Hi,
Those two methods were added in JDK 10 and they are not very optimal.
Map.copyOf(Map map) 1st dumps the source map into an array of
Map.Entry(s) (map.entrySet().toArray(new Entry[0])), which typically
creates new Map.Entry objects, then passes the array to
Map.ofEntries(Map.Entry[] entries) factory method that iterates the
array and constructs a key/value interleaved array from it which is
passed to ImmutableCollections.MapN constructor, which then constructs a
linear-probing hash table from it. So each key and value is copied 3
times, while several temporary objects are created in the process.
One copying step could be eliminated and construction of temporary
Map.Entry objects too:
http://cr.openjdk.java.net/~plevart/jdk-dev/UnmodifiableMap_copyOf/webrev.01/
Collecting stream(s) using Collectors.toUnmodifiableMap() 1st collects
key/value pairs into a HashMap, then it performs equivalent operations
as Map.copyOf(hashMap) at the end. Using Map.copyOf() directly benefits
this collection operation too.
The following benchmark:
http://cr.openjdk.java.net/~plevart/jdk-dev/UnmodifiableMap_copyOf/UnmodifiableMapBench.java
Shows up to 30% improvement of .copyOf operation with this patch applied:
Original:
Benchmark (size) Mode Cnt Score
Error Units
UnmodifiableMapBench.copyOf 10 avgt 10 403.633 ±
2.640 ns/op
UnmodifiableMapBench.copyOf 100 avgt 10 3489.623 ±
44.590 ns/op
UnmodifiableMapBench.copyOf 1000 avgt 10 40030.572 ±
277.075 ns/op
UnmodifiableMapBench.toUnmodifiableMap 10 avgt 10 831.221 ±
3.816 ns/op
UnmodifiableMapBench.toUnmodifiableMap 100 avgt 10 9783.519 ±
43.097 ns/op
UnmodifiableMapBench.toUnmodifiableMap 1000 avgt 10 96524.536 ±
670.818 ns/op
Patched:
Benchmark (size) Mode Cnt Score
Error Units
UnmodifiableMapBench.copyOf 10 avgt 10 264.172 ±
1.882 ns/op
UnmodifiableMapBench.copyOf 100 avgt 10 2318.974 ±
15.877 ns/op
UnmodifiableMapBench.copyOf 1000 avgt 10 29291.782 ±
3139.737 ns/op
UnmodifiableMapBench.toUnmodifiableMap 10 avgt 10 771.221 ±
65.432 ns/op
UnmodifiableMapBench.toUnmodifiableMap 100 avgt 10 9275.016 ±
725.722 ns/op
UnmodifiableMapBench.toUnmodifiableMap 1000 avgt 10 82204.342 ±
851.741 ns/op
Production of garbage is also reduced, since no Map.Entry temporary
objects are constructed:
Original:
Benchmark (size) Mode Cnt Score Error Units
UnmodifiableMapBench.copyOf:·gc.alloc.rate.norm 10 avgt 10 416.001
± 0.002 B/op
UnmodifiableMapBench.copyOf:·gc.alloc.rate.norm 100 avgt 10
2936.005 ± 0.019 B/op
UnmodifiableMapBench.copyOf:·gc.alloc.rate.norm 1000 avgt 10
28136.059 ± 0.199 B/op
UnmodifiableMapBench.toUnmodifiableMap:·gc.alloc.rate.norm 10 avgt
10 1368.001 ± 0.004 B/op
UnmodifiableMapBench.toUnmodifiableMap:·gc.alloc.rate.norm 100 avgt
10 10208.139 ± 0.045 B/op
UnmodifiableMapBench.toUnmodifiableMap:·gc.alloc.rate.norm 1000 avgt
10 93025.923 ± 0.573 B/op
Patched:
Benchmark (size) Mode Cnt Score Error Units
UnmodifiableMapBench.copyOf:·gc.alloc.rate.norm 10 avgt 10 304.000
± 0.001 B/op
UnmodifiableMapBench.copyOf:·gc.alloc.rate.norm 100 avgt 10
2464.004 ± 0.012 B/op
UnmodifiableMapBench.copyOf:·gc.alloc.rate.norm 1000 avgt 10
24064.040 ± 0.137 B/op
UnmodifiableMapBench.toUnmodifiableMap:·gc.alloc.rate.norm 10 avgt
10 1256.001 ± 0.003 B/op
UnmodifiableMapBench.toUnmodifiableMap:·gc.alloc.rate.norm 100 avgt
10 9720.153 ± 0.055 B/op
UnmodifiableMapBench.toUnmodifiableMap:·gc.alloc.rate.norm 1000 avgt
10 88905.688 ± 0.574 B/op
So what do you think? Is this an improvement?
Regards, Peter
