Add Gson.DEFAULT constant

[MukjepScarlet]

Problem solved by the feature

No actual problem

Feature description

This is in order to provide a DEFAULT singleton instance for Gson. I saw so many programmers write new Gson() anywhere. We might directly provide a default constant to prevent this. Maybe also make Gson.<init>() deprecated because we can use Gson.DEFAULT instead.

Alternatives / workarounds

No actual bug

[Marcono1234]

You are right that it is preferable to not always create a new Gson instance because it internally has a typeTokenCache and therefore subsequent usage will be faster.

However, I am not sure if one single Gson instance for the complete JVM is ideal: It might lead to higher contention between threads, and possibly completely unrelated dependencies of an application would then share the same typeTokenCache. I am afraid this might not only decrease lookup time because the classes used by one dependency (especially for the reflective adapter) might be irrelevant for another dependency, but it can also lead to memory leaks because typeTokenCache has 'strong' instead of 'weak' references¹.

This is only my personal opinion on this though.

Footnotes

1. For separate, per-dependency, Gson instances this might not be a big problem because they are then only referenced within the dependency and garbage collected if the objects or classes of that dependency become unreachable. ↩

[Marcono1234]

We can definitely improve the Javadoc and User Guide though, mentioning that reusing a Gson instance leads to better performance because previously created type adapters are cached.