The table contains timings in milliseconds for doing 2**28 get_metadata_domain_list calls. The first column contains timings for performing noop operations to establish a baseline. The second column contains timings for doing the get_metadata_domain_list call (in the native context) and using the Java get_metadata_domain_list with built-in thread local storage (the latter mechanism is introduced in this pull request). The next three columns show results from using the Java get_metadata_domain_list method without built-in thread local storage (the previously-existing mechanism) in various ways. The last column gives timings obtained from using GDALGetMetadataDomainList directly. The bindings themselves were compiled at -O0 and also at -O4 for these tests.

These numbers do not really show it, but I would still suspect that large-enough, preallocated, thread-local byte arrays would be the best for lowest latency.

@metasim
@pomadchin

@jamesmcclain My 2¢
If I understand what's happening here, this approach concerns me as it uses effectively uses a string array as a mutable call-by-reference variable in a way that's not typically seen in JVM code. It makes the end user think about memory management through the use of arrays in a way that feels unconventional.

What I'd prefer to see is these methods just return the data that was requested, without any pre-allocated array being passed in... IOW:

String[] get_metadata_domain_list(long token, int dataset, int attempts, int band_number)
// Not quire abou this one... really want a Map<String, String>...
String[][] get_metadata(long token, int dataset, int attempts, int band_number, String domain)
String get_metadata_item(long token, int dataset, int attempts, int band_number, String key, String domain)

I would like to go ahead a merge this since it does address the metadata performance concerns. We can revisit the issue if the API is not what is wanted (please keep in mind that this library is intended to be a down-and-dirty interface, not an API per se [GDALRasterSource is the API]).