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MicroProfile OpenAPI Specification

1. Introduction

Exposing APIs has become an essential part of all modern applications. At the center of this revolution known as the API Economy we find RESTful APIs, which can transform any application into language agnostic services that can be called from anywhere: on-premises, private cloud, public cloud, etc.

For the clients and providers of these services to connect there needs to be a clear and complete contract. Similar to the WSDL contract for legacy Web Services, the OpenAPI v3 specification is the contract for RESTful Services.

This MicroProfile specification, called OpenAPI 1.0, aims to provide a set of Java interfaces and programming models which allow Java developers to natively produce OpenAPI v3 documents from their JAX-RS applications.

2. Architecture

There are different ways to augment a JAX-RS application in order to produce an OpenAPI document, which are described in Documentation Mechanisms. The picture below provides a quick overview of the different types of components that make up the MP OpenAPI specification:

Architecture Diagram

The remaining sections of this specification will go into the details of each component.

3. Configuration

Configuration of various parts of this specification is provided via the MicroProfile Config mechanism, which means that vendors implementing the MP OpenAPI specification must also implement the MP Config specification.

There are various ways to inject these configuration values into an MP OpenAPI framework, including the default ConfigSource as well as custom ConfigSource.

Vendors implementing the MP OpenAPI specification can optionally provide additional native ways for these configuration values to be injected into the framework (e.g. via a server configuration file), as long as they also implement the MP Config specification.

3.1. List of configurable items

Vendors must support all the Core configurations of this specification. Optionally, they may also support Vendor extensions that allow the configuration of framework-specific values for configurations that affect implementation behavior.

For convenience of vendors (and application developers using custom ConfigSources), the full list of supported configuration keys is available as constants in the OASConfig class.

3.1.1. Core configurations

The following is a list of configuration values that every vendor must support.

Config key Value description

mp.openapi.model.reader

Configuration property to specify the fully qualified name of the OASModelReader implementation.

mp.openapi.filter

Configuration property to specify the fully qualified name of the OASFilter implementation.

mp.openapi.scan.disable

Configuration property to disable annotation scanning. Default value is false.

mp.openapi.scan.packages

Configuration property to specify the list of packages to scan. For example, mp.openapi.scan.packages=com.xyz.PackageA,com.xyz.PackageB

mp.openapi.scan.classes

Configuration property to specify the list of classes to scan. For example, mp.openapi.scan.classes=com.xyz.MyClassA,com.xyz.MyClassB

mp.openapi.scan.exclude.packages

Configuration property to specify the list of packages to exclude from scans. For example, mp.openapi.scan.exclude.packages=com.xyz.PackageC,com.xyz.PackageD

mp.openapi.scan.exclude.classes

Configuration property to specify the list of classes to exclude from scans. For example, mp.openapi.scan.exclude.classes=com.xyz.MyClassC,com.xyz.MyClassD

mp.openapi.servers

Configuration property to specify the list of global servers that provide connectivity information. For example, mp.openapi.servers=https://xyz.com/v1,https://abc.com/v1

mp.openapi.servers.path.

Prefix of the configuration property to specify an alternative list of servers to service all operations in a path. For example, mp.openapi.servers.path./airlines/bookings/{id}=https://xyz.io/v1

mp.openapi.servers.operation.

Prefix of the configuration property to specify an alternative list of servers to service an operation. Operations that want to specify an alternative list of servers must define an operationId, a unique string used to identify the operation. For example, mp.openapi.servers.operation.getBooking=https://abc.io/v1

3.1.2. Vendor extensions

Vendors that wish to provide vendor-specific configuration via MP Config (instead of another native configuration framework) must use the prefix mp.openapi.extensions.

4. Documentation Mechanisms

There are many different ways to provide input for the generation of the resulting OpenAPI document.

The MP OpenAPI specification requires vendors to produce a valid OpenAPI document from pure JAX-RS 2.0 applications. This means that vendors must process all the relevant JAX-RS annotations (such as @Path and @Consumes) as well as Java objects (POJOs) used as input or output to JAX-RS operations. This is a good place to start for application developers that are new to OpenAPI: just deploy your existing JAX-RS application into a MP OpenAPI vendor and check out the output from /openapi!

The application developer then has a few choices:

  1. Augment those JAX-RS annotations with the OpenAPI Annotations. Using annotations means developers don’t have to re-write the portions of the OpenAPI document that are already covered by the JAX-RS framework (e.g. the HTTP method of an operation).

  2. Take the initial output from /openapi as a starting point to document your APIs via Static OpenAPI files. It’s worth mentioning that these static files can also be written before any code, which is an approach often adopted by enterprises that want to lock-in the contract of the API. In this case, we refer to the OpenAPI document as the "source of truth", by which the client and provider must abide.

  3. Use the Programming model to provide a bootstrap (or complete) OpenAPI model tree.

Additionally, a Filter is described which can update the OpenAPI model after it has been built from the previously described documentation mechanisms.

4.1. Annotations

Many of these OpenAPI v3 annotations were derived from the Swagger Core library, which allows for a mostly-mechanical transformation of applications that are using that library and wish to take advantage to the official MP OpenAPI interfaces.

4.1.1. Quick overview of annotations

The following annotations are found in the org.eclipse.microprofile.openapi.annotations package.

Annotation Description

@Callback

Represents a callback URL that will be invoked.

@Callbacks

Represents an array of Callback URLs that can be invoked.

@CallbackOperation

Represents an operation that will be invoked during the callback.

@Components

A container that holds various reusable objects for different aspects of the OpenAPI Specification.

@Explode

Enumeration used to define the value of the explode property.

@ParameterIn

Enumeration representing the parameter’s in property.

@ParameterStyle

Enumeration for the parameter’s style property.

@SecuritySchemeIn

Enumeration for the security scheme’s in property.

@SecuritySchemeType

Enumeration for the security scheme’s type property.

@Extension

Adds an extension with contained properties.

@Extensions

Adds custom properties to an extension.

@ExternalDocumentation

References an external resource for extended documentation.

@Header

Describes a single header object.

@Contact

Contact information for the exposed API.

@Info

This annotation encapsulates metadata about the API.

@License

License information for the exposed API.

@Link

Represents a design-time link for a response.

@LinkParameter

Represents a parameter to pass to the linked operation.

@Content

Provides schema and examples for a particular media type.

@DiscriminatorMapping

Used to differentiate between other schemas which may satisfy the payload description.

@Encoding

Single encoding definition to be applied to single Schema Object.

@ExampleObject

Illustrates an example of a particular content.

@Schema

Allows the definition of input and output data types.

@OpenAPIDefinition

General metadata for an OpenAPI definition.

@Operation

Describes an operation or typically a HTTP method against a specific path.

@Parameter

Describes a single operation parameter.

@Parameters

Encapsulates input parameters.

@RequestBody

Describes a single request body.

@APIResponse

Describes a single response from an API operation.

@APIResponses

A container for multiple responses from an API operation.

@OAuthFlow

Configuration details for a supported OAuth Flow.

@OAuthFlows

Allows configuration of the supported OAuth Flows.

@OAuthScope

Represents an OAuth scope.

@SecurityRequirement

Specifies a security requirement for an operation.

@SecurityRequirements

Represents an array of security requirements where only one needs to be satisfied.

@SecurityRequirementsSet

Represents an array of security requirements that need to be satisfied.

@SecurityScheme

Defines a security scheme that can be used by the operations.

@SecuritySchemes

Represents an array of security schemes that can be specified.

@Server

Represents a server used in an operation or used by all operations in an OpenAPI document.

@Servers

A container for multiple server definitions.

@ServerVariable

Represents a server variable for server URL template substitution.

@Tag

Represents a tag for the API endpoint.

@Tags

A container of multiple tags.

Overrides

When the same annotation is used on a class and a method, the values from the method instance will take precedence for that particular method. This commonly occurs with the @Server and @Tag annotations.

In other cases, such as with @Parameter and @RequestBody, the annotation values from the method’s parameters takes precedence over corresponding annotation values from the method itself - in this scenario the combined usage of these annotations is allowed but discouraged, as it is error prone.

The @Schema annotation has a complex set of possible combinations. It can placed on POJOs (and their fields / methods) and referenced from many other annotations. In the event that a @Schema#implementation value points to a POJO that also contains a @Schema annotation, the values are merged but with precedence given to the referrer annotation (i.e. the one that contains the implementation key). This allows POJO models to be reusable and configurable.

4.1.2. Detailed usage of key annotations

Operation
Sample 1 - Simple operation description
@GET
@Path("/findByStatus")
@Operation(summary = "Finds Pets by status",
           description = "Multiple status values can be provided with comma separated strings")
public Response findPetsByStatus(...) { ... }
Output for Sample 1
/pet/findByStatus:
  get:
    summary: Finds Pets by status
    description: Multiple status values can be provided with comma separated strings
    operationId: findPetsByStatus
Sample 2 - Operation with different responses
@GET
@Path("/{username}")
@Operation(summary = "Get user by user name")
@APIResponse(description = "The user",
             content = @Content(mediaType = "application/json",
                                schema = @Schema(implementation = User.class))),
@APIResponse(responseCode = "400", description = "User not found")
public Response getUserByName(
        @Parameter(description = "The name that needs to be fetched. Use user1 for testing. ", required = true) @PathParam("username") String username)
 {...}
Output for Sample 2
/user/{username}:
    get:
      summary: Get user by user name
      operationId: getUserByName
      parameters:
      - name: username
        in: path
        description: 'The name that needs to be fetched. Use user1 for testing. '
        required: true
        schema:
          type: string
      responses:
        default:
          description: The user
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/User'
        400:
          description: User not found
RequestBody
Sample 1 - Simple RequestBody
@POST
@Path("/user")
@Operation(summary = "Create user",
           description = "This can only be done by the logged in user.")
public Response methodWithRequestBody(
          @RequestBody(description = "Created user object", required = true,
                       content = @Content(schema = @Schema(implementation = User.class))) User user,
          @QueryParam("name") String name, @QueryParam("code") String code)
          { ... }
Output for Sample 1
post:
    summary: Create user
      description: This can only be done by the logged in user.
      operationId: methodWithRequestBody
    parameters:
    - name: name
      in: query
      schema:
      type: string
    - name: code
      in: query
      schema:
      type: string
    requestBody:
      description: Created user object
      content:
        '*/*':
          schema:
            $ref: '#/components/schemas/User'
            required: true
    responses:
      default:
        description: no description
Servers
Sample 1 - Extended Server scenarios
@OpenAPIDefinition(
    servers = {
        @Server(
            description = "definition server 1",
            url = "http://{var1}.definition1/{var2}",
            variables = {
                @ServerVariable(name = "var1",
                                description = "var 1",
                                defaultValue = "1",
                                enumeration = {"1", "2"}),
                @ServerVariable(name = "var2",
                                description = "var 2",
                                defaultValue = "1",
                                enumeration = {"1", "2"})})})
@Server(
    description = "class server 1",
    url = "http://{var1}.class1/{var2}",
    variables = {
            @ServerVariable(
                      name = "var1",
                      description = "var 1",
                      defaultValue = "1",
                      enumeration = {"1", "2"}),
            @ServerVariable(
                      name = "var2",
                      description = "var 2",
                      defaultValue = "1",
                      enumeration = {"1", "2"})})
@Server(
    description = "class server 2",
    url = "http://{var1}.class2",
    variables = {
            @ServerVariable(
                       name = "var1",
                       description = "var 1",
                       defaultValue = "1",
                       enumeration = {"1", "2"})})
public class ServersResource {

    @GET
    @Path("/")
    @Server(
        description = "method server 1",
        url = "http://{var1}.method1",
        variables = {
            @ServerVariable(
                      name = "var1",
                      description = "var 1",
                      defaultValue = "1",
                      enumeration = {"1", "2"})})
    @Server(
        description = "method server 2",
        url = "http://method2"
    )
    public Response getServers() {
        return Response.ok().entity("ok").build();
    }
}
Output for Sample 1
openapi: 3.0.2
servers:
- url: http://{var1}.definition1/{var2}
  description: definition server 1
  variables:
    var1:
      description: var 1
      enum:
      - "1"
      - "2"
      default: "1"
    var2:
      description: var 2
      enum:
      - "1"
      - "2"
      default: "1"
paths:
  /:
    get:
      operationId: getServers
      responses:
        default:
          description: default response
      servers:
      - url: http://{var1}.class1/{var2}
        description: class server 1
        variables:
          var1:
            description: var 1
            enum:
            - "1"
            - "2"
            default: "1"
          var2:
            description: var 2
            enum:
            - "1"
            - "2"
            default: "1"
      - url: http://{var1}.class2
        description: class server 2
        variables:
          var1:
            description: var 1
            enum:
            - "1"
            - "2"
            default: "1"
      - url: http://{var1}.method1
        description: method server 1
        variables:
          var1:
            description: var 1
            enum:
            - "1"
            - "2"
            default: "1"
      - url: http://method2
        description: method server 2
        variables: {}
Schema
Sample 1 - Schema POJO
@Schema(name="MyBooking", description="POJO that represents a booking.")
public class Booking {
    @Schema(required = true, example = "32126319")
    private String airMiles;

    @Schema(required = true, example = "window")
    private String seatPreference;
}
Output for Sample 1
components:
  schemas:
    MyBooking:
      description: POJO that represents a booking.
      required:
      - airMiles
      - seatPreference
      type: object
      properties:
        airMiles:
          type: string
          example: "32126319"
        seatPreference:
          type: string
          example: window
Sample 2 - Schema POJO reference
@POST
public Response createBooking(
     @RequestBody(description = "Create a new booking.",
                  content = @Content(mediaType = "application/json",
                                     schema = @Schema(implementation = Booking.class))) Booking booking) {
Output for Sample 2
post:
  operationId: createBooking
  requestBody:
    description: Create a new booking.
    content:
      application/json:
        schema:
          $ref: '#/components/schemas/MyBooking'

For more samples please see the MicroProfile Wiki.

4.2. Static OpenAPI files

Application developers may wish to include a pre-generated OpenAPI document that was written separately from the code (e.g. with an editor such as this).

Depending on the scenario, the document may be fully complete or partially complete. If a document is fully complete then the application developer will want to set the mp.openapi.scan.disable configuration property to true. If a document is partially complete, then the application developer will need to augment the OpenAPI snippet with annotations, programming model, or via the filter.

4.2.1. Location and formats

Vendors are required to fetch a single document named openapi with an extension of yml, yaml or json, inside the application module’s root META-INF folder. If there is more than one document found that matches one of these extensions the behavior of which file is chosen is undefined (i.e. each vendor may implement their own logic), which means that application developers should only place a single openapi document into that folder.

For convenience, you may also place your microprofile-config.properties in the root META-INF folder, if you wish to keep both documents in the same directory. This is in addition to the default locations defined by MicroProfile Config.

4.3. Programming model

Application developers are able to provide OpenAPI elements via Java POJOs. The complete set of models are found in the org.eclipse.microprofile.openapi.models package.

4.3.1. OASFactory

The OASFactory is used to create all of the elements of an OpenAPI tree.

For example, the following snippet creates a simple Info element that contains a title, description, and version.

OASFactory.createObject(Info.class).title("Airlines").description("Airlines APIs").version("1.0.0");

4.3.2. OASModelReader

The OASModelReader interface allows application developers to bootstrap the OpenAPI model tree used by the processing framework. To use it, simply create an implementation of this interface and register it using the mp.openapi.model.reader configuration key, where the value is the fully qualified name of the reader class.

Sample META-INF/microprofile-config.properties
mp.openapi.model.reader=com.mypackage.MyModelReader

Similar to static files, the model reader can be used to provide either complete or partial model trees. If providing a complete OpenAPI model tree, application developers should set the mp.openapi.scan.disable configuration to true. Oherwise this partial model will be used as the base model during the processing of the other Documentation Mechanisms.

Vendors are required to call the OASReader a single time, in the order defined by the Processing rules section. Only a single OASReader instance is allowed per application.

4.4. Filter

There are many scenarios where application developers may wish to update or remove certain elements and fields of the OpenAPI document. This is done via a filter, which is called once after all other documentation mechanisms have completed.

4.4.1. OASFilter

The OASFilter interface allows application developers to receive callbacks for various key OpenAPI elements. The interface has a default implementation for every method, which allows application developers to only override the methods they care about. To use it, simply create an implementation of this interface and register it using the mp.openapi.filter configuration key, where the value is the fully qualified name of the filter class.

Sample META-INF/microprofile-config.properties
mp.openapi.filter=com.mypackage.MyFilter

Vendors are required to call the registered filter once for each filtered element. For example, the method filterPathItem is called for each corresponding PathItem element in the model tree. This allows application developers to filter the element and any of its descendants.

The order of filter methods called is undefined, with two exceptions:

  1. All filterable descendant elements of a filtered element must be called before its ancestor.

  2. The filterOpenAPI method must be the last method called on a filter (which is just a specialization of the first exception).

4.5. Processing rules

The processed document available from the OpenAPI Endpoint is built from a variety of sources, which were outlined in the sub-headings of Documentation Mechanisms. Vendors are required to process these different sources in the following order:

  1. Fetch configuration values from mp.openapi namespace

  2. Call OASModelReader

  3. Fetch static OpenAPI file

  4. Process annotations

  5. Filter model via OASFilter

Example processing:

  • A vendor starts by fetching all available Configuration. If an OASModelReader was specified in that configuration list, its buildModel method is called to form the starting OpenAPI model tree for this application.

  • Any Vendor extensions are added on top of that starting model (overriding conflicts), or create a new model if an OASModelReader was not registered.

  • The vendor searches for a file as defined in the section Static OpenAPI files. If found, it will read that document and merge with the model produced by previous processing steps (if any), where conflicting elements from the static file will override the values from the original model.

  • If annotation scanning was not disabled, the JAX-RS and OpenAPI annotations from the application will be processed, further overriding any conflicting elements from the current model.

  • The final model is filtered by walking the model tree and invoking all registered OASFilter classes.

5. OpenAPI Endpoint

5.1. Overview

A fully processed OpenAPI document must be available at the root URL /openapi, as a HTTP GET operation.

For example, GET http://myHost:myPort/openapi.

This document represents the result of the applied Processing rules.

The protocol required is http. Vendors are encouraged, but not required, to support the https protocol as well, to enable a secure connection to the OpenAPI endpoint.

5.2. Content format

The default format of the /openapi endpoint is YAML.

Vendors must also support the JSON format if the request contains an Accept header with a value of application/json, in which case the response must contain a Content-Type header with a value of application/json.

5.3. Query parameters

No query parameters are required for the /openapi endpoint. However, one suggested but optional query parameter for vendors to support is format, where the value can be either JSON or YAML, to facilitate the toggle between the default YAML format and JSON format.

5.4. Context root behavior

Vendors are required to ensure that the combination of each global server element and pathItem element resolve to the absolute backend URL of that particular path. If that pathItem contains a servers element , then this list of operation-level server elements replaces the global list of servers for that particular pathItem.

For example: an application may have an ApplicationPath annotation with the value of /, but is assigned the context root of /myApp during deployment. In this case, the server elements (either global or operation-level) must either end with /myApp or a corresponding proxy. Alternatively it is valid, but discouraged, to add that context root (/myApp) to every pathItem defined in that application.

5.5. Multiple applications

The 1.0 version of the MicroProfile OpenAPI specification does not define how the /openapi endpoint may be partitioned in the event that the MicroProfile runtime supports deployment of multiple applications. If an implementation wishes to support multiple applications within a MicroProfile runtime, the semantics of the /openapi endpoint are expected to be the logical union of all the applications in the runtime, which would imply merging multiple OpenAPI documents into a single valid document (handling conflicting IDs and unique names).

6. Integration with other MicroProfile specifications

This section will outline specific integrations between MicroProfile OpenAPI and other MicroProfile specifications.

6.1. MicroProfile Rest Client

It is common that a microservice (A) using MicroProfile OpenAPI will also use MicroProfile Rest Client to make outbound calls into another microservice (B). In this case, we do not want the interface for microservice (B) to appear in microservice (A)'s OAS3 document.

Therefore, vendors are required to exclude from the final OAS3 document any interface annotated with org.eclipse.microprofile.rest.client.inject.RegisterRestClient.

7. Limitations

7.1. Internationalization

The 1.0 version of the MicroProfile OpenAPI spec does not require vendors to support multiple languages based on the Accept-Language. One reasonable approach is for vendors to support unique keys (instead of hardcoded text) via the various Documentation Mechanisms, so that the implementing framework can perform a global replacement of the keys with the language-specific text that matches the Accept-Language request for the /openapi endpoint. A cache of processed languages can be kept to improve performance.

7.2. Validation

The MP OpenAPI 1.0 specification does not mandate vendors to validate the resulting OpenAPI v3 model (after processing the 5 steps previously mentioned), which means that the behavior of invalid models is vendor specific (i.e. vendors may choose to ignore, reject, or pass-through invalid inputs).

7.3. Cross Origin Resource Sharing (CORS)

The MP OpenAPI 1.0 specification does not require vendors to support CORS for the /openapi endpoint. The behavior of CORS requests is implementation dependent.

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