спецификации, руководства, описания, API

Creating the Web Page That Launches the Application

This chapter includes the following topics:

Introduction
Detecting if Java Web Start is installed on Netscape
Detecting if Java Web Start is installed on IE, and if so, the version number
Launching the application if Java Web Start is Installed—or providing a link for auto-install or general download page
Creating an auto-install page

Introduction

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

If so, launch the application.
If not, detect if user is running IE on Window.
- If so, provide link to a page that can auto-install the JRE for Windows
- If not, provide a link to the general download page for the JDK/JRE.

The scripts, and the HTML for the auto-install page, are discussed below.

Detecting if Java Web Start is installed on Netscape

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;
var javaws160Installed = 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;
      javaws160Installed = 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, javaws150Installed and javaws160Installed 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.

Detecting if JavaWeb Start is installed on IE, and if so, the version

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  
  If Not(IsObject(CreateObject("JavaWebStart.isInstalled.1.6.0.0"))) Then
     javaws160Installed = 0
  Else
     javaws160Installed = 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:

whether the client machine has any version of Java Web Start installed;
whether the client machine has version 1.2 of Java Web Start installed;
whether the client machine has versions 1.4.2 of Java Web Start installed;
whether the client machine has versions 1.5.0 of Java Web Start installed.
whether the client machine has versions 1.6.0 of Java Web Start installed.

After the above two scripts have been executed, the variables javawsInstalled, javaws142Installed, javawsInstalled150 and javawsInstalled160 will be set to either 1 or 0, as follows:

Browser	`javawsInstalled`	`javaws142Installed`	`javaws150Installed`	`javaws160Installed`
Internet 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.	1 if Java Web Start 1.6.0 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.	1 if any version of Java Web Start is installed; 0 otherwise.

Launching the application if Java Web Start is Installed—or providing a link for auto-install or general download page

An additional JavaScript can be used to decide whether to:

provide a link to the application's jnlp file (i.e., Java Web Start is installed);
initiate auto-download of JRE 6.0, which includes Java Web Start (i.e., Java Web Start is not installed and the user is running IE on Windows);
or provide a link to the general download page for the 6.0 JDK/JRE (i.e., Java Web Start is not installed and the user is not running IE on Windows).

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/javase/downloads/ea.jsp>here</a> ");
    document.write("to download and install JRE 5.0 and 
        the application.");
}
</SCRIPT>

Notes:

The script only uses javawsInstalled—not javaws142Installed or javaws150Installed or javaws160Installed.
The line breaks following '?' and '&' are for readability purposes only; in an actual script there should be no breaks in the href string.

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 6.0 JRE general download page on java.sun.com.

Creating an auto-install page

Note:

The.cab file in the codebase attribute below will not be available until the GA release of the 6.0 JDK/JRE.

For a complete list of JRE releases that can be autodownloaded via a .cab file, as mentioned below, see Autodownload Files (Windows Only).

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 6.0 . 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/1.6.0/jinstall-6-windows-i586.cab#Version=6,0,0,0" 
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/javase/downloads/ea.jsp">
Download Java Web Start</A>
</OBJECT>
</BODY>
</HTML>

The OBJECT tag fetches a .cab file that contains an auto-installer for JRE 6.0.

The URL: http://java.sun.com/update/1.6.0/jinstall-6-windows-i586.cab will return the latest update release available for the Java SE 6 release.

The string: Version=6,0,0,0 will be used to compare with the wsdetect.dll version, which is registered with the CLSID above. If the local machine has the dll installed and the version is greater than or equal to the Version specified, we will just load the dll in the local machine. Otherwise it will download and install Java from the codebase specified.

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.

app: Once the Java Web Start Active-X control is installed/loaded, it will invoke Java Web Start to launch the application specified by this URL.

back: This controls the behavior of the browser after launching the application. It can remain on the current page (download.html in this case) or navigate back to the previous page.

For issues relating to application development see the next chapter, Application Development Considerations.