Compatibility With Earlier Releases

WebObjects 4.0 is backward compatible to WebObjects 3.5. However, you must keep in mind the following:

WebObjects 4.0 is the first release of WebObjects that runs on Mac OS X Server and Yellow Box for Windows NT. It does not run on OpenStep 4.2. Because of this change, the locations of WebObjects files have changed. See File Location Changes.
Because Mac OS X Server and Yellow Box are not upgrades to OpenStep for Mach and OpenStep Enterprise, respectively, their underlying frameworks are not necessarily upwardly-compatible. In particular, the Foundation framework supplied with Mac OS X Server and Yellow Box isn't upwardly-compatible. As one example, NSArray's makeObjectsPerform: method doesn't exist in the versions of Foundation that ship with Mac OS X Server and Yellow Box.
The file location changes require some changes to your Project Builder projects. See Converting an Existing WebObjects Application.
Yellow Box uses a different version of the Java-wrapped APIs. The package names, class names, and some method names have changed. There are scripts to help you convert your Java code. See Converting an Existing WebObjects Application.
WebObjects applications are launched differently in WebObjects 4.0, as described in Running an Application on WebObjects 4.0.
The WebObjects Framework contains many new optimizations that should greatly improve your application's performance. However, you may find your code relied on some part of the request-response loop or template parsing code that is no longer always performed. If so, there are compatibility flags to disable these optimizations. These flags are described in Converting an Existing WebObjects Application.
The default adaptor now supports multithreaded request handling. This change should have no effect on your running application other than to make resource loading faster.

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