openapi-to-application-java-spring-boot

Spring Boot application from an OpenAPI spec

Spec-first: the OpenAPI file is the source of truth. Generate interfaces and DTOs only at build time (never commit generated code), then hand-write the implementation. Verified with Spring Boot 4.0.8 and 4.1.1, openapi-generator 7.26.0 (Maven plugin, Gradle Groovy and Kotlin DSL), JDK 21+.

Step 0: Detect and gather

• Existing pom.xml / build.gradle(.kts): keep its build tool and Spring Boot version, never upgrade implicitly, add the generator to it. Otherwise create the project with the create-spring-boot-java-project skill (dependencies: web, validation, actuator; add data-jpa, postgresql, security only if needed).
• Get the spec (file, URL or pasted) and save it as src/main/resources/openapi/<name>.yaml. Accept 3.0.x and 3.1.x; Swagger 2.0 must be converted first.
• Ask only what the spec does not answer: persistence (in-memory vs database), authentication, package name.

Step 1: Review the spec

Check unique operationIds (they become method names), tags (they become one interface each), reusable components/schemas, required lists, validation constraints (minLength, maximum, pattern), error responses and securitySchemes. Flag gaps and fix the spec rather than the generated code.

Step 2: Configure the generator

Pin the version in a property; check the latest on Maven Central or the generator docs before changing it.

Maven (pom.xml, in <build><plugins>, before spring-boot-maven-plugin):

<plugin>
  <groupId>org.openapitools</groupId>
  <artifactId>openapi-generator-maven-plugin</artifactId>
  <version>${openapi-generator.version}</version> <!-- 7.26.0 -->
  <executions>
    <execution>
      <goals><goal>generate</goal></goals>
      <configuration>
        <inputSpec>${project.basedir}/src/main/resources/openapi/petstore.yaml</inputSpec>
        <generatorName>spring</generatorName>
        <apiPackage>com.example.petstore.api</apiPackage>
        <modelPackage>com.example.petstore.api.model</modelPackage>
        <generateSupportingFiles>false</generateSupportingFiles>
        <configOptions>
          <interfaceOnly>true</interfaceOnly>
          <requestMappingMode>api_interface</requestMappingMode>
          <useSpringBoot4>true</useSpringBoot4>
          <useJackson3>true</useJackson3>
          <useSpringBuiltInValidation>true</useSpringBuiltInValidation>
          <useTags>true</useTags>
          <skipDefaultInterface>true</skipDefaultInterface>
          <openApiNullable>false</openApiNullable>
          <documentationProvider>none</documentationProvider>
          <annotationLibrary>none</annotationLibrary>
        </configOptions>
      </configuration>
    </execution>
  </executions>
</plugin>

Gradle (Groovy DSL; plugin id 'org.openapi.generator' version '7.26.0'):

openApiGenerate {
  generatorName = 'spring'
  inputSpec = "$projectDir/src/main/resources/openapi/petstore.yaml".toString()
  outputDir = layout.buildDirectory.dir('generated/openapi').get().asFile.path
  apiPackage = 'com.example.petstore.api'
  modelPackage = 'com.example.petstore.api.model'
  generateApiTests = false
  generateModelTests = false
  generateSupportingFiles = false
  configOptions = [interfaceOnly: 'true', requestMappingMode: 'api_interface', useSpringBoot4: 'true', useJackson3: 'true', useSpringBuiltInValidation: 'true', useTags: 'true',
    skipDefaultInterface: 'true', openApiNullable: 'false', documentationProvider: 'none', annotationLibrary: 'none']
}
sourceSets.main.java.srcDir layout.buildDirectory.dir('generated/openapi/src/main/java')
tasks.named('compileJava') { dependsOn 'openApiGenerate' }

Gradle (Kotlin DSL; plugin id("org.openapi.generator") version "7.26.0", resolved from the Gradle plugin portal, so keep gradlePluginPortal() in pluginManagement):

openApiGenerate {
    generatorName.set("spring")
    inputSpec.set("$projectDir/src/main/resources/openapi/petstore.yaml")
    outputDir.set(layout.buildDirectory.dir("generated/openapi").get().asFile.path)
    apiPackage.set("com.example.petstore.api")
    modelPackage.set("com.example.petstore.api.model")
    generateApiTests.set(false)
    generateModelTests.set(false)
    generateSupportingFiles.set(false)
    configOptions.set(mapOf("interfaceOnly" to "true", "requestMappingMode" to "api_interface", "useSpringBoot4" to "true", "useJackson3" to "true", "useSpringBuiltInValidation" to "true", "useTags" to "true",
        "skipDefaultInterface" to "true", "openApiNullable" to "false", "documentationProvider" to "none", "annotationLibrary" to "none"))
}
sourceSets.main { java.srcDir(layout.buildDirectory.dir("generated/openapi/src/main/java")) }
tasks.compileJava { dependsOn("openApiGenerate") }

Option notes:

• Spring Boot 3.x: drop useSpringBoot4 and useJackson3 (useSpringBoot3=true is the default). Boot 4 needs both options; Jackson 3 is only allowed with useSpringBoot4.
• Request-parameter validation: for Boot 3.2+/4 (Framework 6.1+), keep useSpringBuiltInValidation=true in all build-tool configurations. It omits type-level @Validated from generated API interfaces so MVC handles constrained query/path parameters through HandlerMethodValidationException. For Boot 3.0/3.1, leave this option false to retain AOP validation and explicitly map request-parameter ConstraintViolationException to HTTP 400 with the spec's error body. Do not blanket-map service-layer constraint violations to 400. Never patch generated interfaces to change validation behavior.
• Base paths: requestMappingMode=api_interface puts the class-level base-path mapping on the interface when no controller is generated. For servers: [{url: /v1}] and path /pets, verify /v1/pets; do not add the same /v1 prefix again through controller mappings, server.servlet.context-path or proxy rewriting.
• interfaceOnly=true generates <Tag>Api interfaces (with @RequestMapping/validation annotations) and models. delegatePattern=true (instead) generates controllers calling a <Tag>ApiDelegate you implement; prefer interfaceOnly, it is simpler. If you use delegatePattern (verified on Boot 4.1.1): drop skipDefaultInterface (it breaks the generated delegate, compile errors); generate only ApiUtil.java as supporting file, otherwise the default delegate methods fail with cannot find symbol ApiUtil and a full set of supporting files adds an OpenApiGeneratorApplication main class (Unable to find a single main class in bootJar): Maven <supportingFilesToGenerate>ApiUtil.java</supportingFilesToGenerate> instead of <generateSupportingFiles>false</generateSupportingFiles>, Gradle globalProperties = [apis: '', models: '', supportingFiles: 'ApiUtil.java']. The generated <Tag>ApiController is a @Controller in apiPackage: it must be under the @SpringBootApplication package (e.g. com.example.demo.api), otherwise it is not scanned and the endpoints return 404.
• documentationProvider=none and annotationLibrary=none avoid pulling springdoc/swagger annotations into the generated code. Serving the spec/Swagger UI is optional: add org.springdoc:springdoc-openapi-starter-webmvc-ui (3.x line for Boot 4, 2.8.x for Boot 3) only if requested; verified with springdoc 3.1.1 on Boot 4.1.1 (/v3/api-docs returns OpenAPI 3.1 JSON, /swagger-ui/index.html returns 200).
• openApiNullable=false avoids the jackson-databind-nullable dependency; keep it true only if the spec has PATCH semantics with nullable fields.
• Generated models use jakarta.validation: spring-boot-starter-validation must be on the classpath. Required properties become constructor arguments in generated order; read the generated class in target/generated-sources/openapi (or build/generated/openapi) before calling constructors.
• Use <importMappings>/<typeMappings> only when a schema must map to an existing class.

Step 3: Implement

• One @RestController per generated <Tag>Api interface, implements it, no extra mapping annotations (they are inherited from the interface). Keep controllers thin; put logic in @Service classes and return DTOs, not entities. Map entities to generated DTOs explicitly (a MapStruct mapper if the project already uses it).
• Persistence: in-memory only when asked; otherwise Spring Data JPA entities and repositories separate from the generated DTOs, PostgreSQL via Docker Compose for dev, and schema handling per the schema management rule of the java-hibernate-jpa skill (Flyway or Liquibase for production). See the java-springboot skill.
• Errors: use @RestControllerAdvice to preserve each operation's declared error status, media type and response schema, including validation failures. Handle MethodArgumentNotValidException for request bodies and, on Boot 3.2+/4, HandlerMethodValidationException for query/path parameters; override the corresponding handlers when extending ResponseEntityExceptionHandler. Return RFC 9457 ProblemDetail only when the spec declares it; otherwise return the declared error DTO instead of Spring's default validation response. If error responses are missing or ambiguous, ask before changing the contract rather than imposing Problem Details or a blanket 400/404/409 policy.
• Security: map securitySchemes with the spring-security skill (JWT resource server, API key filter, method security). Do not generate useSpringSecurityPreAuthorize unless scopes are declared.
• Never edit generated sources; change the spec or the generator options and rebuild.

Step 4: Test and verify

• Follow the spring-boot-testing skill: @WebMvcTest slices per controller (mock the service), @SpringBootTest + MockMvc for the happy path and one validation failure per write operation, Testcontainers with @ServiceConnection for database code (see java-testcontainers).
• Cover every operation in the spec: success, not found, invalid input. Test invalid constrained query/path parameters as well as invalid request bodies. For error responses, including validation failures, assert the declared status, content type and body schema, not just the status code. Optionally assert the spec itself stays valid in CI with openapi-generator validate -i <spec> or by letting the build fail on generation errors.
• Run ./mvnw verify or ./gradlew build, then start the app and curl one endpoint plus /actuator/health. Include an endpoint from a spec with a non-root server URL and assert its full path, so a missing or duplicated base prefix is caught.
• Report: spec issues found, generator version and options, files written, anything deferred.