RFR: 8368825: Change the DateTimeFormatterBuilder::FIELD_MAP type from Map to Array

[Chen Liang]

On Tue, 5 Aug 2025 01:42:53 GMT, Shaojin Wen <swen at openjdk.org> wrote:

> The DateTimeFormatterBuilder::FIELD_MAP is of type Map<Character, TemporalField>, with key values in the range 'A-Z' and 'a-z'.
> 
> We can change the type of FIELD_MAP to an array of length 128 and then use char as an index to access the array.
> 
> This will reduce the memory usage of DateTimeFormatterBuilder::FIELD_MAP and improve performance slightly.
> 
> By analyzing the jmap dump memory, we can see that the memory size used by DateTimeFormatterBuilder::FIELD_MAP has dropped from 1.12kb to 528b, the number of objects has decreased and the memory has become more flat. as shown below:
> * before
> <img width="2018" height="1016" alt="image" src="https://github.com/user-attachments/assets/57d4c513-55c7-472e-95a8-9f7a4b47662d" />
> 
> * after
> <img width="1894" height="352" alt="image" src="https://github.com/user-attachments/assets/89cbfb6c-360a-475f-85ea-4365d3b66871" />

We might just remove this map when we optimize that huge method, which is this map's only user.

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PR Comment: https://git.openjdk.org/jdk/pull/26634#issuecomment-3349631832