Question 1

How do I register the JavaTimeModule in Jackson?

Accepted Answer

Register it explicitly with mapper.registerModule(new JavaTimeModule()) or use auto-discovery via mapper.findAndRegisterModules(), but note that before Jackson 2.10, auto-registration defaults to the older JSR310Module, so explicit registration is safer for JavaTimeModule.

Question 2

Jackson Java 8 modules or custom serializers – which is better?

Accepted Answer

The official modules are recommended because they are maintained, cover edge cases, and integrate seamlessly with Jackson. Custom serializers require more effort and might not handle all scenarios, like Optional or complex date/time formats.

Question 3

Why is my Optional field not showing in JSON output with Jackson?

Accepted Answer

You likely haven't registered the Jdk8Module, which handles Optional types. Add it via mapper.registerModule(new Jdk8Module()) or use findAndRegisterModules() to include it automatically.

Question 4

Jackson 2.x vs 3.x for Java 8 support – what's the difference?

Accepted Answer

In Jackson 3.x, Java 8 support is built into jackson-databind, so these modules aren't needed. For Jackson 2.x, you must use these modules to serialize Java 8 types, making them essential for legacy codebases.

Question 5

What's the difference between JavaTimeModule and JSR310Module?

Accepted Answer

JavaTimeModule is the newer, recommended module with updated configuration defaults for JSR-310 date/time types, while JSR310Module is legacy. Auto-registration in Jackson 2.9 and earlier uses JSR310Module, so explicit registration is advised for JavaTimeModule.

Question 6

How to auto-discover all Jackson modules without issues?

Accepted Answer

Use mapper.findAndRegisterModules(), but be cautious: it may register older modules like JSR310Module in pre-2.10 versions. For consistency, explicitly register modules after calling it or check your Jackson version to avoid surprises.

jackson-modules-java8

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