Using the Conventional Applet Tag - Java 2 SDK v1.4.2, Java 2 SDK 英文文档

Using the Conventional `APPLET` Tag

This section includes the following topics:

Support for the APPLET Tag
Compatibility with the OBJECT Tag
Supported Platforms and Browsers
Updating Old Class Files
Other Enhancements

Support for the `APPLET` Tag

Java^TM Plug-in now supports the HTML APPLET tag for launching applets. Users may configure their browsers so that Sun's JRE is the default runtime environment for handling APPLET tags.

Notes

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
With the conventional applet format, use of an attribute named object with a PARAM element will cause an exception with the Sun VM: 'Either "code" or "object" should be specified, but not both.'. With the Microsoft VM, no exception will be thrown. This is a compatibility issue. To avoid the exception with the Sun VM, do not use an attribute named object with a PARAM element in an applet.

Compatibility with the `OBJECT` Tag (Internet Explorer on Windows Platform)

This release of Java Plug-in supports the use of APPLET tags for launching applets. However, it is also fully backward compatible with previous Java Plug-in releases in its support of the OBJECT tag for launching applets in Internet Explorer running on the Windows platform. As before, developers have the option of using the HTML Converter to set up their applet web pages to use the OBJECT tag.

For more information about using OBJECT tags to launch applets on Java Plug-in, see Using OBJECT, EMBED and APPLET Tags in Java Plug-in and Using the HTML Converter to Convert Applet Tags for Java Plug-in.

Supported Platforms and Browsers

The support for the APPLET tag in Java Plug-in is intended for use on the following operating systems:

Windows 95
Windows 98 (1st and 2nd editions)
Windows ME
Windows NT 4.0
Windows 2000
Windows XP
Unix
Linux

Java Plug-in 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

Note

Applet tag support currently does not work on Netscape 4.

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 applets are compiled with compilers that don't generate proper class file format which conforms to 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 bad class files 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.