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  1. Lucene - Core
  2. LUCENE-6742

Fix SnowballFilter for Lovins & Finnish (and others that use reflection)


    • Bug
    • Status: Closed
    • Major
    • Resolution: Fixed
    • None
    • 5.4, 6.0
    • modules/analysis
    • None
    • New


      While reviewing the warnings (LUCENE-6740) I noticed the following:
      The LovinsStemmer and the FinnishStemmer 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 don't 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 so 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 the broken static SnowballProgram instance 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 of the implementing class containing the private method. 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 nothing security sensitive (all is our own stuff, no setAccessible).

      And finally: LovinsStemmer and FinnishStemmer got insanely fast (and correct, of course).


        1. LUCENE-6742-Java8.patch
          521 kB
          Uwe Schindler
        2. LUCENE-6742.patch
          32 kB
          Uwe Schindler
        3. LUCENE-6742.patch
          32 kB
          Uwe Schindler
        4. LUCENE-6742.patch
          35 kB
          Uwe Schindler



            uschindler Uwe Schindler
            uschindler Uwe Schindler
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