JSON Schema Support dùng Jackson (Java)

JSON Schema định nghĩa cấu trúc, validation, documentation cho JSON data. Jackson module jackson-module-jsonSchema generate schema từ Java class.

Dependency

<dependency>
    <groupId>com.fasterxml.jackson.module</groupId>
    <artifactId>jackson-module-jsonSchema</artifactId>
    <version>2.15.2</version> <!-- match Jackson version -->
</dependency>

Ví dụ generate Schema từ POJO

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

public class JsonSchemaTest {
    public static void main(String[] args) throws Exception {
        ObjectMapper mapper = new ObjectMapper();
        JsonSchemaGenerator schemaGen = new JsonSchemaGenerator(mapper);
        
        JsonSchema schema = schemaGen.generateSchema(Person.class);
        String schemaJson = mapper.writerWithDefaultPrettyPrinter().writeValueAsString(schema);
        System.out.println(schemaJson);
    }
}

class Person {
    private String name;
    private int age;
    private List<String> courses;
    private Address address;
    // getters...
}

class Address {
    private String firstLine, secondLine, thirdLine;
    // getters...
}

Output (JSON Schema Draft v4)

{
  "type" : "object",
  "id" : "urn:jsonschema:Person",
  "properties" : {
    "name" : { "type" : "string" },
    "age" : { "type" : "integer" },
    "address" : {
      "type" : "object",
      "id" : "urn:jsonschema:Address",
      "properties" : {
        "firstLine" : { "type" : "string" },
        "secondLine" : { "type" : "string" },
        "thirdLine" : { "type" : "string" }
      }
    },
    "course" : { 
      "type" : "array", 
      "items" : { "type" : "string" } 
    }
  }
}

Ứng dụng JSON Schema

• Validation: Validate JSON input trước khi deserialize (dùng library networknt/json-schema-validator hoặc everit-org/json-schema).
• Documentation: Tự động generate API docs (OpenAPI/Swagger dùng JSON Schema).
• Code generation: Generate POJO từ schema (jsonschema2pojo).
• Contract testing: Consumer-driven contract (Pact).

Validation Example (Networknt)

JsonSchemaFactory factory = JsonSchemaFactory.getInstance(SpecVersion.VersionFlag.V4);
JsonSchema schema = factory.getSchema(schemaJsonString);
Set<ValidationMessage> errors = schema.validate(jsonNode);
if (!errors.isEmpty()) { /* handle validation errors */ }

Lưu ý

• Jackson module generate **Draft v4** (cũ). JSON Schema hiện tại là **Draft 2020-12**.
• Annotation Jackson ảnh hưởng schema: @JsonProperty, @JsonIgnore, @NotNull (Bean Validation), @Size, @Pattern.
• Polymorphic type: dùng @JsonTypeInfo để generate oneOf/anyOf.