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Java
Chapter 3
Browsing Java
by Bryan Morgan and Michael Morrison
CONTENTS
Java applets wouldn't be very useful without a client Web browser
to run them in. Sure, we could all download applets and run them
using the JDK (Java Developers Kit) applet viewer tool but that
wouldn't be quite the same. Because Java applets are so dependent
on quality Java-enabled Web browsers, this entire chapter explores
the available Web browsers that currently offer Java support.
The three big browsers that support Java as of this writing are
Sun's HotJava 1.0, Netscape Navigator 3.0, and Microsoft Internet
Explorer 3.0.
The HotJava 1.0 browser is actually still in pre-beta form, but
it will be an important browser in the near future because it
is being developed by JavaSoft. The other two Web browsers, Netscape
Navigator 3.0 and Microsoft Internet Explorer 3.0, are currently
battling it out over who dominates the Web client software market.
This chapter doesn't bother trying to rate one browser over another
because all we're concerned with is Java. Instead, you learn about
what each browser has to offer in terms of Java support.
The HotJava browser is a product of JavaSoft, the subsidiary of
Sun Microsystems that is responsible for Java. It is the only
Web browser that not only supports Java applets but also is actually
written in Java. Although the browser market is dominated at the
present time by Netscape Navigator and Microsoft Internet Explorer,
HotJava stands a chance to rock the boat a little when it ships
in final form.
What is now known as the HotJava product was actually introduced
as a standalone Web browser back in the spring of 1995. As Java's
potential was recognized by the public and its popularity skyrocketed,
HotJava came to be much more than a Web browser. The HotJava product
now also refers to a set of Java class libraries that simplify
the creation of Internet-aware applications. The HotJava browser
is provided as a showcase of these class libraries' capabilities.
The HotJava class library will be available from JavaSoft as a
licensable product in the near future, possibly at the release
of Java 1.1.
At this time, HotJava can best be explored by downloading and
installing the HotJava Web browser. You can download the HotJava
browser from JavaSoft's Web site:
http://www.javasoft.com.
In the following sections, you learn about some of the features
in HotJava that make it unique among other Web browsers.
HotJava Features
The HotJava Web browser supports many of the most popular browser
features. More important to this discussion is its support for
the execution of Java applets. HotJava also has the following
features that make it somewhat unique among other browsers:
- Security-Because HotJava was written using the Java
language, it provides a very secure environment for Java applets
to run in.
- External Viewers-Although the HotJava browser will
natively display a number of file formats such as HTML, GIF, and
JPEG, its designers realized the need to occasionally view documents
of other types. HotJava allows the user to configure viewers based
on the file's MIME (Multipurpose Internet Mail Extensions) type.
- Configurable User Interface-The HotJava user interface
is completely contained within files included with the HotJava
installation. These files can be changed or replaced, allowing
users to completely modify the appearance of the HotJava browser.
Navigating in HotJava
Figure 3.1 shows the navigation buttons available to the HotJava
user.
Figure 3.1: The HotJava navigator buttons.
These buttons, from left to right, perform the following functions:
- Go back a page
- Go forward a page
- Go to home page
- Reload a page
- Stop a page's loading
- Show HTML errors
The last button bears some special mention because this feature
is unique to the HotJava Web browser. HotJava features a sophisticated
HTML parser that can detect errors in HTML pages. The last button
on the HotJava toolbar is used to display any HTML errors found
within a page.
Setting Preferences
HotJava provides a means to alter its preferences, which means
that you can configure the browser display and applet security,
among other things. You access these preferences by selecting
Preferences from the Edit menu. The following list briefly explains
the submenu items of interest available under the Preferences
menu:
Figure 3.2: Contents of the Preferences/Display form.
- Display-This option loads a form that contains several
entry fields (see Figure 3.2). This form allows the user to change
default font sizes, set a home page, and control the display of
the HotJava welcome screen.
- Applet Security-This is the most interesting of the
Edit menu options. HotJava actually allows the user to disable
(or weaken) the default security precautions to allow applets
complete file access on your local machine, as well as the ability
to communicate with other computers across the Internet (in addition
to the server from which the applet was sent). Unless you have
good reason for not doing so, these security settings should be
set to the following:
Network Access = Applet Host
Class Access = Restricted
- If, for some reason, you want to configure your HotJava browser
to allow applets to read and write files on your local drive,
you can do so by modifying the Access Control List in the HotJava
properties file. This file is located in the ./.hotjava
directory. Although the Access Control List is blank by default,
it can be modified by adding the acl_read
and acl_write properties,
using the following syntax:
acl_read=[directory_name1 or file_name1]:[directory_nameN
or file_nameN]
acl_write=[directory_name1 or file_name1]:[directory_nameN or
file_nameN]
| Note |
These security settings are completely browser dependent and do not rely in any way on Java or the Java virtual machine. Developers who complain that Java is too restrictive are generally misinformed (or underinformed). In general, it is the Java default implementations that are designed to be restrictive-and for good reason!
|
Setting View Options
HotJava's View menu contains a set of options that apply specifically
to the current page being viewed. The following list briefly explains
some of the more important menu items:
- Monitor-The Monitor option contains the following submenus:
Progress, Memory, and Thread. These submenus load HTML pages that
can be used to see the current state of various operations. Remember
to press the Shift key when selecting any of these options to
load a separate HotJava window. Otherwise, these pages
replace the current HTML page you are viewing.
- Monitor Progress-This submenu selection loads a form
that shows the progress of the current form load process. Note
that a miniature version of the progress bar is located in the
right corner of the HotJava browser window. When the progress
bar is completely filled, the loading process is finished.
- Monitor Memory-This selection loads a form that shows
a bar graph illustrating the amount of total free memory and the
amount of memory that HotJava is currently using. At the bottom
of this page is a button that allows you to clean up memory previously
allocated by HotJava.
- Monitor Thread-This submenu selection loads a form
showing all active threads in HotJava with their respective priorities
and thread groups. It includes options that allow the user to
raise and lower thread priorities as well as kill active threads
(see Figure 3.3).
Figure 3.3: The HotJava Thread Monitor page.
Content and Protocol Handlers
HotJava is the only Web browser with direct support for dynamic
content and protocol handlers. In case you aren't familiar with
these concepts, a protocol handler is a piece of code that
handles the details of transferring different types of information
between a server and client across an Internet connection. A content
handler takes information that has been transmitted or received
with a protocol handler and determines how that information is
processed.
A good example of a protocol is HTTP (HyperText Transfer Protocol),
which defines the means in which hypertext documents are transferred
between a Web server and client browser. Both the server and client
in this case have HTTP protocol handlers that determine how the
hypertext information is packaged and transferred. Likewise, an
example of a content handler is the component of a Web browser
that interprets and displays Web pages based on HTML content sent
with the HTTP protocol. All Web browsers must support content
and protocol handlers at this level.
HotJava offers a whole new level of abstraction by supporting
dynamic content and protocol handlers-meaning that you can add
support for new protocol and content types to the HotJava browser
without getting a new version of the browser. This modularization
of protocol and content types allows the structure of the Web
to evolve and change without requiring significant reworking of
the browser itself; if a new protocol or content type emerges,
you just plug in the handler for it and go!
You learn all about writing your own content and protocol handlers
in Chapter 24, "Developing Content
and Protocol Handlers."
If you are like the average Web user, chances are extremely good
that you use or have used the Netscape Navigator Web browser.
Netscape Navigator is by far the most popular Web browser. In
fact, Netscape claims to currently control around 80 percent of
the Web browser market with Navigator, although this number is
no doubt dropping as Microsoft aggressively pursues Netscape with
its Internet Explorer. However, one number Microsoft can't compete
with at present is the number of platforms for which Navigator
is available. Currently, Navigator is available for a whopping
16 different platforms including Windows 3.1, Windows 95/NT, Apple
Macintosh 68K/PowerPC, and Sun Solaris, among others.
Because Navigator-and Internet Explorer, for that matter-has features
similar to those discussed earlier in the HotJava Web browser,
there's no need to spend a great deal of time discussing each
individual feature. Instead, let's turn our attention to its support
of Java and related technologies.
Java Support
The most recent version of Netscape Navigator, version 3.0, represents
Netscape's most complete support for Java to date. Navigator 3.0
includes full support for both Java applets and JavaScript programs.
It even supports the latest version of JavaScript (version 1.1).
I guess this doesn't come as too much of a surprise considering
that Netscape developed JavaScript! Fig-ure 3.4 shows Netscape
Navigator 3.0 in action. You can download Netscape Navigator 3.0
from Netscape's Web site, which is located at http://www.netscape.com.
Figure 3.4: Netscape Navigator 3.0.
Even though core support for Java and JavaScript is extremely
important, one of the more exciting features of Navigator is its
inclusion of a just-in-time Java compiler in the Windows 95/NT
version. If you aren't familiar with just-in-time compilers, they
compile Java executables into native machine code on the fly to
allow much faster execution speeds. For the details about just-in-time
Java compilers, check out Chapter 44,
"Just-in-Time Compilers."
Along with all this Java support behind the scenes, Navigator
also has a nice feature not found in many other Web browsers:
the Java console (see Figure 3.5).
Figure 3.5: Netscape Navigator's Java console.
The Java console allows the user to see exactly which Java applets
are being loaded and executed without having to examine the HTML
source directly. This feature can come in handy when you are trying
to debug Java and HTML forms.
You just learned that Navigator supports JavaScript 1.1, which
is a scripting language that allows you to embed scripted programs
directly into HTML code. JavaScript was initially introduced by
Netscape in an earlier version of Navigator as a feature known
as LiveScript. Because Java applets generally do not interact
with other elements on HTML pages, JavaScript is useful in providing
a means to tie these types of elements together. Although much
of the syntax is similar to Java, there are some major differences.
Most notably, JavaScript does not allow you to define new types
of objects. Additionally, inheritance is not allowed and JavaScript
code is not compiled. Instead, it is embedded within the <SCRIPT>
and </SCRIPT> tags
in an HTML form. Nevertheless, JavaScript is still a very viable
alternative for people who want to add a little interactivity
to their Web pages without becoming Java experts.
Security
The approach Navigator 3.0 takes in handling applet security is
what separates its Java support from the HotJava browser. HotJava
allows the user to set security options, but Navigator does not.
With Navigator, Java applets are not allowed to read or write
to the local file system under any circumstances; applets are
restricted to communicating only with the computer from which
they came. In addition, unlike users of HotJava, users of Navigator
can turn off Java applet support altogether. This feature was
provided primarily to pacify users who were concerned over security
problems with early versions of Java. Figure 3.6 shows Navigator's
Java options.
Figure 3.6: Netscape Navigator's Preferences dialog box.
Although late to the game, Microsoft's latest version of the Web
browser, Internet Explorer 3.0, is actually full of features-not
the least of which is its support for Java and other interactive
technologies. Even though Netscape beat Microsoft to the punch
with Java support in earlier versions of Netscape Navigator, Microsoft
has managed to make up ground fast and even to throw in a few
related extras. Internet Explorer 3.0 includes full support for
Java applets and JavaScript, including a just-in-time Java compiler.
However, Internet Explorer doesn't support the latest version
of JavaScript (version 1.1). Figure 3.7 shows Internet Explorer
3.0 in action. You can download Internet Explorer 3.0 from Microsoft's
Web site, which is located at http://www.microsoft.com.
Figure 3.7: Microsoft Internet Explorer 3.0.
Perhaps more exciting than Internet Explorer's direct Java support
is the inclusion of Microsoft's own ActiveX technology. ActiveX
is a powerful new technology built on Microsoft's popular OLE
component technology. ActiveX allows Web developers to embed prebuilt
software components, called controls, directly in Web pages,
much like Java applets are embedded. Also included in ActiveX
is a scripting language called VBScript, which is a scaled-down
version of the popular Visual Basic programming language. Both
ActiveX controls and VBScript programs can be integrated with
Java applets, resulting in an interesting mix of new technologies.
For more information about the specifics regarding Java and ActiveX,
refer to Chapter 38, "Integrating
Java and ActiveX."
Java Support
Similar to Netscape Navigator, Internet Explorer supports Java,
JavaScript, and a just-in-time Java compiler. Unlike Navigator,
Internet Explorer currently supports only version 1.0 of JavaScript
(it does not support the newer version 1.1). Also unlike Navigator,
Internet Explorer allows you to disable the just-in-time Java
compiler. This is done through the Advanced tab of the Options
dialog box, shown in Figure 3.8.
Figure 3.8: Microsoft Internet Explorer's Advanced Options dialog box.
In the Advanced Options dialog box, you may also notice that you
can enable or disable Java logging, which is a feature that tracks
all Java program activity. In a way, this feature is much like
the Java console in Navigator, except that it writes information
to a file rather than presenting it in a window.
Security
Internet Explorer, like Navigator, takes a hard line when it comes
to Java security. As is true in Navigator, Java applets executing
in Internet Explorer are not allowed to read or write to the local
file system and are restricted to communicating only with the
host computer from which they came. Internet Explorer also provides
a means to completely disable Java applets. You set this option
using the Security tab in the Options dialog box, shown in Figure
3.9.
Figure 3.9: Microsoft Internet Explorer's Security Options dialog box.
In the Security Options dialog box, you may also notice that Internet
Explorer allows you to disable ActiveX controls and plug-ins,
as well as ActiveX scripts, which includes both JavaScript and
VBScript.
Because Java applets are only as useful as the Web browsers that
support them, this chapter introduced you to the latest Java-enabled
browsers. Even though Java is still very young, Web browsers have
already come a long way in their internal support for Java. And
with the competition heating up over which browser will rule the
Web, we can only expect better and faster Java support from all
these browsers in subsequent versions.
Each browser you learned about in this chapter took a different
approach in how it positioned Java next to its other features.
HotJava, which is itself entirely written in Java, is the browser
most closely wedded to Java. Netscape Navigator, which was the
only commercial browser to adopt Java early on, is probably second
to HotJava in its support for Java because it includes the latest
release of JavaScript. Internet Explorer, although not as firmly
linked to the Java camp, includes a very competitive implementation
of Java, JavaScript, and a just-in-time Java compiler. Couple
this support with ActiveX and VBScript, and Internet Explorer
ultimately provides the most options for Web developers.
Now that you have an idea of what's out there in regard to Java
browsers, you're probably eager to find out what's available in
terms of Java programming tools. Chapter 4
explores the Java Developers Kit, which is JavaSoft's set of tools
for Java development.
Contact
reference@developer.com with questions or comments.
Copyright 1998
EarthWeb Inc., All rights reserved.
PLEASE READ THE ACCEPTABLE USAGE STATEMENT.
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
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