CODEWITHSUNDEEP
jsonjackson
Ash
Ash

Reputation: 2134

Jackson json deserialization, ignore root element from json

How to ignore parent tag from json??

Here is my json

String str = "{\"parent\": {\"a\":{\"id\": 10, \"name\":\"Foo\"}}}";

And here is the class to be mapped from json.

public class RootWrapper {
  private List<Foo> foos;

  public List<Foo> getFoos() {
    return foos;
  }

  @JsonProperty("a")
  public void setFoos(List<Foo> foos) {
    this.foos = foos;
  }
 }

Here is the test public class JacksonTest {

@Test
public void wrapRootValue() throws Exception {
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE, true);
    mapper.configure(DeserializationConfig.Feature.FAIL_ON_UNKNOWN_PROPERTIES, false);

    String str = "{\"parent\": {\"a\":{\"id\": 10, \"name\":\"Foo\"}}}";

    RootWrapper root = mapper.readValue(str, RootWrapper.class);

    Assert.assertNotNull(root);
}

I get the error ::

 org.codehaus.jackson.map.JsonMappingException: Root name 'parent' does not match expected ('RootWrapper') for type [simple type, class MavenProjectGroup.mavenProjectArtifact.RootWrapper]

I found the solution given by Jackson annotation::

  (a) Annotate you class as below

  @JsonRootName(value = "parent")
  public class RootWrapper {

  (b) It will only work if and only if ObjectMapper is asked to wrap.
    ObjectMapper mapper = new ObjectMapper();
mapper.configure(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE, true);

Job Done!!

Another hiccup with Jackson way of Deserialization :(

if 'DeserializationConfig.Feature.UNWRAP_ROOT_VALUE configured', it unwrap all jsons, eventhough my class in not annotated with @JsonRootName(value = "rootTagInJson"), isn't weired.

I want to unwrap root tag only if the class is annotated with @JsonRootName otherwise, don't unwrap.

So below is the usecase for unwrap root tag.

  ###########################################################
     Unwrap only if the class is annotated with @JsonRootName.
  ############################################################

I did a small change in ObjectMapper of Jackson source code and created a new version of jar. 1. Place this method in ObjectMapper

// Ash:: Wrap json if the class being deserialized, are annotated
// with @JsonRootName else do not wrap.
private boolean hasJsonRootName(JavaType valueType) {
    if (valueType.getRawClass() == null)
        return false;

    Annotation rootAnnotation =  valueType.getRawClass().getAnnotation(JsonRootName.class);
    return rootAnnotation != null;
}


    2. Edit ObjectMapper method :: 
    Replace 
       cfg.isEnabled(DeserializationConfig.Feature.UNWRAP_ROOT_VALUE)
    with
       hasJsonRootName(valueType)

    3. Build your jar file and use it.

Upvotes: 47

Views: 88913

Answers (5)

benathon
benathon

Reputation: 7653

An example taken from TestRootName.java in https://github.com/FasterXML/jackson-databind may give a better way of doing this. Specifically using withRootName(""):

private ObjectMapper rootMapper()
{
    ObjectMapper mapper = new ObjectMapper();
    mapper.configure(SerializationFeature.WRAP_ROOT_VALUE, true);
    mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, true);
    return mapper;
}

public void testRootUsingExplicitConfig() throws Exception
{
    ObjectMapper mapper = new ObjectMapper();
    ObjectWriter writer = mapper.writer().withRootName("wrapper");
    String json = writer.writeValueAsString(new Bean());
    assertEquals("{\"wrapper\":{\"a\":3}}", json);

    ObjectReader reader = mapper.reader(Bean.class).withRootName("wrapper");
    Bean bean = reader.readValue(json);
    assertNotNull(bean);

    // also: verify that we can override SerializationFeature as well:
    ObjectMapper wrapping = rootMapper();
    json = wrapping.writer().withRootName("something").writeValueAsString(new Bean());
    assertEquals("{\"something\":{\"a\":3}}", json);
    json = wrapping.writer().withRootName("").writeValueAsString(new Bean());
    assertEquals("{\"a\":3}", json);

    bean = wrapping.reader(Bean.class).withRootName("").readValue(json);
    assertNotNull(bean);
}

Upvotes: 40

Desi boys
Desi boys

Reputation: 29

I was also facing this type issue. and I added below line of codes in configuration class where I defined the configuration of RestTemplate and its MessageConverters.

Annotation rootAnnotation = mapper.getTypeFactory().getClass().getAnnotation(JsonRootName.class); mapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false); mapper.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, Objects.nonNull(rootAnnotation));

Basically this configuration will help Mapper to identify if provided class is annotated with '@JsonRootName', if it is not then no need to search for root. Hope It will help you and others.

Upvotes: 0

Ilya Sereb
Ilya Sereb

Reputation: 2571

Its very simple:

Create an instance of object mapper and enable wrapping and unwrapping root value

ObjectMapper mapper = new ObjectMapper();
mapper.enable(SerializationFeature.WRAP_ROOT_VALUE);
mapper.enable(DeserializationFeature.UNWRAP_ROOT_VALUE);

Add @JsonRootName("yourname") annotation to you DTO

@JsonRootName("root")
public class YourDto {
    // ...
}

Upvotes: 4

wdb
wdb

Reputation: 1

As an update Seagabond's post, if you want to have the same effect when you write parameter values you can override the additional write methods.

@Component
public class ObjectMapper extends com.fasterxml.jackson.databind.ObjectMapper {
    private void autoconfigureFeatures(Object value) {
        JavaType javaType = _typeFactory.constructType(value.getClass());
        autoconfigureFeatures(javaType);
    }
    private void autoconfigureFeatures(JavaType javaType) {
        Annotation rootAnnotation = javaType.getRawClass().getAnnotation(JsonRootName.class);
        this.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, rootAnnotation != null);
    }

    @Override
    public void writeValue(DataOutput out, Object value) throws IOException {
        autoconfigureFeatures(value);
        super.writeValue(out, value);
    }

    @Override
    public void writeValue(Writer w, Object value) throws IOException, JsonGenerationException, JsonMappingException {
        autoconfigureFeatures(value);
        super.writeValue(w, value);
    }

    @Override
    public byte[] writeValueAsBytes(Object value) throws JsonProcessingException {
        autoconfigureFeatures(value);
        return super.writeValueAsBytes(value);
    }

    @Override
    public String writeValueAsString(Object value) throws JsonProcessingException {
        autoconfigureFeatures(value);
        return super.writeValueAsString(value);
    }

    @Override
    protected Object _readMapAndClose(JsonParser jsonParser, JavaType javaType) throws IOException, JsonParseException, JsonMappingException {
        autoconfigureFeatures(javaType);
        return super._readMapAndClose(jsonParser, javaType);
    }

}

Upvotes: 0

Segabond
Segabond

Reputation: 1143

I experienced a similar problem developing a restful application in Spring. I had to support a very heterogeneous API, some of it had root elements, another did not. I could not find a better solution than to configure this property realtime. It's a great pity there is no support for per-class root element unwrapping in Jackson. Anyway, somebody may find this helpful.

@Component
public class ObjectMapper extends com.fasterxml.jackson.databind.ObjectMapper {
    private void autoconfigureFeatures(JavaType javaType) {
        Annotation rootAnnotation = javaType.getRawClass().getAnnotation(JsonRootName.class);
        this.configure(DeserializationFeature.UNWRAP_ROOT_VALUE, rootAnnotation != null);
    }

    @Override
    protected Object _readMapAndClose(JsonParser jsonParser, JavaType javaType) throws IOException, JsonParseException, JsonMappingException {
        autoconfigureFeatures(javaType);
        return super._readMapAndClose(jsonParser, javaType);
    }

}

Upvotes: 15

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