Week 10: Java Applets and Thread Example

Java Applets Overview

• Platform-independent class libraries
• Java's ability to "activite the Internet"
• Special Java program called Java applet
• Full-featured applets depend on mastering Java's networking abilities along with its ability to handle multiple threads (Info bus concept, RMI, etc)
• Usually applet will do some graphics or UI operations, so you might have to write relatively lot of code
• Tell the Java-enabled browser the following:

1. The name of the class file
2. The location of the class file
3. How the applet sits on the Web page (size, location, and so on)

Basic Applet Concept

• The Applet class
• java.applet package
• Not a part of AWT package, but is an AWT component as the inheritance chain

• Object <- Component <- Container <- Panel <- Applet
                                   <- Window <- Frame

• In order to have the applet working, you will need to have a .java file and a html file

Example:

import java.awt.*;
import java.applet.*;

public class ByeWorld extends Applet
{
	// Override the paint method
	public void paint(Graphics g)
	{
		Font f = new Font("System", Font.BOLD, 18);
		g.setFont(f);
		g.drawString("This is the final Intro to Java class", 25, 50);
	}
}


... Compile the .java file into .class file, then in the same directory, have a 
... html file the contains at least the following line: (More detail later on)

<APPLET CODE="ByeWorld.class" WIDTH=100 HEIGHT=100> </APPLET>

• We can used different Java enabled browsers, or the JDK appletviewer to test the applet
• Remember that there are different version of browsers in the field, different version of JDK (1.0 vs 1.1) and different version of Java (Sun Java vs MS Java)
• HotJava browser from Sun will be a good choice as well (Download from http://java.sun.com ... yes, it is slow)

Security Basics

• Two ways to run an applet

1. Open HTML files as local files from the browsers,
2. Download the HTML file from the net and then run with it

• Using option 1 or appletviewer will give applet almost no security restriction (P469 for detail)
• Option 2 might cause the applet security manager throws a SecurityException whenever an applet attempts to violate one of the access rule
• Actually, a security manager is loaded when the browser first tries to run a Java applet. And Java restricts that only one security manager is loaded and cannot be resetted.
• Iin theory, there is not way for an applet to override the security manager in order to access, say, local file system
  (But applications can set their own security manager)
• In most browsers

1. Applets can never run any local executable program
2. Applets can only communciate with the originating host (the host where the applet is downloaded)
3. On local file system can be accessed by the applet
4. Applet can only find out Java version, the name and version of the OS, file separator and line separator. Applet cannot find out user's name, email, etc.
   (JavaScript on the other hand can find out browser's history information, brower's version, etc)
5. Applets can only pop up windows with warning message (saying something like "Untrusted Java Applet Window")
6. Applets is said to be running in the "sandbox"

• To sum up, Java enforces security by the following

1. All program code is interpreted by the Java Virtual Machine, security is enforced within the java interpreter (as well as JIT compiler is you can converting into native code)
2. A security manager (just anothe Java class) checks all sensitive operations (operations defined in some interfaces) in the Java run-time library
3. Applets can be signed to identify their origin (supported by IE 4.0, Netscape 4.0 ?)
   (This is what Active X is using)

Life Cycle of an Applet

There are four conceptal "phases" in the life cycle (init, start, stop, destroy)

init

• Do initialization for the applet
• Like a constructor (called by the system/browser when the applet is launched for the first item)
• Process parameter (PARM from the HTML file)
• Declare all the UI components in here

start

• Call automatically after the init method
• Automatically call if user returned to the page with applet
• start can be called repeatly VS init only gets called once
• Used to restart threads
• Only implemented if your applet if doing something specials when user leave the page

stop

• Call automatically when user move off the current html page with applet(s)
• Used to stop time-consuming event (audio files, animation, calculations, etc)
• Only implemented if your applet if doing something specials when user leave the page

destroy

• Call automatically when the browser shuts down normally
• Used to reclaim non-memory-dependent resources such as graphics context
• Memory resource will be collected by the Java under the cover
• Normally, Java will call stop then destroy when the browser is shutting down

HTML Tag for Java

• The HTML file will look something like this:

<HTML>
<HEAD>
<TITLE>Week 10</TITLE>
</HEAD>

<BODY>
<H1>Java Test</H1>

<APPLET CODE="ByeWorld.class" WIDTH=100 HEIGHT=100></APPLET>

</BODY>
</HTML>

• WIDTH and HEIGHT are required in your HTML file
• It defines the size of the applet in pixels
• CODE is also a required tag which gives the name of the class, it could be a URL or a local directory
• Browsers usually do not allow you to resize the applet, so make sure you write once and test everywhere
• Other common tags are CODEBASE and ARCHIVE

<!-- If your applet is called ByeWorld.class and is in directory myDir and myDir is
     one level down from your current web page, then the following HTML code will be
     able to use to call the applet -->

<APPLET CODE="ByeWorld.class" CODEBASE="myDir" WIDTH=100 HEIGHT=100></APPLET>

...
...
...
<!-- If you have a jar file need to be loaded, you can use the ARCHIVE ...
     think about a jar files as a compressed file that will contains a bunch of 
     files for your applet -->
<APPLET CODE="Hi.class" ARCHIVE="lotus.jar, sinn/mySrc.jar" WIDTH=888 HEIGHT=188></APPLET>

<!-- For non java enabled browser, we can use the same old ALT tag to display information/error message -->

<APPLET CODE="hi.class" WIDTH=888 HEIGHT=188 ALT="Your browser is not java enabled, please download a new one"></APPLET>

JAR Files

For normal loading method of applet:
<APPLET CODE="hi.class" WIDTH = 400 HEIGHT=300></APPLET>

• The browser makes a connection to the web server and fetches hi.class file
• The class loader (with JVM in the browser) of the Java interpreter that is built into the browser then loads the hi.class from that file
• The class loader then, resolve and load other classes (say five more). Each additional class will be a connection to the Web server
• A very time consuming process

We can use the following to load all the class files at once:
<APPLET CODE="hi.class" ARCHIVE="hi2.jar" WIDTH=300 HEIGHT=500></APPLET>

• Only one http request will be made to the server (to load hi2.jar)
• All the class files is packed into hi2.jar
• JAR files can contain both class files and other types of files such as image and sound files
• JAR can be compressed again into ZIP file using PKWARE (The browser will unzip that for you ... better test it well)

In order to make a JAR file, use the jar tool under \jdk\bin

jar <options> File1 File2 ...

For example: jar cf hi2.jar *.java logo.gif

// c - create new archive, f - use 2nd parameter as JAR file name

Resources

• Common data files such as Image and sound files, Text message and binary data (mapping data) can be viewed as resource in Java
• Another good example will be the Copyright statement
• The resource file can be named, for example, copyright.txt
• It can then be located under

• The local file system, relative to the class path
• In a JAR file
• Or on a Web server

• Then, we can use the following to read in resource

URL myUrl = hi.class.getResource("copyright.txt");
InputStream in = url.openStream();

... or, put everything in one line ...

InputStream in = hi.class.getResourceAsStream("copyright.txt");

• Resource is often used in making internationalized applets and applications
• Since program logic is separated with messages
• Messages can be translated into different languages

Passing Information Into Applet

Applets have the ability to use parameters that are embedded into the HTML file. This is done by using the PARAM tag

<APPLET CODE="Hi.class" WIDTH=300, HEIGHT=200>
<PARM NAME=hidear VALUE="This is Week10">
</APPLET>

....
.... // Then, in the java code, we have ...

import java.applet.*;
import java.awt.*;

public class Hi extends Applet
{
	public void paint(Graphics g)
	{
		String inputString = getParameter("hidear");
		g.drawString("Finally ..." + inputString, 25, 50);
	}
}

Lab 1 (Applet)

Use the lab exercise from Week 9 and then convert that into an applet. Use Week10.java as the main class for this week and name the HTML file as week10.html

Note:

• There is no need to have a menu bar
• There is no need to have an inner class for window events
• There is no need to create a frame for main window, we are using the browser
• Only ButtonTextChanger.java ItemColorChanger.java, Week10.java and week10.html are needed
• Since we are using the lastest model for AWT in JDK 1.1. Netscape 4.0 will not be able to view our applet correctly. You can either use appletviewer from the JDK or MS IE4.0 to view the applet. (Now, you know one of the reasons why product reviews rank IE 4.0 higher then Netscape 4.0...)

Lab 2 - A Threaded Server Java Application

A Thread is a single sequential flow of control within a program. In Java, thread is in the user level (vs operating system level). Threads can be created easily in order to provide base for concurrent applications.

In this lab, we will use the java classes from Week 7 and create a threaded echo server that will be able to accept multiple clients.

Note:

• Extends the Thread class in order to achieve the purpose
• Overwrite the run method in the Thread class to put the main logic
• Build your own network protocol, so that a shut down command can be sent to the EchoServer
  (For example, runing "java EchoClient sinn" will shut down the server. Thus, the client will need to be changed as well)
• For each threaded server, if the input is QUIT, the thread should exit
• Three java classes EchoServer EchoClient and ThreadedServerHandler should be enough