Validating mutually exclusive properties (like xs:choice) in Jackson for XML

[marcieldeg]

In XML schemas, there is a "choice" type. This type indicates that one and only one of the specified values should be provided in the XML. It is similar to the UNION type in C/C++.

Since Java does not have support for a UNION type, classes are usually created with all possible properties.

For example, I have the XSD file below:

<xs:complexType name="TCEmitente">
  <xs:sequence>
    <xs:choice>
      <xs:element name="cnpj" type="xs:string">
        <xs:annotation>
          <xs:documentation>
            CNPJ number of the NFS-e issuer.
          </xs:documentation>
        </xs:annotation>
      </xs:element>
      <xs:element name="cpf" type="xs:string">
        <xs:annotation>
          <xs:documentation>
            CPF number of the NFS-e issuer.
          </xs:documentation>
        </xs:annotation>
      </xs:element>
    </xs:choice>
    <xs:element name="xNome" type="xs:string">
      <xs:annotation>
        <xs:documentation>
          Name / Company Name of the issuer.
        </xs:documentation>
      </xs:annotation>
    </xs:element>
  </xs:sequence>
</xs:complexType>

For this, the generated Java class is as follows:

public static class TCEmitente {
  private String cnpj;
  private String cpf;
  private String xNome;
  ...
}

It may cause inconsistencies in generated XML file, as the "cpf" and "cnpj" fields should never be filled in simultaneously.

Is it possible to perform this validation in Jackson?

I think an annotation like the one below would be interesting:

public static class TCEmitente {
  @JsonChoice(group="document")
  private String cnpj;

  @JsonChoice(group="document")
  private String cpf;

  private String xNome;
  ...
}

And Jackson would prevent two or more properties with the same annotation from being filled at the time of serialization. This would guarantee the integrity of the generated XML with the schema.

[pjfanning]

I don't think that any of the Jackson volunteers has much time to spend on features like this. If you want to try building it yourself, we can review it.
If this was my problem, I would be looking at XML specific libraries, maybe JAXB.
Jackson is, in essence, a JSON oriented set of libs which has been extended to support other formats so will not always cover some scenarios for the other formats.

[pjfanning]

Is there not a way of creating multiple classes instead of having a generated class that is a horrible mishmash of all the possible classes?

sealed class TCEmitente
class Cnpj extend TCEmitente // with cnpj field
class Cpf extend TCEmitente // with cpf field
class XNome extend TCEmitente // with xNome field

Jackson does support polymorphism like the class structure that I suggest. https://www.baeldung.com/java-jackson-polymorphic-deserialization

[pjfanning]

Jackson also let's you write custom serializers and deserializers. https://www.baeldung.com/jackson-deserialization

[cowtowncoder]

Quick answer: no, Jackson does not have support for validation across properties -- 2 areas Jackson explicitly defines as sort of off-limits (or, maybe, limited support area):

1. Structural transformation (exception: @JsonUnwrapped)
2. Content Validation (beyond simple datatype matching)

For (2) the idea is to instead of use separate validation library like Bean Validation API.