Object-Oriented Programming with Java Lecture Notes

17 April 2008 • Applets

Outline

Applets
- Java
- HTML
- Browsers
Web Start: network delivery and launch.

History

Java was becomming an orphan, but...
Hey, you got your JVM in my web browser!
- The result was HotJava.
- And thus a revolution was born.
Set-top box Java had, almost, what browser Java needed.
- Unfortunately, browser Java hasn't been able to catch up.

Broswer Java

The idea behind broswer Java was to run Java programs in a browser.
- Much more capable web pages (Java vs HTML).
- Elegant delivery (HTTP).
- A universal application platform (ha! ha! ha!).
Security was the sticking point.
- “What? You’ve made it easy for anybody to download and run arbitrary code on my system?”

Applets

Applets were the answer to the security sticking point.
- “Yes, but the code’s been neutered so it can’t do harmful things.”
- “And the code’s verified at the browser to make sure that’s so.”
An applet is a Java program customized for browsers.
- Web-page real estate.
- Security limited execution environment.

The Applet Class

applets in context

Applet abstracts web-page real estate.
- In this Applet correspond roughly to frames.
Applet contains AWT components; JApplet contains Swing and AWT components.
- Don't confuse Applet and JApplet.

Applets And Applets

An applet is a Java program in an Applet.
- Every browser Java program is an applet.
- Background applets have 0×0 real estate.
With a bit of careful structuring
- Applets can be programs.
- Programs can be applets, excepting dangerous operations.
- Code can be both applet and program.

Applet Life-Cycle

Applet defines four methods relating to instance life-cycle:

public void init() Applet loaded.

public void start() Applet visible.

public void stop() Applet occluded.

public void destroy() Applet gone.

Life-Cycle Example.

import java.applet.Applet

public class sa
extends Applet

  @Override public void init()
    System.out.println("in init()")

  @Override public void start()
    System.out.println("in start()")

  @Override public void stop()
    System.out.println("in stop()")

  @Override public void destroy()
    System.out.println("in destroy()")

Life-Cycle Example..

Page loaded.
Page unloded.

Life-Cycle Parts

init(): Called once after the applet's been loaded. Set-up the applet, acquire resources. Favor init() over a default constructor.
start(): Called when the applet becomes visible. Start doing whatever the applet does (if anything).
stop(): Called when the applet is no longer visible. Stop doing whatever the applet does (if anything).
destroy(): Called once when the applet's no longer needed. Release resources acquired by the applet.

Applet vs Program Structure

Assuming a graphical program, the corresponding applet has
- a web page, not a command line,
- a public JApplet subclass, not a JFrame,
- an init(), not a constructor,
- a start(), not a main(),
- a destroy(), not a setDefaultCloseOperation(),

Applet vs Program Attributes

Assuming a graphical program, the corresponding applet has
- height and width attributes, not setSize(),
- the <title></title> tag, not setTitle(),
- start() and stop(), not setVisible().
Also, these are under control of the browser, not the applet, which can only react.

Applet HTML

An applet is embedded in a standard HTML file using the applet tag.

<html>
  <head> <title></title> </head>
<body>
This is a background
<applet code = "sa.class" 
 height = 0 width = 0>
</applet>
applet.
</body></html>

Applet tags should appear in the body of an HTML document.

Applet Tag Attributes

There are twelve attributes specific to the applet tag.
Three important ones are:
- code: a URL to an applet class.
- height and width: the applet real-estate height and width in pixels (required).
```
<applet code = "sa.class" 
 height = 0 width = 0>
</applet>
```

Applet Class Loading

Class files are retrieved from the directory containing the HTML page.

$ls public-html
dragon-curve.html
DragonCurve.class                    
DragonCurve$LineSegmentIterator.class
DragonPanel$1.class                  
DragonPanel.class                    
DragonPanel$MouseHandler.class       
DragonPanel$KeyHandler.class       

$

Alternate Load Directories

The codebase attribute specifies an alternative directory for class files.
- codebase is like a one-entry class path.

Relative codebase paths start at the HTML page directory.

<applet code = "DragonCurve.class" 
 height = "500" width = "500"
 codebase = "../applets">

Class File Downloading

As is its wont, HTTP spends a connection per class file during download.
- This makes downloading expensive.

The archive attribute is a list of URLs for jar file containing supporting class files.

<applet code = "EaglesWing.main" 
 height = 500 width = 500
 archive = 
   "model.jar,view.jar,controller.jar">

Applet Parameters

An applet can read parameter values from the body of its applet tag:

<applet code = "show-quotes.class" 
 height = 10 width = 10>

  <param name = "count" value = "10"/>
  <param name = "ibm" value = "123.23"/>
  <!- blah blah blah ->
  <param name = "red hat" value = "19.41"/>

</applet>

Reading Parameters

The getParameter() method reads paramter values from the applet-tag body.
- It returns the string value or null for no non-existent parameters.
Parameters can be set by the server or by JavaScript working on the DOM.
- Although applets and JavaScript can communicate more directly.

The Object Tag

HTML 4.0 defines the object tag as a more general replacement for the applet tag.
- Being more general, it’s a bit more complicated than the applet tag.
- However, it generally operates the same way.
```
<object codetype = "application/java"
 classid = "java:DragonCurve.class"
 height = 100 width = 100>
</object>
```

Java And Browsers

Almost no broswers support Java natively (Hot Java was maybe the last one).
Instead, most browsers us a plug-in (dynamic library) to add the Java JVM.
If you installed a JRE (or JDK) the plug-in was installed along with the rest.
- You may have to configure the browser to find it.

Checking Browser Plugins

Use the about:plugins url to check your browser's set of installed plugins.
Other browesers should have similar capabilities.

Enabling Java

Once installed, Java usually has to be enabled.
I find enabling Java slows the browser down.

Java Console

The Java console is the brower equivalent of System.out and err.

JVM Caching

For performance, the JVM caches classes.
- Reloads can be satisfied out of the cache.
New versions of cached classes doesn't automatically invalidate the cache.
You have to
1. remember the cache is in the way, and
2. clear the cache manually with the x Java-console command.

Non-Java Browsers

If a browser doesn't have a Java plug-in, the applet tag is ignored.
- Add a helpful reminder to the applet-tag body.
If a browser has Java disabled, the applet tag is replaced by the alt-attribute value, if any.
- Add a helpful reminder as an alt-tag value.

Example

<applet code = ...
 alt = "[Enable Java to see this applet.]"
>

[Install the Java plug-in 
to see this applet.]

</applet>

Appletviewer

The appletviewer tool is a convenient alternative to using a browser to develop an applet.
- Appletviewer is part of the JDK.
However, you still need an HTML page to use appletviewer.
See the appletviewer man page for more details.

Applet Security

Applet security is based on two mechanisms:
- A sandbox to restrict applet access to resources.
- The JVM to check for and enforce sandbox restrictions.

Sandboxing

A sandbox is a constrained run-time environment.
The constraints prevent potentially harmful operations in the local environment.
- Limited or no access to local resources.
  - No file systems, device, or direct CPU access.
- Limited or no access to external communications.
- Clearly labeled responsibilities.

Applet Resources

What can a sandboxed applet access?
- Parts of the browser environment.
  - The keyboard and mouse.
  - The DOM and status bar.
- The serving host.
These are useful, but nearly all of what a useful applet might want to access.

The Security Profile and Manager

The security profile determines what the sandbox allows and disallows.
The security manager is responsible for ensuring behavior in the sandbox satisfies the security profile.
- The manager answers the question “can I do this?”
- The profile tells the manager the answer.
Both the security profile and security manager are Java classes, replacable with custom subclasses.

Signed Applets

Relaxed security should apply only to known (implies trusted) applets.
Java authentication is based on X.500 standards.
Jar files can be signed with a cryptographically secure certificate.
- The certificate reliably asserts the identity of the jar file’s issuer.
- The certificate says nothing about the jar file’s contents.

Web Start

Applets have failed, both as web-page enliveners and as an application delivery system.
Plug-ins have failed applets.
- It's a special-case requirement on both sides of the browser.
External Plug-ins, however, have been more successful.

External Plug-Ins

External plug-ins are just host-system programs run by the browser to deal with particular document types.

JNLP

Sun, not beating them, joined them.
The Java Network Launch Protocol (JNLP) is a
- File type identifying application loading and launching information.
- A browser-independent implementation of the Java sandbox.
JNLP programs can be more capable and browser-independent than applets can be.

JNLP Files

A JNLP file is an XML document containing information about application load and lanuch.
JNLP files are identified by
- A .jnlp extension, or
- An application/x-java-jnlp-file MIME type.
JNLP files are offered up by Web servers as just another document.

Web Start

javaws (Java Web Start) is the externally implemented sandbox.
The javaws sandbox is a bit less constrained than the in-browser sandbox.
- Greater local file access as well as network connectivity.
- Access to the clipboard, printer, and the browser.
- Signed code gets full access.

This page last modified on 10 April 2008.

`public void init()`		Applet loaded.
`public void start()`		Applet visible.
`public void stop()`		Applet occluded.
`public void destroy()`		Applet gone.