Java Web Start allows developers to deploy Swing desktop applications over the web. It works by packaging the application code and resources into JAR files along with a deployment descriptor (JNLP file). When end users access the JNLP file through a web browser, Java Web Start automatically installs and launches the application similar to a desktop program. This allows Zhara Tech to develop their POS application as a rich client using Java/Swing but deploy it via a web application for centralized updates and management. They faced challenges around database synchronization and logging which they overcame through custom servlets and configuration.

1. Zhara Tech Talks – Session 03 19th May 2011
Java Web Start
How Zhara POS Works
Yohan Liyanage
BSc (Hons) MBCS MIET MIEEE SCJP SCWCD

2. Zhara POS - What We Needed
Initial Prototype Design for POS – A Web Application

3. And We Needed More !
• A desktop-like application, with high responsiveness
• Direct integration with local resources (ex. Printers)
• 100% keyboard driven interface
• High availability
• Support for independent operations in offline mode
when connection to PMS is not available
• Centralized Management
– Automated Upgrades
– User Accounts
– Settings
– All financial data captured to be transferred to PMS

4. What we thought of…
• A pure web application will not be able to address the
requirement.
• The initial design in the prototype is complex. We
decided to re-design it, breaking down to smaller, task
specific UIs.
• We looked at…
– HTML 5 / Google Gears
– Flex / Flash Applications
– Java Applets
– Java Web Start

5. We thought of… HTML 5 & Gears
• HTML 5 supports local databases in browser to reduce
network overhead and to support fail-over, etc.
• It’s a web application, it runs on browser, as if it’s a local
application.
• But it is not yet materialized properly, and browser
compliance is minimal.
• It is still web based and the responsiveness issue and
printer integration is in question.
• Google Gears is a similar technology, but it is no longer
under active development due to HTML 5’s arrival.

6. We thought of… Flex & Flash
• Flash & Flex based applications could provide highly
responsive GUIs.
• Flash supports writing to local files, which we could use
to facilitate failover.
• Can integrate with a Java back-end through Spring Flex /
BlazeDS Integration.
• Requires Flash to be installed on client machine.
• Our teams are Java based. Lack of development
expertise, and on-going maintenance (development) will
be a problem.

7. We thought of… Java Applets
• Java Applets are capable of being integrated into the
web browser (thus it supports web based deployment),
and also to deliver near desktop application GUIs.
• Signed Applets can obtain access to local disk and
printers.
• Developers who are proficient in Java can easily learn
and use applets
• But applets are executed in a restricted environment
– sandbox – signed applets can get more privileges, but still it is
not like a desktop application.
• It requires that the browsers have Java plug-in installed.

8. Meet Java Web Start
• Develop a full-blown desktop application with Swing, and
deploy it over the Web !
• Supports automated installation
– Start Menu, Desktop shortcuts.
• Supports automatic updates
• No restrictions in execution environment (for a signed
application), it run’s as a standalone application.
• Only difference is in the way we deploy / install our
application.
• But it requires Java Runtime Environment on client
machines.

9. Zhara POS & Web Start
 A Swing based desktop application with high
responsiveness
 Direct integration with local resources (ex. Printers)
 100% keyboard driven interface
 High availability
 Supports offline execution
 Centralized Management
 Deployment through a Web Application (WAR). Updates can be
also deployed through this.
 Communicate with PMS through a Web Service for
synchronization of data.
 Installation of JRE is feasible for our requirement

10. Java Web Start – How it works - 1
• A Java Web Start application is same as a normal Java
application bundled in a JAR file.
• But it is deployed with a special deployment descriptor
called the JNLP file.
– JNLP – Java Network Launching Protocol
• We need to bundle the JAR file and the JNLP file into a
WAR (Web Archive), and then we can deploy it to any
Web Container / Application Server.
• The user is given a URL to access the JNLP file.
• When the user points the browser to that URL, the
application gets installed automagically !

11. JWS – Deployment Structure
• The structure of the WAR file is as follows (can be
different).
• jwsapp.war
− index.jsp (not required. Can be used to give a friendly link to JNLP)
− jwsapp.jnlp
− jwsapp.jar (this is the main application JAR file)
− lib
− lib1.jar (not required – if there are any libraries, we can place those like this)

12. The JNLP File
• It’s a XML document like the well known web.xml, but
much simpler !

13. Steps to make & run a Java WS Application
• Write your application like you would normally, and
make a JAR file.
• Optionally, if you want un-restricted environment, you
need to sign all your JAR files, including the libraries you
use.
– You can use the jarsigner tool in JDK for this. Simply, execute
signjar as follows for all your JAR files.
• jarsigner –keystore mystore –storepass password jar-file.jar alias
• Read more at
http://java.sun.com/developer/Books/javaprogramming/JAR/sign/signing.html
• Write your JNLP file.
• Deploy it as a WAR file in web container.
• Point your browser to the JNLP URL.

14. Running a Java WS Application
• Point your browser to the JNLP file URL. Make sure you
open the file with Java Web Start Launcher.
• Then the application will get installed (if not installed
already), and application will start to run.

15. Under-the-Hood
• How does Java Web Start do it’s magic ?
• A special application bundled with JRE called javaws.exe
(or just javaws in *nix) gets invoked with the URL of the
JNLP File.
• The javaws application reads the JNLP file, and starts
downloading each JAR file mentioned in the JNLP
document.
• Once all dependencies are satisfied, javaws executes the
jar file marked with main=true flag in JNLP, with the
system properties given (if any).
• There’s much more to this (including security / sandbox,
etc), but this is what happens in a nutshell.

16. There’s More – The JnlpDownloadServlet
• There’s a special servlet called JnlpDownloadServlet in a
jar file distributed with JDK called jnlp-servlet.jar.
• If you add this servlet to your web application (through
web.xml), you can use special place-holder values in the
JNLP file.
– $$codebase – URL of the web application
– $$name – Name of the JNLP file
• Automatic JAR Diff Generation
– When a JAR file is updated, and a client requests for the
updated JAR file, only the difference between the version client
has and the new version (the diff) will be transferred to save
network bandwidth.

17. There’s More – The JnlpDownloadServlet (2)
• Pack200 Compression
– The servlet supports compressed JAR files using Pack200
compression algorithm, which is geared towards compressing
Java Byte Code.
– Pack200 compression can reduce the JAR file size up to 50%
less. The program is included with JDK since 5.0 (pack200.exe).
– You need to manually compress and include the file into the
WAR file, along with the uncompressed version.

18. Zhara POS & Web Start
• We developed Zhara POS as a normal desktop
application, and we execute an embedded H2 database
to keep the local data.
• POS interfaces with the PMS through two web services in
PMS, one for uploading data from POS, and one for
synchronizing PMS changes.
• Once the application was ready, we signed all JAR files
with a custom certificate, and deployed it to JBoss after
bundling it to a WAR file.
• Now the POS application can be installed via Web Start.
Any updates can be deployed to JBoss with updated
WAR file, and all clients will be updated on next access.

19. But…. We faced challenges on the way !
• How to send and update the database ?
– We need a mechanism to transfer the blank database when a
POS client is initially installed.
– Also, any database changes related to a update also need to be
pushed when the application update happens. Web Start auto-
update is only for the Java code in JAR files.
• Log4J Log Files
– We need to store the log files in a specific location. Current
directory is not reliable under Java WS execution.
• POS relies on special set of parameters, which are
managed in PMS, and synchronized to clients.
– All settings of the POS is stored in this way, and we need to
send these parameters upon initial startup of a POS client.
– Also, we need to ensure that people from outside cannot install
and run POS clients (our application is exposed to Internet).

20. Challenges - Database
• We stored a blank database with in the WAR file, and
wrote a servlet to transfer this database to POS client.
– When the POS client starts up for the first time, it checks for the
existence of database in a predefined location (C:ZharaPOS in
Windows, /opt/zharapos in Linux).
– If it is not available, the client talks to this servlet, and
automatically downloads the DB to that location. If the location
does not exist at all, it will be created on the fly.
• We needed to transfer database changes to client.
– We maintain a file called update-scripts.xml in the WAR file. This
XML file contains SQL scripts related to each database update,
along with the version number of the update.

21. Challenges – Database (2)
• The update-scripts.xml is as follows.

22. Challenges – Database (3)
– In each POS database, we maintain the database version in a
table called _DB_VERSION.
– When the POS client starts up in Web Start Mode, the required
database version is passed in as a System Property specified in
the JNLP file.
– The client checks the version in the database with the version
required.
– If the version required is new, it contacts a special servlet in the
WAR file passing the current version of the database.
– The servlet then reads the update-scripts.xml file, and creates a
single SQL script for updating the database to latest version and
sends to client.
– The client then executes this script before starting up the Spring
Container with Hibernate.

23. Challenges – Log4J
• POS uses Log4J for logging purposes. By default it writes
to a /log directory in the POS application directory.
• But the application directory under Java Web Start is
unknown (not reliable).
• So we created a special Java based Log4J configurator,
which triggers when application runs in Web Start Mode.
– This configurator configures Log4J to write to log files into the
same directory mentioned before (C:ZharaPOS or
/opt/zharapos).

24. Challenges – Initial Sync & Setup
• For initial synchronization, we decided to ask the user to
enter a special code which is unique to a POS instance
configured in PMS.
– This code is randomly generated at the creation of POS
instances.
– The code is visible to authorized administrators to create POS
instances.
– When the administrator enters this code, all parameters related
to POS is downloaded, and a initial synchronization sequence is
triggered.
• This basic security mechanism also ensures that only
authorized users can install and run Zhara POS.
– This can be improved later on to add on more security.

25. Things to be done…
• We are currently working on the following
– Creation of a Control Panel in PMS side, which allows to issue
commands to POS clients. For example,
• Send log files since a given time
– When the command is issued, the POS client will upload the selected
log files in compressed form to PMS. Then PMS will email the log files
to a defined address.
• Change log levels
– Dynamically change log levels, for example from INFO to DEBUG, etc.
• Request Restart
– The POS user will be notified to restart the POS client.
• Etc.
– These commands will be ‘piggy-backed’ when the POS client
synchronizes with PMS (periodic synchronization).
– All POS Parameters are already getting synchronized when
updated centrally in PMS.

26. Questions ?

27. Thank You
References
 http://download.oracle.com/javase/6/docs/technotes/guides/javaws/developersguide/downloadservletguide.html
 http://en.wikipedia.org/wiki/Java_Web_Start
 http://download.oracle.com/javase/tutorial/deployment/webstart/