[Spring] Generate javax.validation annotations for @RequestHeader method arguments

Description

Swagger specification allows us to enforce validation constraints on request headers such as maxLength, minLength, pattern etc. However, when we generate server code using Spring as a language, the validation annotations are missing from the generated code. This is not the case for request parameters though where validation constraints are indeed generated.

Swagger-codegen version

I use 2.2.2 but have verified that I can still reproduce it in 2.3.1

Swagger declaration file content or url

"customHeader": {
    "name": "X-Custom-Header",
    "in": "header",
    "required": true,
    "type": "string",
    "minLength": 3
}

Command line used for generation

I use a Gradle plugin to generate code and here is the configuration:

swagger {
    inputSpec = file('swagger.json').absolutePath
    outputDir = "${buildDir}/generated-sources"
    lang = 'spring'

    addDynamicProperty 'sourceFolder', 'src/swagger/java'

    additionalProperties = [
        basePackage: 'com.example.foo',
        invokerPackage: 'com.example.foo',
        modelPackage: 'com.example.foo.api.model',
        apiPackage: 'com.example.foo.api',
        configPackage: 'com.example.foo.config',
        interfaceOnly: true,
        java8: true
    ]

    systemProperties = [
        apis: '',
        models: ''
    ]
}

Steps to reproduce

Just generate code and look at the generated code. For the above header, I expect a @Size(min = 1) constraint to be added to the method signature but it's clearly missing.

Start by generating the Spring server code from the provided Swagger declaration and compare the @RequestHeader method signature with the request-parameter output. Trace the Spring generator templates or entry point responsible for parameter annotations; done means header constraints such as minLength, maxLength, and pattern appear as javax.validation annotations in the generated signature.