Github user afs commented on the issue:
https://github.com/apache/jena/pull/431
The diff may not make this clear but the old `GraphPrefixesProjection` is
replaced by `GraphPrefixesProjectionTDB` as it noted "Despite the name "TDB"
this is general replacement for DatasetPrefixStorage".
`PrefixMappingBase` is a new `PrefixMapping` base class:
`PrefixMappingImpl` has hidden state and its extension mechanism is too
limited, causing measurably expensive usage due to
[JENA-81|https://issues.apache.org/jira/browse/JENA-81] when working with a lot
of prefixes (100+).
The changes in `PrefixMappingImpl` are limited to using `ConcurrentHashMap`
instead of `HashMap` - that does not make the class multi-thread-consistent but
does address multi-thread access to `HashMap`s.
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