Improve performance

[tobiasdiez]

Improve performance by:

• Caching fields as KeywordList
• Remove duplicate change listener
• Remove preference access in loop

With these changes, the mega huge library of @AEgit works without too much delays (the initial loading of the database is a bit slow but still ok). Thus, this PR is a potential fix for #4430 and for #5071.

---

• Change in CHANGELOG.md described
• Tests created for changes
• Manually tested changed features in running JabRef
• Screenshots added in PR description (for bigger UI changes)
• Ensured that the git commit message is a good one
• Check documentation status (Issue created for outdated help page at help.jabref.org?)

[koppor]

Looks OK. I like Google Guava - therefore, I commented it at two places.

[koppor]

Can't you use Google Guava's Cache? 😇 Maybe, with Eclipse Collections Twin

LoadingCache<Pair<Field, Character>, KeywordList> storedKeywordList = CacheBuilder.newBuilder()
  .build(new CacheLoader<Pair<Field, Character>, KeywordList>() {
    @Override
    public String load(final Pair<Field, Character> pair) throws Exception {
      return getField(pair.getOne().map(... pair.getTwo());
    }
});

return storedKeywordList.get(Tuples.pair(field, keywordSeparator));

OK, invalidation is worse (as it is not possible to invalidate all pairs having the same first value (easily)).

storedKeywordList.invalidateAll();

(I now that Tuples.pair(...) might be slower than your approach. Howver, I would like that the cache can be tweaked ^^).

[tobiasdiez]

That looks complicated 😄 https://github.com/google/guava/wiki/CachesExplained#applicability has the nice explanation

Note: If you do not need the features of a Cache, ConcurrentHashMap is more memory-efficient -- but it is extremely difficult or impossible to duplicate most Cache features with any old ConcurrentMap.

I guess we do not need these cache features (but I've also never worked with Guave Caches...so I cannot appreciate them).

[koppor]

This could also be a good place for Guava's Cache.

[AEgit]

@tobiasdiez Wonderful! Let me know, when I can start testing this fix.