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

user_72437 0
Viewed 196725

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();
}
jackson json java
6 Answers

A trick I learned from this PR if you don't want to define it as a static final variable but want to save a bit of overhead and guarantee thread safe.

private static final ThreadLocal<ObjectMapper> om = new ThreadLocal<ObjectMapper>() {
    @Override
    protected ObjectMapper initialValue() {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        return objectMapper;
    }
};

public static ObjectMapper getObjectMapper() {
    return om.get();
}

credit to the author.

user_43042980

This question may be old, but here's what I do.

Hold the ObjectMapper instance in a thread-safe singleton:

public final class JacksonObjectMapperHolder {

  private static volatile JacksonObjectMapperHolder INSTANCE;

  private static final Object MUTEX = new Object();

  public static JacksonObjectMapperHolder getInstance() {
    JacksonObjectMapperHolder instance = INSTANCE;

    if(instance == null) {
      synchronized(MUTEX) {
        instance = INSTANCE;

        if(instance == null) {
          INSTANCE = instance = new JacksonObjectMapperHolder();
        }
      }
    }

    return instance;
  }

  private final ObjectMapper objectMapper = new ObjectMapper();

  private JacksonObjectMapperHolder() {
    super();
  }

  public final ObjectMapper getObjectMapper() {
    return objectMapper;
  }

}
user_152240520
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