{{announcement.body}}
{{announcement.title}}

Consuming Variable Responses in Jackson

DZone 's Guide to

Consuming Variable Responses in Jackson

In this article, we demonstrate using the Jackson Library for the same solution used previously to handle variable responses in Gson.

· Java Zone ·
Free Resource

Check out this post where we further demonstrate using the Jackson Library to handle variable responses.

In addition to being another popular library of JSON in Java, the Jackson Library is another well-known library, primarily recognized for its ability to offer deep customization in an opinionated way.

In this article, we are going to show an alternate way to use the Jackson Library for the same solution that was demonstrated in this post on how to handle variable responses in Gson.

You may also like: Processing JSON With Jackson

Adding Dependencies

Before we start, we need to add the Maven dependencies into our project'spom.xml:

XML




x
11


 
1
<dependency>
2
    <groupId>com.fasterxml.jackson.core</groupId>
3
    <artifactId>jackson-core</artifactId>
4
    <version>2.9.9</version>
5
</dependency>
6
 
           
7
<dependency>
8
    <groupId>com.fasterxml.jackson.core</groupId>
9
    <artifactId>jackson-databind</artifactId>
10
    <version>2.9.9</version>
11
</dependency>



We should note that if we already use Spring Boot and Spring Web Starter module enabled in our project, then we simply don't need to add any extra dependencies for Jackson.

Sample Response

Let's use this sample API response:

JSON




xxxxxxxxxx
1


 
1
{
2
    "id": 1,
3
    "name": "sample article",
4
    "comments": [
5
        {"id":1, "text":"some comment text"},
6
        {"id":2, "text":"some another comment text"}
7
    ]
8
}



Also, the response changes from Array to Object notation when there is only a single comment:

JSON




xxxxxxxxxx
1


 
1
{
2
    "id": 1,
3
    "name": "sample article",
4
    "comments": {"id":1, "text":"some comment text"}
5
}



Response Models

We'll also use the same models from the previous article:

Java




xxxxxxxxxx
1
21


 
1
public class ArticleModel {
2
 
           
3
    private Long id;
4
 
           
5
    private String name;
6
 
           
7
    private List<CommentModel> comments;
8
 
           
9
    // standard getters and setters
10
 
           
11
}
12
 
           
13
public class CommentModel {
14
 
           
15
    private Long id;
16
 
           
17
    private String text;
18
 
           
19
    // standard getters and setters
20
 
           
21
}



Consuming the Response

We defined a collection type for the comment field as List<CommentModel>. Thus, we can map multiple comments into a single field automatically because Jackson already does the heavy lifting for us:

Java




xxxxxxxxxx
1
24


 
1
String jsonString = "{
2
    "id": 1,
3
    "name": "sample article",
4
    "comments": [
5
        {"id":1, "text":"some comment text"},
6
        {"id":2, "text":"some another comment text"}
7
    ]
8
}";
9
 
           
10
ObjectMapper mapper = new ObjectMapper();
11
ArticleModel model = mapper.readValue(this.mockResponseMultiValue, ArticleModel.class);
12
 
           
13
Assert.assertEquals(ArticleModel.class, model.getClass());
14
Assert.assertEquals(model.getComments().getClass(), CommentList.class);
15
Assert.assertEquals(model.getComments().get(0).getClass(), CommentModel.class);
16
 
           
17
Assert.assertEquals(1, model.getId().longValue());
18
Assert.assertEquals("sample article", model.getName());
19
 
           
20
Assert.assertEquals(2, model.getComments().size());
21
Assert.assertEquals(1, model.getComments().get(0).getId().longValue());
22
Assert.assertEquals("some comment text", model.getComments().get(0).getText());
23
Assert.assertEquals(2, model.getComments().get(1).getId().longValue());
24
Assert.assertEquals("some another comment text", model.getComments().get(1).getText());



Certainly, this configuration resolves the multiple comment values unless the response changes to a single value structure. Otherwise, it simply doesn't work.

Consequently, we need to define a custom deserializer to handle both single and multiple values at once.

Custom Deserializers for Jackson

 StdDeserializer is an abstract type and also the base class for common deserializers in Jackson.

Therefore, to implement a custom deserialization behavior, we will create an implementation of  StdDeserializer. This behavior will be responsible for adding any single value to the list as the same as multiple values are being added automatically by Jackson.

So, let's create CommentListDeserializer, which inherits the base class StdDeserializer:

Java




xxxxxxxxxx
1
18


 
1
public class CommentListDeserializer extends StdDeserializer<List> {
2
 
           
3
    public CommentListDeserializer() {
4
        this(null);
5
    }
6
 
           
7
    private CommentListDeserializer(JavaType valueType) {
8
        super(valueType);
9
    }
10
 
           
11
    @Override
12
    public List deserialize(JsonParser p, DeserializationContext ctxt)
13
      throws IOException, JsonProcessingException {
14
 
           
15
        return null;
16
    }
17
 
           
18
}



We need a TypeReference for Jackson to specify the Collection type and the type of elements in which are involved as well.

So, let's define a constant for a type of  List<CommentModel>:

Java




xxxxxxxxxx
1


 
1
private static final TypeReference<List<CommentModel>> LIST_OF_COMMENTS_TYPE =
2
  new TypeReference<List<CommentModel>>() {};



Next, we implement the deserialize method to come up with the expected behavior:

Java




xxxxxxxxxx
1
20


 
1
@Override
2
public List deserialize(JsonParser p, DeserializationContext ctxt)
3
  throws IOException, JsonProcessingException {
4
 
           
5
    List<CommentModel> commentList = new ArrayList<>();
6
 
           
7
    JsonToken token = p.currentToken();
8
 
           
9
    if (JsonToken.START_ARRAY.equals(token)) {
10
        List<CommentModel> listOfArticles = p.readValueAs(LIST_OF_COMMENTS_TYPE);
11
        commentList.addAll(listOfArticles);
12
    }
13
 
           
14
    if (JsonToken.START_OBJECT.equals(token)) {
15
        CommentModel article = p.readValueAs(CommentModel.class);
16
        commentList.add(article);
17
    }
18
 
           
19
    return commentList;
20
}



Deserialization Features in Jackson

Although writing a custom deserializer offers us great flexibility, there are lots of features bundled in the Jackson Library for conversions. Jackson's predefined features provide practical ways to customize both serialization and deserialization.

Hopefully, the ACCEPT_SINGLE_VALUE_AS_ARRAY feature does the exact job for our use case. Certainly, we could use it for the sake of simplicity:

Java




xxxxxxxxxx
1


 
1
ObjectMapper mapper = new ObjectMapper();
2
mapper.enable(DeserializationFeature.ACCEPT_SINGLE_VALUE_AS_ARRAY);



Also, an alternate way is to configure the @JsonFormat annotation. In our example, we'll go with the  @JsonFormat annotation to set the ACCEPT_SINGLE_VALUE_AS_ARRAY feature.

Let's remove our custom deserializer and add a simple @JsonFormat annotation to our comments field in ArticleModel:

Java




xxxxxxxxxx
1


 
1
@JsonFormat(with = Feature.ACCEPT_SINGLE_VALUE_AS_ARRAY)
2
private List<CommentModel> comments;



Thus, this will automatically add any single value of CommentModel to our list as if it were already inside an array. Unless we have any further requirements, using Jackson features can be the most elegant solution for our use-case.

We can always have a chance to check the full list of features from the source code.

Going Further With Generic Deserializers

We can write generic deserializers for different requirements that we couldn't handle by predefined features. Besides, for the benefit of reusability, we can go further to refactor our code not to create new deserializers for each element type.

So, we'll implement a new class as SingleAwareListDeserializer to handle the deserialization in a more customizable way:

Java




xxxxxxxxxx
1
51


 
1
public class SingleAwareListDeserializer extends StdDeserializer<List> 
2
  implements ContextualDeserializer {
3
 
           
4
    private Class<?> contentClassType;
5
 
           
6
    public SingleAwareListDeserializer() {
7
        this(null);
8
    }
9
 
           
10
    private SingleAwareListDeserializer(JavaType valueType) {
11
        super(valueType);
12
    }
13
 
           
14
    @Override
15
    public JsonDeserializer<?> createContextual(
16
      DeserializationContext ctxt, BeanProperty property) throws JsonMappingException {
17
        // we use ContextualDeserializer to obtain content class type
18
        contentClassType = property.getType().getContentType().getRawClass();
19
        return this;
20
    }
21
 
           
22
    @Override
23
    public List deserialize(JsonParser p, DeserializationContext ctxt) 
24
      throws IOException, JsonProcessingException {
25
 
           
26
        List list = new ArrayList<>();
27
 
           
28
        JsonToken token = p.currentToken();
29
        // if token is array type then perform object deserialization to each element element
30
        if (JsonToken.START_ARRAY.equals(token)) {
31
            while (p.nextToken() != null) {
32
                if (JsonToken.START_OBJECT.equals(p.currentToken())) {
33
                    list.add(deserializeObject(p));
34
                }
35
            }
36
        }
37
 
           
38
        // if token is object type
39
        if (JsonToken.START_OBJECT.equals(token)) {
40
            list.add(deserializeObject(p));
41
        }
42
 
           
43
        return list;
44
    }
45
 
           
46
    private Object deserializeObject(JsonParser p) throws IOException {
47
        // just use jackson default object deserializer by using element type
48
        return p.readValueAs(contentClassType);
49
    }
50
 
           
51
}



Now, we can use the same deserializer for different types as well.

Certainly, we should notice the challenge here, which is about how to obtain the class type of the element inside our collection. Without the element's type information, it is not possible for Jackson to initialize the concrete type but only LinkedHashMap.

However, there is a workaround to obtain type-related information inside our deserializer.

We used the ContextualDeserializer interface to inform Jackson that we need information about the bean we are dealing with. Thus, an instance of BeanProperty is automatically injected and we obtained the class type of the collection element.

Finally, to use our generic deserializer, let's change our comments field:

Java
xxxxxxxxxx
1
 
1
@JsonDeserialize(using = SingleAwareListDeserializer.class)
2
private List<CommentModel> comments;


As a result, we can provide the same behavior by the  SingleAwareListDeserializer generically for all types as well.

Finally

In this tutorial, we learned how to handle variable responses in Jackson by using the builtin features and custom deserializers as well.

All the source code for the examples shown in this tutorial are available over on GitHub.

Further Reading

 Processing JSON With Jackson

Solving the XML Problem With Jackson

Jackson Annotations for JSON (Part 1): Serialization and Deserialization

Topics:
java ,jackson ,variable responses ,variable ,json

Published at DZone with permission of Yavuz Tas . See the original article here.

Opinions expressed by DZone contributors are their own.

{{ parent.title || parent.header.title}}

{{ parent.tldr }}

{{ parent.urlSource.name }}