Contents
Support for the APPLET Tag
Compatibility with the OBJECT Tag
Supported Platforms and Browsers
Class Loader Issues
Updating Old Class Files
Other Enhancements
Fixed Bugs
Support for the APPLET Tag
Java Plug-in now supports
the HTML APPLET tag for launching applets on Microsoft
Windows platforms. Users may configure their browsers so that
JRE 1.3.1_01a is the default runtime environment for handling
APPLET tags.
For developers, the Java Plug-in enhancements enable you to
deploy your applet web pages without the need for an HTML Converter
or the use of OBJECT tags, while still being able to
ensure that your users will have the latest JRE/Java Plug-in
for running your code. In outline form, the new model for
launching an applet is as follows.
- User accesses web page which employes APPLET tags
(not OBJECT tags) to launch an applet.
- The web page invokes a script that determines whether the
latest, enhanced JRE/Java Plug-in is installed on the user's machine.
- If the latest, enhanced JRE/Java Plug-in is not installed on the
user's machine, the script initiates an auto-download process for
the JRE/Java Plug-in if the user's machine is a Microsoft Windows
platform. For other platforms, the script sends the user to a web
page that directs the user to manually download JRE/Java Plug-in.
- If the latest JRE/Java Plug-in from Sun is installed on the
user's machine,
Java Plug-in checks whether the user has selected it as the
default runtime environment for running applets. If so, then
the applet is launched on Sun's JRE. If not, the
browser's internal Java runtime environment (if it has one) will
be used to run the applet.
Even though Java Plug-in now supports the APPLET tag, it
does not support applets that make use of proprietary technologies.
Examples of such technologies are
- CAB files
- Authenticode signing
- Java Moniker
Compatibility with the OBJECT Tag
This release of Java Plug-in supports the use of APPLET
tags for launching applets from web pages on Microsoft Windows
platforms. However, it is also
fully backward compatible with previous Java Plug-in releases in its
support of the OBJECT tag for launching applets. Developers
have the option of using the 1.3.1_01a HTML Converter to set up their
applet web pages to use the OBJECT tag as before.
For more information about using OBJECT tags to launch applets
on Java Plug-in, see Java Plug-in HTML Specification and
Using the HTML Converter.
Supported Platforms and Browsers
The support for the APPLET tag in JRE/Java Plug-in
1.3.1_01a is intended for use on the following Microsoft Windows
operating systems:
- Windows 95
- Windows 98 (1st and 2nd editions)
- Windows ME
- Windows NT 4.0
- Windows 2000
In addition, JRE/Java Plug-in 1.3.1_01a has been tested on pre-releases
of Windows XP and is expected to work with the final release
of Windows XP.
JRE/Java Plug-in 1.3.1_01a provides support for the APPLET tag
on the following web browsers:
- Internet Explorer 4.0 (4.01 recommended), 5.0 (5.01 recommended),
5.5 (Service Pack 2 recommended), 6.0
- Netscape 6.0, 6.1
Class Loader Issues
Prior to version 1.3.1_01a, Java Plug-in would use a single
class loader to load multiple applets during a browser session.
This practice allowed the multiple applets to share information
with each other through static variables.
In order to maximize compatibility with the Java virtual machines
embedded in Microsoft Internet Explorer browsers, beginning with
version 1.3.1_01a Java Plug-in uses separate class loaders for
applets that differ in their codebase and/or the value of the
ARCHIVE parameter in the APPLET tags that invoke
the applets. Only if two or more applets share the same codebase
and have the same value for their ARCHIVE parameter
will the same class loader be used.
Applets that rely explicitly on the old Java Plug-in behavior
with respect to class loaders may be incompatible with the new
Java Plug-in. That is, applets that rely on the ability to
share information afforded by a single class loader may fail
to work properly if the new Java Plug-in uses separate class
loaders for each applet. If this compatibility issue applies
to your applets, you can force Java Plug-in to use a single
class loader for multiple applets by simply ensuring that all
applets have the same codebase and that the APPLET
tags used to launch the applets each has the same value for
the ARCHIVE parameter.
Updating Old Class Files
In some cases, the new Java Runtime Environment associated with this
Java Plug-in release will not run class files that were generated with
old compilers. The usual symptom is a java.lang.ClassFormatError
that the virtual machine throws when it attempts to load such a class
file. This failure has nothing specifically to do with
the changes in this release. Rather, old bytecode compilers did not
strictly adhere to proper class-file format in all situations when
generating class files. Recent virtual machines are
implemented to be strict in enforcing proper class file format,
and this can lead to errors when they attempt to load old, improperly
formatted class files. Some typical problems in some older class files
are (this list is not exhaustive):
- There are extra bytes at the end of the class file;
- The class file contains method or field names that do not begin with a letter;
- The class attempts to access private members of another class;
- The class file has other format errors, including illegal constant pool indices and illegal UTF-8 strings;
- Some early (third-party) bytecode obfuscators produced class files
that violated proper class-file format.
You can avoid this type of problem by
recompiling your classes with the Javac bytecode compiler from the
current Java 2 SDK. If you choose to use a third-party obfuscator, be sure
to use one that produces class files that respect proper class-file format.
Other Enhancements
In addition to the support for the APPLET tag described
above, the Java Plug-in has many performance
and architecture enhancements that will make it more suitable for
widespread use on consumer client machines that typically are not
as powerful as client platforms in an enterprise environment.
Some of these enhancements are summarized below.
-
Memory management
-
Dynamic maximum heap size is scaled down from 128 MB to 96MB
to avoid unnecessary paging on the system.
-
Classloader implementation has been tuned to allow memory
to be reclaimed more often by the garbage collector.
-
Potential memory leak issue is addressed by using JNI/COM smart pointers
in implementation.
-
Performance
-
Applet download time is significant reduced by relying on browser caching
when possible. No connection will be opened on the server side unless it is
absolutely necessary.
-
Applet lifecycle is controlled asynchronously to allow very fast page
switch.
-
Sped up classloader object reclaim by removing its
finalize() method.
-
HTTPS read has been made significantly faster by increasing buffer size.
-
HTTPS calls are significant faster by statically linking Microsoft's Wininet
instead of dynamic function lookup every time.
-
JavaScript performance is greatly enhanced by eliminating
BeanInfo lookup over the network.
-
Java Console performance is enhanced by using the Console Writer thread to
avoid blocking System.out and System.err when possible.
-
Design
-
Default JavaScript/Java interaction is changed from "not-scriptable"
to "scriptable", allowing any applets running through the
APPLET tag to be used from JavaScript without modification.
-
JDK 1.1 compatibility
- Java Plug-in can now access the sun.audio package.
- Some of applets are compiled with compilers that don't generate
proper class file format which conforms the class-file specification.
Such appletss may result in a ClassFormatError when Java
Plug-in attempts to load them. To help address this problem,
PluginClassLoader provides a bytecode scanner
to transform bad class files to conforming on-the-fly to allow
the applets to run. Currently, only the bad class filse with the
following errors may be transformed
- Local variable name has bad constant pool index
- Extra bytes at the end of the class file
- Code segment has wrong length
- Illegal Field/Method name
- Illegal field/method modifiers
- Invalid start_pc/length in local var table
See also Updating Old Class Files above.
Fixed Bugs
For a list of bugs fixed in this release of Java Plug-in, see
http://java.sun.com/j2se/1.3.1/ReleaseNotes.html
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