Deserialize JSON to ArrayList POJO using Jackson

Question

I have a Java class MyPojo that I am interested in deserializing from JSON  I have configured a special MixIn class  MyPojoDeMixIn  to assist me with the deserialization  MyPojo has only int and String instance variables combined with proper getters and setters  MyPojoDeMixIn looks something like this   public abstract class MyPojoDeMixIn     MyPojoDeMixIn         JsonProperty  JsonName1   int prop1         JsonProperty  JsonName2   int prop2         JsonProperty  JsonName3   String prop3         In my test client I do the following  but of course it does not work at compile time because there is a JsonMappingException related to a type mismatch   ObjectMapper m   new ObjectMapper    m getDeserializationConfig   addMixInAnnotations MyPojo class MyPojoDeMixIn class   try   ArrayList lt MyPojo gt  arrayOfPojo   m readValue response  MyPojo class     catch  Exception e    System out println e      I am aware that I could alleviate this issue by creating a  Response  object that has only an ArrayList lt MyPojo gt  in it  but then I would have to create these somewhat useless objects for every single type I want to return   I also looked online at JacksonInFiveMinutes but had a terrible time understanding the stuff about Map lt A B gt  and how it relates to my issue  If you cannot tell  I m entirely new to Java and come from an Obj-C background  They specifically mention       In addition to binding to POJOs and  simple  types  there is one   additional variant  that of binding to generic  typed  containers    This case requires special handling due to so-called Type Erasure    used by Java to implement generics in somewhat backwards compatible   way   which prevents you from using something like   Collection class  which does not compile        So if you want to bind data into a Map you will need to use    Map lt String User gt  result   mapper readValue src  new TypeReference lt Map lt String User gt  gt            How can I deserialize directly to ArrayList

User · Answer

This works for me    Test public void cloneTest         List lt Part gt  parts   new ArrayList lt Part gt         Part part1   new Part 1       parts add part1       Part part2   new Part 2       parts add part2       try           ObjectMapper objectMapper   new ObjectMapper            String jsonStr   objectMapper writeValueAsString parts            List lt Part gt  cloneParts   objectMapper readValue jsonStr  new TypeReference lt ArrayList lt Part gt  gt               catch  Exception e              fail  failed             e printStackTrace                 TODO  Assert  compare both list values

User · Answer

You can deserialize directly to a list by using the TypeReference wrapper  An example method   public static  lt T gt  T fromJSON final TypeReference lt T gt  type        final String jsonPacket       T data   null      try         data   new ObjectMapper   readValue jsonPacket  type        catch  Exception e             Handle the problem         return data      And is used thus   final String json       Set lt POJO gt  properties   fromJSON new TypeReference lt Set lt POJO gt  gt        json     TypeReference Javadoc

User · Answer

This variant looks more simple and elegant   CollectionType typeReference       TypeFactory defaultInstance   constructCollectionType List class  Dto class   List lt Dto gt  resultDto   objectMapper readValue content  typeReference

User · Answer

I am also having the same problem  I have a json which is to be converted to ArrayList    Account looks like this   Account    Person p     Related r       Person      String Name       Address a       All of the above classes have been annotated properly  I have tried TypeReference        but is not working   It gives me Arraylist but ArrayList has a linkedHashMap which contains some more linked hashmaps containing final values   My code is as Follows   public T unmarshal String responseXML String c        ObjectMapper mapper   new ObjectMapper         AnnotationIntrospector introspector   new JacksonAnnotationIntrospector         mapper getDeserializationConfig   withAnnotationIntrospector introspector        mapper getSerializationConfig   withAnnotationIntrospector introspector       try             this targetclass    T  mapper readValue responseXML   new TypeReference lt ArrayList lt T gt  gt                   catch  JsonParseException e              e printStackTrace              catch  JsonMappingException e          e printStackTrace          catch  IOException e          e printStackTrace               return this targetclass      I finally solved the problem  I am able to convert the List in Json String directly to ArrayList as follows   JsonMarshallerUnmarshaller lt T gt         T targetClass         public ArrayList lt T gt  unmarshal String jsonString                 ObjectMapper mapper   new ObjectMapper             AnnotationIntrospector introspector   new JacksonAnnotationIntrospector             mapper getDeserializationConfig   withAnnotationIntrospector introspector            mapper getSerializationConfig   withAnnotationIntrospector introspector           JavaType type   mapper getTypeFactory                        constructCollectionType ArrayList class  targetclass getClass              try                   Class c1   this targetclass getClass             Class c2   this targetclass1 getClass                 ArrayList lt T gt  temp    ArrayList lt T gt   mapper readValue jsonString   type           return temp                    catch  JsonParseException e                   e printStackTrace                    catch  JsonMappingException e               e printStackTrace             catch  IOException e              e printStackTrace                   return null

User · Answer

Another way is to use an array as a type  e g    ObjectMapper objectMapper   new ObjectMapper    MyPojo   pojos   objectMapper readValue json  MyPojo   class     This way you avoid all the hassle with the Type object  and if you really need a list you can always convert the array to a list by   List lt MyPojo gt  pojoList   Arrays asList pojos     IMHO this is much more readable   And to make it be an actual list  that can be modified  see limitations of Arrays asList    then just do the following   List lt MyPojo gt  mcList   new ArrayList lt  gt  Arrays asList pojos

[java] Deserialize JSON to ArrayList<POJO> using Jackson

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