Json Schema Generator with inheritance and references

Question

I am trying to generate a JSON schema using POJOs with deep inheritance structure.

Using jackson-module-jsonSchema library I am able to generate a schema.

Given a simplified Java example:

public interface I {...}
public class A implements I {
    public int varA;
}
public class B implements I {
    public int varB;
}
public class C {
    public I varC;
}

Below is my code to generate the schema:

import com.fasterxml.jackson.databind.*
import com.fasterxml.jackson.module.jsonSchema.*

// ...

ObjectMapper mapper = new ObjectMapper();

SchemaFactoryWrapper visitor = new SchemaFactoryWrapper();

mapper.acceptJsonFormatVisitor(mapper.constructType(C.class), visitor);

JsonSchema schema = visitor.finalSchema();

String outputSchemaJson = mapper.writerWithDefaultPrettyPrinter()
                                .writeValueAsString(schema);

Actual Json Schema:

{
  "type" : "object",
  "id" : "urn:jsonschema:com:mycompany:GenerateSchemas:C",
  "properties" : {
    "varC" : {
      "type" : "any"
    }
  }
}

Desired Json Schema:

{
  "definitions": {
    "A": {
        "type" : "object",
        "id" : "urn:jsonschema:com:mycompany:GenerateSchemas:A",
        "properties" : {
          "varA" : {
            "type" : "integer"
          }
        }
      },
    "B": {
        "type" : "object",
        "id" : "urn:jsonschema:com:mycompany:GenerateSchemas:B",
        "properties" : {
          "varB" : {
            "type" : "integer"
          }
        }
      }
  },
  "type" : "object",
  "id" : "urn:jsonschema:com:mycompany:GenerateSchemas:C",
  "properties" : {
    "varC" : {
      "type" : "object",
      "oneOf": [
        { "$ref": "urn:jsonschema:com:mycompany:GenerateSchemas:A" },
        { "$ref": "urn:jsonschema:com:mycompany:GenerateSchemas:B" }
      ]
    }
  }
}

I have tried overriding core classes from Json Schema library. This answer from stack overflow was helpful to generate a schema with references.

Now I am trying to understand what I need to override such that I can use reflection to get all inheriting-classes of an interface and add oneOf references to it.

score 1 · Answer 1 · answered May 21 '19 at 21:59

I was finally able to figure out which classes I needed to override.

Notes:

Java does not support dynamically finding sub-classes via reflection through a simple out-of-box api. A workaround was to annotate classes with @JsonSubType which I was able to extract at run-time.
In version 2.9.8 of the json-module-schema library (where revision 1 of solution is written), there is no support yet for object definitions. Just to get the work done, I had to override a few extra classes to make this possible.
definitions need to be defined in the json schema only once at root level because there can be cases of recursive references.

With updated POJO code:

@JsonSubTypes({
    @JsonSubTypes.Type(name = "A", value = A.class),
    @JsonSubTypes.Type(name = "B", value = B.class)
})
public interface I {}

public class A implements I {
    public int varA;
}
public class B implements I {
    public int varB;
}
public class C {
    public I varC;
}

The desired json schema output is produced successfully.

Given the following code: https://gist.github.com/rrmistry/2246c959d1c9cc45894ecf55305c61fd, I imported GenerateSchema class to make schema generation code more simplified:

public void generate() throws Exception {
    generateSchemasFromJavaSubTypes(C.class);
}

private void generateSchemasFromJavaSubTypes(Class<?> classToGenerate) throws Exception {

    JsonSchema schema = GenerateSchemas.generateSchemaFromJavaClass(classToGenerate);

    ObjectMapper mapper = new ObjectMapper();

    String jsonSchemaStr = mapper.writerWithDefaultPrettyPrinter()
                                 .writeValueAsString(schema);
}

GitHub issue has been created to request native support: https://github.com/FasterXML/jackson-module-jsonSchema/issues/135

Json Schema Generator with inheritance and references

1 Answers1