Make QParserPlugin and related classes extendible
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Key: SOLR-1149
URL: https://issues.apache.org/jira/browse/SOLR-1149
Project: Solr
Issue Type: Improvement
Components: search
Reporter: Kaktu Chakarabati
Fix For: 1.4
In a recent attempt to create a QParserPlugin which extends
DisMaxQParser/FunctionQParser functionality,
it became apparent that in the current state of these classes, it is not
straight forward and in fact impossible to seriously build
upon the existing code.
To this end, I've refactored some of the involved classes which enabled me to
reuse existing logic to great results.
I thought I will share these changes and comment on their nature in the hope
these will make sense to other solr developers/users, and
at the very least cultivate a fruitful discussion about this particular area of
the solr codebase.
The relevant changes are as follows:
* Renamed DismaxQParser class to DisMaxQParser ( in accordance with the
apparent naming convention, e.g DisMaxQParserPlugin )
* Moved DisMaxQParser to its own .java file, making it a public class rather
than its previous package-private visibility. This makes
it possible for users to build upon its logic, which is considerable, and to
my mind is a good place to start alot of custom
QParser implementations.
* Changed access modifiers for the QParser abstract base class to protected
(were package-private). Again as above, it makes this
object usable by user-defined classes that wish to define custom QParser
classes. More generally, and on the philosophy-of-code
side of things, it seems misleading to define some class members as having
the default access modifier (package-private) and then
letting other package-scope derived classes use these while not explicitly
allowing user-defined derived classes to make use of these members.
In specific i'm thinking of how DisMaxQParser makes use of these members:
**not because it is derived from QParser, but because it
simply resides in the same namespace**
* Changed access modifier for the QueryParsing.StrParser inner class and its
constructors to public. Again as in above, same issue
of having same-package classes enjoy the benefit of being in the same
namespace (FunctionQParser.parse() uses it like so),
while user defined classes cannot. Particulary in this case it is pretty bad
since this class advertises itself as a collection of utilities
for query parsing in general - great resource, should probably even live
elsewhere (common.utils?)
* Changed Function.FunctionWeight inner class data member modifiers to
protected (were default - package-private). This allowed me
to inherit from FunctionQuery as well as make use of its original
FunctionWeight inner class while overriding some of the latter's
methods. This is in the same spirit of the changes above. Please also note
this follows the common Query/Weight implementation pattern
in the lucene codebase, see for example the BooleanQuery/BooleanWeight code.
All in all these are relatively minor changes which unlock a great deal of
functionality to 3rd party developers, which i think is
ultimately a big part of what solr is all about - extendability. It is also
perhaps a cue for a more serious refactoring of the
QParserPlugin hierarchy, although i will leave such bold exclamations to
another occasion.
Attached is a patch file, having passed the usual coding-style/unit testing
cycle.
-Chak
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