Java 2 Ada

Generating a REST Ada client with OpenAPI and Swagger Codegen

By stephane.carrez

The OpenAPI initiative aims at defining a standard for the specification of REST API. The OpenAPI Specification (OAS) defines a programming language-agnostic interface to describe a REST API. The Swagger Codegen generator supports more than 28 different languages (including Ada) and it is able to read an OpenAPI document and generate either the documentation or the client and server REST code for several target languages.


Writing an OpenAPI document

The OpenAPI document is either a JSON or a YAML file that describes the REST API operations. The document can be used both for the documentation of the API and for the code generation in several programming language. We will see briefly through the Petstore example how the OpenAPI document is organized. The full OpenAPI document is available in petstore.yaml.

General description

A first part of the OpenAPI document provides a general description of the API. This includes the general description, the terms of service, the license and some contact information.

swagger: '2.0'
  description: 'This is a sample server Petstore server.  You can find out more about Swagger at [](http://s or on [, #swagger](  For this sample, you can use the api key `special-key
` to test the authorization filters.'
  version: 1.0.0
  title: Swagger Petstore
  termsOfService: ''
    name: Apache 2.0
    url: ''
basePath: /v2

Type description

The OpenAPI document can also describe types which are used by the REST operations. These types provide a description of how the data is organized and passed through the API operations.

It is possible to describe almost all possible types from simple properties, group of properties up to complex types including arrays. For example a Pet type is made of several properties each of them having a name, a type and other information to describe how the type is serialized.

    title: a Pet
    description: A pet for sale in the pet store
    type: object
      - name
      - photoUrls
        type: integer
        format: int64
        $ref: '#/definitions/Category'
        type: string
        example: doggie
        type: array
          name: photoUrl
          wrapped: true
          type: string
        type: array
          name: tag
          wrapped: true
          $ref: '#/definitions/Tag'
        type: string
        description: pet status in the store
          - available
          - pending
          - sold
      name: Pet

In this example, the Pet type contains 6 properties (id, category, name, photoUrls, tags, status) and refers to two other types Category and Tag.

Operation description

Operations are introduced by the paths object in the OpenAPI document. This section describes the possible paths that can be used by URL and the associated operation. Some operations receive their parameter within the path and this is represented by the {name} notation.

The operation description indicates the HTTP method that is used get, post, put or delete.

The following definition describes the getPetById operation.

        - pet
      summary: Find pet by ID
      description: Returns a single pet
      operationId: getPetById
        - application/xml
        - application/json
        - name: petId
          in: path
          description: ID of pet to return
          required: true
          type: integer
          format: int64
          description: successful operation
            $ref: '#/definitions/Pet'
          description: Invalid ID supplied
          description: Pet not found
        - api_key: []

The summary and description are used for the documentation purposes. The operationId is used by code generators to provide an operation name that a target programming language can use. The produces section indicates the media types that are supported by the operation and which are generated for the response. The parameters section represents all the operation parameters. Some parameters can be extracted from the path (which is the case for the petId parameter) and some others can be passed as query parameter.

The responses section describes the possible responses for the operation as well as the format used by the response. In this example, the operation returns an object described by the Pet type.

Using Swagger Codegen

The documentation and the Ada client are generated from the OpenAPI document by using the Swagger Codegen generator. The generator is a Java program that is packaged within a jar file. It must be launched by the Java 7 or Java 8 runtime.

Generating the documentation

The HTML documentation is generated from the OpenAPI document by using the following command:

 java -jar swagger-codegen-cli.jar generate -l html -i petstore.yaml -o doc

Generating the Ada client

To generate the Ada client, you will use the -l ada option to use the Ada code generator. The OpenAPI document is passed with the -i option.

 java -jar swagger-codegen-cli.jar generate -l ada -i petstore.yaml -o client \
       -DprojectName=Petstore --model-package Samples.Petstore

The Ada generator uses two options to control the generation. The -DprojectName=Petstore option allows to control the name of the generated GNAT project and the --model-package option controls the name of the Ada package for the generated code.

The Ada generator will create the following Ada packages:

  • Samples.Petstore.Models is the package that contains all the types described in the OpenAPI document. Each OpenAPI type is represented by an Ada record and it is also completed by an instantiation of the Ada.Containers.Vectors package for the representation of arrays of the given type. The Models package also provides Serialize and Deserialize procedures for the serialization and deserialization of the data over JSON or XML streams.
  • Samples.Petstore.Clients is the package that declares the Client_Type tagged record which provides all the operations for the OpenAPI document.

For the Pet type describe previously, the Ada generator produces the following code extract:

package Samples.Petstore.Models is
   type Pet_Type is
       Id : Swagger.Long;
       Category : Samples.Petstore.Models.Category_Type;
       Name : Swagger.UString;
       Photo_Urls : Swagger.UString_Vectors.Vector;
       Tags : Samples.Petstore.Models.Tag_Type_Vectors.Vector;
       Status : Swagger.UString;
     end record;
end Samples.Petstore.Models;

and for the operation it generates the following code:

package Samples.Petstore.Clients is
   type Client_Type is new Swagger.Clients.Client_Type with null record;
   procedure Get_Pet_By_Id
      (Client : in out Client_Type;
       Pet_Id : in Swagger.Long;
       Result : out Samples.Petstore.Models.Pet_Type);
end Samples.Petstore.Clients;

Using the REST Ada client


The HTTP/REST support is provided by Ada Util and encapsulated by Swagger Ada. The Ada Util library also takes care of the JSON and XML serialization and deserialization. If you want to use Curl, you should initialize with the following:

with Util.Http.Clients.Curl;

But if you want to use AWS, you will initialize with:

with Util.Http.Clients.Web;

After the initialization is done, you will declare a client instance to access the API operations:

with Samples.Petstore.Clients;
   C : Samples.Petstore.Clients.Client_Type;

And you should initialize the server base URL you want to connect to. To use the live Swagger Petstore service you can set the server base URL as follows:

  C.Set_Server ("");

At this stage, you can use the generated operation by calling operations on the client.

Calling a REST operation

Let's retrieve some pet information by calling the Get_Pet_By_Id operation described previously. This operation needs an integer as input parameter and returns a Pet_Type object that contains all the pet information. You will first declare the pet instance as follows:

with Samples.Petstore.Models;
  Pet  : Samples.Petstore.Models.Pet_Type;

And then call the Get_Pet_By_Id operation:

  C.Get_Pet_By_Id (768, Pet);

At this stage, you can access information from the Pet instance:

with Ada.Text_IO;
  Ada.Text_IO.Put_Line ("Id      : " & Swagger.Long'Image (Pet.Id));
  Ada.Text_IO.Put_Line ("Name    : " & Swagger.To_String (Pet.Name));
  Ada.Text_IO.Put_Line ("Status  : " & Swagger.To_String (Pet.Status));

The Swagger Ada Petstore illustrates other uses of the generated operations. It allows to list the inventory, list the pets with a given status, add a pet and so on...

Conclusion and references

The OpenAPI Specification provides a standard way to describe REST operations. The Swagger Codegen is the generator to be used to simplify the implementation of REST clients in many programming languages and to generate the documentation of the API. The Ada code generator only supports the client side but the server code generation is under work.

The sources of the petstore samples are available:

The lists more than 550 API descriptions from various providers such as Amazon, Google, Microsoft and many other online services. They are now available to the Ada community!

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Dynamo 0.7.0 is available

By stephane.carrez

Dynamo is a code generator used to generate Ada Web Application or database mappings.

  • New project template to generate Gtk Ada application
  • Register the new module in the application when they are added
  • Update the current testsuite when new tests are added
  • New stereotype <<Limited_Bean>> for Ada bean generation
  • Support for the creation of Debian packages
  • New command add-form and add-module-operation

You can download the new version at

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Dynamo 0.6.0 is available

By stephane.carrez

Dynamo is a tool to help developers write some types of Ada Applications which use the Ada Server Faces or Ada Database Objects frameworks. Dynamo provides several commands to perform one specific task in the development process: creation of an application, generation of database model, generation of Ada model, creation of database.

The new version of Dynamo provides:

  • A new command build-doc to extract some documentation from the sources,
  • The generation of MySQL and SQLite schemas from UML models,
  • The generation of Ada database mappings from UML models,
  • The generation of Ada beans from the UML models,
  • A new project template for command line tools using ADO,
  • A new distribution command to merge the resource bundles.

The most important feature is probably the Ada code generation from a UML class diagram. With this, you can design the data model of an application using ArgoUML and generate the Ada model files that will be used to access the database easily through the Ada Database Objects library. The tool will also generate the SQL database schema so that everything is concistent from your UML model, to the Ada implementation and the database tables.

The short tutorial below indicates how to design a UML model with ArgoUML, generate the Ada model files, the SQL files and create the MySQL database.

The Dynamo tool is available at

To build Dynamo, you will need:

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Dynamo 0.5.0 is available

By stephane.carrez

Dynamo is a tool to help developers write an Ada Web Application using the Ada Server Faces and the Ada Database Objects frameworks. Dynamo provides several commands to perform one specific task in the development process: creation of an application, generation of database model, generation of Ada model, creation of database.

The new version of Dynamo provides:

  • Support multi-line comments in XML mappings,
  • Generate List_Bean types for the XML mapped queries,
  • Add support for Ada enum generation,
  • Add test template generation,
  • Add AWA service template generation,
  • Add support for blob model mapping,
  • New command 'add-ajax-form', 'add-query', 'dist', 'create-plugin'

The Dynamo tool is available at To build Dynamo, you will need:

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Ada perfect hash generation with gperfhash

By stephane.carrez

A perfect hash function is a function that returns a distinct hash number for each keyword of a well defined set. gperf is famous and well known perfect hash generator used for C or C++ languages. Ada is not supported.

The gperfhash is a sample from the Ada Utility Library which generates an Ada package that implements such perfect hash operation. It is not as complete as gperf but allows to easily get a hash operation. The gperfhash tool uses the GNAT package GNAT.Perfect_Hash_Generators.

Pre requisite

Since the gperfhash tool is provided by the Ada Util samples, you must build these samples with the following command:

$ gnatmake -Psamples

Define a keyword file

First, create a file which contains one keyword on each line. For example, let's write a keywords.txt file which contains the following three keywords:


Generate the package

Run the gperfhash tool and give it the package name.

$ gperfhash -p Hashing keywords.txt

The package defines a Hash and an Is_Keyword function. The Hash function returns a hash number for each string passed as argument. The hash number will be different for each string that matches one of our keyword. You can give a string not in the keyword list, in that case the hash function will return a number that collides with a hash number of one or our keyword.

The Is_Keyword function allows to check whether a string is a keyword of the list. This is very useful when you just want to know whether a string is a reserved keyword in some application.

The package specification is the following:

--  Generated by gperfhash
package Hashing is

   function Hash (S : String) return Natural;

   --  Returns true if the string <b>S</b> is a keyword.
   function Is_Keyword (S : in String) return Boolean;

   type Name_Access is access constant String;
   type Keyword_Array is array (Natural range <>) of Name_Access;
   Keywords : constant Keyword_Array;
end Hashing;

How to use the hash

Using the perfect hash generator is simple:

with Hashing;

  if Hashing.Is_Keyword (S) then
     -- 'S' is one of our keyword
     -- No, it's not a keyword
  end if;
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