Specification: Rest Client for MicroProfile

Version: 1.0-RC3

Status: Draft

Release: December 19, 2017

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Microprofile Rest Client

MicroProfile Rest Client Definition Examples

MicroProfile TypeSafe Rest Clients are defined as Java interfaces.

Sample Definitions

public interface MyServiceClient {
    Response greet();

This simple API exposes one API call, located at /greet from the base URL of the client. Invoking this endpoint returns a javax.ws.rs.core.Response object that represents the raw response from invoking the API. Below is a more comprehensive example of a client.

public interface UsersClient {
    Response options();

    Response head();

    List<User> getUsers();

    User getUser(@PathParam("userId") String userId);

    Response headUser(@PathParam("userId") String userId);

    Response createUser(@HeaderParam("Authorization") String authorization, User user);

    Response updateUser(@BeanParam PutUser putUser, User user);

    Response deleteUser(@CookieParam("AuthToken") String authorization, @PathParam("userId") String userId);

public class PutUser {
    private String authorization;
    private String userId;
    // getters, setters, constructors omitted

All built in HTTP methods are supported by the client API. Likewise, all base parameter types (query, cookie, matrix, path, form and bean) are supported. If you only need to inspect the body, you can provide a POJO can be processed by the underlying MessageBodyReader or MessageBodyWriter. Otherwise, you can receive the entire Response object for parsing the body and header information from the server invocation.

Invalid Client Interface Examples

Invalid client interfaces will result in a RestClientDefinitionException (which may be wrapped in a DefinitionException if using CDI). Invalid interfaces can include:

  • Using multiple HTTP method annotations on the same method

A client interface method may contain, at most, one HTTP method annotation (such as javax.ws.rs.GET, javax.ws.rs.PUT, javax.ws.rs.OPTIONS, etc.). If a method is annotated with more than one HTTP method, the implementation must throw a RestClientDefinitionException.

public interface MultipleVerbsClient {
    Response ambiguousClientMethod()
  • Invalid URI templates

A client interface that accepts parameters based on the URI path must ensure that the path parameter is defined correctly in the @Path annotation. For example:

public interface GoodInterfaceOne {
    public Response deleteEntry(@PathParam("someParam") String entryNameToDelete);

public interface GoodInterfaceTwo {
    public Response quickCheck(@PathParam("someOtherParam") String entryNameToCheck);

Both of these interfaces show valid usage of the @PathParam annotation. In GoodInterfaceOne, the URI template is specified at the class-level @Path annotation; in GoodInterfaceTwo, the template is specified at the method-level.

Implementations must throw a RestClientDefinitionException if a @Path annotation specifies an unresolved URI template or if a @PathParam annotations specifies a template that is not specified in a @Path annotation on the enclosing method or interface. For example, the following three interfaces will result in a RestClientDefinitionException:

public interface BadInterfaceOne {
    public Response deleteEntry();

public interface BadInterfaceTwo {
    public Response quickCheck(@PathParam("someOtherParam") String entryNameToCheck);

public interface BadInterfaceThree {
    public Response quickCheck(@PathParam("notTheSameParam") String entryNameToCheck);

BadInterfaceOne declares a URI template named "someParam" but the deleteEntry method does not specify a @PathParam("someParam") annotation. BadInterfaceTwo does not declare a URI template, but the quickCheck method specifies a @PathParam annotation on a parameter. BadInterfaceThree has a mismatch. The @Path annotation declares a URI template named "someOtherParam" but the @PathParam annotation specifies a template named "notTheSameParam". All three interfaces will result in a RestClientDefinitionException.

MicroProfile Rest Client Programmatic Lookup

Type Safe Rest Clients support both programmatic look up and CDI injection approaches for usage. An implementation of MicroProfile Rest Client is expected to support both use cases.

Sample Builder Usage

public class SomeService {
   public Response doWorkAgainstApi(URL apiUrl, ApiModel apiModel) {
       RemoteApi remoteApi = RestClientBuilder.newBuilder()
       return remoteApi.execute(apiModel);

Specifying the baseUrl is the URL to the remote service. The proxy method takes an interface that defines one or more API methods to be invoked, returning back an instance of that interface that can be used to perform API calls.

MicroProfile Rest Client Provider Registration

The RestClientBuilder interface extends the Configurable interface from JAX-RS, allowing a user to register custom providers while its being built. The behavior of the providers supported is defined by the JAX-RS Client API specification. Below is a list of provider types expected to be supported by an implementation:


Filters of type ClientResponseFilter are invoked in order when a response is received from a remote service.


Filters of type ClientRequestFilter are invoked in order when a request is made to a remote service.


The MessageBodyReader interface defined by JAX-RS allows the entity to be read from the API response after invocation.


The MessageBodyWriter interface defined by JAX-RS allows a request body to be written in the request for @POST, @PUT operations, as well as other HTTP methods that support bodies.


The ParamConverter interface defined by JAX-RS allows a parameter in a resource method to be converted to a format to be used in a request or a response.


The ReaderInterceptor interface is a listener for when a read occurs against the response received from a remote service call.


The WriterInterceptor interface is a listener for when a write occurs to the stream to be sent on the remote service invocation.


The ResponseExceptionMapper is specific to MicroProfile Rest Client. This mapper will take a Response object retrieved via an invocation of a client and convert it to a Throwable, if applicable. The runtime should scan all of the registered mappers, sort them ascending based on getPriority(), find the ones that can handle the given status code and response headers, and invoke them. The first one discovered where toThrowable returns a non-null Throwable that can be thrown given the client method’s signature will be thrown by the runtime.

How to Implement ResponseExceptionMapper

The specification provides default methods for getPriority() and handles(int status, MultivaluedMap<String,Object> headers) methods. Priority is meant to be derived via a @Priority annotation added to the ResponseExceptionMapper implementation. The runtime will sort ascending, taking the one with the lowest numeric value first to check if it can handle the Response object based on it’s status code and headers. The usage of ascending sorting is done to be consistent with JAX-RS behavior.

Likewise, the handles method by default will handle any response status code >= 400. You may override this behavior if you so choose to handle other response codes (both a smaller ranger and a larger range are expected) or base the decision on the response headers.

The toThrowable(Response) method actually does the conversion work. This method should not raise any Throwable, instead just return a Throwable if it can. This method may return null if no throwable should be raised. If this method returns a non-null throwable that is a sub-class of RuntimeException or Error (i.e. unchecked throwables), then this exception will be thrown to the client. Otherwise, the (checked) exception will only be thrown to the client if the client method declares that it throws that type of exception (or a super-class). For example, assume there is a client interface like this:

public interface SomeService {
   public String get() throws SomeException;

   public String put(String someValue);

and assume that the following ResponseExceptionMapper has been registered:

public class MyResponseExceptionMapper implements ResponseExceptionMapper<SomeException> {

   public SomeException toThrowable(Response response) {
       return new SomeException();

In this case, if the get method results in an exception (response status code of 400 or higher), SomeException will be thrown. If the put method results in an exception, SomeException will not be thrown because the method does not declare that it throws SomeException. If another ResponseExceptionMapper (such as the default mapper, see below) is registered that returns a subclass of RuntimeException or Error, then that exception will be thrown.

Any methods that read the response body as a stream must ensure that they reset the stream.

Provider Declaration

In addition to defining providers via the client definition, interfaces may use the @RegisterProvider annotation to define classes to be registered as providers in addition to providers registered via the RestClientBuilder

Provider Priority

Providers may be registered via both annotations and the builder pattern. Providers registered via a builder will take precedence over the @RegisterProvider annotation. The @RegisterProvider annotation takes precedence over the @Priority annotation on the class.

Provider priorities can be overridden using the various register methods on Configurable, which can take a provider class, provider instance as well as priority and mappings of those priorities.

Feature Registration

If the type of provider registered is a Feature, then the priority set by that Feature will be a part of the builder as well. Implementations must maintain the overall priority of registered providers, regardless of how they are registered. A Feature will be used to register additional providers at runtime, and may be registered via @RegisterProvider, configuration or via RestClientBuilder. A Feature will be executed immediately, as a result its priority is not taken into account (features are always executed).

Automatic Provider Registration

Implementations may provide any number of providers registered automatically, but the following providers must be registered by the runtime.

JSON-P Provider

When an interface is registered that contains:

  • @Produces("*/json") or

  • @Consumes("*/json") or

  • a method that declares input or output of type javax.json.JsonValue or any subclass therein

Then a JSON-P MessageBodyReader and MessageBodyWriter will be registered automatically by the implementation. This is in alignment with the JAX-RS 2.0 specification. The provider registered will have a priority of Integer.MAX_VALUE, allowing a user to register a custom provider to be used instead.

Default Message Body Readers and Writers

For the following types, and any media type, the runtime must support `MessageBodyReader`s and `MessageBodyWriter`s being automatically registered.

  • byte[]

  • String

  • InputStream

  • Reader

  • File

Values supported with text/plain

The following types are supported for automatic conversion, only when the media type is text/plain.

  • Number

  • Character and char

  • Long and long

  • Integer and int

  • Double and double

  • Float and float

  • Boolean and boolean (literal value of true and false only)

Default ResponseExceptionMapper

Each implementation will provide out of the box a ResponseExceptionMapper implementation that will map the response into a WebApplicationException whenever the response status code is >= 400. It has a priority of Integer.MAX_VALUE. It is meant to be used as a fall back whenever an error is encountered. This mapper will be registered by default to all client interfaces.

This behavior can be disabled by adding a configuration property microprofile.rest.client.disable.default.mapper with value true that will be resolved as a boolean via MicroProfile Config.

It can also be disabled on a per client basis by using the same property when building the client, RestClientBuilder.newBuilder().property("microprofile.rest.client.disable.default.mapper",true)

MicroProfile Rest Client CDI Support

Rest Client interfaces may be injected as CDI beans. The runtime must create a CDI bean for each interface annotated with RegisterRestClient. The bean created will include a qualifier @RestClient to differentiate the use as an API call against any other beans registered of the same type. Based on the rules of how CDI resolves bean, you are only required to use the qualifier if you have multiple beans of the same type. Any injection point or programmatic look up that uses the qualifier RestClient is expected to be resolved by the MicroProfile Rest Client runtime. Below is an example of said interface, with its matching injection point:

package com.mycompany.remoteServices;

public interface MyServiceClient {
    Response greet();
public class MyService {
    private MyServiceClient client;

Likewise, a user can perform programmatic look up of the interface. Here is one example, but any CDI look up should work:

public class MyService {
    public void execute() {
        MyServiceClient client = CDI.current().select(MyServiceClient.class, RestClient.LITERAL).get();

The qualifier is used to differentiate use cases of the interface that are managed by this runtime, versus use cases that may be managed by other runtimes.

Interfaces are assumed to have a scope of @Dependent unless there is another scope defined on the interface. Implementations are expected to support all of the built in scopes for a bean. Support for custom registered scopes should work, but is not guaranteed.

Support for MicroProfile Config

For CDI defined interfaces, it is possible to use MicroProfile Config properties to define additional behaviors of the rest interface. Assuming this interface:

package com.mycompany.remoteServices;

public interface MyServiceClient {
    Response greet();

The values of the following properties will be provided via MicroProfile Config:

  • com.mycompany.remoteServices.MyServiceClient/mp-rest/url: The base URL to use for this service, the equivalent of the baseUrl method. This property is considered required, however implementations may have other ways to define these URLs.

  • com.mycompany.remoteServices.MyServiceClient/mp-rest/scope: The fully qualified classname to a CDI scope to use for injection, defaults to javax.enterprise.context.Dependent as mentioned above.

  • com.mycompany.remoteServices.MyServiceClient/mp-rest/providers: A comma separated list of fully-qualified provider classnames to include in the client, the equivalent of the register method or the @RegisterProvider annotation.

  • com.mycompany.remoteServices.MyServiceClient/mp-rest/providers/com.mycompany.MyProvider/priority will override the priority of the provider for this interface.

Implementations may support other custom properties registered in similar fashions or other ways.

The url property must resolve to a value that can be parsed by the URL converter required by the MicroProfile Config spec.

The providers property is not aggregated, the value will be read from the highest property ConfigSource

Release Notes for MicroProfile Rest Client 1.0