This is especially a problem when you want to use DirectSolrSpellChecker with FileBasedSpellChecker. What make things interesting is that WordBreakSpellChecker works fine beacuse its getStringDistance() method throws UnsupportedOperationException, and in ConjunctionSolrSpellChecker there is that work-around for this:

try {
      if (stringDistance == null) {
        stringDistance = checker.getStringDistance();
      } else if (stringDistance != checker.getStringDistance()) {
        throw new IllegalArgumentException(
            "All checkers need to use the same StringDistance.");
      }
    } catch (UnsupportedOperationException uoe) {
      // ignore
    }

GitHub user gmcoringa opened a pull request:

https://github.com/apache/lucene-solr/pull/135

SOLR-6271: ConjunctionSolrSpellChecker wrong check for same string distance

ConjunctionSolrSpellChecher now uses equals method to check for same string distance.

You can merge this pull request into a Git repository by running:

$ git pull https://github.com/elo7/lucene-solr SOLR-6271

Alternatively you can review and apply these changes as the patch at:

https://github.com/apache/lucene-solr/pull/135.patch

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

This closes #135

commit 1856fc9bf18dd3d2528c1200e11b15ec122b9b2c
Author: Fabiano V. Santos <tech+gmcoringa@elo7.com>
Date: 2015-03-17T16:44:09Z

ConjunctionSolrSpellChecher now uses equals method to check for same string distance

Patch based on trunk version from git commit 1e79536e2371737a3732369a0db4022a257be1fe

Also was created a pull request: https://github.com/apache/lucene-solr/pull/135

Here is an updated patch with a slightly different unit test.

This is a trivial fix, but important if we ever implement multiple dictionaries: SOLR-1074 / SOLR-2106 .

Commit 1717999 from jdyer@apache.org in branch 'dev/trunk'
[ https://svn.apache.org/r1717999 ]

SOLR-6271: fix StringDistance comparison in CSSC. ( This closes #135 )

Commit 1718000 from jdyer@apache.org in branch 'dev/branches/branch_5x'
[ https://svn.apache.org/r1718000 ]

SOLR-6271: fix StringDistance comparison in CSSC. ( This closes #135 )

Github user asfgit closed the pull request at:

https://github.com/apache/lucene-solr/pull/135

Thanks Igor & Fabiano for reporting this one.

This issue is still occuring in my case, wherein I'm using DirectSolrSpellChecker and FileBasedSpellChecker . The problem is that DirectSolrSpellChecker is using LuceneLavensteinDistance while FileBasedSpellChecker is using LavensteinDistance as the StringDistance.
This is throwing the IllegalArgumentException( "All checkers need to use the same StringDistance."); .

What can be the fix to this?

As of Solr 6.5, this has been changed to stringDistance.equals(checker.getStringDistance()) .

However, LuceneLevenshteinDistance does not even override equals method.
This does not solve the problem yet, because this default equals method anyway compares references.