Transcript Chapter 23 Applets and Java Web Start

Java How to Program, 9/e
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© Copyright 1992-2012 by Pearson
Education, Inc. All Rights Reserved.
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Education, Inc. All Rights Reserved.
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This chapter introduces applets—Java programs that are
typically embedded in XHTML (Extensible HyperText
Markup Language) documents.
When a Java-enabled web browser loads a web page
containing an applet, the applet downloads into the browser
and executes.
The application in which an applet executes is known as the
applet container.
The applet container loads the applet’s class(es), creates an
instance of the applet and manages its life cycle.
◦ The Java Development Kit (JDK) includes one called the
appletviewer for testing applets as you develop them and before
you embed them in web pages.
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Some browsers don’t come with the plug-in by default.
You can visit java.com to determine whether your
browser is ready to execute Java applets.
If not, you can click the Free Java Download button to
install Java for your browser.
We demonstrate applets using both the appletviewer
and web browsers, which execute Java applets via the Java
Plug-In.
Java Web Start and the Java Network Launch Protocol
(JNLP) enable you to package your applets and applications
so that they can be installed onto the user’s desktop.
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Demonstration applets provided with the JDK.
◦ Each sample applet comes with its source code.
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The demo directory contains several subdirectories.
The applets directory contains demonstration applets.
The jfc (Java Foundation Classes) directory contains
applets and applications that demonstrate Java’s powerful
graphics and GUI capabilities.
Change to the applets directory and list its contents to
see the directory names for the demonstration applets.
Figure 23.1 provides a brief description of each demo
applet.
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This TicTacToe demonstration applet allows you to play TicTac-Toe against the computer.
Change directories to subdirectory TicTacToe, where you’ll
find the file example1.html that loads the applet.
Execute the applet with the command
appletviewer example1.html
◦ Loads the XHTML document example1.html specified as its
command-line argument.
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Can also open the XHTML document in your browser.
To play again, click the appletviewer’s Applet menu and
select the Reload menu item (Fig. 23.3), or click the applet again
when the game is over.
To terminate the appletviewer, click the appletviewer’s
Applet menu and select the Quit menu item.
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The DrawTest applet allows you to draw lines and
points in different colors.
In the command window, change directories to
directory applets, then to subdirectory DrawTest.
In the command window, type the command
◦ appletviewer example1.html
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The Java2D applet demonstrates many features of the
Java 2D API (which we introduced in Chapter 14).
Change directories to the jfc directory in the JDK’s
demo directory, then change to the Java2D directory.
In the command window, type the command
◦ appletviewer Java2Demo.html
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Every Java applet is a graphical user interface on which
you can place GUI components using the techniques
introduced in Chapter 14 or draw using the techniques
demonstrated in Chapter 14.
In this chapter, we’ll demonstrate drawing on an applet.
Examples in Chapters 24 and 27 demonstrate building
an applet’s graphical user interface.
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Class JApplet (package javax.swing) is used to create
applets.
Every applet contains at least one public class declaration.
An applet container can create only objects of classes that are
public and extend JApplet (or its superclass Applet).
An applet container expects every applet to have methods named
init, start, paint, stop and destroy, each of which is
declared in class JApplet.
◦ These methods can be overridden (redefined) to perform tasks that are
specific to your applet.
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When an applet container loads class WelcomeApplet, the
container creates a WelcomeApplet object, then calls its
methods init, start and paint in sequence.
Inheriting the “default” versions of these methods guarantees that
the applet container can execute each applet uniformly.
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Override method paint to draw on an applet.
Method paint receives a parameter of type
Graphics, which is used to draw graphics on the
applet.
The applet container calls paint to tell the applet
when to draw, and the applet container is responsible
for passing a Graphics object as an argument.
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Applets are embedded in web pages for execution in an
applet container (appletviewer or a browser).
You must create an XHTML document that specifies
which applet to execute in the applet container.
An applet element tells the applet container to load a
specific applet and defines the size of its display area
(its width and height in pixels) in the applet container.
The applet and its corresponding XHTML document
are normally stored in the same directory on disk.
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When an applet container encounters an applet
element in an XHTML document, it loads the applet’s
.class file (or files) from the same location that
contains the XHTML document.
The applet element has several attributes.
Attribute code indicates the applet’s .class file.
Attributes width and height specify the dimensions
of the applet.
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To execute an applet in Firefox, perform the following
steps:
◦ Select Open File… from the File menu.
◦ In the dialog box that appears, locate the
directory containing the XHTML document
for the applet you wish to execute.
◦ Select the XHTML document.
◦ Click the Open button.
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The steps for executing applets in other web browsers
are similar.
In most browsers, you can simply type Ctrl + O to
open a dialog that enables you to select an XHTML
document from your local hard disk.
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Five applet methods are called by the applet container
from the time the applet is loaded into the browser to
the time it’s terminated by the browser.
These methods correspond to various aspects of an
applet’s life cycle.
Figure 23.8 lists these methods, which are inherited
into your applet classes from class JApplet.
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Our next applet (Fig. 23.9) computes the sum of two
values entered into input dialogs by the user and
displays the result by drawing a String inside a
rectangle on the applet.
The sum is stored in an instance variable of the
AdditionApplet class, so it can be used in both the
init method and the paint method.
The XHTML document that you can use to load this
applet into an applet container (i.e., the
appletviewer or a web browser) is shown in
Fig. 23.10.
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For security reasons, it’s generally considered
dangerous to allow applets or any other program that
you execute from a web browser to access your local
computer.
So, you must decide whether you trust the source.
Most of what you do with your web browsers requires
you to trust the sites you visit and to trust the
organizations that maintain those sites.
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Applets are typically downloaded from the Internet.
What would happen if you downloaded a malicious applet?
A browser downloads and executes a Java applet automatically—the
user is not asked for approval.
In fact, an applet typically downloads without the user’s knowledge—
it’s just another element of the web page the user happens to be
visiting.
To combat malicious code, the Java platform uses a so-called sandbox
security model that provides a mechanism for executing downloaded
code safely.
Such code executes in the “sandbox” and is not allowed to “play
outside the sandbox.”
By default, downloaded code cannot access local system resources, and
an applet can interact only with the server from which the applet was
downloaded.
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The sandbox makes it difficult for applets to perform useful
tasks.
It’s possible, however, for an organization that wishes to create
applets with access to the local system to obtain a security
certificate (also called a digital certificate) from one of several
certificate authorities.
Can then use tools provided with the JDK to “digitally sign” an
applet that requires access to local system resources.
When a user downloads a digitally signed applet, a dialog
prompts the user asking whether he or she trusts the applet’s
source.
If so, only then will the applet be able to access to the local
computer’s resources.
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Java Web Start is a framework for running downloaded
applets and applications outside the browser.
Typically, stored on a web server for access via the
Internet, but can also be stored on an organization’s
network for internal distribution, or even on CDs,
DVDs or other media.
Java Web Start enables you to ask the user if a
downloaded program can have access to the resources
of the user’s computer.
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Desktop integration: Users can launch robust applets and
applications by clicking a hyperlink in a web page, and can
quickly and easily install the programs on their computers.
◦ Can be configured to ask the user if a desktop icon should be created so
the user can launch the program directly from the desktop.
◦ Downloaded programs can also have an “offline mode” for execution
when the computer is not connected to the Internet.
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Automatic updating: Java Web Start programs are downloaded
and cached (stored) on the user’s computer. The next time the
user executes that program, Java Web Start launches it from the
cache. If the program has been updated, Java Web Start can
automatically download the updates, so a user always has the
most up-to-date version.
◦ This makes installation and updating software simple and seamless to
the user.
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Draggable applets: New in Java SE 6 Update 10. With
a small change to the applet element that invokes an
applet from an XHTML document, you can allow users
to execute an applet in its own window by holding the
Alt key and dragging the applet out of the web browser.
The applet continues to execute even after the web
browser closes.
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A Java Network Launch Protocol (JNLP) document
provides the information that Java Web Start needs in
order to download and run a program.
Must package your program in one or more Java
archive (JAR) files that contain the program’s code and
resources.
By default, programs launched via Java Web Start
execute using the sandbox security model.
If the user gives permission, such programs can access
the local file system, the clipboard and other services
via the JNLP APIs of package javax.jnlp.
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To package the JDK’s DrawTest demonstration applet so
that you can execute it via Java Web Start, you must first
wrap the applet’s .class files and the resources it uses (if
any) into a Java archive (JAR) file.
In a command window, change to the DrawTest directory,
as you did in Section 23.2.
Once in that folder, execute the following command:
◦ jar cvf DrawTest.jar *.class
creates a JAR file in the current directory named
DrawTest.jar containing the applet’s .class files.
If the program had other resources, you’d simply add the
file names or the folder names in which those resources are
stored to the end of the preceding command.
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The letters cvf are command-line options to the jar
command.
The c option indicates that the command should create
a new JAR file.
The v option indicates that the command should
produce verbose output so you can see the list of files
and directories being included in the JAR file.
The f option indicates that the next argument in the
command line (DrawTest.jar) is the new JAR
file’s name.
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Next, create a JNLP document that describes the
contents of the JAR file and specifies which file in the
JAR is the so-called main-class that begins the
program’s execution.
For an applet, the main-class is the one that
extends JApplet.
For an application, the main-class is the one that
contains the main method.
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JNLP documents are written in Extensible Markup
Language (XML).
JNLP is a so-called XML vocabulary that describes the
information Java Web Start needs to launch a program.
In Fig. 23.12, the jnlp element (lines 2–26) is the root
element-.
The jnlp element’s start tag (lines 2–4) has two
attributes—codebase and href.
◦ The codebase attribute’s value is a URL that specifies the path
where the JNLP document and the JAR file are stored.
◦ The href attribute specifies the JNLP file that launches the
program.
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Typically, the codebase references a directory on a
web server with an http:// URL.
If you’d like to serve your applet or application from a
web server so users can access it online, you’ll need to
configure your web server correctly, as described at
java.sun.com/javase/6/docs/technotes/
guides/javaws/developersguide/setup.h
tml.
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The information element provides details about the program.
The title element specifies a title for the program.
The vendor element specifies who created the program.
◦ The values of these elements appear in Java Web Start’s security
warnings and errors that are presented to the user.
◦ The title’s value also appears in the title bar of the window in which
the program executes.
The desktop element that is nested in the shortcut element
tells Java Web Start to ask whether the user wishes to install a
desktop shortcut.
◦ If the user accepts, an icon will appear on the desktop.
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The offline-allowed element indicates that once the
program is installed on the user’s computer, it can be launched
via Java Web Start—even when the computer is not connected to
the Internet.
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The resources element contains two nested elements.
◦ The java element lists the minimum version of Java required to
execute the program
◦ The jar element specifies the location of the JAR file that contains
the program and whether that JAR file contains the class that
launches the program
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The applet-desc element is similar to the applet
element in XHTML.
◦ The name attribute specifies the applet’s name.
◦ The main-class attribute specifies the main applet class (the one
that extends JApplet).
◦ The width and height attributes specify the width and height in
pixels, respectively, of the window in which the applet will execute.
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There are several ways to to launch the applet via Java Web
Start.
The javaws command in a command window from the
folder that contains the JNLP document, as in
 javaws DrawTest.jnlp
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Can also use your operating system’s file manager to locate
the JNLP on your computer and double click its file name.
The JNLP file can be referenced from a web page via a
hyperlink.
Clicking the hyperlink in the web page downloads the JNLP
file and executes the corresponding applet.
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You can view the installed Java Web Start programs in
the Java Cache Viewer by typing the following
command in a command window:
 javaws -viewer
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The Java Cache Viewer enables you to manage the
Java Web Start programs on your system.
You can run a selected program, create a desktop
shortcut for a program (if there is not one already),
delete installed programs, and more.
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