Quarkus - Writing REST Services with RESTEasy Reactive

This guide explains how to write REST Services with RESTEasy Reactive in Quarkus.

What is RESTEasy Reactive?

RESTEasy Reactive is a new JAX-RS implementation written from the ground up to work on our common Vert.x layer and is thus fully reactive, while also being very tightly integrated with Quarkus and consequently moving a lot of work to build time.

You should be able to use it in place of any JAX-RS implementation, but on top of that it has great performance for both blocking and non-blocking endpoints, and a lot of new features on top of what JAX-RS provides.

Writing endpoints

Getting started

Add the following import to your pom.xml file:

<dependency>
    <groupId>io.quarkus</groupId>
    <artifactId>quarkus-resteasy-reactive</artifactId>
</dependency>

You can now write your first endpoint in the org.acme.rest.Endpoint class:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.Path;

@Path("")
public class Endpoint {

    @GET
    public String hello(){
        return "Hello, World!";
    }
}

Terminology

REST: REpresentational State Transfer
Endpoint: Java method which is called to serve a REST call
URL / URI (Uniform Resource Locator / Identifier): Used to identify the location of REST resources (specification)
Resource: Represents your domain object. This is what your API serves and modifies. Also called an entity in JAX-RS.
Representation: How your resource is represented on the wire, can vary depending on content types
Content type: Designates a particular representation (also called a media type), for example text/plain or application/json
HTTP: Underlying wire protocol for routing REST calls (see HTTP specifications)
HTTP request: the request part of the HTTP call, consisting of an HTTP method, a target URI, headers and an optional message body
HTTP response: the response part of the HTTP call, consisting of an HTTP response status, headers and an optional message body

Declaring endpoints: URI mapping

Any class annotated with a @Path annotation can have its methods exposed as REST endpoints, provided they have an HTTP method annotation (see below).

That @Path annotation defines the URI prefix under which those methods will be exposed. It can be empty, or contain a prefix such as rest or rest/V1.

Each exposed endpoint method can in turn have another @Path annotation which adds to its containing class annotation. For example, this defines a rest/hello endpoint:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.Path;

@Path("rest")
public class Endpoint {

    @Path("hello")
    @GET
    public String hello(){
        return "Hello, World!";
    }
}

See URI parameters for more information about URI mapping.

You can set the root path for all rest endpoints using the @ApplicationPath annotation, as shown below.

package org.acme.rest;

import javax.ws.rs.ApplicationPath;
import javax.ws.rs.core.Application;

@ApplicationPath("/api")
public static class MyApplication extends Application {

}

This will cause all rest endpoints to be resolve relative to /api, so the endpoint above with @Path("rest") would be accessible at /api/rest/. You can also set the `quarkus.rest.path build time property to set the root path if you don’t want to use an annotation.

Declaring endpoints: HTTP methods

Each endpoint method must be annotated with one of the following annotations, which defines which HTTP method will be mapped to the method:

Table 1. Table HTTP method annotations
Annotation	Usage
`@GET`	Obtain a resource representation, should not modify state, idempotent (HTTP docs)
`@HEAD`	Obtain metadata about a resource, similar to `GET` with no body (HTTP docs)
`@POST`	Create a resource and obtain a link to it (HTTP docs)
`@PUT`	Replace a resource or create one, should be idempotent (HTTP docs)
`@DELETE`	Delete an existing resource, idempotent (HTTP docs)
`@OPTIONS`	Obtain information about a resource, idempotent (HTTP docs)
`@PATCH`	Update a resource, or create one, not idempotent (HTTP docs)

You can also declare other HTTP methods by declaring them as an annotation with the @HttpMethod annotation:

package org.acme.rest;

import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

import javax.ws.rs.HttpMethod;
import javax.ws.rs.Path;

@Retention(RetentionPolicy.RUNTIME)
@HttpMethod("FROMAGE")
@interface FROMAGE {
}

@Path("")
public class Endpoint {

    @FROMAGE
    public String hello(){
        return "Hello, Cheese World!";
    }
}

Declaring endpoints: representation / content types

Each endpoint method may consume or produce specific resource representations, which are indicated by the HTTP Content-Type header, which in turn contains MIME (Media Type) values, such as the following:

text/plain which is the default for any endpoint returning a String.
text/html for HTML (such as with Qute templating)
application/json for a JSON REST endpoint
text/* which is a sub-type wildcard for any text media type
*/* which is a wildcard for any media type

You may annotate your endpoint class with the @Produces or @Consumes annotations, which allow you to specify one or more media types that your endpoint may accept as HTTP request body or produce as HTTP response body. Those class annotations apply to each method.

Any method may also be annotated with the @Produces or @Consumes annotations, in which case they override any eventual class annotation.

The MediaType class has many constants you can use to point to specific pre-defined media types.

See Negotiation for more information.

Accessing request parameters

don’t forget to configure your compiler to generate parameter name information with -parameters (javac) or <parameters> or <maven.compiler.parameters> (Maven).

The following HTTP request elements may be obtained by your endpoint method:

Table 2. Table HTTP request parameter annotations
HTTP element	Annotation	Usage
Path parameter	`@RestPath` (or nothing)	URI template parameter (simplified version of the URI Template specification), see URI parameters for more information.
Query parameter	`@RestQuery`	The value of an URI query parameter
Header	`@RestHeader`	The value of an HTTP header
Cookie	`@RestCookie`	The value of an HTTP cookie
Form parameter	`@RestForm`	The value of an HTTP URL-encoded FORM
Matrix parameter	`@RestMatrix`	The value of an URI path segment parameter

For each of those annotations, you may specify the name of the element they refer to, otherwise they will use the name of the annotated method parameter.

If a client made the following HTTP call:

POST /cheeses;variant=goat/tomme?age=matured HTTP/1.1
Content-Type: application/x-www-form-urlencoded
Cookie: level=hardcore
X-Cheese-Secret-Handshake: fist-bump

smell=strong

Then you could obtain all the various parameters with this endpoint method:

package org.acme.rest;

import javax.ws.rs.POST;
import javax.ws.rs.Path;

import org.jboss.resteasy.reactive.RestCookie;
import org.jboss.resteasy.reactive.RestForm;
import org.jboss.resteasy.reactive.RestHeader;
import org.jboss.resteasy.reactive.RestMatrix;
import org.jboss.resteasy.reactive.RestPath;
import org.jboss.resteasy.reactive.RestQuery;

@Path("/cheeses/{type}")
public class Endpoint {

    @POST
    public String allParams(@RestPath String type,
                            @RestMatrix String variant,
                            @RestQuery String age,
                            @RestCookie String level,
                            @RestHeader("X-Cheese-Secret-Handshake")
                            String secretHandshake,
                            @RestForm String smell){
        return type + "/" + variant + "/" + age + "/" + level + "/" + secretHandshake + "/" + smell;
    }
}

the @RestPath annotation is optional: any parameter whose name matches an existing URI template variable will be automatically assumed to have @RestPath.

You can also use any of the JAX-RS annotations @PathParam, @QueryParam, @HeaderParam, @CookieParam, @FormParam or @MatrixParam for this, but they require you to specify the parameter name.

See Parameter mapping for more advanced use-cases.

Declaring URI parameters

You can declare URI parameters and use regular expressions in your path, so for instance the following endpoint will serve requests for /hello/stef/23 and /hello but not /hello/stef/0x23:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.Path;

@Path("hello")
public class Endpoint {

    @Path("{name}/{age:\\d+}")
    @GET
    public String personalisedHello(String name, int age){
        return "Hello " + name + " is your age really " + age + "?";
    }

    @GET
    public String genericHello(){
        return "Hello stranger";
    }
}

Accessing the request body

Any method parameter with no annotation will receive the method body.^[1], after it has been mapped from its HTTP representation to the Java type of the parameter.

The following parameter types will be supported out of the box:

Table 3. Table Request body parameter type
Type	Usage
`File`	The entire request body in a temporary file
`byte[]`	The entire request body, not decoded
`char[]`	The entire request body, decoded
`String`	The entire request body, decoded
`InputStream`	The request body in a blocking stream
`Reader`	The request body in a blocking stream
All Java primitives and their wrapper classes	Java primitive types
`BigDecimal`, `BigInteger`	Large integers and decimals.
`JsonArray`, `JsonObject`, `JsonStructure`, `JsonValue`	JSON value types
`Buffer`	Vert.x Buffer
any other type	Will be mapped from JSON to that type

You can add support for more body parameter types.

Handling Multipart Form data

To handle HTTP requests that have multipart/form-data as their content type, RESTEasy Reactive introduces the @org.jboss.resteasy.reactive.MultipartForm annotation. Let us look at an example of its use.

Assuming an HTTP request containing a file upload and a form value containing a string description need to be handled, we could write a POJO that will hold this information like so:

import java.util.Map;

import javax.ws.rs.core.MediaType;

import org.jboss.resteasy.reactive.PartType;
import org.jboss.resteasy.reactive.RestForm;
import org.jboss.resteasy.reactive.multipart.FileUpload;

public class FormData {

    @RestForm
    @PartType(MediaType.TEXT_PLAIN)
    public String description;

    @RestForm("image")
    public FileUpload file;
}

The name field will contain the data contained in the part of HTTP request called description (because @RestForm does not define a value, the field name is used), while the file field will contain data about the uploaded file in the image part of HTTP request.

FileUpload provides access to various metadata of the uploaded file. If however all you need is a handle to the uploaded file, java.nio.file.Path or java.io.File could be used.

@PartType is used to aid in deserialization of the corresponding part of the request into the desired Java type. It is very useful when for example the corresponding body part is JSON and needs to be converted to a POJO.

This POJO could be used in a Resource method like so:

import javax.ws.rs.Consumes;
import javax.ws.rs.POST;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

import org.jboss.resteasy.reactive.MultipartForm;

@Path("multipart")
public class Endpoint {

    @POST
    @Produces(MediaType.APPLICATION_JSON)
    @Consumes(MediaType.MULTIPART_FORM_DATA)
    @Path("form")
    public String form(@MultipartForm FormData formData) {
        // return something
    }
}

The use of @MultipartForm as method parameter makes RESTEasy Reactive handle the request as a multipart form request.

When handling file uploads, it is very important to move the file to permanent storage (like a database, a dedicated file system or a cloud storage) in your code that handles the POJO. Otherwise, the file will no longer be accessible when the request terminates. Moreoever if quarkus.http.body.delete-uploaded-files-on-end is set to true, Quarkus will delete the uploaded file when the HTTP response is sent. If the setting is disabled, the file will reside on the file system of the server (in the directory defined by the quarkus.http.body.uploads-directory configuration option), but as the uploaded files are saved with a UUID file name and no additional metadata is saved, these files are essentially a random dump of files.

Returning a response body

In order to return an HTTP response, simply return the resource you want from your method. The method return type and its optional content type will be used to decide how to serialise it to the HTTP response (see Negotiation for more advanced information).

You can return any of the pre-defined types that you can read from the HTTP response, and any other type will be mapped from that type to JSON.

Alternately, you can also return a reactive type such as Uni, Multi or CompletionStage that resolve to one of the mentioned return types.

Setting other response properties

If you need to set more properties on the HTTP response than just the body, such as the status code or headers, you can make your method return the Response type:

package org.acme.rest;

import java.time.Duration;
import java.time.Instant;
import java.util.Date;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.NewCookie;
import javax.ws.rs.core.Response;

@Path("")
public class Endpoint {

    @GET
    public Response hello() {
        // HTTP OK status with text/plain content type
        return Response.ok("Hello, World!", MediaType.TEXT_PLAIN_TYPE)
         // set a response header
         .header("X-FroMage", "Camembert")
         // set the Expires response header to two days from now
         .expires(Date.from(Instant.now().plus(Duration.ofDays(2))))
         // send a new cookie
         .cookie(new NewCookie("Flavour", "praliné"))
         // end of builder API
         .build();
    }
}

Async/reactive support

If your endpoint method needs to accomplish an asynchronous or reactive task before being able to answer, you can declare your method to return the Uni type (from Mutiny), in which case the current HTTP request will be automatically suspended after your method, until the returned Uni instance resolves to a value, which will be mapped to a response exactly according to the previously described rules:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.Path;

import io.smallrye.mutiny.Uni;

@Path("escoffier")
public class Endpoint {

    @GET
    public Uni<Book> culinaryGuide(){
        return Book.findByIsbn("978-2081229297");
    }
}

This allows you to not block the event-loop thread while the book is being fetched from the database, and allows Quarkus to serve more requests until your book is ready to be sent to the client and terminate this request. Check out our Execution Model documentation for more information.

The CompletionStage return type is also supported.

Streaming support

If you want to stream your response element by element, you can make your endpoint method return a Multi type (from Mutiny). This is especially useful for streaming text or binary data.

This example, using Reactive Messaging shows how to stream text data:

package org.acme.rest;

import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;

import org.eclipse.microprofile.reactive.messaging.Channel;

import io.smallrye.mutiny.Multi;

@Path("logs")
public class Endpoint {

    @Inject
    @Channel("log-out")
    Multi<String> logs;

    @GET
    public Multi<String> streamLogs() {
        return logs;
    }
}

Server-Sent Event (SSE) support

If you want to stream JSON objects in your response, you can use Server-Sent Events by just annotating your endpoint method with @Produces(MediaType.SERVER_SENT_EVENTS) and specifying that each element should be serialised to JSON with @RestSseElementType(MediaType.APPLICATION_JSON).

package org.acme.rest;

import javax.inject.Inject;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

import org.jboss.resteasy.reactive.RestSseElementType;

import io.smallrye.mutiny.Multi;
import io.smallrye.mutiny.Uni;

import io.smallrye.reactive.messaging.annotations.Channel;

@Path("escoffier")
public class Endpoint {

    // Inject our Book channel
    @Inject
    @Channel("book-out")
    Multi<Book> books;

    @GET
    // Send the stream over SSE
    @Produces(MediaType.SERVER_SENT_EVENTS)
    // Each element will be sent as JSON
    @RestSseElementType(MediaType.APPLICATION_JSON)
    public Multi<Book> stream() {
        return books;
    }
}

Accessing context objects

There are a number of contextual objects that the framework will give you, if your endpoint method takes parameters of the following type:

Table 4. Table Context object
Type	Usage
`HttpHeaders`	All the request headers
`ResourceInfo`	Information about the current endpoint method and class (requires reflection)
`SecurityContext`	Access to the current user and roles
`SimpleResourceInfo`	Information about the current endpoint method and class (no reflection required)
`UriInfo`	Provides information about the current endpoint and application URI
`Application`	Advanced: Current JAX-RS application class
`Configuration`	Advanced: Configuration about the deployed JAX-RS application
`Providers`	Advanced: Runtime access to JAX-RS providers
`Request`	Advanced: Access to the current HTTP method and Preconditions
`ResourceContext`	Advanced: access to instances of endpoints
`ServerRequestContext`	Advanced: RESTEasy Reactive access to the current request/response
`Sse`	Advanced: Complex SSE use-cases

For example, here is how you can return the name of the currently logged-in user:

package org.acme.rest;

import java.security.Principal;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.core.SecurityContext;

@Path("user")
public class Endpoint {

    @GET
    public String userName(SecurityContext security) {
        Principal user = security.getUserPrincipal();
        return user != null ? user.getName() : "<NOT LOGGED IN>";
    }
}

You can also inject those context objects using @Inject on fields of the same type:

package org.acme.rest;

import java.security.Principal;

import javax.inject.Inject;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.core.SecurityContext;

@Path("user")
public class Endpoint {

    @Inject
    SecurityContext security;

    @GET
    public String userName() {
        Principal user = security.getUserPrincipal();
        return user != null ? user.getName() : "<NOT LOGGED IN>";
    }
}

Or even on your endpoint constructor:

package org.acme.rest;

import java.security.Principal;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.core.SecurityContext;

@Path("user")
public class Endpoint {

    SecurityContext security;

    Endpoint(SecurityContext security){
        this.security = security;
    }

    @GET
    public String userName() {
        Principal user = security.getUserPrincipal();
        return user != null ? user.getName() : "<NOT LOGGED IN>";
    }
}

JSON serialisation

Instead of importing io.quarkus:quarkus-resteasy-reactive, you can import either of the following modules to get support for JSON:

Table 5. Table Context object
GAV	Usage
`io.quarkus:quarkus-resteasy-reactive-jackson`	Jackson support
`io.quarkus:quarkus-resteasy-reactive-jsonb`	JSONB support

In both cases, importing those modules will allow HTTP message bodies to be read from JSON and serialised to JSON, for all the types not already registered with a more specific serialisation.

Advanced Jackson-specific features

When using the quarkus-resteasy-reactive-jackson extension there are some advanced features that RESTEasy Reactive supports.

@JsonView support

JAX-RS methods can be annotated with @JsonView in order to customize the serialization of the returned POJO, on a per method-basis. This is best explained with an example.

A typical use of @JsonView is to hide certain fields on certain methods. In that vein, let’s define two views:

public class Views {

    public static class Public {
    }

    public static class Private extends Public {
    }
}

Let’s assume we have the User POJO on which we want to hide some field during serialization. A simple example of this is:

public class User {

    @JsonView(Views.Private.class)
    public int id;

    @JsonView(Views.Public.class)
    public String name;
}

Depending on the JAX-RS method that returns this user, we might want to exclude the id field from serialization - for example you might want an insecure method to not expose this field. The way we can achieve that in RESTEasy Reactive is shown in the following example:

@JsonView(Views.Public.class)
@GET
@Path("/public")
public User userPublic() {
    return testUser();
}

@JsonView(Views.Private.class)
@GET
@Path("/private")
public User userPrivate() {
    return testUser();
}

When the result the userPublic method is serialized, the id field will not be contained in the response as the Public view does not include it. The result of userPrivate however will include the id as expected when serialized.

Completely customized per method serialization

There are times when you need to completely customize the serialization of a POJO on a per JAX-RS method basis. For such use cases, the @io.quarkus.resteasy.reactive.jackson.CustomSerialization annotation is a great tool, as it allows you to configure a per-method com.fasterxml.jackson.databind.ObjectWriter which can be configured at will.

Here is an example use case:

@CustomSerialization(UnquotedFields.class)
@GET
@Path("/invalid-use-of-custom-serializer")
public User invalidUseOfCustomSerializer() {
    return testUser();
}

where UnquotedFields is a BiFunction defined as so:

public static class UnquotedFields implements BiFunction<ObjectMapper, Type, ObjectWriter> {

    @Override
    public ObjectWriter apply(ObjectMapper objectMapper, Type type) {
        return objectMapper.writer().without(JsonWriteFeature.QUOTE_FIELD_NAMES);
    }
}

Essentially what this class does is force Jackson to not include quotes in the field names.

It is important to note that this customization is only performed for the serialization of the JAX-RS methods that use @CustomSerialization(UnquotedFields.class).

More advanced usage

Here are some more advanced topics that you may not need to know about initially, but could prove useful for more complex use cases.

Execution model, blocking, non-blocking

RESTEasy Reactive is implemented using two main thread types:

Event-loop threads: which are responsible, among other things, for reading bytes from the HTTP request and writing bytes back to the HTTP response
Worker threads: they are pooled and can be used to offload long-running operations

The event-loop threads (also called IO threads) are responsible for actually performing all the IO operations in an asynchronous way, and to trigger any listener interested in the completion of those IO operations.

By default, RESTEasy Reactive will run endpoint methods on the event-loop threads, on the assumption that they are going to be fast and only invoke non-blocking operations.

This is the model of execution that leads to best performance if your endpoints do not do any blocking operation (such as blocking IO, blocking on an asynchronous operation, or sleeping).

If your endpoint method needs to do any of those blocking operations, you should add the @Blocking annotation on your endpoint and it will instead be invoked on a worker thread. Your endpoint method code can remain exactly the same, and it will be allowed to block:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.Path;

import io.smallrye.common.annotation.Blocking;

@Path("yawn")
public class Endpoint {

    @Blocking
    @GET
    public String blockingHello() throws InterruptedException {
        // do a blocking operation
        Thread.sleep(1000);
        return "Yaaaawwwwnnnnnn…";
    }
}

Most of the time, there are ways to achieve the same blocking operations in an asynchronous/reactive way, using Mutiny, Hibernate Reactive or any of the Quarkus Reactive extensions for example:

package org.acme.rest;

import java.time.Duration;

import javax.ws.rs.GET;
import javax.ws.rs.Path;

import io.smallrye.mutiny.Uni;

@Path("yawn")
public class Endpoint {

    @GET
    public Uni<String> blockingHello() throws InterruptedException {
        return Uni.createFrom().item("Yaaaawwwwnnnnnn…")
                // do a non-blocking sleep
                .onItem().delayIt().by(Duration.ofSeconds(2));
    }
}

Exception mapping

If your application needs to return non-nominal HTTP codes in error cases, the best is to throw exceptions that will result in the proper HTTP response being sent by the framework using WebApplicationException or any of its subtypes:

package org.acme.rest;

import javax.ws.rs.BadRequestException;
import javax.ws.rs.GET;
import javax.ws.rs.NotFoundException;
import javax.ws.rs.Path;

@Path("fromages/{fromage}")
public class Endpoint {

    @GET
    public String findFromage(String fromage) {
        if(fromage == null)
            // send a 400
            throw new BadRequestException();
        if(!fromage.equals("camembert"))
            // send a 404
            throw new NotFoundException("Unknown cheese: " + fromage);
        return "Camembert is a very nice cheese";
    }
}

If your endpoint method is delegating calls to another service layer which does not know of JAX-RS, you need a way to turn service exceptions to an HTTP response, and you can do that using the @ServerExceptionMapper annotation on a method, with one parameter of the exception type you want to handle, and turning that exception into a Response (or a Uni<Response>):

package org.acme.rest;

import java.util.Map;

import javax.enterprise.context.ApplicationScoped;
import javax.inject.Inject;
import javax.ws.rs.BadRequestException;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.core.Response;

import org.jboss.resteasy.reactive.server.ServerExceptionMapper;

class UnknownCheeseException extends RuntimeException {
    public final String name;

    public UnknownCheeseException(String name) {
        this.name = name;
    }
}

@ApplicationScoped
class CheeseService {
    private static final Map<String, String> cheeses =
            Map.of("camembert", "Camembert is a very nice cheese",
                   "gouda", "Gouda is acceptable too, especially with cumin");

    public String findCheese(String name) {
        String ret = cheeses.get(name);
        if(ret != null)
            return ret;
        throw new UnknownCheeseException(name);
    }
}

@Path("fromages/{fromage}")
public class Endpoint {

    @Inject
    CheeseService cheeses;

    @ServerExceptionMapper
    public Response mapException(UnknownCheeseException x) {
        return Response.status(Response.Status.NOT_FOUND)
                       .entity("Unknown cheese: " + x.name)
                       .build();
    }

    @GET
    public String findFromage(String fromage) {
        if(fromage == null)
            // send a 400
            throw new BadRequestException();
        return cheeses.findCheese(fromage);
    }
}

exception mappers defined in REST endpoint classes will only be called if the exception is thrown in the same class. If you want to define global exception mappers, simply define them outside a REST endpoint class:

package org.acme.rest;

import javax.ws.rs.core.Response;

import org.jboss.resteasy.reactive.server.ServerExceptionMapper;

class ExceptionMappers {
    @ServerExceptionMapper
    public Response mapException(UnknownCheeseException x) {
        return Response.status(Response.Status.NOT_FOUND)
                       .entity("Unknown cheese: " + x.name)
                       .build();
    }
}

You can also declare exception mappers in the JAX-RS way.

Your exception mapper may declare any of the following parameter types:

Table 6. Table Exception mapper parameters
Type	Usage
An exception type	Defines the exception type you want to handle
Any of the Context objects
`ContainerRequestContext`	A context object to access the current request

It may declare any of the following return types:

Table 7. Table Exception mapper return types
Type	Usage
`Response`	The response to send to the client when the exception occurs
`Uni<Response>`	An asynchronous response to send to the client when the exception occurs

Request or response filters

You can declare functions which are invoked in the following phases of the request processing:

Before the endpoint method is identified: pre-routing request filter
After routing, but before the endpoint method is called: normal request filter
After the endpoint method is called: response filter

These filters allow you to do various things such as examine the request URI, HTTP method, influence routing, look or change request headers, abort the request, or modify the response.

Request filters can be declared with the @ServerRequestFilter annotation:

class Filters {

    @ServerRequestFilter(preMatching = true)
    public void preMatchingFilter(ContainerRequestContext requestContext) {
        // make sure we don't lose cheese lovers
        if("yes".equals(requestContext.getHeaderString("Cheese"))) {
            requestContext.setRequestUri(URI.create("/cheese"));
        }
    }

    @ServerRequestFilter
    public void getFilter(ContainerRequestContext ctx) {
        // only allow GET methods for now
        if(ctx.getMethod().equals(HttpMethod.GET)) {
            ctx.abortWith(Response.status(Response.Status.METHOD_NOT_ALLOWED).build());
        }
    }
}

Similarly, response filters can be declared with the @ServerResponseFilter annotation:

class Filters {
    @ServerResponseFilter
    public void getFilter(ContainerResponseContext responseContext) {
        Object entity = responseContext.getEntity();
        if(entity instanceof String) {
            // make it shout
            responseContext.setEntity(((String)entity).toUpperCase());
        }
    }
}

You can also declare request and response filters in the JAX-RS way.

Your filters may declare any of the following parameter types:

Table 8. Table Filter parameters
Type	Usage
Any of the Context objects
`ContainerRequestContext`	A context object to access the current request
`ContainerResponseContext`	A context object to access the current response
`Throwable`	Any thrown exception, or `null` (only for response filters)

It may declare any of the following return types:

Table 9. Table Filter return types
Type	Usage
`Response`	The response to send to the client instead of continuing the filter chain, or `null` if the filter chain should proceed
`Optional<Response>`	An optional response to send to the client instead of continuing the filter chain, or an empty value if the filter chain should proceed
`Uni<Response>`	An asynchronous response to send to the client instead of continuing the filter chain, or `null` if the filter chain should proceed

Readers and Writers: mapping entities and HTTP bodies

Whenever your endpoint methods return a object (of when they return a Response with an entity), RESTEasy Reactive will look for a way to map that into an HTTP response body.

Similarly, whenever your endpoint method takes an object as parameter, we will look for a way to map the HTTP request body into that object.

This is done via a pluggable system of MessageBodyReader and MessageBodyWriter interfaces, which are responsible for defining which Java type they map from/to, for which media types, and how they turn HTTP bodies to/from Java instances of that type.

For example, if we have our own FroMage type on our endpoint:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.PUT;
import javax.ws.rs.Path;

class FroMage {
    public String name;

    public FroMage(String name) {
        this.name = name;
    }
}

@Path("cheese")
public class Endpoint {

    @GET
    public FroMage sayCheese() {
        return new FroMage("Cheeeeeese");
    }

    @PUT
    public void addCheese(FroMage fromage) {
        System.err.println("Received a new cheese: " + fromage.name);
    }
}

Then we can define how to read and write it with our body reader/writers, annotated with @Provider:

package org.acme.rest;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.nio.charset.StandardCharsets;

import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.MultivaluedMap;
import javax.ws.rs.ext.MessageBodyReader;
import javax.ws.rs.ext.MessageBodyWriter;
import javax.ws.rs.ext.Provider;

@Provider
public class FroMageBodyHandler implements MessageBodyReader<FroMage>,
                                           MessageBodyWriter<FroMage> {

    @Override
    public boolean isWriteable(Class<?> type, Type genericType,
                               Annotation[] annotations, MediaType mediaType) {
        return type == FroMage.class;
    }

    @Override
    public void writeTo(FroMage t, Class<?> type, Type genericType,
                        Annotation[] annotations, MediaType mediaType,
                        MultivaluedMap<String, Object> httpHeaders,
                        OutputStream entityStream)
            throws IOException, WebApplicationException {
        entityStream.write(("[FroMageV1]" + t.name)
                           .getBytes(StandardCharsets.UTF_8));
    }

    @Override
    public boolean isReadable(Class<?> type, Type genericType,
                              Annotation[] annotations, MediaType mediaType) {
        return type == FroMage.class;
    }

    @Override
    public FroMage readFrom(Class<FroMage> type, Type genericType,
                            Annotation[] annotations, MediaType mediaType,
                            MultivaluedMap<String, String> httpHeaders,
                            InputStream entityStream)
            throws IOException, WebApplicationException {
        String body = new String(entityStream.readAllBytes(), StandardCharsets.UTF_8);
        if(body.startsWith("[FroMageV1]"))
            return new FroMage(body.substring(11));
        throw new IOException("Invalid fromage: " + body);
    }

}

If you want to get the most performance our of your writer, you can extend ServerMessageBodyWriter instead of MessageBodyWriter where you will be able to use less reflection and bypass the blocking IO layer:

package org.acme.rest;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.nio.charset.StandardCharsets;

import javax.ws.rs.WebApplicationException;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.MultivaluedMap;
import javax.ws.rs.ext.MessageBodyReader;
import javax.ws.rs.ext.Provider;

import org.jboss.resteasy.reactive.server.spi.ResteasyReactiveResourceInfo;
import org.jboss.resteasy.reactive.server.spi.ServerMessageBodyWriter;
import org.jboss.resteasy.reactive.server.spi.ServerRequestContext;

@Provider
public class FroMageBodyHandler implements MessageBodyReader<FroMage>,
                                           ServerMessageBodyWriter<FroMage> {

    // …

    @Override
    public boolean isWriteable(Class<?> type, ResteasyReactiveResourceInfo target,
                               MediaType mediaType) {
        return type == FroMage.class;
    }

    @Override
    public void writeResponse(FroMage t, ServerRequestContext context)
      throws WebApplicationException, IOException {
        context.serverResponse().end("[FroMageV1]" + t.name);
    }
}

You can restrict which content-types your reader/writer apply to by adding Consumes/Produces annotations on your provider class.

Reader and Writer interceptors

Just as you can intercept requests and responses, you can also intercept readers and writers, by extending the ReaderInterceptor or WriterInterceptor on a class annotated with @Provider.

If we look at this endpoint:

package org.acme.rest;

import javax.ws.rs.GET;
import javax.ws.rs.PUT;
import javax.ws.rs.Path;

@Path("cheese")
public class Endpoint {

    @GET
    public String sayCheese() {
        return "Cheeeeeese";
    }

    @PUT
    public void addCheese(String fromage) {
        System.err.println("Received a new cheese: " + fromage);
    }
}

We can add reader and writer interceptors like this:

package org.acme.rest;

import java.io.IOException;

import javax.ws.rs.WebApplicationException;
import javax.ws.rs.ext.Provider;
import javax.ws.rs.ext.ReaderInterceptor;
import javax.ws.rs.ext.ReaderInterceptorContext;
import javax.ws.rs.ext.WriterInterceptor;
import javax.ws.rs.ext.WriterInterceptorContext;

@Provider
public class FroMageIOInterceptor implements ReaderInterceptor, WriterInterceptor {

    @Override
    public void aroundWriteTo(WriterInterceptorContext context)
      throws IOException, WebApplicationException {
        System.err.println("Before writing " + context.getEntity());
        context.proceed();
        System.err.println("After writing " + context.getEntity());
    }

    @Override
    public Object aroundReadFrom(ReaderInterceptorContext context)
      throws IOException, WebApplicationException {
        System.err.println("Before reading " + context.getGenericType());
        Object entity = context.proceed();
        System.err.println("After reading " + entity);
        return entity;
    }
}

Parameter mapping

All Request Parameters can be declared as String, but also any of the following types:

Types for which a ParamConverter is available via a registered ParamConverterProvider.
Primitive types.
Types that have a constructor that accepts a single String argument.
Types that have a static method named valueOf or fromString with a single String argument that return an instance of the type. If both methods are present then valueOf will be used unless the type is an enum in which case fromString will be used.
List<T>, Set<T>, or SortedSet<T>, where T satisfies any above criterion.

The following example illustrates all those possibilities:

package org.acme.rest;

import java.lang.annotation.Annotation;
import java.lang.reflect.Type;
import java.util.List;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.ext.ParamConverter;
import javax.ws.rs.ext.ParamConverterProvider;
import javax.ws.rs.ext.Provider;

import org.jboss.resteasy.reactive.RestQuery;

@Provider
class MyConverterProvider implements ParamConverterProvider {

    @Override
    public <T> ParamConverter<T> getConverter(Class<T> rawType, Type genericType,
                                              Annotation[] annotations) {
        // declare a converter for this type
        if(rawType == Converter.class) {
            return (ParamConverter<T>) new MyConverter();
        }
        return null;
    }

}

// this is my custom converter
class MyConverter implements ParamConverter<Converter> {

    @Override
    public Converter fromString(String value) {
        return new Converter(value);
    }

    @Override
    public String toString(Converter value) {
        return value.value;
    }

}

// this uses a converter
class Converter {
    String value;
    Converter(String value) {
        this.value = value;
    }
}

class Constructor {
    String value;
    // this will use the constructor
    public Constructor(String value) {
        this.value = value;
    }
}

class ValueOf {
    String value;
    private ValueOf(String value) {
        this.value = value;
    }
    // this will use the valueOf method
    public static ValueOf valueOf(String value) {
        return new ValueOf(value);
    }
}

@Path("hello")
public class Endpoint {

    @Path("{converter}/{constructor}/{primitive}/{valueOf}")
    @GET
    public String convertions(Converter converter, Constructor constructor,
                              int primitive, ValueOf valueOf,
                              @RestQuery List<Constructor> list){
        return converter + "/" + constructor + "/" + primitive
               + "/" + valueOf + "/" + list;
    }
}

Preconditions

HTTP allows requests to be conditional, based on a number of conditions, such as:

Date of last resource modification
A resource tag, similar to a hash code of the resource to designate its state or version

Let’s see how you can do conditional request validation using the Request context object:

package org.acme.rest;

import java.time.Instant;
import java.time.temporal.ChronoUnit;
import java.time.temporal.TemporalUnit;
import java.util.Date;

import javax.ws.rs.GET;
import javax.ws.rs.PUT;
import javax.ws.rs.Path;
import javax.ws.rs.core.EntityTag;
import javax.ws.rs.core.Request;
import javax.ws.rs.core.Response;
import javax.ws.rs.core.Response.ResponseBuilder;

@Path("conditional")
public class Endpoint {

    // It's important to keep our date on seconds because that's how it's sent to the
    // user in the Last-Modified header
    private Date date = Date.from(Instant.now().truncatedTo(ChronoUnit.SECONDS));
    private int version = 1;
    private EntityTag tag = new EntityTag("v1");
    private String resource = "Some resource";

    @GET
    public Response get(Request request){
        // first evaluate preconditions
        ResponseBuilder conditionalResponse = request.evaluatePreconditions(date, tag);
        if(conditionalResponse != null)
            return conditionalResponse.build();
        // preconditions are OK
        return Response.ok(resource)
                .lastModified(date)
                .tag(tag)
                .build();
    }

    @PUT
    public Response put(Request request, String body){
        // first evaluate preconditions
        ResponseBuilder conditionalResponse = request.evaluatePreconditions(date, tag);
        if(conditionalResponse != null)
            return conditionalResponse.build();
        // preconditions are OK, we can update our resource
        resource = body;
        date = Date.from(Instant.now().truncatedTo(ChronoUnit.SECONDS));
        version++;
        tag = new EntityTag("v" + version);
        return Response.ok(resource)
                .lastModified(date)
                .tag(tag)
                .build();
    }
}

When we call GET /conditional the first time, we will get this response:

HTTP/1.1 200 OK
Content-Type: text/plain;charset=UTF-8
ETag: "v1"
Last-Modified: Wed, 09 Dec 2020 16:10:19 GMT
Content-Length: 13

Some resource

So now if we want to check if we need to fetch a new version, we can make the following request:

GET /conditional HTTP/1.1
Host: localhost:8080
If-Modified-Since: Wed, 09 Dec 2020 16:10:19 GMT

And we would get the following response:

HTTP/1.1 304 Not Modified

Because the resource has not been modified since that date. This saves on sending the resource, but can also help your users detect concurrent modification, for example, let’s suppose that one client wants to update the resource, but another user has modified it since. You can follow the previous GET request with this update:

PUT /conditional HTTP/1.1
Host: localhost:8080
If-Unmodified-Since: Wed, 09 Dec 2020 16:25:43 GMT
If-Match: v1
Content-Length: 8
Content-Type: text/plain

newstuff

And if some other user has modified the resource between your GET and your PUT you would get this answer back:

HTTP/1.1 412 Precondition Failed
ETag: "v2"
Content-Length: 0

Negotiation

One of the main ideas of REST (and HTTP) is that your resource is independent from its representation, and that both the client and server are free to represent their resources in as many media types as they want. This allows the server to declare support for multiple representations and let the client declare which ones it supports and get served something appropriate.

The following endpoint supports serving cheese in plain text or JSON:

package org.acme.rest;

import javax.ws.rs.Consumes;
import javax.ws.rs.GET;
import javax.ws.rs.PUT;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

import com.fasterxml.jackson.annotation.JsonCreator;

class FroMage {
    public String name;
    @JsonCreator
    public FroMage(String name) {
        this.name = name;
    }
    @Override
    public String toString() {
        return "Cheese: " + name;
    }
}

@Path("negotiated")
public class Endpoint {

    @Produces({MediaType.APPLICATION_JSON, MediaType.TEXT_PLAIN})
    @GET
    public FroMage get(){
        return new FroMage("Morbier");
    }

    @Consumes(MediaType.TEXT_PLAIN)
    @PUT
    public FroMage putString(String cheese){
        return new FroMage(cheese);
    }

    @Consumes(MediaType.APPLICATION_JSON)
    @PUT
    public FroMage putJson(FroMage fromage){
        return fromage;
    }
}

The user will be able to select which representation it gets with the Accept header, in the case of JSON:

> GET /negotiated HTTP/1.1
> Host: localhost:8080
> Accept: application/json

< HTTP/1.1 200 OK
< Content-Type: application/json
< Content-Length: 18
<
< {"name":"Morbier"}

And for text:

> GET /negotiated HTTP/1.1
> Host: localhost:8080
> Accept: text/plain
>
< HTTP/1.1 200 OK
< Content-Type: text/plain
< Content-Length: 15
<
< Cheese: Morbier

Similarly, you can PUT two different representations. JSON:

> PUT /negotiated HTTP/1.1
> Host: localhost:8080
> Content-Type: application/json
> Content-Length: 16
>
> {"name": "brie"}

< HTTP/1.1 200 OK
< Content-Type: application/json;charset=UTF-8
< Content-Length: 15
<
< {"name":"brie"}

Or plain text:

> PUT /negotiated HTTP/1.1
> Host: localhost:8080
> Content-Type: text/plain
> Content-Length: 9
>
> roquefort

< HTTP/1.1 200 OK
< Content-Type: application/json;charset=UTF-8
< Content-Length: 20
<
< {"name":"roquefort"}

1. Unless it is a URI template parameter or a context object.