Use empty array constants instead of repeatedly creating new ones

[kamilkrzywanski]

Refactor: use predefined empty arrays instead of creating new ones in places identified during runtime where unnecessary objects were frequently created.

[donalmurtagh]

I don't see the point in this myself. I doubt it will ever make any noticeable difference to performance and the empty array declarations are avoidable clutter.

[kamilkrzywanski]

@donalmurtagh Your suggested approach will produce a ClassCastException.
Here is a benchmark to compare these two approaches. I know it’s not a game-changer, but it seems to be a good practice, too.

ToArrayBenchmark.toArrayWithEmptyConst                     thrpt   20  67323.479 ± 515.563  ops/ms
ToArrayBenchmark.toArrayWithEmptyConst:gc.alloc.rate       thrpt   20   3081.736 ±  23.608  MB/sec
ToArrayBenchmark.toArrayWithEmptyConst:gc.alloc.rate.norm  thrpt   20     48.000 ±   0.001    B/op
ToArrayBenchmark.toArrayWithEmptyConst:gc.count            thrpt   20   1268.000            counts
ToArrayBenchmark.toArrayWithEmptyConst:gc.time             thrpt   20    882.000                ms
ToArrayBenchmark.toArrayWithNew                            thrpt   20  62202.810 ± 102.498  ops/ms
ToArrayBenchmark.toArrayWithNew:gc.alloc.rate              thrpt   20   3796.461 ±   6.257  MB/sec
ToArrayBenchmark.toArrayWithNew:gc.alloc.rate.norm         thrpt   20     64.000 ±   0.001    B/op
ToArrayBenchmark.toArrayWithNew:gc.count                   thrpt   20   1671.000            counts
ToArrayBenchmark.toArrayWithNew:gc.time                    thrpt   20   1090.000                ms

[sbrannen]

We do in fact tend to cache empty arrays in static fields if the empty arrays are reused.

For example, if you search the code base for [] EMPTY_ you'll see that we have over 45 such constants in use in production code (i.e., excluding tests and test fixtures).

In other words, there is precedent for this form of micro optimization within Spring Framework.

[sbrannen]

This has been merged into main in 9483670 and revised in 0fbebd8.

Thanks