Jackson and generic type reference

Question

I want to use jackson json library for a generic method as follows   public MyRequest lt T gt  tester         TypeReference lt MyWrapper lt T gt  gt  typeRef   new TypeReference lt MyWrapper lt T gt  gt           MyWrapper lt T gt  requestWrapper    MyWrapper lt T gt   JsonConverter fromJson jsonRequest  typeRef       return requestWrapper getRequest             public class MyWrapper lt T gt         private MyRequest lt T gt  request       public MyRequest lt T gt  getRequest             return request             public void setRequest MyRequest lt T gt  request            this request   request             public class MyRequest       private List lt T gt  myobjects        public void setMyObjects List lt T gt  ets             this myobjects   ets                NotNull       JsonIgnore      public T getMyObject              return myobjects get 0              Now the problem is that when I call getMyObject   which is inside the request object jackson returns the nested custom object as a LinkedHashMap  Is there any way in which I specify that T object needs to be returned   For example  if I sent object of type Customer then Customer should be returned from that List    Thanks

User · Accepted Answer

This is a well-known problem with Java type erasure: T is just a type variable, and you must indicate actual class, usually as Class argument. Without such information, best that can be done is to use bounds; and plain T is roughly same as 'T extends Object'. And Jackson will then bind JSON Objects as Maps.

In this case, tester method needs to have access to Class, and you can construct

JavaType type = mapper.getTypeFactory().
  constructCollectionType(List.class, Foo.class)

and then

List<Foo> list = mapper.readValue(new File("input.json"), type);

User · Answer

I modified rushidesai1 s answer to include a working example   JsonMarshaller java  import java io    import java util     public class JsonMarshaller lt T gt        private static ClassLoader loader   JsonMarshaller class getClassLoader         public static void main String   args            try               JsonMarshallerUnmarshaller lt Station gt  marshaller   new JsonMarshallerUnmarshaller lt  gt  Station class               String jsonString   read loader getResourceAsStream  data json                 List lt Station gt  stations   marshaller unmarshal jsonString               stations forEach System out  println               System out println marshaller marshal stations              catch  IOException e                e printStackTrace                          SuppressWarnings  resource       public static String read InputStream ios            return new Scanner ios  useDelimiter    A   next       Read the entire file           Output  Station  id 123  title my title  name my name  Station  id 456  title my title 2  name my name 2     id  123  title   my title   name   my name     id  456  title   my title 2   name   my name 2      JsonMarshallerUnmarshaller java  import java io    import java util List   import com fasterxml jackson core    import com fasterxml jackson databind    import com fasterxml jackson databind introspect JacksonAnnotationIntrospector   public class JsonMarshallerUnmarshaller lt T gt        private ObjectMapper mapper      private Class lt T gt  targetClass       public JsonMarshallerUnmarshaller Class lt T gt  targetClass            AnnotationIntrospector introspector   new JacksonAnnotationIntrospector             mapper   new ObjectMapper            mapper getDeserializationConfig   with introspector           mapper getSerializationConfig   with introspector            this targetClass   targetClass             public List lt T gt  unmarshal String jsonString  throws JsonParseException  JsonMappingException  IOException           return parseList jsonString  mapper  targetClass              public String marshal List lt T gt  list  throws JsonProcessingException           return mapper writeValueAsString list              public static  lt E gt  List lt E gt  parseList String str  ObjectMapper mapper  Class lt E gt  clazz              throws JsonParseException  JsonMappingException  IOException           return mapper readValue str  listType mapper  clazz               public static  lt E gt  List lt E gt  parseList InputStream is  ObjectMapper mapper  Class lt E gt  clazz              throws JsonParseException  JsonMappingException  IOException           return mapper readValue is  listType mapper  clazz               public static  lt E gt  JavaType listType ObjectMapper mapper  Class lt E gt  clazz            return mapper getTypeFactory   constructCollectionType List class  clazz             Station java  public class Station       private long id      private String title      private String name       public long getId             return id             public void setId long id            this id   id             public String getTitle             return title             public void setTitle String title            this title   title             public String getName             return name             public void setName String name            this name   name              Override     public String toString             return String format  Station  id  s  title  s  name  s    id  title  name             data json        id   123     title    my title      name    my name          id   456     title    my title 2      name    my name 2

User · Answer

JavaType  works     I was trying to unmarshall  deserialize  a List in json String to ArrayList java Objects and was struggling to find a solution since days  Below is the code that finally gave me solution   Code    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

[java] Jackson and generic type reference

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