CustomDeserializer has no default (no arg) constructor

问题

I am consuming a REST Api with RestTemplate. The response I'm getting from the API has lots of nested objects. Here's a little snippet as an example:

"formularios": [
  {
    "form_data_id": "123006",
    "form_data": {
      "form_data_id": "123006",
      "form_id": "111",
      "efs": {
        "1": {},
        "2": "{\"t\":\"c\",\"st\":\"m\",\"v\":[{\"id\":\"3675\",\"l\":\"a) Just an example\",\"v\":\"1\"},{\"id\":\"3676\",\"l\":\"b) Another example.\",\"v\":\"2\"}]}"
      }
    }

The problem I'm having is that most of the times the "1" actually has content, just like "2", and the jackson just parses it as a String on the object "efs". But sometimes, just like in the code snippet, the API sends it empty, and jackson takes it as an Object, which gives me an error that says something about START_OBJECT (can't remember the exact error, but it's not important for this question).

So I decided to make a custom deserializer so when jackson reads "1", it ignores the empty object and just parses it as a null string.

Here's my custom deserializer:

public class CustomDeserializer extends StdDeserializer<Efs> {

 public CustomDeserializer(Class<Efs> t) {
     super(t);
 }

 @Override
 public Efs deserialize(JsonParser jp, DeserializationContext dc)
         throws IOException, JsonProcessingException {

     String string1 = null;
     String string2 = null;
     JsonToken currentToken = null;

     while ((currentToken = jp.nextValue()) != null) {
         if (currentToken.equals(JsonToken.VALUE_STRING)) {
             if (jp.getCurrentName().equals("1")) {
                 string1 = jp.getValueAsString();
             } else {
                 string2 = jp.getValueAsString();
             }

         } else {
             if (jp.getCurrentName().equals("2")) {
                 string2 = jp.getValueAsString();
             }

         }
     }
     return new Efs(string1, string2);

  }
 }

And this is the way I'm using it when receiving the response from the API:

    ObjectMapper mapper = new ObjectMapper();  
    SimpleModule mod = new SimpleModule("EfsModule");
    mod.addDeserializer(Efs.class, new CustomDeserializer(Efs.class));
    mapper.registerModule(mod);


    List<HttpMessageConverter<?>> messageConverters = new ArrayList<>();
    MappingJackson2HttpMessageConverter jsonMessageConverter = new MappingJackson2HttpMessageConverter();
    jsonMessageConverter.setObjectMapper(mapper);
    messageConverters.add(jsonMessageConverter);


    RestTemplate restTemplate = new RestTemplate();
    restTemplate.setMessageConverters(messageConverters);

I'm getting the error:

 CustomDeserializer has no default (no arg) constructor

But I don't know exactly what I'm doing wrong nor how to solve it. Thanks for the help and apologies for the long question, I wanted to give as much context as possible.

回答1:

It is required that you have a default constructor without arguments. What you can do is create one (or replace the other one if you don't really need it):

public class CustomDeserializer extends StdDeserializer<Efs> {

   public CustomDeserializer() {
       super(Efs.class);
   }
   ...
}

回答2:

There is also one trap that users can fall into (like my self). If you declare deserializer as a inner class (not a static nested class) like:

@JsonDeserialize(using = DomainObjectDeserializer.class)
public class DomainObject {
    private String key;

    public class DomainObjectDeserializer extends StdDeserializer<DomainObject> {
        public DomainObjectDeserializer() {
            super(DomainObject.class);
        }

        @Override
        public DomainObject deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
            // code
        }
    }
}

Jackson uses the Class#getDeclaredConstructor() with no argument (method accepts vararg) witch means: give me a default (no argument) constructor. Code above will throw exception when Jackson tries to create DomainObjectDeserializer because javac does generate the constructor that does accept enclosing class reference. Technically speaking DomainObjectDeserializer does not have a default constructor.

For a curiosity sake you can execute DomainObjectDeserializer.class.getDeclaredConstructors() and ensure that method does return single element array containing constructor definition with enclosing class reference.

The DomainObjectDeserializer should be declared as a static class.

Here is a good answer to read in more details.

来源：https://stackoverflow.com/questions/43214545/customdeserializer-has-no-default-no-arg-constructor