See Java_1.5_Migration for information on our move to Java 1.5

Core Lucene Functionality

Backwards Compatibility in Lucene is all about version numbers.

APIs and Compilation

Minor versions should always have complete back-compatiblity for supported (i.e. public and protected) APIs. That's to say, any code developed against X.0 should continue to run without alteration and without recompilation (ie, simply drop in the new JAR) against all X.N releases, as long as only supported APIs are used. This is no longer the case though - back compat breaks have become more and more common over time, and you will need to refer to the "Changes in backwards compatibility policy" section of CHANGES to see all of the breaks between minor releases.

A major release may also introduce incompatible API changes. The transition strategy is to introduce new APIs in release X.N, deprecating old APIs, then remove all deprecated APIs in release X+1.0.

Package-private APIs (e.g. methods without any modifier such as public or protected) are not supported and thus exempt from these back-compatibility requirements. They can change without warning or intermediate deprecation.

File Formats

File formats are back-compatible between major versions. Version X.N should be able to read indexes generated by any version after and including version X-1.0, but may-or-may-not be able to read indexes generated by version X-2.N.

Note: Older releases are never guaranteed to be able to read indexes generated by newer releases. When this is attempted, a predictable error should be generated.

Changes to Runtime Behavior

From time to time, the Lucene Developers may decide that a particular bug fix or enhancement (which does not require or involve and API change) should result in a change to the default runtime behavior in some Lucene functionality because the "new" behavior is deemed "better" or more "correct" then the existing behavior. These changes will not necessarily be backwards compatible for some Lucene clients who may have dependencies or expectations on the "old" behavior.

If a runtime behavior change is included in a release, this will be clearly noted in the CHANGES.txt file.

If a runtime behavior change is included in Minor releases (ie: X.N to X.N+1) then there will be two mechanisms available for people to "force" the old behavior:

  1. A JVM system property will be available for clients to set. (this will allow clients to force the old behavior without any code changes)
  2. A static method will be introduced which client code can call to set some internal state prior to using any Lucene code. (this will allow clients in environments that do not permit system properties to be set to upgrade by only adding one line of code)

Contrib Packages

"All contribs are not created equal."

The compatibility commitments of a contrib package can vary based on it's maturity and intended usage. The README.txt file for each contrib should identify it's approach to compatibility. If the README.txt file for a contrib package does not address it's backwards compatibility commitments users should assume it does not make any compatibility commitments.