How To - Java Web Start for Alma


Java Web Start is a technology to deploy applications to end-users through a Web Browser Plug-in. By clicking a link the user can install and run applications on their local host. Java Web Start is smart enough to download newer versions of an application from the server whenever they are available. The Acs team uses it to deploy a Java-only Acs and various Acs tools (see AcsWebStart).


I. Porting

From Sun’s official FAQ:

Do I need to change my application to work with Java Web Start?

If your application is written to the Java 2 platform, and is delivered as a set of JAR files1, there should be no need to revise your application.
Make sure that all your application resources (such as images and resource bundles, for example) are retrieved from a JAR file2, since Java Web Start launches an application by invoking the public static void main(String[] args) method. 
And if your application needs unrestricted access to the system, (for example, network or disk access), you will need to sign your code.3


  1. All required jars will need to be listed in a jnlp-file. See II below.
  2. Put simple: Do not use java.io.File, instead use getClassLoader.getResource()
    For a full-flegded solution, see the resource loading toolkit Rachel in the References section below
  3. Explained in III below


II. Preparing a Jnlp file

The jnlp-file tells the Web Start Plug-in, besides some descriptive info, what jars need to be loaded from the server

Example: SuperApp.jnlp

<?xml version="1.0" encoding="utf-8"?>
<jnlp
  spec="1.0+"
  codebase="http://www.example.com/path/to/superapp">1
 
  <information>
    <title>Shortdescription</title>
    <vendor>Us, the Alma Team</vendor>
    <description>Longdescription shown in Web Start Manager</description>
   
    <offline-allowed/>2
 </information>
 
  <security>
    <all-permissions/>3
  </security>
 
  <resources>
    <j2se version="1.4+"/>

    <jar href="lib/myApplication.jar"/>4
    <jar href="lib/someLibrary.jar"/>

    <property name="SuperApp.isWebstarted" value="true"/>5

    <extension href="./Libraries.jnlp">6
     <ext-download ext-part="Libraries"/>
    </extension>

  </resources>

  <application-desc main-class="com.example.SuperApplication">
      <argument> -doExample </argument>
      <argument> -verbose </argument>
  </application-desc>
</jnlp>


The example is pretty straightforward, however, some points require attention:
  1. The codebase must match the location of the jnlp-file itself (due to how Web Start works). If this jnlp-file cannot be found under www.example.com/path/to/superapp/SuperApp.jnlp, it will not work.
  2. If your app doesn't require a network connection to run, put in <offline-allowed/>
  3. If your app needs access to the filesystem or to servers other than the one it was loaded from, put in <all-permissions/>**
  4. The jar myApplication.jar that contains the main-class com.example.SuperApplication has to be first in the sequence of <jar>s.
  5. You can define your own system property values as you like. Most java.* system properties are write-protected however for security reasons.
  6. If you have many libraries and if you like it modular, use sub-jnlp-files and include them as <extension>s

** In that case you'll also need to sign the jars. See III below.


III. Preparing the jars

If your app is considered security-critical by Java Web Start, you are required to sign your application jar files.

Things to consider:
  1. All jars specified in the same jnlp-file must use the same security signature1
  2. Each of the jars must have one and only one signature

Viewing existing signatures

To get information about the security signatures of one or more jars, use the command acsJarSignInfo coming with ACS.

> acsJarSignInfo
Scanning all jars in this directory tree for signatures...
1 (ESO) in ./acscomponent.jar
1 (ESO) in ./acsjlog.jar
1 (ESO) in ./archive_database.jar
1 (ESO) in ./archive.jar
1 (ESO) in ./archive_xmlstore_if.jar
1 (ESO) in ./cdbDAL.jar

Each jarfile in this directory has 1 signature and the names of the used certificates (ESO) are all the same - looks healthy.

Signing

To sign jars yourself, you first need a keystore. You can create one using the Java keytool in interactive mode:

> keytool  -genkey  -keystore MyKeyStore  -alias SuperAppDeveloper

Second, you sign each jar using the Java jarsigner:

> jarsigner  -keystore MyKeyStore  -storepass MySecretPassword   myApplication.jar   SuperAppDeveloper


  1. If this imposes a problem, think of modularizing your jnlp files into sub-jnlp-files using the <extension> tag.


IV. Create a nice Html page

... with a link to SuperApp.jnlp



V. Troubleshooting

Make your Java Web Start Manager log its output to a file. This will be valuable info for troubleshooting.
The following jnlp-file will launch the Manager so you can edit its configuration:

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




References

Java Web Start Home :  http://java.sun.com/products/javawebstart
Official FAQ :  http://java.sun.com/products/javawebstart/faq.html
Unofficial FAQ :  http://lopica.sourceforge.net/faq.html
Resource Loading Toolkit :  http://rachel.sourceforge.net
AcsWebStart :  http://www.eso.org/projects/alma/AcsWebStart



Version
 Author
Changes






2009-11-27
 Marcus Schilling
 Some info was outdated
2004-08-05
 Marcus Schilling
 Created