javajsonjackson

Should I declare Jackson's ObjectMapper as a static field?

The Jackson library's ObjectMapper class seems to be thread safe.

Does this mean that I should declare my ObjectMapper as a static field like this

class Me {
    private static final ObjectMapper mapper = new ObjectMapper();
}

instead of as an instance-level field like this?

class Me {
    private final ObjectMapper mapper = new ObjectMapper();
}
Solution

  • Yes, that is safe and recommended.

    The only caveat from the page you referred is that you can't be modifying configuration of the mapper once it is shared; but you are not changing configuration so that is fine. If you did need to change configuration, you would do that from the static block and it would be fine as well.

    EDIT: (2013/10)

    With 2.0 and above, above can be augmented by noting that there is an even better way: use ObjectWriter and ObjectReader objects, which can be constructed by ObjectMapper. They are fully immutable, thread-safe, meaning that it is not even theoretically possible to cause thread-safety issues (which can occur with ObjectMapper if code tries to re-configure instance).