun.com
web site. PluginBrowserCheck checks whether the client uses Internet
Explorer on a Microsoft Windows platform. If so, PluginBrowserCheck
sends the user to the auto-install page http://www.MySite.com/download.html.
(See the next section, Creating an auto-install page,
for how to create an auto-install page for IE running on Windows.) If PluginBrowserCheck
determines the user is not using Internet Explorer on Microsoft Windows, the
user is redirected to the 5.0 JRE general download page on java.sun.com.
Note: The For a complete list of JRE releases that can be autodownloaded via a
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The download.html file should be staged on the server side. It contains special OBJECT and PARAM tags that will download to the client an auto-installer for JRE 5.0 . Along with Java Web Start, an ActiveX control will be downloaded to the client. The ActiveX control will launch the application using the newly installed Java Web Start. Here is a sample download.html file:
<HTML> <BODY> <OBJECT codebase="http://java.sun.com/update/1.5.0/jinstall-1_5_0-windows-i586.cab" classid="clsid:5852F5ED-8BF4-11D4-A245-0080C6F74284" height=0 width=0> <PARAM name="app" value="http://www.MySite.com/app.jnlp"> <PARAM name="back" value="true"> <!-- Alternate HTML for browsers which cannot instantiate the object --> <A href="http://java.sun.com/j2se/1.5.0/download.html"> Download Java Web Start</A> </OBJECT> </BODY> </HTML>
This OBJECT tag fetches a .cab file that contains an
auto-installer for JRE 5.0. The PARAM tags specify the location of
the application's jnlp file so that it may be automatically launched
after the JRE is installed on the client.
For issues relating to application development see the next chapter, Application Development Considerations.