Should I declare Jackson s ObjectMapper as a static field

Question

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

User · Answer

Although ObjectMapper is thread safe  I would strongly discourage from declaring it as a static variable  especially in multithreaded application  Not even because it is a bad practice  but because you are running a heavy risk of deadlocking  I am telling it from my own experience  I created an application with 4 identical threads that were getting and processing JSON data from web services  My application was frequently stalling on the following command  according to the thread dump   Map aPage   mapper readValue reader  Map class     Beside that  performance was not good  When I replaced static variable with the instance based variable  stalling disappeared and performance quadrupled  I e  2 4 millions JSON documents were processed in 40min 56sec   instead of 2 5 hours previously

User · Answer

Although it is safe to declare a static ObjectMapper in terms of thread safety  you should be aware that constructing static Object variables in Java is considered bad practice  For more details  see Why are static variables considered evil   and if you d like  my answer   In short  statics should be avoided because the make it difficult to write concise unit tests  For example  with a static final ObjectMapper  you can t swap out the JSON serialization for dummy code or a no-op   In addition  a static final prevents you from ever reconfiguring ObjectMapper at runtime  You might not envision a reason for that now  but if you lock yourself into a static final pattern  nothing short of tearing down the classloader will let you re-initialize it   In the case of ObjectMapper its fine  but in general it is bad practice and there is no advantage over using a singleton pattern or inversion-of-control to manage your long-lived objects

User · Answer

com fasterxml jackson databind type TypeFactory  hashMapSuperInterfaceChain HierarchicType   com fasterxml jackson databind type TypeFactory  findSuperInterfaceChain Type  Class    com fasterxml jackson databind type TypeFactory  findSuperTypeChain Class  Class       com fasterxml jackson databind type TypeFactory findTypeParameters Class  Class  TypeBindings          com fasterxml jackson databind type TypeFactory findTypeParameters JavaType  Class             com fasterxml jackson databind type TypeFactory  fromParamType ParameterizedType  TypeBindings                com fasterxml jackson databind type TypeFactory  constructType Type  TypeBindings                   com fasterxml jackson databind type TypeFactory constructType TypeReference                      com fasterxml jackson databind ObjectMapper convertValue Object  TypeReference    The method  hashMapSuperInterfaceChain in class com fasterxml jackson databind type TypeFactory is synchronized   Am seeing contention on the same at high loads   May be another reason to avoid a static ObjectMapper

User · Answer

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 lt ObjectMapper gt  om   new ThreadLocal lt ObjectMapper gt           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 · Answer

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

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

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