将 JSON 反序列化为 ArrayList<POJO> 使用杰克逊

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<MyPojo> 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<MyPojo> 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<A,B> 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<String,User> result = mapper.readValue(src, new TypeReference<Map<String,User>>() { });

How can I deserialize directly to ArrayList ?

Answer 1

You can deserialize directly to a list by using the TypeReference wrapper. An example method:

public static <T> T fromJSON(final TypeReference<T> 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<POJO> properties = fromJSON(new TypeReference<Set<POJO>>() {}, json);

TypeReference Javadoc

Answer 2

Another way is to use an array as a type, eg:

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<MyPojo> 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<MyPojo> mcList = new ArrayList<>(Arrays.asList(pojos));

Answer 3

This variant looks more simple and elegant.

//import com.fasterxml.jackson.core.JsonProcessingException;
//import com.fasterxml.jackson.databind.ObjectMapper;
//import com.fasterxml.jackson.databind.type.CollectionType;
//import com.fasterxml.jackson.databind.type.TypeFactory;

//import java.util.List;


CollectionType typeReference =
    TypeFactory.defaultInstance().constructCollectionType(List.class, Dto.class);
List<Dto> resultDto = objectMapper.readValue(content, typeReference);

Answer 4

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<ArrayList<T>>() {});
    }
    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<T>{

     T targetClass ;

     public ArrayList<T> 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<T> temp = (ArrayList<T>) mapper.readValue(jsonString,  type);
        return temp ;
        }
       catch (JsonParseException e)
       {
        e.printStackTrace();
       }
       catch (JsonMappingException e) {
           e.printStackTrace();
       } catch (IOException e) {
          e.printStackTrace();
       }

     return null ;
    }  

}

Answer 5

This works for me.

@Test
public void cloneTest() {
    List<Part> parts = new ArrayList<Part>();
    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<Part> cloneParts = objectMapper.readValue(jsonStr, new TypeReference<ArrayList<Part>>() {});
    } catch (Exception e) {
        //fail("failed.");
        e.printStackTrace();
    }

    //TODO: Assert: compare both list values.
}