Reactive Web Applications

Spring Boot simplifies development of reactive web applications by providing auto-configuration for Spring Webflux.

The “Spring WebFlux Framework”

Spring WebFlux is the new reactive web framework introduced in Spring Framework 5.0. Unlike Spring MVC, it does not require the servlet API, is fully asynchronous and non-blocking, and implements the Reactive Streams specification through the Reactor project.

Spring WebFlux comes in two flavors: functional and annotation-based. The annotation-based one is quite close to the Spring MVC model, as shown in the following example:

Java
Kotlin

import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;

import org.springframework.web.bind.annotation.DeleteMapping;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
@RequestMapping("/users")
public class MyRestController {

	private final UserRepository userRepository;

	private final CustomerRepository customerRepository;

	public MyRestController(UserRepository userRepository, CustomerRepository customerRepository) {
		this.userRepository = userRepository;
		this.customerRepository = customerRepository;
	}

	@GetMapping("/{userId}")
	public Mono<User> getUser(@PathVariable Long userId) {
		return this.userRepository.findById(userId);
	}

	@GetMapping("/{userId}/customers")
	public Flux<Customer> getUserCustomers(@PathVariable Long userId) {
		return this.userRepository.findById(userId).flatMapMany(this.customerRepository::findByUser);
	}

	@DeleteMapping("/{userId}")
	public Mono<Void> deleteUser(@PathVariable Long userId) {
		return this.userRepository.deleteById(userId);
	}

}

import org.springframework.web.bind.annotation.DeleteMapping
import org.springframework.web.bind.annotation.GetMapping
import org.springframework.web.bind.annotation.PathVariable
import org.springframework.web.bind.annotation.RequestMapping
import org.springframework.web.bind.annotation.RestController
import reactor.core.publisher.Flux
import reactor.core.publisher.Mono

@RestController
@RequestMapping("/users")
class MyRestController(private val userRepository: UserRepository, private val customerRepository: CustomerRepository) {

	@GetMapping("/{userId}")
	fun getUser(@PathVariable userId: Long): Mono<User?> {
		return userRepository.findById(userId)
	}

	@GetMapping("/{userId}/customers")
	fun getUserCustomers(@PathVariable userId: Long): Flux<Customer> {
		return userRepository.findById(userId).flatMapMany { user: User? ->
			customerRepository.findByUser(user)
		}
	}

	@DeleteMapping("/{userId}")
	fun deleteUser(@PathVariable userId: Long): Mono<Void> {
		return userRepository.deleteById(userId)
	}

}

WebFlux is part of the Spring Framework and detailed information is available in its reference documentation.

“WebFlux.fn”, the functional variant, separates the routing configuration from the actual handling of the requests, as shown in the following example:

Java
Kotlin

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.web.reactive.function.server.RequestPredicate;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.ServerResponse;

import static org.springframework.web.reactive.function.server.RequestPredicates.accept;
import static org.springframework.web.reactive.function.server.RouterFunctions.route;

@Configuration(proxyBeanMethods = false)
public class MyRoutingConfiguration {

	private static final RequestPredicate ACCEPT_JSON = accept(MediaType.APPLICATION_JSON);

	@Bean
	public RouterFunction<ServerResponse> monoRouterFunction(MyUserHandler userHandler) {
		return route()
				.GET("/{user}", ACCEPT_JSON, userHandler::getUser)
				.GET("/{user}/customers", ACCEPT_JSON, userHandler::getUserCustomers)
				.DELETE("/{user}", ACCEPT_JSON, userHandler::deleteUser)
				.build();
	}

}

import org.springframework.context.annotation.Bean
import org.springframework.context.annotation.Configuration
import org.springframework.http.MediaType
import org.springframework.web.reactive.function.server.RequestPredicates.DELETE
import org.springframework.web.reactive.function.server.RequestPredicates.GET
import org.springframework.web.reactive.function.server.RequestPredicates.accept
import org.springframework.web.reactive.function.server.RouterFunction
import org.springframework.web.reactive.function.server.RouterFunctions
import org.springframework.web.reactive.function.server.ServerResponse

@Configuration(proxyBeanMethods = false)
class MyRoutingConfiguration {

	@Bean
	fun monoRouterFunction(userHandler: MyUserHandler): RouterFunction<ServerResponse> {
		return RouterFunctions.route(
			GET("/{user}").and(ACCEPT_JSON), userHandler::getUser).andRoute(
			GET("/{user}/customers").and(ACCEPT_JSON), userHandler::getUserCustomers).andRoute(
			DELETE("/{user}").and(ACCEPT_JSON), userHandler::deleteUser)
	}

	companion object {
		private val ACCEPT_JSON = accept(MediaType.APPLICATION_JSON)
	}

}

Java
Kotlin

import reactor.core.publisher.Mono;

import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;

@Component
public class MyUserHandler {

	public Mono<ServerResponse> getUser(ServerRequest request) {
		...
	}

	public Mono<ServerResponse> getUserCustomers(ServerRequest request) {
		...
	}

	public Mono<ServerResponse> deleteUser(ServerRequest request) {
		...
	}

}

import org.springframework.stereotype.Component
import org.springframework.web.reactive.function.server.ServerRequest
import org.springframework.web.reactive.function.server.ServerResponse
import reactor.core.publisher.Mono

@Component
class MyUserHandler {

	fun getUser(request: ServerRequest?): Mono<ServerResponse> {
		...
	}

	fun getUserCustomers(request: ServerRequest?): Mono<ServerResponse> {
		...
	}

	fun deleteUser(request: ServerRequest?): Mono<ServerResponse> {
		...
	}

}

“WebFlux.fn” is part of the Spring Framework and detailed information is available in its reference documentation.

You can define as many RouterFunction beans as you like to modularize the definition of the router. Beans can be ordered if you need to apply a precedence.

To get started, add the spring-boot-starter-webflux module to your application.

Adding both spring-boot-starter-web and spring-boot-starter-webflux modules in your application results in Spring Boot auto-configuring Spring MVC, not WebFlux. This behavior has been chosen because many Spring developers add spring-boot-starter-webflux to their Spring MVC application to use the reactive WebClient. You can still enforce your choice by setting the chosen application type to SpringApplication.setWebApplicationType(WebApplicationType.REACTIVE).

Spring WebFlux Auto-configuration

Spring Boot provides auto-configuration for Spring WebFlux that works well with most applications.

The auto-configuration adds the following features on top of Spring’s defaults:

Configuring codecs for HttpMessageReader and HttpMessageWriter instances (described later in this document).
Support for serving static resources, including support for WebJars (described later in this document).

If you want to keep Spring Boot WebFlux features and you want to add additional WebFlux configuration, you can add your own @Configuration class of type WebFluxConfigurer but without @EnableWebFlux.

If you want to add additional customization to the auto-configured HttpHandler, you can define beans of type WebHttpHandlerBuilderCustomizer and use them to modify the WebHttpHandlerBuilder.

If you want to take complete control of Spring WebFlux, you can add your own @Configuration annotated with @EnableWebFlux.

Spring WebFlux Conversion Service

If you want to customize the ConversionService used by Spring WebFlux, you can provide a WebFluxConfigurer bean with an addFormatters method.

Conversion can also be customized using the spring.webflux.format.* configuration properties. When not configured, the following defaults are used:

Property DateTimeFormatter Formats

Property	`DateTimeFormatter`	Formats
`spring.webflux.format.date`	`ofLocalizedDate(FormatStyle.SHORT)`	`java.util.Date` and `java.time.LocalDate`
`spring.webflux.format.time`	`ofLocalizedTime(FormatStyle.SHORT)`	java.time’s `LocalTime` and `OffsetTime`
`spring.webflux.format.date-time`	`ofLocalizedDateTime(FormatStyle.SHORT)`	java.time’s `LocalDateTime`, `OffsetDateTime`, and `ZonedDateTime`

spring.webflux.format.date

ofLocalizedDate(FormatStyle.SHORT)

java.util.Date and java.time.LocalDate

spring.webflux.format.time

ofLocalizedTime(FormatStyle.SHORT)

java.time’s LocalTime and OffsetTime

spring.webflux.format.date-time

ofLocalizedDateTime(FormatStyle.SHORT)

java.time’s LocalDateTime, OffsetDateTime, and ZonedDateTime

HTTP Codecs with HttpMessageReaders and HttpMessageWriters

Spring WebFlux uses the HttpMessageReader and HttpMessageWriter interfaces to convert HTTP requests and responses. They are configured with CodecConfigurer to have sensible defaults by looking at the libraries available in your classpath.

Spring Boot provides dedicated configuration properties for codecs, spring.codec.*. It also applies further customization by using CodecCustomizer instances. For example, spring.jackson.* configuration keys are applied to the Jackson codec.

If you need to add or customize codecs, you can create a custom CodecCustomizer component, as shown in the following example:

Java
Kotlin

import org.springframework.boot.web.codec.CodecCustomizer;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.codec.ServerSentEventHttpMessageReader;

@Configuration(proxyBeanMethods = false)
public class MyCodecsConfiguration {

	@Bean
	public CodecCustomizer myCodecCustomizer() {
		return (configurer) -> {
			configurer.registerDefaults(false);
			configurer.customCodecs().register(new ServerSentEventHttpMessageReader());
			// ...
		};
	}

}

import org.springframework.boot.web.codec.CodecCustomizer
import org.springframework.context.annotation.Bean
import org.springframework.http.codec.CodecConfigurer
import org.springframework.http.codec.ServerSentEventHttpMessageReader

class MyCodecsConfiguration {

	@Bean
	fun myCodecCustomizer(): CodecCustomizer {
		return CodecCustomizer { configurer: CodecConfigurer ->
			configurer.registerDefaults(false)
			configurer.customCodecs().register(ServerSentEventHttpMessageReader())
		}
	}

}

You can also leverage Boot’s custom JSON serializers and deserializers.

Static Content

By default, Spring Boot serves static content from a directory called /static (or /public or /resources or /META-INF/resources) in the classpath. It uses the ResourceWebHandler from Spring WebFlux so that you can modify that behavior by adding your own WebFluxConfigurer and overriding the addResourceHandlers method.

By default, resources are mapped on /**, but you can tune that by setting the spring.webflux.static-path-pattern property. For instance, relocating all resources to /resources/** can be achieved as follows:

Properties
YAML

spring.webflux.static-path-pattern=/resources/**

spring:
  webflux:
    static-path-pattern: "/resources/**"

You can also customize the static resource locations by using spring.web.resources.static-locations. Doing so replaces the default values with a list of directory locations. If you do so, the default welcome page detection switches to your custom locations. So, if there is an index.html in any of your locations on startup, it is the home page of the application.

In addition to the “standard” static resource locations listed earlier, a special case is made for Webjars content. By default, any resources with a path in /webjars/** are served from jar files if they are packaged in the Webjars format. The path can be customized with the spring.webflux.webjars-path-pattern property.

Spring WebFlux applications do not strictly depend on the servlet API, so they cannot be deployed as war files and do not use the src/main/webapp directory.

Welcome Page

Spring Boot supports both static and templated welcome pages. It first looks for an index.html file in the configured static content locations. If one is not found, it then looks for an index template. If either is found, it is automatically used as the welcome page of the application.

This only acts as a fallback for actual index routes defined by the application. The ordering is defined by the order of HandlerMapping beans which is by default the following:

RouterFunctionMapping

Endpoints declared with RouterFunction beans

RequestMappingHandlerMapping

Endpoints declared in @Controller beans

RouterFunctionMapping for the Welcome Page

The welcome page support

Template Engines

As well as REST web services, you can also use Spring WebFlux to serve dynamic HTML content. Spring WebFlux supports a variety of templating technologies, including Thymeleaf, FreeMarker, and Mustache.

Spring Boot includes auto-configuration support for the following templating engines:

When you use one of these templating engines with the default configuration, your templates are picked up automatically from src/main/resources/templates.

Error Handling

Spring Boot provides a WebExceptionHandler that handles all errors in a sensible way. Its position in the processing order is immediately before the handlers provided by WebFlux, which are considered last. For machine clients, it produces a JSON response with details of the error, the HTTP status, and the exception message. For browser clients, there is a “whitelabel” error handler that renders the same data in HTML format. You can also provide your own HTML templates to display errors (see the next section).

Before customizing error handling in Spring Boot directly, you can leverage the RFC 9457 Problem Details support in Spring WebFlux. Spring WebFlux can produce custom error messages with the application/problem+json media type, like:

{
	"type": "https://example.org/problems/unknown-project",
	"title": "Unknown project",
	"status": 404,
	"detail": "No project found for id 'spring-unknown'",
	"instance": "/projects/spring-unknown"
}

This support can be enabled by setting spring.webflux.problemdetails.enabled to true.

The first step to customizing this feature often involves using the existing mechanism but replacing or augmenting the error contents. For that, you can add a bean of type ErrorAttributes.

To change the error handling behavior, you can implement ErrorWebExceptionHandler and register a bean definition of that type. Because an ErrorWebExceptionHandler is quite low-level, Spring Boot also provides a convenient AbstractErrorWebExceptionHandler to let you handle errors in a WebFlux functional way, as shown in the following example:

Java
Kotlin

import reactor.core.publisher.Mono;

import org.springframework.boot.autoconfigure.web.WebProperties;
import org.springframework.boot.autoconfigure.web.reactive.error.AbstractErrorWebExceptionHandler;
import org.springframework.boot.web.reactive.error.ErrorAttributes;
import org.springframework.context.ApplicationContext;
import org.springframework.http.HttpStatus;
import org.springframework.http.MediaType;
import org.springframework.http.codec.ServerCodecConfigurer;
import org.springframework.stereotype.Component;
import org.springframework.web.reactive.function.server.RouterFunction;
import org.springframework.web.reactive.function.server.RouterFunctions;
import org.springframework.web.reactive.function.server.ServerRequest;
import org.springframework.web.reactive.function.server.ServerResponse;
import org.springframework.web.reactive.function.server.ServerResponse.BodyBuilder;

@Component
public class MyErrorWebExceptionHandler extends AbstractErrorWebExceptionHandler {

	public MyErrorWebExceptionHandler(ErrorAttributes errorAttributes, WebProperties webProperties,
			ApplicationContext applicationContext, ServerCodecConfigurer serverCodecConfigurer) {
		super(errorAttributes, webProperties.getResources(), applicationContext);
		setMessageReaders(serverCodecConfigurer.getReaders());
		setMessageWriters(serverCodecConfigurer.getWriters());
	}

	@Override
	protected RouterFunction<ServerResponse> getRoutingFunction(ErrorAttributes errorAttributes) {
		return RouterFunctions.route(this::acceptsXml, this::handleErrorAsXml);
	}

	private boolean acceptsXml(ServerRequest request) {
		return request.headers().accept().contains(MediaType.APPLICATION_XML);
	}

	public Mono<ServerResponse> handleErrorAsXml(ServerRequest request) {
		BodyBuilder builder = ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR);
		// ... additional builder calls
		return builder.build();
	}

}

import org.springframework.boot.autoconfigure.web.WebProperties
import org.springframework.boot.autoconfigure.web.reactive.error.AbstractErrorWebExceptionHandler
import org.springframework.boot.web.reactive.error.ErrorAttributes
import org.springframework.context.ApplicationContext
import org.springframework.http.HttpStatus
import org.springframework.http.MediaType
import org.springframework.http.codec.ServerCodecConfigurer
import org.springframework.stereotype.Component
import org.springframework.web.reactive.function.server.RouterFunction
import org.springframework.web.reactive.function.server.RouterFunctions
import org.springframework.web.reactive.function.server.ServerRequest
import org.springframework.web.reactive.function.server.ServerResponse
import reactor.core.publisher.Mono

@Component
class MyErrorWebExceptionHandler(
		errorAttributes: ErrorAttributes, webProperties: WebProperties,
		applicationContext: ApplicationContext, serverCodecConfigurer: ServerCodecConfigurer
) : AbstractErrorWebExceptionHandler(errorAttributes, webProperties.resources, applicationContext) {

	init {
		setMessageReaders(serverCodecConfigurer.readers)
		setMessageWriters(serverCodecConfigurer.writers)
	}

	override fun getRoutingFunction(errorAttributes: ErrorAttributes): RouterFunction<ServerResponse> {
		return RouterFunctions.route(this::acceptsXml, this::handleErrorAsXml)
	}

	private fun acceptsXml(request: ServerRequest): Boolean {
		return request.headers().accept().contains(MediaType.APPLICATION_XML)
	}

	fun handleErrorAsXml(request: ServerRequest): Mono<ServerResponse> {
		val builder = ServerResponse.status(HttpStatus.INTERNAL_SERVER_ERROR)
		// ... additional builder calls
		return builder.build()
	}

}

For a more complete picture, you can also subclass DefaultErrorWebExceptionHandler directly and override specific methods.

In some cases, errors handled at the controller level are not recorded by web observations or the metrics infrastructure. Applications can ensure that such exceptions are recorded with the observations by setting the handled exception on the observation context.

Custom Error Pages

If you want to display a custom HTML error page for a given status code, you can add views that resolve from error/*, for example by adding files to a /error directory. Error pages can either be static HTML (that is, added under any of the static resource directories) or built with templates. The name of the file should be the exact status code, a status code series mask, or error for a default if nothing else matches. Note that the path to the default error view is error/error, whereas with Spring MVC the default error view is error.

For example, to map 404 to a static HTML file, your directory structure would be as follows:

src/
 +- main/
     +- java/
     |   + <source code>
     +- resources/
         +- public/
             +- error/
             |   +- 404.html
             +- <other public assets>

To map all 5xx errors by using a Mustache template, your directory structure would be as follows:

src/
 +- main/
     +- java/
     |   + <source code>
     +- resources/
         +- templates/
             +- error/
             |   +- 5xx.mustache
             +- <other templates>

Web Filters

Spring WebFlux provides a WebFilter interface that can be implemented to filter HTTP request-response exchanges. WebFilter beans found in the application context will be automatically used to filter each exchange.

Where the order of the filters is important they can implement Ordered or be annotated with @Order. Spring Boot auto-configuration may configure web filters for you. When it does so, the orders shown in the following table will be used:

Web Filter Order

Web Filter	Order
`WebFilterChainProxy` (Spring Security)	`-100`
`HttpExchangesWebFilter`	`Ordered.LOWEST_PRECEDENCE - 10`

WebFilterChainProxy (Spring Security)

-100

HttpExchangesWebFilter

Ordered.LOWEST_PRECEDENCE - 10

Embedded Reactive Server Support

Spring Boot includes support for the following embedded reactive web servers: Reactor Netty, Tomcat, Jetty, and Undertow. Most developers use the appropriate starter to obtain a fully configured instance. By default, the embedded server listens for HTTP requests on port 8080.

Customizing Reactive Servers

Common reactive web server settings can be configured by using Spring Environment properties. Usually, you would define the properties in your application.properties or application.yaml file.

Common server settings include:

Network settings: Listen port for incoming HTTP requests (server.port), interface address to bind to (server.address), and so on.
Error management: Location of the error page (server.error.path) and so on.
SSL
HTTP compression

Spring Boot tries as much as possible to expose common settings, but this is not always possible. For those cases, dedicated namespaces such as server.netty.* offer server-specific customizations.

See the ServerProperties class for a complete list.

Programmatic Customization

If you need to programmatically configure your reactive web server, you can register a Spring bean that implements the WebServerFactoryCustomizer interface. WebServerFactoryCustomizer provides access to the ConfigurableReactiveWebServerFactory, which includes numerous customization setter methods. The following example shows programmatically setting the port:

Java
Kotlin

import org.springframework.boot.web.reactive.server.ConfigurableReactiveWebServerFactory;
import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.stereotype.Component;

@Component
public class MyWebServerFactoryCustomizer implements WebServerFactoryCustomizer<ConfigurableReactiveWebServerFactory> {

	@Override
	public void customize(ConfigurableReactiveWebServerFactory server) {
		server.setPort(9000);
	}

}

import org.springframework.boot.web.server.WebServerFactoryCustomizer
import org.springframework.boot.web.reactive.server.ConfigurableReactiveWebServerFactory
import org.springframework.stereotype.Component

@Component
class MyWebServerFactoryCustomizer : WebServerFactoryCustomizer<ConfigurableReactiveWebServerFactory> {

	override fun customize(server: ConfigurableReactiveWebServerFactory) {
		server.setPort(9000)
	}

}

JettyReactiveWebServerFactory, NettyReactiveWebServerFactory, TomcatReactiveWebServerFactory, and UndertowReactiveWebServerFactory are dedicated variants of ConfigurableReactiveWebServerFactory that have additional customization setter methods for Jetty, Reactor Netty, Tomcat, and Undertow respectively. The following example shows how to customize NettyReactiveWebServerFactory that provides access to Reactor Netty-specific configuration options:

Java
Kotlin

import java.time.Duration;

import org.springframework.boot.web.embedded.netty.NettyReactiveWebServerFactory;
import org.springframework.boot.web.server.WebServerFactoryCustomizer;
import org.springframework.stereotype.Component;

@Component
public class MyNettyWebServerFactoryCustomizer implements WebServerFactoryCustomizer<NettyReactiveWebServerFactory> {

	@Override
	public void customize(NettyReactiveWebServerFactory factory) {
		factory.addServerCustomizers((server) -> server.idleTimeout(Duration.ofSeconds(20)));
	}

}

import org.springframework.boot.web.embedded.netty.NettyReactiveWebServerFactory
import org.springframework.boot.web.server.WebServerFactoryCustomizer
import org.springframework.stereotype.Component
import java.time.Duration

@Component
class MyNettyWebServerFactoryCustomizer : WebServerFactoryCustomizer<NettyReactiveWebServerFactory> {

	override fun customize(factory: NettyReactiveWebServerFactory) {
		factory.addServerCustomizers({ server -> server.idleTimeout(Duration.ofSeconds(20)) })
	}

}

Customizing ConfigurableReactiveWebServerFactory Directly

For more advanced use cases that require you to extend from ReactiveWebServerFactory, you can expose a bean of such type yourself.

Setters are provided for many configuration options. Several protected method “hooks” are also provided should you need to do something more exotic. See the ConfigurableReactiveWebServerFactory API documentation for details.

Auto-configured customizers are still applied on your custom factory, so use that option carefully.

Reactive Server Resources Configuration

When auto-configuring a Reactor Netty or Jetty server, Spring Boot will create specific beans that will provide HTTP resources to the server instance: ReactorResourceFactory or JettyResourceFactory.

By default, those resources will be also shared with the Reactor Netty and Jetty clients for optimal performances, given:

the same technology is used for server and client
the client instance is built using the WebClient.Builder bean auto-configured by Spring Boot

Developers can override the resource configuration for Jetty and Reactor Netty by providing a custom ReactorResourceFactory or JettyResourceFactory bean - this will be applied to both clients and servers.

You can learn more about the resource configuration on the client side in the WebClient Runtime section.