Uwe Schindler created LUCENE-6742:
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Summary: Fix SnowballFilter for Lovins & Finish (and others that
use reflection)
Key: LUCENE-6742
URL: https://issues.apache.org/jira/browse/LUCENE-6742
Project: Lucene - Core
Issue Type: Bug
Components: modules/analysis
Reporter: Uwe Schindler
Assignee: Uwe Schindler
Fix For: 5.4
While reviewing the warnings I noticed the following:
The LovinsStemmer and the FinishStemmer from the snowball package cannot work
at all. Indeed Robert Muir added a comment to the test referring to:
http://article.gmane.org/gmane.comp.search.snowball/1139
The bug is the following: The Among class looks up the method to call via
reflection. But the stemmer compiler creates all these methods as private. As
AccessibleObject#setAccessible() is called nowhere (we also donÄt want this),
the SnowballProgram main class cannot call the method. Unfortunately it
completely supresses all reflection errors and returns false. SnowballProgram
then thinks, the called {{private boolean r_A()}} method returned false and s
the whole snowball algorithm breaks.
Also the snowball stemmer classes had a bug: methodObject is a static instance
of the Stemmer, and Among calls the private method then on wrong object. So
when fixing the above, this would fail again (because the stemmer changes a
static singleton object, not itsself!). We also need to change the object in
SnowballProgram to use "this" when calling the method.
There are several possibilities to solve the private methods problem:
- call AccessibleObject.setAccessible(true) -> we don't want this! Never Ever!
:-)
- patch the whole Snowball-generated classes to make those methods "public".
This is a huge patch and shows many internals => no way
- Since Java 7 we can use a cool trick, my favourite: MethodHandles
The MethodHandles trick can be done the following way:
- Change the Among class to get a MethodHandles.Lookup in its ctor (instead of
methodObject). This lookup is used to lookup the method by name. This Lookup
Object must be private to our implementing class (the one that defines the
private methods). If this is the case, we can get a method handle and call the
method (from anywhere, because we "own" it).
- replace the (already broken) {{methodObject}} in every stemmer to instead be
the correct MethodHandles.Lookup. This has 2 effects: The class to get the
lookup gets all access rights to the class calling it (so we can access private
methods) and also sees all methods.
The second part is done by a new Ant task: {{ant patch-snowball}}. Whenever you
add a new Snowball stemmer, copy the java file generated by the snowball
compiler to the ext directory and call the ant task. It will patch the
{{methodObject}} declaration (and also add a {{@SuppressWarnings("unused")}}
for convenience).
I reenabled the dictionary tests and the whole stemmer works. There are also no
issues with security managers, because we do nonthing security sensitive (all
is our own stuff, no setAccessible).
And finally: LovinsStemmer and FinishStemmer got insanely fast (and correct, of
course).
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