Parts of this sound like the PERFECT candidate for a framework for a CJAN.
It probably sucks though. Anyone use it?
-jon
------ Forwarded Message
From: "JDC Tech Tips" <[EMAIL PROTECTED]>
Date: Wed, 30 May 2001 14:13:43 PDT
To: [EMAIL PROTECTED]
Subject: JDC Tech Tips May 30, 2001
J D C T E C H T I P S
TIPS, TECHNIQUES, AND SAMPLE CODE
WELCOME to the Java Developer Connection(sm) (JDC) Tech Tips,
May 30, 2001. This issue covers the Java(tm) Network Launching
Protocol (JNLP) and its reference implementation, Java(tm)
Web Start.
This tip was developed using Java 2 SDK, Standard Edition,
v 1.3.
This issue of the JDC Tech Tips is written by Stuart Halloway,
a Java specialist at DevelopMentor (http://www.develop.com/java).
You can view this issue of the Tech Tips on the Web at
http://java.sun.com/jdc/JDCTechTips/2001/tt0530.html
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THE JAVA NETWORK LAUNCHING PROTOCOL (JNLP) AND JAVA WEB START
During the early days of the Java Programming Language, much
emphasis was placed on client-side development. Support in the
language for applets and for the secure downloading of code
seemed ideal for delivering function over the World Wide Web.
However the reality is that the Java Programming Language's
greatest success has been on the server side. The language's
power and flexibility has won the hearts and minds of
server-side developers. Meanwhile, development on the client
side has lagged. Tricky deployment problems have limited the
utility of applets, and developers have been forced to turn to
browser-based "thin" clients.
The Java Network Launching Protocol (JNLP) promises to change all
that. Developed through the Java Community Process as JSR-56,
JNLP solves many of the previous problems in deploying Java
function to the client. A JNLP client is an application or
service that can launch applications from resources hosted on
a network. If you package an application with JNLP, then a JNLP
client can:
o Detect, install, and use the correct version of the Java
Runtime Environment for the application
o Launch the application from the browser or the desktop
o Automatically download newer versions of the application as
they become available
o Cache classes used by the application locally for fast startup
o Run as either an applet or an application
o Download native libraries if necessary
o Use local resources, such as the filesystem, in a secure way
o Automatically locate and load external dependencies
Sun Microsystems provides a reference implementation of JNLP
called Java Web Start. Let's use it to deploy a simple
application that uses JFC Swing classes. To do this, you will
need to download Java Web Start from
http://java.sun.com/products/javawebstart.
Here's the code for the application:
//File HelloJNLP.java
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
public class HelloJNLP extends JFrame {
public HelloJNLP() {
super("Hello JNLP");
String loadedFrom = this.getClass().getClassLoader().toString();
JLabel jl = new JLabel("loaded by " + loadedFrom);
JEditorPane jtp = new JEditorPane("text/plain",
"Edit this text");
getContentPane().add(jl, BorderLayout.NORTH);
getContentPane().add(jtp, BorderLayout.CENTER);
}
public static void main(String [] args) {
JFrame f = new HelloJNLP();
f.setBounds(100,100,325,250);
f.setDefaultCloseOperation(DISPOSE_ON_CLOSE);
f.setVisible(true);
f.addWindowListener(new WindowAdapter() {
public void windowClosed(WindowEvent e) {
System.out.println("Shutting down...");
System.exit(0);
}
});
}
}
At the core of JNLP is a deployment manifest. The deployment
manifest is an XML file with the extension .jnlp (the JNLP
specification simply calls it the "JNLP file"). To deploy the
HelloJNLP application, you need to describe it with a JNLP file,
like this:
<?xml version="1.0" encoding="UTF-8"?>
<!-- file Hello.jnlp -->
<jnlp codebase="http://staff.develop.com/halloway/TechTips/May2001";
href="http://staff.develop.com/halloway/TechTips/May2001/Hello.jnlp";>
<information>
<title>Hello</title>
<vendor>Tech Tips Sample May 2001</vendor>
<icon href="HelloJNLP.jpg"/>
</information>
<resources>
<j2se version="1.2+"/>
<jar href="HelloJNLP.jar"/>
</resources>
<application-desc main-class="HelloJNLP"/>
</jnlp>
This manifest contains all the information that a client needs
to download and use the HelloJNLP application:
o The jnlp element's codebase attribute specifies the top-level
URL to search for application resources.
o The information element specifies information that a JNLP user
interface can display to the client.
o The j2se element specifies that the client must have
version 1.2 or later of J2SE(tm). (This is a big improvement
over applet deployment, which is often stuck with whatever
VM the browser supplies.)
o The jar element specifies the location of the application JAR
file, relative to the jnlp codebase.
o The application-desc element specifies the class to run. You
can add subelements to specify command line arguments or
system properties.
To deploy the application to a web server, you need to execute
the following steps:
1. Change the URLs in the jnlp codebase and href attributes to
appropriate URLs for your web server.
2. Deploy the JNLP file to the web server.
3. Compile and JAR the HelloJNLP.java file, and deploy it to the
web server. For example:
jar cvf HelloJNLP.jar HelloJNLP.class HelloJNLP$1.class
4. Create an icon HelloJNLP.jpg and install it on the web server.
You can use the one at
http://staff.develop.com/halloway/TechTips/May2001/HelloJNLP.jpg
5. Set your web server's mime-type associations to map .jnlp to
the mime-type application/x-java-jnlp-file. For example, with
Apache, add the following to mime.types:
application/x-java-jnlp-file jnlp
6. Restart the web server.
To execute the application from a client machine, first make sure
that you have installed Java Web Start. Then simply point your
browser to the URL for the jnlp file. The Java Web Start client
will download the jnlp file, download the necessary resources,
and launch the application. What you should see is the text "Edit
this text" displayed in an editor. If you are having trouble
configuring the web server, or do not have access to a web server,
you can also launch this application from
http://staff.develop.com/halloway/TechTips/May2001/Hello.jnlp
Notice that HelloJNLP is not running as an applet inside the
browser, but as a separate, standalone application.
When you close the application, HelloJNLP uses System.out to
print the message "Shutting down..." However, no console is
visible. The console is one of many settings that Java Web Start
sets to "off" by default. This is one of a pair of settings that
you should change, as follows:
1. Edit the javaws.cfg file in the Java Web Start install
directory. Add the line "javaws.cfg.forceUpdate=true". This
causes Java Web Start to automatically check for newer
versions before starting an application.
2. Run the Java Web Start application. Under File->Preferences,
go to the Advanced Tab and select "Show Java Console". Also,
select "Log Output" and log output to a file of your choice.
This is very helpful when you are debugging and need to catch
System.out and System.err.
The HelloJNLP application displays an editor, but the editor's
contents are lost when you close the application. Add the
following code to HelloJNLP to automatically save the editor
state to the client hard drive:
//changes to HelloJNLP.java
import java.io.*;
import java.net.*;
import javax.jnlp.*;
//replace the constructor with this new version:
JEditorPane jtp;
public HelloJNLP() {
super("Hello JNLP, Second Version");
String loadedFrom = this.getClass().getClassLoader().toString();
JLabel jl = new JLabel("loaded by " + loadedFrom);
jtp = new JEditorPane("text/plain", "Edit this text");
readEditorContents();
getContentPane().add(jl, BorderLayout.NORTH);
getContentPane().add(jtp, BorderLayout.CENTER);
addWindowListener(new WindowAdapter() {
public void windowClosed(WindowEvent e) {
System.out.println("Shutting down...");
try {
writeEditorContents();
}
catch (Exception ex) {
System.out.println("Yoinks!");
ex.printStackTrace();
}
System.exit(0);
}
});
}
//add these helper methods
private void writeEditorContents() throws
UnavailableServiceException, IOException {
System.out.println("writeEditorContents");
PersistenceService ps = (PersistenceService)
ServiceManager.lookup("javax.jnlp.PersistenceService");
BasicService bs = (BasicService)
ServiceManager.lookup("javax.jnlp.BasicService");
URL baseURL = bs.getCodeBase();
System.out.println("CodeBase was " + baseURL);
URL editorURL = new URL(baseURL, "Editor");
try {
ps.create(editorURL, 1024);
}
catch (Exception e) {
e.printStackTrace();
}
FileContents fc = ps.get(editorURL);
DataOutputStream os = new DataOutputStream(
fc.getOutputStream(false));
String s = jtp.getText();
os.writeUTF(s);
os.flush();
os.close();
}
private void readEditorContents() {
try {
PersistenceService ps = (PersistenceService)
ServiceManager.lookup("javax.jnlp.PersistenceService");
BasicService bs = (BasicService)
ServiceManager.lookup("javax.jnlp.BasicService");
URL baseURL = bs.getCodeBase();
URL editorURL = new URL(baseURL, "Editor");
FileContents fc = ps.get(editorURL);
DataInputStream is = new DataInputStream(fc.getInputStream());
jtp.setText(is.readUTF());
is.close();
}
catch (Exception e) {
e.printStackTrace();
}
}
The JNLP API defines a set of services that bypass the security
sandbox to enable some common client operations. In the
writeEditorContents method, the BasicService discovers the
application's codebase. Then, the PersistenceService caches the
edit pane's contents on the local hard drive, keyed to a URL that
is relative to the application's codebase. The name/value pairs
provided by the PersistenceService are similar to browser
cookies. The Java Web Start implementation honors this legacy by
naming the pairs "muffins." Muffins are not appropriate for large
data storage; they should be used to cache small identifiers on
the client. These identifiers can then be used to locate larger
pieces of information on the server.
Redeploy the changed application to the web server, and try
running it from the client -- again by browsing to the URL. If
you do not have a web server, you can run this version from the
URL http://staff.develop.com/halloway/TechTips/May2001/Hello2.jnlp
Java Web Start automatically detects that that application has
changed and runs the newer version. You can confirm this by
checking the title bar string, which now should say "Hello JNLP,
Second Version." Make some changes to the contents of the editor,
and then close it. When you launch the application again, your
changes should be visible. (You may see an exception in the
console output the first time you run the new version. This is
expected behavior because readEditorContents will not find
a muffin the first time.)
JNLP provides many more services than those shown here. For
example, you can:
o Finely control how applications are downloaded
o Describe dependency relationships between JARs
o Download and run native code installers
o Grant additional permissions to signed code
o Request specific versions of applications or applets
To learn more about JNLP, download the JNLP specification at
http://java.sun.com/products/javawebstart/download-spec.html
To learn more about Java Web Start, see the Java Web Start page
at http://java.sun.com/products/javawebstart/
. . . . . . . . . . . . . . . . . . . . . . .
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May 30, 2001
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