22. WebSocket Support

This part of the reference documentation covers Spring Framework’s support for WebSocket-style messaging in web applications including use of STOMP as an application level WebSocket sub-protocol.

Section 22.1, “Introduction” establishes a frame of mind in which to think about WebSocket, covering adoption challenges, design considerations, and thoughts on when it is a good fit.

Section 22.2, “WebSocket API” reviews the Spring WebSocket API on the server-side, while Section 22.3, “SockJS Fallback Options” explains the SockJS protocol and shows how to configure and use it.

Section 22.4.1, “Overview of STOMP” introduces the STOMP messaging protocol. Section 22.4.2, “Enable STOMP over WebSocket” demonstrates how to configure STOMP support in Spring. Section 22.4.4, “Annotation Message Handling” and the following sections explain how to write annotated message handling methods, send messages, choose message broker options, as well as work with the special "user" destinations. Finally, Section 22.4.17, “Testing Annotated Controller Methods” lists three approaches to testing STOMP/WebSocket applications.

22.1 Introduction

The WebSocket protocol RFC 6455 defines an important new capability for web applications: full-duplex, two-way communication between client and server. It is an exciting new capability on the heels of a long history of techniques to make the web more interactive including Java Applets, XMLHttpRequest, Adobe Flash, ActiveXObject, various Comet techniques, server-sent events, and others.

A proper introduction to the WebSocket protocol is beyond the scope of this document. At a minimum however it’s important to understand that HTTP is used only for the initial handshake, which relies on a mechanism built into HTTP to request a protocol upgrade (or in this case a protocol switch) to which the server can respond with HTTP status 101 (switching protocols) if it agrees. Assuming the handshake succeeds the TCP socket underlying the HTTP upgrade request remains open and both client and server can use it to send messages to each other.

Spring Framework 4 includes a new spring-websocket module with comprehensive WebSocket support. It is compatible with the Java WebSocket API standard (JSR-356) and also provides additional value-add as explained in the rest of the introduction.

22.1.1 WebSocket Fallback Options

An important challenge to adoption is the lack of support for WebSocket in some browsers. Notably the first Internet Explorer version to support WebSocket is version 10 (see http://caniuse.com/websockets for support by browser versions). Furthermore, some restrictive proxies may be configured in ways that either preclude the attempt to do an HTTP upgrade or otherwise break connection after some time because it has remained opened for too long. A good overview on this topic from Peter Lubbers is available in the InfoQ article "How HTML5 Web Sockets Interact With Proxy Servers".

Therefore to build a WebSocket application today, fallback options are required in order to simulate the WebSocket API where necessary. The Spring Framework provides such transparent fallback options based on the SockJS protocol. These options can be enabled through configuration and do not require modifying the application otherwise.

22.1.2 A Messaging Architecture

Aside from short-to-midterm adoption challenges, using WebSocket brings up important design considerations that are important to recognize early on, especially in contrast to what we know about building web applications today.

Today REST is a widely accepted, understood, and supported architecture for building web applications. It is an architecture that relies on having many URLs (nouns), a handful of HTTP methods (verbs), and other principles such as using hypermedia (links), remaining stateless, etc.

By contrast a WebSocket application may use a single URL only for the initial HTTP handshake. All messages thereafter share and flow on the same TCP connection. This points to an entirely different, asynchronous, event-driven, messaging architecture. One that is much closer to traditional messaging applications (e.g. JMS, AMQP).

Spring Framework 4 includes a new spring-messaging module with key abstractions from the Spring Integration project such as Message, MessageChannel, MessageHandler, and others that can serve as a foundation for such a messaging architecture. The module also includes a set of annotations for mapping messages to methods, similar to the Spring MVC annotation based programming model.

22.1.3 Sub-Protocol Support in WebSocket

WebSocket does imply a messaging architecture but does not mandate the use of any specific messaging protocol. It is a very thin layer over TCP that transforms a stream of bytes into a stream of messages (either text or binary) and not much more. It is up to applications to interpret the meaning of a message.

Unlike HTTP, which is an application-level protocol, in the WebSocket protocol there is simply not enough information in an incoming message for a framework or container to know how to route it or process it. Therefore WebSocket is arguably too low level for anything but a very trivial application. It can be done, but it will likely lead to creating a framework on top. This is comparable to how most web applications today are written using a web framework rather than the Servlet API alone.

For this reason the WebSocket RFC defines the use of sub-protocols. During the handshake, the client and server can use the header Sec-WebSocket-Protocol to agree on a sub-protocol, i.e. a higher, application-level protocol to use. The use of a sub-protocol is not required, but even if not used, applications will still need to choose a message format that both the client and server can understand. That format can be custom, framework-specific, or a standard messaging protocol.

The Spring Framework provides support for using STOMP — a simple, messaging protocol originally created for use in scripting languages with frames inspired by HTTP. STOMP is widely supported and well suited for use over WebSocket and over the web.

22.1.4 Should I Use WebSocket?

With all the design considerations surrounding the use of WebSocket, it is reasonable to ask, "When is it appropriate to use?".

The best fit for WebSocket is in web applications where the client and server need to exchange events at high frequency and with low latency. Prime candidates include, but are not limited to, applications in finance, games, collaboration, and others. Such applications are both very sensitive to time delays and also need to exchange a wide variety of messages at a high frequency.

For other application types, however, this may not be the case. For example, a news or social feed that shows breaking news as it becomes available may be perfectly okay with simple polling once every few minutes. Here latency is important, but it is acceptable if the news takes a few minutes to appear.

Even in cases where latency is crucial, if the volume of messages is relatively low (e.g. monitoring network failures) the use of long polling should be considered as a relatively simple alternative that works reliably and is comparable in terms of efficiency (again assuming the volume of messages is relatively low).

It is the combination of both low latency and high frequency of messages that can make the use of the WebSocket protocol critical. Even in such applications, the choice remains whether all client-server communication should be done through WebSocket messages as opposed to using HTTP and REST. The answer is going to vary by application; however, it is likely that some functionality may be exposed over both WebSocket and as a REST API in order to provide clients with alternatives. Furthermore, a REST API call may need to broadcast a message to interested clients connected via WebSocket.

The Spring Framework allows @Controller and @RestController classes to have both HTTP request handling and WebSocket message handling methods. Furthermore, a Spring MVC request handling method, or any application method for that matter, can easily broadcast a message to all interested WebSocket clients or to a specific user.

22.2 WebSocket API

The Spring Framework provides a WebSocket API designed to adapt to various WebSocket engines. Currently the list includes WebSocket runtimes such as Tomcat 7.0.47+, Jetty 9.1+, GlassFish 4.1+, WebLogic 12.1.3+, and Undertow 1.0+ (and WildFly 8.0+). Additional support may be added as more WebSocket runtimes become available.

[Note]Note

As explained in the introduction, direct use of a WebSocket API is too low level for applications — until assumptions are made about the format of a message there is little a framework can do to interpret messages or route them via annotations. This is why applications should consider using a sub-protocol and Spring’s STOMP over WebSocket support.

When using a higher level protocol, the details of the WebSocket API become less relevant, much like the details of TCP communication are not exposed to applications when using HTTP. Nevertheless this section covers the details of using WebSocket directly.

22.2.1 Create and Configure a WebSocketHandler

Creating a WebSocket server is as simple as implementing WebSocketHandler or more likely extending either TextWebSocketHandler or BinaryWebSocketHandler:

import org.springframework.web.socket.WebSocketHandler;
import org.springframework.web.socket.WebSocketSession;
import org.springframework.web.socket.TextMessage;

public class MyHandler extends TextWebSocketHandler {

    @Override
    public void handleTextMessage(WebSocketSession session, TextMessage message) {
        // ...
    }

}

There is dedicated WebSocket Java-config and XML namespace support for mapping the above WebSocket handler to a specific URL:

import org.springframework.web.socket.config.annotation.EnableWebSocket;
import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Override
    public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
        registry.addHandler(myHandler(), "/myHandler");
    }

    @Bean
    public WebSocketHandler myHandler() {
        return new MyHandler();
    }

}

XML configuration equivalent:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:handlers>
        <websocket:mapping path="/myHandler" handler="myHandler"/>
    </websocket:handlers>

    <bean id="myHandler" class="org.springframework.samples.MyHandler"/>

</beans>

The above is for use in Spring MVC applications and should be included in the configuration of a DispatcherServlet. However, Spring’s WebSocket support does not depend on Spring MVC. It is relatively simple to integrate a WebSocketHandler into other HTTP serving environments with the help of WebSocketHttpRequestHandler.

22.2.2 Customizing the WebSocket Handshake

The easiest way to customize the initial HTTP WebSocket handshake request is through a HandshakeInterceptor, which exposes "before" and "after" the handshake methods. Such an interceptor can be used to preclude the handshake or to make any attributes available to the WebSocketSession. For example, there is a built-in interceptor for passing HTTP session attributes to the WebSocket session:

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Override
    public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
        registry.addHandler(new MyHandler(), "/myHandler")
            .addInterceptors(new HttpSessionHandshakeInterceptor());
    }

}

And the XML configuration equivalent:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:handlers>
        <websocket:mapping path="/myHandler" handler="myHandler"/>
        <websocket:handshake-interceptors>
            <bean class="org.springframework.web.socket.server.support.HttpSessionHandshakeInterceptor"/>
        </websocket:handshake-interceptors>
    </websocket:handlers>

    <bean id="myHandler" class="org.springframework.samples.MyHandler"/>

</beans>

A more advanced option is to extend the DefaultHandshakeHandler that performs the steps of the WebSocket handshake, including validating the client origin, negotiating a sub-protocol, and others. An application may also need to use this option if it needs to configure a custom RequestUpgradeStrategy in order to adapt to a WebSocket server engine and version that is not yet supported (also see Section 22.2.4, “Deployment Considerations” for more on this subject). Both the Java-config and XML namespace make it possible to configure a custom HandshakeHandler.

22.2.3 WebSocketHandler Decoration

Spring provides a WebSocketHandlerDecorator base class that can be used to decorate a WebSocketHandler with additional behavior. Logging and exception handling implementations are provided and added by default when using the WebSocket Java-config or XML namespace. The ExceptionWebSocketHandlerDecorator catches all uncaught exceptions arising from any WebSocketHandler method and closes the WebSocket session with status 1011 that indicates a server error.

22.2.4 Deployment Considerations

The Spring WebSocket API is easy to integrate into a Spring MVC application where the DispatcherServlet serves both HTTP WebSocket handshake as well as other HTTP requests. It is also easy to integrate into other HTTP processing scenarios by invoking WebSocketHttpRequestHandler. This is convenient and easy to understand. However, special considerations apply with regards to JSR-356 runtimes.

The Java WebSocket API (JSR-356) provides two deployment mechanisms. The first involves a Servlet container classpath scan (Servlet 3 feature) at startup; and the other is a registration API to use at Servlet container initialization. Neither of these mechanism makes it possible to use a single "front controller" for all HTTP processing — including WebSocket handshake and all other HTTP requests — such as Spring MVC’s DispatcherServlet.

This is a significant limitation of JSR-356 that Spring’s WebSocket support addresses by providing a server-specific RequestUpgradeStrategy even when running in a JSR-356 runtime.

[Note]Note

A request to overcome the above limitation in the Java WebSocket API has been created and can be followed at WEBSOCKET_SPEC-211. Also note that Tomcat and Jetty already provide native API alternatives that makes it easy to overcome the limitation. We are hopeful that more servers will follow their example regardless of when it is addressed in the Java WebSocket API.

A secondary consideration is that Servlet containers with JSR-356 support are expected to perform a ServletContainerInitializer (SCI) scan that can slow down application startup, in some cases dramatically. If a significant impact is observed after an upgrade to a Servlet container version with JSR-356 support, it should be possible to selectively enable or disable web fragments (and SCI scanning) through the use of the <absolute-ordering /> element in web.xml:

<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://java.sun.com/xml/ns/javaee
        http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">

    <absolute-ordering/>

</web-app>

You can then selectively enable web fragments by name, such as Spring’s own SpringServletContainerInitializer that provides support for the Servlet 3 Java initialization API, if required:

<web-app xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="
        http://java.sun.com/xml/ns/javaee
        http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    version="3.0">

    <absolute-ordering>
        <name>spring_web</name>
    </absolute-ordering>

</web-app>

22.2.5 Configuring the WebSocket Engine

Each underlying WebSocket engine exposes configuration properties that control runtime characteristics such as the size of message buffer sizes, idle timeout, and others.

For Tomcat, WildFly, and GlassFish add a ServletServerContainerFactoryBean to your WebSocket Java config:

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Bean
    public ServletServerContainerFactoryBean createWebSocketContainer() {
        ServletServerContainerFactoryBean container = new ServletServerContainerFactoryBean();
        container.setMaxTextMessageBufferSize(8192);
        container.setMaxBinaryMessageBufferSize(8192);
        return container;
    }

}

or WebSocket XML namespace:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <bean class="org.springframework...ServletServerContainerFactoryBean">
        <property name="maxTextMessageBufferSize" value="8192"/>
        <property name="maxBinaryMessageBufferSize" value="8192"/>
    </bean>

</beans>
[Note]Note

For client side WebSocket configuration, you should use WebSocketContainerFactoryBean (XML) or ContainerProvider.getWebSocketContainer() (Java config).

For Jetty, you’ll need to supply a pre-configured Jetty WebSocketServerFactory and plug that into Spring’s DefaultHandshakeHandler through your WebSocket Java config:

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Override
    public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
        registry.addHandler(echoWebSocketHandler(),
            "/echo").setHandshakeHandler(handshakeHandler());
    }

    @Bean
    public DefaultHandshakeHandler handshakeHandler() {

        WebSocketPolicy policy = new WebSocketPolicy(WebSocketBehavior.SERVER);
        policy.setInputBufferSize(8192);
        policy.setIdleTimeout(600000);

        return new DefaultHandshakeHandler(
                new JettyRequestUpgradeStrategy(new WebSocketServerFactory(policy)));
    }

}

or WebSocket XML namespace:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:handlers>
        <websocket:mapping path="/echo" handler="echoHandler"/>
        <websocket:handshake-handler ref="handshakeHandler"/>
    </websocket:handlers>

    <bean id="handshakeHandler" class="org.springframework...DefaultHandshakeHandler">
        <constructor-arg ref="upgradeStrategy"/>
    </bean>

    <bean id="upgradeStrategy" class="org.springframework...JettyRequestUpgradeStrategy">
        <constructor-arg ref="serverFactory"/>
    </bean>

    <bean id="serverFactory" class="org.eclipse.jetty...WebSocketServerFactory">
        <constructor-arg>
            <bean class="org.eclipse.jetty...WebSocketPolicy">
                <constructor-arg value="SERVER"/>
                <property name="inputBufferSize" value="8092"/>
                <property name="idleTimeout" value="600000"/>
            </bean>
        </constructor-arg>
    </bean>

</beans>

22.2.6 Configuring allowed origins

As of Spring Framework 4.1.5, the default behavior for WebSocket and SockJS is to accept only same origin requests. It is also possible to allow all or a specified list of origins. This check is mostly designed for browser clients. There is nothing preventing other types of clients from modifying the Origin header value (see RFC 6454: The Web Origin Concept for more details).

The 3 possible behaviors are:

  • Allow only same origin requests (default): in this mode, when SockJS is enabled, the Iframe HTTP response header X-Frame-Options is set to SAMEORIGIN, and JSONP transport is disabled since it does not allow to check the origin of a request. As a consequence, IE6 and IE7 are not supported when this mode is enabled.
  • Allow a specified list of origins: each provided allowed origin must start with http:// or https://. In this mode, when SockJS is enabled, both IFrame and JSONP based transports are disabled. As a consequence, IE6 through IE9 are not supported when this mode is enabled.
  • Allow all origins: to enable this mode, you should provide * as the allowed origin value. In this mode, all transports are available.

WebSocket and SockJS allowed origins can be configured as shown bellow:

import org.springframework.web.socket.config.annotation.EnableWebSocket;
import org.springframework.web.socket.config.annotation.WebSocketConfigurer;
import org.springframework.web.socket.config.annotation.WebSocketHandlerRegistry;

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Override
    public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
        registry.addHandler(myHandler(), "/myHandler").setAllowedOrigins("http://mydomain.com");
    }

    @Bean
    public WebSocketHandler myHandler() {
        return new MyHandler();
    }

}

XML configuration equivalent:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:handlers allowed-origins="http://mydomain.com">
        <websocket:mapping path="/myHandler" handler="myHandler" />
    </websocket:handlers>

    <bean id="myHandler" class="org.springframework.samples.MyHandler"/>

</beans>

22.3 SockJS Fallback Options

As explained in the introduction, WebSocket is not supported in all browsers yet and may be precluded by restrictive network proxies. This is why Spring provides fallback options that emulate the WebSocket API as close as possible based on the SockJS protocol (version 0.3.3).

22.3.1 Overview of SockJS

The goal of SockJS is to let applications use a WebSocket API but fall back to non-WebSocket alternatives when necessary at runtime, i.e. without the need to change application code.

SockJS consists of:

  • The SockJS protocol defined in the form of executable narrated tests.
  • The SockJS JavaScript client - a client library for use in browsers.
  • SockJS server implementations including one in the Spring Framework spring-websocket module.
  • As of 4.1 spring-websocket also provides a SockJS Java client.

SockJS is designed for use in browsers. It goes to great lengths to support a wide range of browser versions using a variety of techniques. For the full list of SockJS transport types and browsers see the SockJS client page. Transports fall in 3 general categories: WebSocket, HTTP Streaming, and HTTP Long Polling. For an overview of these categories see this blog post.

The SockJS client begins by sending "GET /info" to obtain basic information from the server. After that it must decide what transport to use. If possible WebSocket is used. If not, in most browsers there is at least one HTTP streaming option and if not then HTTP (long) polling is used.

All transport requests have the following URL structure:

http://host:port/myApp/myEndpoint/{server-id}/{session-id}/{transport}
  • {server-id} - useful for routing requests in a cluster but not used otherwise.
  • {session-id} - correlates HTTP requests belonging to a SockJS session.
  • {transport} - indicates the transport type, e.g. "websocket", "xhr-streaming", etc.

The WebSocket transport needs only a single HTTP request to do the WebSocket handshake. All messages thereafter are exchanged on that socket.

HTTP transports require more requests. Ajax/XHR streaming for example relies on one long-running request for server-to-client messages and additional HTTP POST requests for client-to-server messages. Long polling is similar except it ends the current request after each server-to-client send.

SockJS adds minimal message framing. For example the server sends the letter o ("open" frame) initially, messages are sent as a["message1","message2"] (JSON-encoded array), the letter h ("heartbeat" frame) if no messages flow for 25 seconds by default, and the letter c ("close" frame) to close the session.

To learn more, run an example in a browser and watch the HTTP requests. The SockJS client allows fixing the list of transports so it is possible to see each transport one at a time. The SockJS client also provides a debug flag which enables helpful messages in the browser console. On the server side enable TRACE logging for org.springframework.web.socket. For even more detail refer to the SockJS protocol narrated test.

22.3.2 Enable SockJS

SockJS is easy to enable through Java configuration:

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Override
    public void registerWebSocketHandlers(WebSocketHandlerRegistry registry) {
        registry.addHandler(myHandler(), "/myHandler").withSockJS();
    }

    @Bean
    public WebSocketHandler myHandler() {
        return new MyHandler();
    }

}

and the XML configuration equivalent:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:handlers>
        <websocket:mapping path="/myHandler" handler="myHandler"/>
        <websocket:sockjs/>
    </websocket:handlers>

    <bean id="myHandler" class="org.springframework.samples.MyHandler"/>

</beans>

The above is for use in Spring MVC applications and should be included in the configuration of a DispatcherServlet. However, Spring’s WebSocket and SockJS support does not depend on Spring MVC. It is relatively simple to integrate into other HTTP serving environments with the help of SockJsHttpRequestHandler.

On the browser side, applications can use the sockjs-client (version 1.0.x) that emulates the W3C WebSocket API and communicates with the server to select the best transport option depending on the browser it’s running in. Review the sockjs-client page and the list of transport types supported by browser. The client also provides several configuration options, for example, to specify which transports to include.

22.3.3 HTTP Streaming in IE 8, 9: Ajax/XHR vs IFrame

Internet Explorer 8 and 9 are and will remain common for some time. They are a key reason for having SockJS. This section covers important considerations about running in those browsers.

The SockJS client supports Ajax/XHR streaming in IE 8 and 9 via Microsoft’s XDomainRequest. That works across domains but does not support sending cookies. Cookies are very often essential for Java applications. However since the SockJS client can be used with many server types (not just Java ones), it needs to know whether cookies matter. If so the SockJS client prefers Ajax/XHR for streaming or otherwise it relies on a iframe-based technique.

The very first "/info" request from the SockJS client is a request for information that can influence the client’s choice of transports. One of those details is whether the server application relies on cookies, e.g. for authentication purposes or clustering with sticky sessions. Spring’s SockJS support includes a property called sessionCookieNeeded. It is enabled by default since most Java applications rely on the JSESSIONID cookie. If your application does not need it, you can turn off this option and the SockJS client should choose xdr-streaming in IE 8 and 9.

If you do use an iframe-based transport, and in any case, it is good to know that browsers can be instructed to block the use of IFrames on a given page by setting the HTTP response header X-Frame-Options to DENY, SAMEORIGIN, or ALLOW-FROM <origin>. This is used to prevent clickjacking.

[Note]Note

Spring Security 3.2+ provides support for setting X-Frame-Options on every response. By default the Spring Security Java config sets it to DENY. In 3.2 the Spring Security XML namespace does not set that header by default but may be configured to do so, and in the future it may set it by default.

See Section 7.1. "Default Security Headers" of the Spring Security documentation for details on how to configure the setting of the X-Frame-Options header. You may also check or watch SEC-2501 for additional background.

If your application adds the X-Frame-Options response header (as it should!) and relies on an iframe-based transport, you will need to set the header value to SAMEORIGIN or ALLOW-FROM <origin>. Along with that the Spring SockJS support also needs to know the location of the SockJS client because it is loaded from the iframe. By default the iframe is set to download the SockJS client from a CDN location. It is a good idea to configure this option to a URL from the same origin as the application.

In Java config this can be done as shown below. The XML namespace provides a similar option via the <websocket:sockjs> element:

@Configuration
@EnableWebSocket
public class WebSocketConfig implements WebSocketConfigurer {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/portfolio").withSockJS()
                .setClientLibraryUrl("http://localhost:8080/myapp/js/sockjs-client.js");
    }

    // ...

}
[Note]Note

During initial development, do enable the SockJS client devel mode that prevents the browser from caching SockJS requests (like the iframe) that would otherwise be cached. For details on how to enable it see the SockJS client page.

22.3.4 Heartbeat Messages

The SockJS protocol requires servers to send heartbeat messages to preclude proxies from concluding a connection is hung. The Spring SockJS configuration has a property called heartbeatTime that can be used to customize the frequency. By default a heartbeat is sent after 25 seconds assuming no other messages were sent on that connection. This 25 seconds value is in line with the following IETF recommendation for public Internet applications.

[Note]Note

When using STOMP over WebSocket/SockJS, if the STOMP client and server negotiate heartbeats to be exchanged, the SockJS heartbeats are disabled.

The Spring SockJS support also allows configuring the TaskScheduler to use for scheduling heartbeats tasks. The task scheduler is backed by a thread pool with default settings based on the number of available processors. Applications should consider customizing the settings according to their specific needs.

22.3.5 Servlet 3 Async Requests

HTTP streaming and HTTP long polling SockJS transports require a connection to remain open longer than usual. For an overview of these techniques see this blog post.

In Servlet containers this is done through Servlet 3 async support that allows exiting the Servlet container thread processing a request and continuing to write to the response from another thread.

A specific issue is that the Servlet API does not provide notifications for a client that has gone away, see SERVLET_SPEC-44. However, Servlet containers raise an exception on subsequent attempts to write to the response. Since Spring’s SockJS Service supports sever-sent heartbeats (every 25 seconds by default), that means a client disconnect is usually detected within that time period or earlier if messages are sent more frequently.

[Note]Note

As a result network IO failures may occur simply because a client has disconnected, which can fill the log with unnecessary stack traces. Spring makes a best effort to identify such network failures that represent client disconnects (specific to each server) and log a minimal message using the dedicated log category DISCONNECTED_CLIENT_LOG_CATEGORY defined in AbstractSockJsSession. If you need to see the stack traces, set that log category to TRACE.

22.3.6 CORS Headers for SockJS

If you allow cross-origin requests (see Section 22.2.6, “Configuring allowed origins”), the SockJS protocol uses CORS for cross-domain support in the XHR streaming and polling transports. Therefore CORS headers are added automatically unless the presence of CORS headers in the response is detected. So if an application is already configured to provide CORS support, e.g. through a Servlet Filter, Spring’s SockJsService will skip this part.

It is also possible to disable the addition of these CORS headers via the suppressCors property in Spring’s SockJsService.

The following is the list of headers and values expected by SockJS:

  • "Access-Control-Allow-Origin" - initialized from the value of the "Origin" request header.
  • "Access-Control-Allow-Credentials" - always set to true.
  • "Access-Control-Request-Headers" - initialized from values from the equivalent request header.
  • "Access-Control-Allow-Methods" - the HTTP methods a transport supports (see TransportType enum).
  • "Access-Control-Max-Age" - set to 31536000 (1 year).

For the exact implementation see addCorsHeaders in AbstractSockJsService as well as the TransportType enum in the source code.

Alternatively if the CORS configuration allows it consider excluding URLs with the SockJS endpoint prefix thus letting Spring’s SockJsService handle it.

22.3.7 SockJS Client

A SockJS Java client is provided in order to connect to remote SockJS endpoints without using a browser. This can be especially useful when there is a need for bidirectional communication between 2 servers over a public network, i.e. where network proxies may preclude the use of the WebSocket protocol. A SockJS Java client is also very useful for testing purposes, for example to simulate a large number of concurrent users.

The SockJS Java client supports the "websocket", "xhr-streaming", and "xhr-polling" transports. The remaining ones only make sense for use in a browser.

The WebSocketTransport can be configured with:

  • StandardWebSocketClient in a JSR-356 runtime
  • JettyWebSocketClient using the Jetty 9+ native WebSocket API
  • Any implementation of Spring’s WebSocketClient

An XhrTransport by definition supports both "xhr-streaming" and "xhr-polling" since from a client perspective there is no difference other than in the URL used to connect to the server. At present there are two implementations:

  • RestTemplateXhrTransport uses Spring’s RestTemplate for HTTP requests.
  • JettyXhrTransport uses Jetty’s HttpClient for HTTP requests.

The example below shows how to create a SockJS client and connect to a SockJS endpoint:

List<Transport> transports = new ArrayList<>(2);
transports.add(new WebSocketTransport(new StandardWebSocketClient()));
transports.add(new RestTemplateXhrTransport());

SockJsClient sockJsClient = new SockJsClient(transports);
sockJsClient.doHandshake(new MyWebSocketHandler(), "ws://example.com:8080/sockjs");
[Note]Note

SockJS uses JSON formatted arrays for messages. By default Jackson 2 is used and needs to be on the classpath. Alternatively you can configure a custom implementation of SockJsMessageCodec and configure it on the SockJsClient.

To use the SockJsClient for simulating a large number of concurrent users you will need to configure the underlying HTTP client (for XHR transports) to allow a sufficient number of connections and threads. For example with Jetty:

HttpClient jettyHttpClient = new HttpClient();
jettyHttpClient.setMaxConnectionsPerDestination(1000);
jettyHttpClient.setExecutor(new QueuedThreadPool(1000));

Consider also customizing these server-side SockJS related properties (see Javadoc for details):

@Configuration
public class WebSocketConfig extends WebSocketMessageBrokerConfigurationSupport {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/sockjs").withSockJS()
            .setStreamBytesLimit(512 * 1024)
            .setHttpMessageCacheSize(1000)
            .setDisconnectDelay(30 * 1000);
    }

    // ...

}

22.4 STOMP Over WebSocket Messaging Architecture

The WebSocket protocol defines two types of messages, text and binary, but their content is undefined. It’s expected that the client and server may agree on using a sub-protocol (i.e. a higher-level protocol) to define message semantics. While the use of a sub-protocol with WebSocket is completely optional either way client and server will need to agree on some kind of protocol to help interpret messages.

22.4.1 Overview of STOMP

STOMP is a simple text-oriented messaging protocol that was originally created for scripting languages such as Ruby, Python, and Perl to connect to enterprise message brokers. It is designed to address a subset of commonly used messaging patterns. STOMP can be used over any reliable 2-way streaming network protocol such as TCP and WebSocket. Although STOMP is a text-oriented protocol, the payload of messages can be either text or binary.

STOMP is a frame based protocol whose frames are modeled on HTTP. The structure of a STOMP frame:

COMMAND
header1:value1
header2:value2

Body^@

Clients can use the SEND or SUBSCRIBE commands to send or subscribe for messages along with a "destination" header that describes what the message is about and who should receive it. This enables a simple publish-subscribe mechanism that can be used to send messages through the broker to other connected clients or to send messages to the server to request that some work be performed.

When using Spring’s STOMP support, the Spring WebSocket application acts as the STOMP broker to clients. Messages are routed to @Controller message-handling methods or to a simple, in-memory broker that keeps track of subscriptions and broadcasts messages to subscribed users. You can also configure Spring to work with a dedicated STOMP broker (e.g. RabbitMQ, ActiveMQ, etc) for the actual broadcasting of messages. In that case Spring maintains TCP connections to the broker, relays messages to it, and also passes messages from it down to connected WebSocket clients. Thus Spring web applications can rely on unified HTTP-based security, common validation, and a familiar programming model message-handling work.

Here is an example of a client subscribing to receive stock quotes which the server may emit periodically e.g. via a scheduled task sending messages through a SimpMessagingTemplate to the broker:

SUBSCRIBE
id:sub-1
destination:/topic/price.stock.*

^@

Here is an example of a client sending a trade request, which the server may handle through an @MessageMapping method and later on, after the execution, broadcast a trade confirmation message and details down to the client:

SEND
destination:/queue/trade
content-type:application/json
content-length:44

{"action":"BUY","ticker":"MMM","shares",44}^@

The meaning of a destination is intentionally left opaque in the STOMP spec. It can be any string, and it’s entirely up to STOMP servers to define the semantics and the syntax of the destinations that they support. It is very common, however, for destinations to be path-like strings where "/topic/.." implies publish-subscribe (one-to-many) and "/queue/" implies point-to-point (one-to-one) message exchanges.

STOMP servers can use the MESSAGE command to broadcast messages to all subscribers. Here is an example of a server sending a stock quote to a subscribed client:

MESSAGE
message-id:nxahklf6-1
subscription:sub-1
destination:/topic/price.stock.MMM

{"ticker":"MMM","price":129.45}^@

It is important to know that a server cannot send unsolicited messages. All messages from a server must be in response to a specific client subscription, and the "subscription-id" header of the server message must match the "id" header of the client subscription.

The above overview is intended to provide the most basic understanding of the STOMP protocol. It is recommended to review the protocol specification in full.

The benefits of using STOMP as a WebSocket sub-protocol:

  • No need to invent a custom message format
  • Use existing stomp.js client in the browser
  • Ability to route messages to based on destination
  • Option to use full-fledged message broker such as RabbitMQ, ActiveMQ, etc. for broadcasting

Most importantly the use of STOMP (vs plain WebSocket) enables the Spring Framework to provide a programming model for application-level use in the same way that Spring MVC provides a programming model based on HTTP.

22.4.2 Enable STOMP over WebSocket

The Spring Framework provides support for using STOMP over WebSocket through the spring-messaging and spring-websocket modules. Here is an example of exposing a STOMP WebSocket/SockJS endpoint at the URL path /portfolio where messages whose destination starts with "/app" are routed to message-handling methods (i.e. application work) and messages whose destinations start with "/topic" or "/queue" will be routed to the message broker (i.e. broadcasting to other connected clients):

import org.springframework.web.socket.config.annotation.EnableWebSocketMessageBroker;
import org.springframework.web.socket.config.annotation.StompEndpointRegistry;

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/portfolio").withSockJS();
    }

    @Override
    public void configureMessageBroker(MessageBrokerRegistry config) {
        config.setApplicationDestinationPrefixes("/app");
        config.enableSimpleBroker("/topic", "/queue");
    }

}

and in XML:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:message-broker application-destination-prefix="/app">
        <websocket:stomp-endpoint path="/portfolio">
            <websocket:sockjs/>
        </websocket:stomp-endpoint>
        <websocket:simple-broker prefix="/topic, /queue"/>
    </websocket:message-broker>

</beans>
[Note]Note

The "/app" prefix is arbitrary. You can pick any prefix. It’s simply meant to differentiate messages to be routed to message-handling methods to do application work vs messages to be routed to the broker to broadcast to subscribed clients.

The "/topic" and "/queue" prefixes depend on the broker in use. In the case of the simple, in-memory broker the prefixes do not have any special meaning; it’s merely a convention that indicates how the destination is used (pub-sub targetting many subscribers or point-to-point messages typically targeting an individual recipient). In the case of using a dedicated broker, most brokers use "/topic" as a prefix for destinations with pub-sub semantics and "/queue" for destinations with point-to-point semantics. Check the STOMP page of the broker to see the destination semantics it supports.

On the browser side, a client might connect as follows using stomp.js and the sockjs-client:

var socket = new SockJS("/spring-websocket-portfolio/portfolio");
var stompClient = Stomp.over(socket);

stompClient.connect({}, function(frame) {
}

Or if connecting via WebSocket (without SockJS):

var socket = new WebSocket("/spring-websocket-portfolio/portfolio");
var stompClient = Stomp.over(socket);

stompClient.connect({}, function(frame) {
}

Note that the stompClient above does not need to specify login and passcode headers. Even if it did, they would be ignored, or rather overridden, on the server side. See the sections Section 22.4.8, “Connections To Full-Featured Broker” and Section 22.4.10, “Authentication” for more information on authentication.

22.4.3 Flow of Messages

When a STOMP endpoint is configured, the Spring application acts as the STOMP broker to connected clients. This section provides a big picture overview of how messages flow within the application.

The spring-messaging module provides the foundation for asynchronous message processing. It contains a number of abstractions that originated in the Spring Integration project and are intended for use as building blocks in messaging applications:

  • Message — a message with headers and a payload.
  • MessageHandler — a contract for handling a message.
  • MessageChannel — a contract for sending a message enabling loose coupling between senders and receivers.
  • SubscribableChannel — extends MessageChannel and sends messages to registered MessageHandler subscribers.
  • ExecutorSubscribableChannel — a concrete implementation of SubscribableChannel that can deliver messages asynchronously via a thread pool.

The @EnableWebSocketMessageBroker Java config and the <websocket:message-broker> XML config both assemble a concrete message flow. Below is a diagram of the part of the setup when using the simple, in-memory broker:

message flow simple broker

The above setup that includes 3 message channels:

  • "clientInboundChannel" for messages from WebSocket clients.
  • "clientOutboundChannel" for messages to WebSocket clients.
  • "brokerChannel" for messages to the broker from within the application.

The same three channels are also used with a dedicated broker except here a "broker relay" takes the place of the simple broker:

message flow broker relay

Messages on the "clientInboundChannel" can flow to annotated methods for application handling (e.g. a stock trade execution request) or can be forwarded to the broker (e.g. client subscribing for stock quotes). The STOMP destination is used for simple prefix-based routing. For example the "/app" prefix could route messages to annotated methods while the "/topic" and "/queue" prefixes could route messages to the broker.

When a message-handling annotated method has a return type, its return value is sent as the payload of a Spring Message to the "brokerChannel". The broker in turn broadcasts the message to clients. Sending a message to a destination can also be done from anywhere in the application with the help of a messaging template. For example, an HTTP POST handling method can broadcast a message to connected clients, or a service component may periodically broadcast stock quotes.

Below is a simple example to illustrate the flow of messages:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/portfolio");
    }

    @Override
    public void configureMessageBroker(MessageBrokerRegistry registry) {
        registry.setApplicationDestinationPrefixes("/app");
        registry.enableSimpleBroker("/topic");
    }

}

@Controller
public class GreetingController {

    @MessageMapping("/greeting") {
    public String handle(String greeting) {
        return "[" + getTimestamp() + ": " + greeting;
    }

}

The following explains the message flow for the above example:

  • WebSocket clients connect to the WebSocket endpoint at "/portfolio".
  • Subscriptions to "/topic/greeting" pass through the "clientInboundChannel" and are forwarded to the broker.
  • Greetings sent to "/app/greeting" pass through the "clientInboundChannel" and are forwarded to the GreetingController. The controller adds the current time, and the return value is passed through the "brokerChannel" as a message to "/topic/greeting" (destination is selected based on a convention but can be overridden via @SendTo).
  • The broker in turn broadcasts messages to subscribers, and they pass through the "clientOutboundChannel".

The next section provides more details on annotated methods including the kinds of arguments and return values supported.

22.4.4 Annotation Message Handling

The @MessageMapping annotation is supported on methods of @Controller classes. It can be used for mapping methods to message destinations and can also be combined with the type-level @MessageMapping for expressing shared mappings across all annotated methods within a controller.

By default destination mappings are treated as Ant-style, slash-separated, path patterns, e.g. "/foo*", "/foo/**". etc. They can also contain template variables, e.g. "/foo/{id}" that can then be referenced via @DestinationVariable-annotated method arguments.

[Note]Note

Applications can also use dot-separated destinations (vs slash). See Section 22.4.9, “Using Dot as Separator in @MessageMapping Destinations”.

The following method arguments are supported for @MessageMapping methods:

  • Message method argument to get access to the complete message being processed.
  • @Payload-annotated argument for access to the payload of a message, converted with a org.springframework.messaging.converter.MessageConverter. The presence of the annotation is not required since it is assumed by default. Payload method arguments annotated with validation annotations (like @Validated) will be subject to JSR-303 validation.
  • @Header-annotated arguments for access to a specific header value along with type conversion using an org.springframework.core.convert.converter.Converter if necessary.
  • @Headers-annotated method argument that must also be assignable to java.util.Map for access to all headers in the message.
  • MessageHeaders method argument for getting access to a map of all headers.
  • MessageHeaderAccessor, SimpMessageHeaderAccessor, or StompHeaderAccessor for access to headers via typed accessor methods.
  • @DestinationVariable-annotated arguments for access to template variables extracted from the message destination. Values will be converted to the declared method argument type as necessary.
  • java.security.Principal method arguments reflecting the user logged in at the time of the WebSocket HTTP handshake.

The return value from an @MessageMapping method is converted with a org.springframework.messaging.converter.MessageConverter and used as the body of a new message that is then sent, by default, to the "brokerChannel" with the same destination as the client message but using the prefix "/topic" by default. An @SendTo message level annotation can be used to specify any other destination instead. It can also be set a class-level to share a common destination.

An @SubscribeMapping annotation can also be used to map subscription requests to @Controller methods. It is supported on the method level, but can also be combined with a type level @MessageMapping annotation that expresses shared mappings across all message handling methods within the same controller.

By default the return value from an @SubscribeMapping method is sent as a message directly back to the connected client and does not pass through the broker. This is useful for implementing request-reply message interactions; for example, to fetch application data when the application UI is being initialized. Or alternatively an @SubscribeMapping method can be annotated with @SendTo in which case the resulting message is sent to the "brokerChannel" using the specified target destination.

[Note]Note

In some cases a controller may need to be decorated with an AOP proxy at runtime. One example is if you choose to have @Transactional annotations directly on the controller. When this is the case, for controllers specifically, we recommend using class-based proxying. This is typically the default choice with controllers. However if a controller must implement an interface that is not a Spring Context callback (e.g. InitializingBean, *Aware, etc), you may need to explicitly configure class-based proxying. For example with <tx:annotation-driven />, change to <tx:annotation-driven proxy-target-class="true" />.

22.4.5 Sending Messages

What if you want to send messages to connected clients from any part of the application? Any application component can send messages to the "brokerChannel". The easiest way to do that is to have a SimpMessagingTemplate injected, and use it to send messages. Typically it should be easy to have it injected by type, for example:

@Controller
public class GreetingController {

    private SimpMessagingTemplate template;

    @Autowired
    public GreetingController(SimpMessagingTemplate template) {
        this.template = template;
    }

    @RequestMapping(path="/greetings", method=POST)
    public void greet(String greeting) {
        String text = "[" + getTimestamp() + "]:" + greeting;
        this.template.convertAndSend("/topic/greetings", text);
    }

}

But it can also be qualified by its name "brokerMessagingTemplate" if another bean of the same type exists.

22.4.6 Simple Broker

The built-in, simple message broker handles subscription requests from clients, stores them in memory, and broadcasts messages to connected clients with matching destinations. The broker supports path-like destinations, including subscriptions to Ant-style destination patterns.

[Note]Note

Applications can also use dot-separated destinations (vs slash). See Section 22.4.9, “Using Dot as Separator in @MessageMapping Destinations”.

22.4.7 Full-Featured Broker

The simple broker is great for getting started but supports only a subset of STOMP commands (e.g. no acks, receipts, etc.), relies on a simple message sending loop, and is not suitable for clustering. As an alternative, applications can upgrade to using a full-featured message broker.

Check the STOMP documentation for your message broker of choice (e.g. RabbitMQ, ActiveMQ, etc.), install the broker, and run it with STOMP support enabled. Then enable the STOMP broker relay in the Spring configuration instead of the simple broker.

Below is example configuration that enables a full-featured broker:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void registerStompEndpoints(StompEndpointRegistry registry) {
        registry.addEndpoint("/portfolio").withSockJS();
    }

    @Override
    public void configureMessageBroker(MessageBrokerRegistry registry) {
        registry.enableStompBrokerRelay("/topic", "/queue");
        registry.setApplicationDestinationPrefixes("/app");
    }

}

XML configuration equivalent:

<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:message-broker application-destination-prefix="/app">
        <websocket:stomp-endpoint path="/portfolio" />
            <websocket:sockjs/>
        </websocket:stomp-endpoint>
        <websocket:stomp-broker-relay prefix="/topic,/queue" />
    </websocket:message-broker>

</beans>

The "STOMP broker relay" in the above configuration is a Spring MessageHandler that handles messages by forwarding them to an external message broker. To do so it establishes TCP connections to the broker, forwards all messages to it, and then forwards all messages received from the broker to clients through their WebSocket sessions. Essentially it acts as a "relay" that forwards messages in both directions.

[Note]Note

Please org.projectreactor:reactor-net and io.netty:netty-all dependencies to your project for TCP connection management.

Furthermore, application components (e.g. HTTP request handling methods, business services, etc.) can also send messages to the broker relay, as described in Section 22.4.5, “Sending Messages”, in order to broadcast messages to subscribed WebSocket clients.

In effect, the broker relay enables robust and scalable message broadcasting.

22.4.8 Connections To Full-Featured Broker

A STOMP broker relay maintains a single "system" TCP connection to the broker. This connection is used for messages originating from the server-side application only, not for receiving messages. You can configure the STOMP credentials for this connection, i.e. the STOMP frame login and passcode headers. This is exposed in both the XML namespace and the Java config as the systemLogin/systemPasscode properties with default values guest/guest.

The STOMP broker relay also creates a separate TCP connection for every connected WebSocket client. You can configure the STOMP credentials to use for all TCP connections created on behalf of clients. This is exposed in both the XML namespace and the Java config as the clientLogin/clientPasscode properties with default values guest/guest.

[Note]Note

The STOMP broker relay always sets the login and passcode headers on every CONNECT frame that it forwards to the broker on behalf of clients. Therefore WebSocket clients need not set those headers; they will be ignored. As the following section explains, instead WebSocket clients should rely on HTTP authentication to protect the WebSocket endpoint and establish the client identity.

The STOMP broker relay also sends and receives heartbeats to and from the message broker over the "system" TCP connection. You can configure the intervals for sending and receiving heartbeats (10 seconds each by default). If connectivity to the broker is lost, the broker relay will continue to try to reconnect, every 5 seconds, until it succeeds.

[Note]Note

A Spring bean can implement ApplicationListener<BrokerAvailabilityEvent> in order to receive notifications when the "system" connection to the broker is lost and re-established. For example a Stock Quote service broadcasting stock quotes can stop trying to send messages when there is no active "system" connection.

The STOMP broker relay can also be configured with a virtualHost property. The value of this property will be set as the host header of every CONNECT frame and may be useful for example in a cloud environment where the actual host to which the TCP connection is established is different from the host providing the cloud-based STOMP service.

22.4.9 Using Dot as Separator in @MessageMapping Destinations

Although slash-separated path patterns are familiar to web developers, in messaging it is common to use a "." as the separator, for example in the names of topics, queues, exchanges, etc. Applications can also switch to using "." (dot) instead of "/" (slash) as the separator in @MessageMapping mappings by configuring a custom AntPathMatcher.

In Java config:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

  // ...

  @Override
  public void configureMessageBroker(MessageBrokerRegistry registry) {
    registry.enableStompBrokerRelay("/queue/", "/topic/");
    registry.setApplicationDestinationPrefixes("/app");
    registry.setPathMatcher(new AntPathMatcher("."));
  }

}

In XML config:

<beans xmlns="http://www.springframework.org/schema/beans"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xmlns:websocket="http://www.springframework.org/schema/websocket"
  xsi:schemaLocation="
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/websocket
    http://www.springframework.org/schema/websocket/spring-websocket.xsd">

  <websocket:message-broker application-destination-prefix="/app" path-matcher="pathMatcher">
    <websocket:stomp-endpoint path="/stomp" />
    <websocket:simple-broker prefix="/topic, /queue"/>
  </websocket:message-broker>

  <bean id="pathMatcher" class="org.springframework.util.AntPathMatcher">
    <constructor-arg index="0" value="." />
  </bean>

</beans>

And below is a simple example to illustrate a controller with "." separator:

@Controller
@MessageMapping("foo")
public class FooController {

  @MessageMapping("bar.{baz}")
  public void handleBaz(@DestinationVariable String baz) {
  }

}

If the application prefix is set to "/app" then the foo method is effectively mapped to "/app/foo.bar.{baz}".

22.4.10 Authentication

In a WebSocket-style application it is often useful to know who sent a message. Therefore some form of authentication is needed to establish the user identity and associate it with the current session.

Existing Web applications already use HTTP based authentication. For example Spring Security can secure the HTTP URLs of the application as usual. Since a WebSocket session begins with an HTTP handshake, that means URLs mapped to STOMP/WebSocket are already automatically protected and require authentication. Moreover the page that opens the WebSocket connection is itself likely protected and so by the time of the actual handshake, the user should have been authenticated.

When a WebSocket handshake is made and a new WebSocket session is created, Spring’s WebSocket support automatically propagates the java.security.Principal from the HTTP request to the WebSocket session. After that every message flowing through the application on that WebSocket session is enriched with the user information. It’s present in the message as a header. Controller methods can access the current user by adding a method argument of type javax.security.Principal.

Note that even though the STOMP CONNECT frame has "login" and "passcode" headers that can be used for authentication, Spring’s STOMP WebSocket support ignores them and currently expects users to have been authenticated already via HTTP.

In some cases it may be useful to assign an identity to a WebSocket session even when the user has not been formally authenticated. For example, a mobile app might assign some identity to anonymous users, perhaps based on geographical location. The do that currently, an application can sub-class DefaultHandshakeHandler and override the determineUser method. The custom handshake handler can then be plugged in (see examples in Section 22.2.4, “Deployment Considerations”).

22.4.11 User Destinations

An application can send messages targeting a specific user, and Spring’s STOMP support recognizes destinations prefixed with "/user/" for this purpose. For example, a client might subscribe to the destination "/user/queue/position-updates". This destination will be handled by the UserDestinationMessageHandler and transformed into a destination unique to the user session, e.g. "/queue/position-updates-user123". This provides the convenience of subscribing to a generically named destination while at the same time ensuring no collisions with other users subscribing to the same destination so that each user can receive unique stock position updates.

On the sending side messages can be sent to a destination such as "/user/{username}/queue/position-updates", which in turn will be translated by the UserDestinationMessageHandler into one or more destinations, one for each session associated with the user. This allows any component within the application to send messages targeting a specific user without necessarily knowing anything more than their name and the generic destination. This is also supported through an annotation as well as a messaging template.

For example, a message-handling method can send messages to the user associated with the message being handled through the @SendToUser annotation (also supported on the class-level to share a common destination):

@Controller
public class PortfolioController {

    @MessageMapping("/trade")
    @SendToUser("/queue/position-updates")
    public TradeResult executeTrade(Trade trade, Principal principal) {
        // ...
        return tradeResult;
    }
}

If the user has more than one session, by default all of the sessions subscribed to the given destination are targeted. However sometimes, it may be necessary to target only the session that sent the message being handled. This can be done by setting the broadcast attribute to false, for example:

@Controller
public class MyController {

    @MessageMapping("/action")
    public void handleAction() throws Exception{
        // raise MyBusinessException here
    }

    @MessageExceptionHandler
    @SendToUser(destinations="/queue/errors", broadcast=false)
    public ApplicationError handleException(MyBusinessException exception) {
        // ...
        return appError;
    }
}
[Note]Note

While user destinations generally imply an authenticated user, it isn’t required strictly. A WebSocket session that is not associated with an authenticated user can subscribe to a user destination. In such cases the @SendToUser annotation will behave exactly the same as with broadcast=false, i.e. targeting only the session that sent the message being handled.

It is also possible to send a message to user destinations from any application component by injecting the SimpMessagingTemplate created by the Java config or XML namespace, for example (the bean name is "brokerMessagingTemplate" if required for qualification with @Qualifier):

@Service
public class TradeServiceImpl implements TradeService {

	private final SimpMessagingTemplate messagingTemplate;

	@Autowired
	public TradeServiceImpl(SimpMessagingTemplate messagingTemplate) {
		this.messagingTemplate = messagingTemplate;
	}

	// ...

	public void afterTradeExecuted(Trade trade) {
		this.messagingTemplate.convertAndSendToUser(
				trade.getUserName(), "/queue/position-updates", trade.getResult());
	}
}
[Note]Note

When using user destinations with an external message broker, check the broker documentation on how to manage inactive queues, so that when the user session is over, all unique user queues are removed. For example, RabbitMQ creates auto-delete queues when destinations like /exchange/amq.direct/position-updates are used. So in that case the client could subscribe to /user/exchange/amq.direct/position-updates. Similarly, ActiveMQ has configuration options for purging inactive destinations.

In a multi-application server scenario a user destination may remain unresolved because the user is connected to a different server. In such cases you can configure a destination to broadcast unresolved messages to so that other servers have a chance to try. This can be done through the userDestinationBroadcast property of the MessageBrokerRegistry in Java config and the user-destination-broadcast attribute of the message-broker element in XML.

22.4.12 Listening To ApplicationContext Events and Intercepting Messages

Several ApplicationContext events (listed below) are published and can be received by implementing Spring’s ApplicationListener interface.

  • BrokerAvailabilityEvent — indicates when the broker becomes available/unavailable. While the "simple" broker becomes available immediately on startup and remains so while the application is running, the STOMP "broker relay" may lose its connection to the full featured broker, for example if the broker is restarted. The broker relay has reconnect logic and will re-establish the "system" connection to the broker when it comes back, hence this event is published whenever the state changes from connected to disconnected and vice versa. Components using the SimpMessagingTemplate should subscribe to this event and avoid sending messages at times when the broker is not available. In any case they should be prepared to handle MessageDeliveryException when sending a message.
  • SessionConnectEvent — published when a new STOMP CONNECT is received indicating the start of a new client session. The event contains the message representing the connect including the session id, user information (if any), and any custom headers the client may have sent. This is useful for tracking client sessions. Components subscribed to this event can wrap the contained message using SimpMessageHeaderAccessor or StompMessageHeaderAccessor.
  • SessionConnectedEvent — published shortly after a SessionConnectEvent when the broker has sent a STOMP CONNECTED frame in response to the CONNECT. At this point the STOMP session can be considered fully established.
  • SessionSubscribeEvent — published when a new STOMP SUBSCRIBE is received.
  • SessionUnsubscribeEvent — published when a new STOMP UNSUBSCRIBE is received.
  • SessionDisconnectEvent — published when a STOMP session ends. The DISCONNECT may have been sent from the client, or it may also be automatically generated when the WebSocket session is closed. In some cases this event may be published more than once per session. Components should be idempotent with regard to multiple disconnect events.
[Note]Note

When using a full-featured broker, the STOMP "broker relay" automatically reconnects the "system" connection in case the broker becomes temporarily unavailable. Client connections however are not automatically reconnected. Assuming heartbeats are enabled, the client will typically notice the broker is not responding within 10 seconds. Clients need to implement their own reconnect logic.

Furthermore, an application can directly intercept every incoming and outgoing message by registering a ChannelInterceptor on the respective message channel. For example to intercept inbound messages:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {

  @Override
  public void configureClientInboundChannel(ChannelRegistration registration) {
    registration.setInterceptors(new MyChannelInterceptor());
  }
}

A custom ChannelInterceptor can extend the empty method base class ChannelInterceptorAdapter and use StompHeaderAccessor or SimpMessageHeaderAccessor to access information about the message.

public class MyChannelInterceptor extends ChannelInterceptorAdapter {

  @Override
  public Message<?> preSend(Message<?> message, MessageChannel channel) {
    StompHeaderAccessor accessor = StompHeaderAccessor.wrap(message);
    StompCommand command = accessor.getStompCommand();
    // ...
    return message;
  }
}

22.4.13 STOMP Client

Spring provides a STOMP over WebSocket client and a STOMP over TCP client.

To begin create and configure WebSocketStompClient:

WebSocketClient webSocketClient = new StandardWebSocketClient();
WebSocketStompClient stompClient = new WebSocketStompClient(webSocketClient);
stompClient.setMessageConverter(new StringMessageConverter());
stompClient.setTaskScheduler(taskScheduler); // for heartbeats

In the above example StandardWebSocketClient could be replaced with SockJsClient since that is also an implementation of WebSocketClient. The SockJsClient can use WebSocket or HTTP-based transport as a fallback. For more details see Section 22.3.7, “SockJS Client”.

Next establish a connection and provide a handler for the STOMP session:

String url = "ws://127.0.0.1:8080/endpoint";
StompSessionHandler sessionHandler = new MyStompSessionHandler();
stompClient.connect(url, sessionHandler);

When the session is ready for use the handler is notified:

public class MyStompSessionHandler extends StompSessionHandlerAdapter {

    @Override
    public void afterConnected(StompSession session, StompHeaders connectedHeaders) {
        // ...
    }
}

Once the session is established any payload can be sent and that will be serialized with the configured MessageConverter:

session.send("/topic/foo", "payload");

You can also subscribe to destinations. The subscribe methods require a handler for messages on the subscription and return a Subscription handle that can be used to unsubscribe. For each received message the handler can specify the target Object type the payload should be deserialized to:

session.subscribe("/topic/foo", new StompFrameHandler() {

    @Override
    public Type getPayloadType(StompHeaders headers) {
        return String.class;
    }

    @Override
    public void handleFrame(StompHeaders headers, Object payload) {
        // ...
    }

});

To enable STOMP heartbeat configure WebSocketStompClient with a TaskScheduler and optionally customize the heartbeat intervals, 10 seconds for write inactivity which causes a heartbeat to be sent and 10 seconds for read inactivity which closes the connection.

[Note]Note

When using WebSocketStompClient for performance tests to simulate thousands of clients from the same machine consider turning off heartbeats since each connection schedules its own heartbeat tasks and that’s not optimized for a a large number of clients running on the same machine.

The STOMP protocol also supports receipts where the client must add a "receipt" header to which the server responds with a RECEIPT frame after the send or subscribe are processed. To support this the StompSession offers setAutoReceipt(boolean) that causes a "receipt" header to be added on every subsequent send or subscribe. Alternatively you can also manually add a "receipt" header to the StompHeaders. Both send and subscribe return an instance of Receiptable that can be used to register for receipt success and failure callbacks. For this feature the client must be configured with a TaskScheduler and the amount of time before a receipt expires (15 seconds by default).

Note that StompSessionHandler itself is a StompFrameHandler which allows it to handle ERROR frames in addition to the handleException callback for exceptions from the handling of messages, and handleTransportError for transport-level errors including ConnectionLostException.

22.4.14 WebSocket Scope

Each WebSocket session has a map of attributes. The map is attached as a header to inbound client messages and may be accessed from a controller method, for example:

@Controller
public class MyController {

    @MessageMapping("/action")
    public void handle(SimpMessageHeaderAccessor headerAccessor) {
        Map<String, Object> attrs = headerAccessor.getSessionAttributes();
        // ...
    }
}

It is also possible to declare a Spring-managed bean in the websocket scope. WebSocket-scoped beans can be injected into controllers and any channel interceptors registered on the "clientInboundChannel". Those are typically singletons and live longer than any individual WebSocket session. Therefore you will need to use a scope proxy mode for WebSocket-scoped beans:

@Component
@Scope(scopeName = "websocket", proxyMode = ScopedProxyMode.TARGET_CLASS)
public class MyBean {

    @PostConstruct
    public void init() {
        // Invoked after dependencies injected
    }

    // ...

    @PreDestroy
    public void destroy() {
        // Invoked when the WebSocket session ends
    }
}

@Controller
public class MyController {

    private final MyBean myBean;

    @Autowired
    public MyController(MyBean myBean) {
        this.myBean = myBean;
    }

    @MessageMapping("/action")
    public void handle() {
        // this.myBean from the current WebSocket session
    }
}

As with any custom scope, Spring initializes a new MyBean instance the first time it is accessed from the controller and stores the instance in the WebSocket session attributes. The same instance is returned subsequently until the session ends. WebSocket-scoped beans will have all Spring lifecycle methods invoked as shown in the examples above.

22.4.15 Configuration and Performance

There is no silver bullet when it comes to performance. Many factors may affect it including the size of messages, the volume, whether application methods perform work that requires blocking, as well as external factors such as network speed and others. The goal of this section is to provide an overview of the available configuration options along with some thoughts on how to reason about scaling.

In a messaging application messages are passed through channels for asynchronous executions backed by thread pools. Configuring such an application requires good knowledge of the channels and the flow of messages. Therefore it is recommended to review Section 22.4.3, “Flow of Messages”.

The obvious place to start is to configure the thread pools backing the "clientInboundChannel" and the "clientOutboundChannel". By default both are configured at twice the number of available processors.

If the handling of messages in annotated methods is mainly CPU bound then the number of threads for the "clientInboundChannel" should remain close to the number of processors. If the work they do is more IO bound and requires blocking or waiting on a database or other external system then the thread pool size will need to be increased.

[Note]Note

ThreadPoolExecutor has 3 important properties. Those are the core and the max thread pool size as well as the capacity for the queue to store tasks for which there are no available threads.

A common point of confusion is that configuring the core pool size (e.g. 10) and max pool size (e.g. 20) results in a thread pool with 10 to 20 threads. In fact if the capacity is left at its default value of Integer.MAX_VALUE then the thread pool will never increase beyond the core pool size since all additional tasks will be queued.

Please review the Javadoc of ThreadPoolExecutor to learn how these properties work and understand the various queuing strategies.

On the "clientOutboundChannel" side it is all about sending messages to WebSocket clients. If clients are on a fast network then the number of threads should remain close to the number of available processors. If they are slow or on low bandwidth they will take longer to consume messages and put a burden on the thread pool. Therefore increasing the thread pool size will be necessary.

While the workload for the "clientInboundChannel" is possible to predict — after all it is based on what the application does — how to configure the "clientOutboundChannel" is harder as it is based on factors beyond the control of the application. For this reason there are two additional properties related to the sending of messages. Those are the "sendTimeLimit" and the "sendBufferSizeLimit". Those are used to configure how long a send is allowed to take and how much data can be buffered when sending messages to a client.

The general idea is that at any given time only a single thread may be used to send to a client. All additional messages meanwhile get buffered and you can use these properties to decide how long sending a message is allowed to take and how much data can be buffered in the mean time. Please review the Javadoc and documentation of the XML schema for this configuration for important additional details.

Here is example configuration:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
        registration.setSendTimeLimit(15 * 1000).setSendBufferSizeLimit(512 * 1024);
    }

    // ...

}
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:message-broker>
        <websocket:transport send-timeout="15000" send-buffer-size="524288" />
        <!-- ... -->
    </websocket:message-broker>

</beans>

The WebSocket transport configuration shown above can also be used to configure the maximum allowed size for incoming STOMP messages. Although in theory a WebSocket message can be almost unlimited in size, in practice WebSocket servers impose limits — for example, 8K on Tomcat and 64K on Jetty. For this reason STOMP clients such as stomp.js split larger STOMP messages at 16K boundaries and send them as multiple WebSocket messages thus requiring the server to buffer and re-assemble.

Spring’s STOMP over WebSocket support does this so applications can configure the maximum size for STOMP messages irrespective of WebSocket server specific message sizes. Do keep in mind that the WebSocket message size will be automatically adjusted if necessary to ensure they can carry 16K WebSocket messages at a minimum.

Here is example configuration:

@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {

    @Override
    public void configureWebSocketTransport(WebSocketTransportRegistration registration) {
        registration.setMessageSizeLimit(128 * 1024);
    }

    // ...

}
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:websocket="http://www.springframework.org/schema/websocket"
    xsi:schemaLocation="
        http://www.springframework.org/schema/beans
        http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/websocket
        http://www.springframework.org/schema/websocket/spring-websocket.xsd">

    <websocket:message-broker>
        <websocket:transport message-size="131072" />
        <!-- ... -->
    </websocket:message-broker>

</beans>

An important point about scaling is using multiple application instances. Currently it is not possible to do that with the simple broker. However when using a full-featured broker such as RabbitMQ, each application instance connects to the broker and messages broadcast from one application instance can be broadcast through the broker to WebSocket clients connected through any other application instances.

22.4.16 Runtime Monitoring

When using @EnableWebSocketMessageBroker or <websocket:message-broker> key infrastructure components automatically gather stats and counters that provide important insight into the internal state of the application. The configuration also declares a bean of type WebSocketMessageBrokerStats that gathers all available information in one place and by default logs it at INFO level once every 30 minutes. This bean can be exported to JMX through Spring’s MBeanExporter for viewing at runtime, for example through JDK’s jconsole. Below is a summary of the available information.

Client WebSocket Sessions
Current
indicates how many client sessions there are currently with the count further broken down by WebSocket vs HTTP streaming and polling SockJS sessions.
Total
indicates how many total sessions have been established.
Abnormally Closed
Connect Failures
these are sessions that got established but were closed after not having received any messages within 60 seconds. This is usually an indication of proxy or network issues.
Send Limit Exceeded
sessions closed after exceeding the configured send timeout or the send buffer limits which can occur with slow clients (see previous section).
Transport Errors
sessions closed after a transport error such as failure to read or write to a WebSocket connection or HTTP request/response.
STOMP Frames
the total number of CONNECT, CONNECTED, and DISCONNECT frames processed indicating how many clients connected on the STOMP level. Note that the DISCONNECT count may be lower when sessions get closed abnormally or when clients close without sending a DISCONNECT frame.
STOMP Broker Relay
TCP Connections
indicates how many TCP connections on behalf of client WebSocket sessions are established to the broker. This should be equal to the number of client WebSocket sessions + 1 additional shared "system" connection for sending messages from within the application.
STOMP Frames
the total number of CONNECT, CONNECTED, and DISCONNECT frames forwarded to or received from the broker on behalf of clients. Note that a DISCONNECT frame is sent to the broker regardless of how the client WebSocket session was closed. Therefore a lower DISCONNECT frame count is an indication that the broker is pro-actively closing connections, may be because of a heartbeat that didn’t arrive in time, an invalid input frame, or other.
Client Inbound Channel
stats from thread pool backing the "clientInboundChannel" providing insight into the health of incoming message processing. Tasks queueing up here is an indication the application may be too slow to handle messages. If there I/O bound tasks (e.g. slow database query, HTTP request to 3rd party REST API, etc) consider increasing the thread pool size.
Client Outbound Channel
stats from the thread pool backing the "clientOutboundChannel" providing insight into the health of broadcasting messages to clients. Tasks queueing up here is an indication clients are too slow to consume messages. One way to address this is to increase the thread pool size to accommodate the number of concurrent slow clients expected. Another option is to reduce the send timeout and send buffer size limits (see the previous section).
SockJS Task Scheduler
stats from thread pool of the SockJS task scheduler which is used to send heartbeats. Note that when heartbeats are negotiated on the STOMP level the SockJS heartbeats are disabled.

22.4.17 Testing Annotated Controller Methods

There are two main approaches to testing applications using Spring’s STOMP over WebSocket support. The first is to write server-side tests verifying the functionality of controllers and their annotated message handling methods. The second is to write full end-to-end tests that involve running a client and a server.

The two approaches are not mutually exclusive. On the contrary each has a place in an overall test strategy. Server-side tests are more focused and easier to write and maintain. End-to-end integration tests on the other hand are more complete and test much more, but they’re also more involved to write and maintain.

The simplest form of server-side tests is to write controller unit tests. However this is not useful enough since much of what a controller does depends on its annotations. Pure unit tests simply can’t test that.

Ideally controllers under test should be invoked as they are at runtime, much like the approach to testing controllers handling HTTP requests using the Spring MVC Test framework. i.e. without running a Servlet container but relying on the Spring Framework to invoke the annotated controllers. Just like with Spring MVC Test here there are two two possible alternatives, either using a "context-based" or "standalone" setup:

  1. Load the actual Spring configuration with the help of the Spring TestContext framework, inject "clientInboundChannel" as a test field, and use it to send messages to be handled by controller methods.
  2. Manually set up the minimum Spring framework infrastructure required to invoke controllers (namely the SimpAnnotationMethodMessageHandler) and pass messages for controllers directly to it.

Both of these setup scenarios are demonstrated in the tests for the stock portfolio sample application.

The second approach is to create end-to-end integration tests. For that you will need to run a WebSocket server in embedded mode and connect to it as a WebSocket client sending WebSocket messages containing STOMP frames. The tests for the stock portfolio sample application also demonstrates this approach using Tomcat as the embedded WebSocket server and a simple STOMP client for test purposes.