ObjectMapper is better to have as a field rather than a local variable

[dgroup]

Summary

Key	Value
Rule	AvoidUsingObjectMapperAsALocalVariable
Ruleset	io/github/dgroup/arch4u/pmd/arch4u-ruleset.xml
Category	Performance
Framework	com.fasterxml.jackson.core:jackson-databind
Since	0.1.0
Incidents in past at my experience	1

Why?

ObjectMapper is better to have as a field rather than a local variable due to performance reasons.
It is allowed to be declared in fields, constructors, and initialization blocks.

class Foo {
    ObjectMapper objectMapper = new ObjectMapper(); //ok

    {
        objectMapper = new ObjectMapper();  //ok
    }

    Foo (ObjectMapper om) {                 //ok
        objectMapper = new ObjectMapper();  //ok
    }

    void bar(ObjectMapper om) {                                  //violation
        ObjectMapper mapper = om;                                //violation
        mapper = new ObjectMapper();                             //violation
        String str = new ObjectMapper().writeValueAsString(obj); //violation
    }
}

[davidrupp]

Hello. Can you share some details about the "incident in past" you've experienced that led to this conversation?

[Cimballi]

What about adding a similar rule but for Gson ?