Generate Code from the Contract

Why do We Want to Generate Code from the Contract?

If we were to write by hand one Primary Adapter that has some few endpoints and its necessary models (called Schemas in the OpenAPI Specification) it may not be that complicated.

But imagine if it was a big project, with a few dozen Primary Adapters… Doing this manually is prone to error and consumes a lot of time.

OpenAPI Generator

Let’s save us some problems by using one of the greatest libraries to ever exist: openapi-generator.

Add the plugin in the plugins section (usually at the start of build.gradle):
build.gradle
```
id 'org.openapi.generator' version '7.12.0'
```

We need Swagger Core Jakarta, JsonNullable Jackson Module, and Spring Boot Starter Validation dependencies. Add them in the build.gradle dependencies section:

build.gradle
implementation 'io.swagger.core.v3:swagger-core-jakarta:2.2.28'
implementation 'org.openapitools:jackson-databind-nullable:0.2.6'
implementation 'org.springframework.boot:spring-boot-starter-validation'

Configure the openapi-generator (bottom of build.gradle):

build.gradle
openApiGenerate {
    apiPackage = "${project.group}.${project.name}.adapter.in.rest.api".toString()
    configOptions = [
        interfaceOnly: "true",
        skipOperationExample: "true",
        useEnumCaseInsensitive: "true",
        useSpringBoot3: "true"
    ]
    generateApiTests = false
    generateApiDocumentation = false
    generateModelTests = false
    generateModelDocumentation = false
    generatorName = "spring"
    inputSpec = "$rootDir/src/main/resources/openapi/users_manager.yaml".toString()
    modelPackage = "${project.group}.${project.name}.adapter.in.rest.dto".toString()
    outputDir = layout.buildDirectory.dir("generated/sources/openapi").get().asFile.toString()
}

You can find more information about the different possible configurations in the OpenAPI Generator Gradle Plugin GitHub page.
It is important to make sure that inputspec is pointing to the desired OpenAPI Specification YAML file.

Add the sourceSets configuration. This tells Gradle where to find the generated Java sources. Place the following code immediately below the plugins section:

build.gradle
sourceSets {
    main {
        java {
            srcDir(layout.buildDirectory.dir("generated/sources/openapi/src/main/java"))
        }
    }
}

Generate code on compilation by adding a new task:

build.gradle
tasks.named('compileJava') {
    dependsOn 'openApiGenerate'
}

Now that everything is set up, run the Build Task. When the task finishes, check the build\generated\sources\openapi folder. You’ll find the representation of the OpenAPI Specification (our contract) in Java code, ready to be used.

Commit the progress so far.

git add .
git commit -m "generated primary adapter code from an openapi specification"

Why do We Want to Generate Code from the Contract?​

OpenAPI Generator​

Why do We Want to Generate Code from the Contract?

OpenAPI Generator