Module openapi

ballerina/openapi Ballerina library


Module Overview

This module provides the Ballerina OpenAPI tooling, which will make it easy to start the development of a service documented in an OpenAPI contract in Ballerina by generating the Ballerina service and client skeletons.

The OpenAPI tools provide the following capabilities.

  1. Generate the Ballerina service or client code for a given OpenAPI definition.
  2. Export the OpenAPI definition of a Ballerina service.
  3. Validate the service implementation of a given OpenAPI contract.

The openapi command in Ballerina is used for OpenAPI to Ballerina and Ballerina to OpenAPI code generations. Code generation from OpenAPI to Ballerina can produce ballerina service stubs and ballerina client stubs. The OpenAPI compiler plugin will allow you to validate a service implementation against an OpenAPI contract during the compile time. This plugin ensures that the implementation of a service does not deviate from its OpenAPI contract.

OpenAPI to Ballerina

Generate Service and Client Stub from an OpenAPI Contract

bal openapi -i <openapi-contract-path> 
               [--tags: tags list]
               [--operations: operationsID list]
               [--mode service|client ]
               [(-o|--output): output file path]

Generates both the Ballerina service and Ballerina client stubs for a given OpenAPI file.

This -i <openapi-contract-path> parameter of the command is mandatory. It will get the path to the OpenAPI contract file (i.e., my-api.yaml or my-api.json) as an input.

You can give the specific tags and operations that you need to document as services without documenting all the operations using these optional --tags and --operations commands.

The (-o|--output) is an optional parameter. You can use this to give the output path of the generated files. If not, it will take the execution path as the output path.


If you want to generate a service only, you can set the mode as service in the OpenAPI tool.

bal openapi -i <openapi-contract-path> --mode service [(-o|--output) output file path]

If you want to generate a client only, you can set the mode as client in the OpenAPI tool. This client can be used in client applications to call the service defined in the OpenAPI file.

bal openapi -i <openapi-contract-path> --mode client [(-o|--output) output file path]

Ballerina to OpenAPI

Service to OpenAPI Export

bal openapi -i <ballerina-file-path> 
           [(-o|--output) output openapi file path]

Export the Ballerina service to an OpenAPI Specification 3.0 definition. For the export to work properly, the input Ballerina service should be defined using the basic service and resource-level HTTP annotations. If you need to document an OpenAPI contract for only one given service, then use this command.

bal openapi -i <ballerina-file-path> (-s | --service) <service-name>

Samples for OpenAPI Commands

Generate Service and Client Stub from OpenAPI

bal openapi -i hello.yaml

This will generate a Ballerina service and client stub for the hello.yaml OpenAPI contract named hello-service and client named hello-client. The above command can be run from within anywhere on the execution path. It is not mandatory to run it from inside the Ballerina project.


The service generation process is complete. The following files were created.
-- hello-service.bal
-- client.bal
-- types.bal

Generate an OpenAPI Contract from a Service

bal openapi -i modules/helloworld/helloService.bal

This will generate the OpenAPI contracts for the Ballerina services, which are in the hello.bal Ballerina file.

al openapi -i modules/helloworld/helloService.bal (-s | --service) helloworld

This command will generate the helloworld-openapi.yaml file that is related to the helloworld service inside the helloService.bal file.

bal openapi -i modules/helloworld/helloService.bal --json

This --json option can be used with the Ballerina to OpenAPI command to generate the helloworld-openapi.json file instead of generating the YAML file.

OpenAPI Validator Compiler Plugin

The OpenAPI Validator Compiler plugin validates a service against a given OpenAPI contract. The Compiler Plugin is activated if a service has the openapi:ServiceInfo annotation. This plugin compares the service and the OpenAPI Contract and validates both against a pre-defined set of validation rules. If any of the rules fail, the plugin will give the result as one or more compilation errors.

Annotation for Validator Plugin

The @openapi:ServiceInfo annotation is used to bind the service with an OpenAPI contract. You need to add this annotation to the service file with the required values for enabling the validations.
The following is an example of the annotation usage.

    contract: “/path/to/openapi.json|yaml”,
    [ tag : “store” ],
    [ operations: [“op1”, “op2”] ] 
    [ failOnErrors]: true/false → default : true
    [ excludeTags ]: [“pets”, “user”]
    [ excludeOperations: [“op1”, “op2”] ]
service greet on new http:Listener(9090) {

Annotation Support for the Following Attributes:

  • Contract (Required) : string : Here, you can provide a path to the OpenAPI contract as a string and the OpenAPI file can either be .yaml or .json. This is a required attribute.

  • Tag (Optional) : string[]? : The compiler will only validate resources against operations, which are tagged with a tag specified in the list. If not specified, the compiler will validate resources against all the operations defined in the OpenAPI contract.

  • Operations (Optional): string[]? : Should contain a list of operation names that need to be validated against the resources in the service. If not specified, the compiler will validate resources against all the operations defined in the OpenAPI contract. If both tags and operations are defined, it will validate against the union set of the resources.

  • ExcludeTags (Optional) : string[]? : This feature is for users to store the tag. It does not need to be validated. At the same time, the excludeTag and Tag cannot store and the plugin will generate warning messages regarding it.

  • ExcludeOperations (Optional) : string[]? : This feature is for users to store the operations that do not need to be validated. At the same time, the excludeOperations and Operations can not store and they will generate warning messages. The Tag feature can store with excludeOperations. Then, all the tag operations will be validated except the exclude operations.

  • FailOnErrors (Optional) : boolean value : If you need to turn off the validation, add this to the annotation with the value as false.


import ballerina/openapi;Copy


Released date: 8 months ago

Version: 1.8.1


Platform: java17

Ballerina version: 2201.8.3

Pull count

Total: 28759

Current verison: 2289

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