JDK 15: Illegal reflective access to java.time classes

JDK 15 produces many Illegal reflective access warnings like following on serializing LocalTime, LocalDateTime, YearMonth, MonthYear and so on:

WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by com.google.gson.internal.reflect.UnsafeReflectionAccessor (file:/home/eldar/.gradle/caches/modules-2/files-2.1/com.google.code.gson/gson/2.8.6/9180733b7df8542621dc12e21e87557e8c99b8cb/gson-2.8.6.jar) to field java.time.MonthDay.month
WARNING: Please consider reporting this to the maintainers of com.google.gson.internal.reflect.UnsafeReflectionAccessor
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

Code example

Gson gson = new GsonBuilder().create();
System.out.println(gson.toJson(LocalDate.now()));
System.out.println(gson.toJson(LocalDateTime.now()));
System.out.println(gson.toJson(YearMonth.now()));
System.out.println(gson.toJson(MonthDay.now()));

[Marcono1234]

Note that it is recommended to prefer TypeAdapter over JsonSerializer and JsonDeserializer since it reads the JSON data in a streaming way instead of parsing it as an in-memory representation of JsonElements. The code for it will be pretty similar to your current adapter.

[Marcono1234]

Can probably be closed as duplicate of #1059.