Jackson - Deserialize using generic class

Question

I have a json string  which I should deSerialize to the following class  class Data  lt T gt        int found      Class lt T gt  hits     How do I do it  This is the usual way  mapper readValue jsonString  Data class     But how do I mention what T stands for

User · Answer

You need to create a TypeReference object for each generic type you use and use that for deserialization  For example -   mapper readValue jsonString  new TypeReference lt Data lt String gt  gt

User · Answer

First thing you do is serialize  then you can do deserialize   so when you do serialize  you should use  JsonTypeInfo to let jackson write class information into your json data  What you can do is like this   Class Data  lt T gt        int found       JsonTypeInfo use JsonTypeInfo Id CLASS  include JsonTypeInfo As PROPERTY  property   class       Class lt T gt  hits     Then when you deserialize  you will find jackson has deserialize your data into a class which your variable hits actually is at runtime

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From Jackson 2 5  an elegant way to solve that is using the  TypeFactory constructParametricType Class parametrized  Class    parameterClasses  method that allows to define straigthly a Jackson JavaType by specifying the parameterized class and its parameterized types          Supposing you want to deserialize to Data lt String gt   you can do       the json variable may be a String  an InputStream and so for    JavaType type   mapper getTypeFactory   constructParametricType Data class  String class   Data lt String gt  data   mapper readValue json  type     Note that if the class declared multiple parameterized types  it would not be really harder     class Data  lt T  U gt        int found      Class lt T gt  hits      List lt U gt  list      We could do    JavaType type   mapper getTypeFactory   constructParametricType Data class  String class  Integer   Data lt String  Integer gt  data   mapper readValue json  type

User · Answer

You can wrap it in another class which knows the type of your generic type   Eg   class Wrapper    private Data lt Something gt  data    mapper readValue jsonString  Wrapper class     Here Something is a concrete type  You need a wrapper per reified type  Otherwise Jackson does not know what objects to create

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You can t do that  you must specify fully resolved type  like Data lt MyType gt   T is just a variable  and as is meaningless   But if you mean that T will be known  just not statically  you need to create equivalent of TypeReference dynamically  Other questions referenced may already mention this  but it should look something like   public Data lt T gt  read InputStream json  Class lt T gt  contentClass       JavaType type   mapper getTypeFactory   constructParametricType Data class  contentClass      return mapper readValue json  type

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public class Data lt T gt  extends JsonDeserializer implements ContextualDeserializer       private Class lt T gt  cls      public JsonDeserializer createContextual DeserializationContext ctx  BeanProperty prop  throws JsonMappingException           cls    Class lt T gt   ctx getContextualType   getRawClass            return this

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if you re using scala and know the generic type at compile time  but don t want to manually pass TypeReference everywhere in all your api l ayers  you can use the following code  with jackson 2 9 5    def read T  entityStream  InputStream  implicit typeTag  WeakTypeTag T    T            nathang  all of this  crazy  scala reflection allows us to handle List Seq Map Int Value     without passing        new TypeReference List Seq Map Int Value          to the function      def recursiveFindGenericClasses t  Type   JavaType           val current   typeTag mirror runtimeClass t         if  t typeArgs isEmpty            val noSubtypes   Seq empty Class             factory constructParametricType current  noSubtypes                    else           val genericSubtypes  Seq JavaType    t typeArgs map recursiveFindGenericClasses          factory constructParametricType current  genericSubtypes                         val javaType   recursiveFindGenericClasses typeTag tpe       json readValue T  entityStream  javaType        which can be used like this   read List Map Int  SomethingToSerialize    inputStream

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Just write a static method in Util class  I am reading a Json from a file  you can give String also to readValue  public static  lt T gt  T convertJsonToPOJO String filePath  Class lt   gt  target  throws JsonParseException  JsonMappingException  IOException  ClassNotFoundException           ObjectMapper objectMapper   new ObjectMapper            return objectMapper readValue new File filePath   objectMapper  getTypeFactory   constructCollectionType List class  Class forName target getName           Usage   List lt TaskBean gt  list    Util  lt List lt TaskBean gt  gt convertJsonToPOJO  E  J2eeWorkspaces az workspace svn az-client-service dir1 dir2 filename json   TaskBean class

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To deserialize a generic JSON-string to Java-object with Jackson you need    To define a JSON class  Perform an attributes mapping    Final code  tested  and ready-to-be used   static class MyJSON        private Map lt String  Object gt  content   new HashMap lt  gt           JsonAnySetter     public void setContent String key  Object value            content put key  value            String json       City     Prague       try        MyPOJO myPOJO   objectMapper readValue json  MyPOJO class        String jsonAttVal   myPOJO content get  City   toString         System out println jsonAttVal      catch  IOException e        e printStackTrace        Important   JsonAnySetter annotation is mandatory  it ensures a generic JSON-parsing and population   For more complicated cases with nested arrays please see the Baeldung reference  https   www baeldung com jackson-mapping-dynamic-object

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For class Data lt     ObjectMapper mapper   new ObjectMapper    JavaType type   mapper getTypeFactory   constructParametrizedType Data class  Data class  Parameter class   Data lt Parameter gt  dataParam   mapper readValue jsonString type

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JSON string that needs to be deserialized will have to contain the type information about parameter T  You will have to put Jackson annotations on every class that can be passed as parameter T to class Data so that the type information about parameter type T can be read from   written to JSON string by Jackson   Let us assume that T can be any class that extends abstract class Result   class Data  lt T extends Result gt        int found      Class lt T gt  hits     JsonTypeInfo use   JsonTypeInfo Id NAME  include   JsonTypeInfo As WRAPPER OBJECT   JsonSubTypes            JsonSubTypes Type value   ImageResult class  name    ImageResult             JsonSubTypes Type value   NewsResult class  name    NewsResult     public abstract class Result       public class ImageResult extends Result       public class NewsResult extends Result        Once each of the class  or their common supertype  that can be passed as parameter T  is annotated  Jackson will include information about parameter T in the JSON  Such JSON can then be deserialized without knowing the parameter T at compile time  This Jackson documentation link talks about Polymorphic Deserialization but is useful to refer to for this question as well

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Example of not very good  but simple decision  not only for Jackson  also for Spring RestTemplate  etc     Set lt MyClass gt  res   new HashSet lt  gt     objectMapper readValue json  res getClass

[java] Jackson - Deserialize using generic class

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