This part of the documentation covers support for Servlet stack, web applications built on the Servlet API and deployed to Servlet containers. Individual chapters include Spring MVC, View Technologies, CORS Support, and WebSocket Support. For reactive stack, web applications, go to Web on Reactive Stack.
1. Spring Web MVC
1.1. Introduction
Spring Web MVC is the original web framework built on the Servlet API and included in the Spring Framework from the very beginning. The formal name "Spring Web MVC" comes from the name of its source module spring-webmvc but it is more commonly known as "Spring MVC".
Parallel to Spring Web MVC, Spring Framework 5.0 introduced a reactive stack, web framework whose name Spring WebFlux is also based on its source module spring-webflux. This section covers Spring Web MVC. The next section covers Spring WebFlux.
For baseline information and compatibility with Servlet container and Java EE version ranges please visit the Spring Framework Wiki.
1.2. DispatcherServlet
Spring MVC, like many other web frameworks, is designed around the front controller
pattern where a central Servlet, the DispatcherServlet, provides a shared algorithm
for request processing while actual work is performed by configurable, delegate components.
This model is flexible and supports diverse workflows.
The DispatcherServlet, as any Servlet, needs to be declared and mapped according
to the Servlet specification using Java configuration or in web.xml.
In turn the DispatcherServlet uses Spring configuration to discover
the delegate components it needs for request mapping, view resolution, exception
handling, and more.
Below is an example of the Java configuration that registers and initializes
the DispatcherServlet. This class is auto-detected by the Servlet container
(see Servlet Config):
public class MyWebApplicationInitializer implements WebApplicationInitializer {
@Override
public void onStartup(ServletContext servletCxt) {
// Load Spring web application configuration
AnnotationConfigWebApplicationContext ac = new AnnotationConfigWebApplicationContext();
ac.register(AppConfig.class);
ac.refresh();
// Create and register the DispatcherServlet
DispatcherServlet servlet = new DispatcherServlet(ac);
ServletRegistration.Dynamic registration = servletCxt.addServlet("app", servlet);
registration.setLoadOnStartup(1);
registration.addMapping("/app/*");
}
}|
In addition to using the ServletContext API directly, you can also extend
|
Below is an example of web.xml configuration to register and initialize the DispatcherServlet:
<web-app>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/app-context.xml</param-value>
</context-param>
<servlet>
<servlet-name>app</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value></param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>app</servlet-name>
<url-pattern>/app/*</url-pattern>
</servlet-mapping>
</web-app>|
Spring Boot follows a different initialization sequence. Rather than hooking into
the lifecycle of the Servlet container, Spring Boot uses Spring configuration to
bootstrap itself and the embedded Servlet container. |
1.2.1. Context Hierarchy
DispatcherServlet expects a WebApplicationContext, an extension of a plain
ApplicationContext, for its own configuration. WebApplicationContext has a link to the
ServletContext and Servlet it is associated with. It is also bound to the ServletContext
such that applications can use static methods on RequestContextUtils to look up the
WebApplicationContext if they need access to it.
For many applications having a single WebApplicationContext is simple and sufficient.
It is also possible to have a context hierarchy where one root WebApplicationContext
is shared across multiple DispatcherServlet (or other Servlet) instances, each with
its own child WebApplicationContext configuration.
See Additional Capabilities of the ApplicationContext
for more on the context hierarchy feature.
The root WebApplicationContext typically contains infrastructure beans such as data repositories and
business services that need to be shared across multiple Servlet instances. Those beans
are effectively inherited and could be overridden (i.e. re-declared) in the Servlet-specific,
child WebApplicationContext which typically contains beans local to the given Servlet:
Below is example configuration with a WebApplicationContext hierarchy:
public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
@Override
protected Class<?>[] getRootConfigClasses() {
return new Class<?>[] { RootConfig.class };
}
@Override
protected Class<?>[] getServletConfigClasses() {
return new Class<?>[] { App1Config.class };
}
@Override
protected String[] getServletMappings() {
return new String[] { "/app1/*" };
}
}|
If an application context hierarchy is not required, applications may return all
configuration via |
And the web.xml equivalent:
<web-app>
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/root-context.xml</param-value>
</context-param>
<servlet>
<servlet-name>app1</servlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
<init-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/app1-context.xml</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>app1</servlet-name>
<url-pattern>/app1/*</url-pattern>
</servlet-mapping>
</web-app>|
If an application context hierarchy is not required, applications may configure a
"root" context only and leave the |
1.2.2. Special Bean Types
The DispatcherServlet delegates to special beans to process requests and render the
appropriate responses. By "special beans" we mean Spring-managed, Object instances that
implement framework contracts. Those usually come with built-in contracts but you can
customize their properties, extend or replace them.
The table below lists the special beans detected by the DispatcherServlet:
| Bean type | Explanation |
|---|---|
HandlerMapping |
Map a request to a handler along with a list of
interceptors for pre- and post-processing.
The mapping is based on some criteria the details of which vary by The two main |
HandlerAdapter |
Help the |
Strategy to resolve exceptions, possibly mapping them to handlers, to HTML error views, or other targets. See Exceptions. |
|
Resolve logical |
|
Resolve the |
|
Resolve themes your web application can use — for example, to offer personalized layouts. See Themes. |
|
Abstraction for parsing a multi-part request (for example, browser form file upload) with the help of some multipart parsing library. See Multipart resolver. |
|
Store and retrieve the "input" and the "output" |
1.2.3. Web MVC Config
Applications can declare the infrastructure beans listed in Special Bean Types
that are required to process requests. The DispatcherServlet checks the
WebApplicationContext for each special bean. If there are no matching bean types,
it falls back on the default types listed in
DispatcherServlet.properties.
In most cases the MVC Config is the best starting point. It declares the required beans in either Java or XML, and provides a higher level configuration callback API to customize it.
|
Spring Boot relies on the MVC Java config to configure Spring MVC and also provides many extra convenient options. |
1.2.4. Servlet Config
In a Servlet 3.0+ environment, you have the option of configuring the Servlet container
programmatically as an alternative or in combination with a web.xml file. Below is an
example of registering a DispatcherServlet:
import org.springframework.web.WebApplicationInitializer;
public class MyWebApplicationInitializer implements WebApplicationInitializer {
@Override
public void onStartup(ServletContext container) {
XmlWebApplicationContext appContext = new XmlWebApplicationContext();
appContext.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
ServletRegistration.Dynamic registration = container.addServlet("dispatcher", new DispatcherServlet(appContext));
registration.setLoadOnStartup(1);
registration.addMapping("/");
}
}WebApplicationInitializer is an interface provided by Spring MVC that ensures your
implementation is detected and automatically used to initialize any Servlet 3 container.
An abstract base class implementation of WebApplicationInitializer named
AbstractDispatcherServletInitializer makes it even easier to register the
DispatcherServlet by simply overriding methods to specify the servlet mapping and the
location of the DispatcherServlet configuration.
This is recommended for applications that use Java-based Spring configuration:
public class MyWebAppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
@Override
protected Class<?>[] getRootConfigClasses() {
return null;
}
@Override
protected Class<?>[] getServletConfigClasses() {
return new Class<?>[] { MyWebConfig.class };
}
@Override
protected String[] getServletMappings() {
return new String[] { "/" };
}
}If using XML-based Spring configuration, you should extend directly from
AbstractDispatcherServletInitializer:
public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {
@Override
protected WebApplicationContext createRootApplicationContext() {
return null;
}
@Override
protected WebApplicationContext createServletApplicationContext() {
XmlWebApplicationContext cxt = new XmlWebApplicationContext();
cxt.setConfigLocation("/WEB-INF/spring/dispatcher-config.xml");
return cxt;
}
@Override
protected String[] getServletMappings() {
return new String[] { "/" };
}
}AbstractDispatcherServletInitializer also provides a convenient way to add Filter
instances and have them automatically mapped to the DispatcherServlet:
public class MyWebAppInitializer extends AbstractDispatcherServletInitializer {
// ...
@Override
protected Filter[] getServletFilters() {
return new Filter[] {
new HiddenHttpMethodFilter(), new CharacterEncodingFilter() };
}
}Each filter is added with a default name based on its concrete type and automatically
mapped to the DispatcherServlet.
The isAsyncSupported protected method of AbstractDispatcherServletInitializer
provides a single place to enable async support on the DispatcherServlet and all
filters mapped to it. By default this flag is set to true.
Finally, if you need to further customize the DispatcherServlet itself, you can
override the createDispatcherServlet method.
1.2.5. Processing
The DispatcherServlet processes requests as follows:
-
The
WebApplicationContextis searched for and bound in the request as an attribute that the controller and other elements in the process can use. It is bound by default under the keyDispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE. -
The locale resolver is bound to the request to enable elements in the process to resolve the locale to use when processing the request (rendering the view, preparing data, and so on). If you do not need locale resolving, you do not need it.
-
The theme resolver is bound to the request to let elements such as views determine which theme to use. If you do not use themes, you can ignore it.
-
If you specify a multipart file resolver, the request is inspected for multiparts; if multiparts are found, the request is wrapped in a
MultipartHttpServletRequestfor further processing by other elements in the process. See Multipart resolver for further information about multipart handling. -
An appropriate handler is searched for. If a handler is found, the execution chain associated with the handler (preprocessors, postprocessors, and controllers) is executed in order to prepare a model or rendering. Or alternatively for annotated controllers, the response may be rendered (within the
HandlerAdapter) instead of returning a view. -
If a model is returned, the view is rendered. If no model is returned, (may be due to a preprocessor or postprocessor intercepting the request, perhaps for security reasons), no view is rendered, because the request could already have been fulfilled.
The HandlerExceptionResolver beans declared in the WebApplicationContext are used to
resolve exceptions thrown during request processing. Those exception resolvers allow
customizing the logic to address exceptions. See Exceptions for more details.
The Spring DispatcherServlet also supports the return of the
last-modification-date, as specified by the Servlet API. The process of determining
the last modification date for a specific request is straightforward: the
DispatcherServlet looks up an appropriate handler mapping and tests whether the
handler that is found implements the LastModified interface. If so, the value of the
long getLastModified(request) method of the LastModified interface is returned to
the client.
You can customize individual DispatcherServlet instances by adding Servlet
initialization parameters (init-param elements) to the Servlet declaration in the
web.xml file. See the following table for the list of supported parameters.
| Parameter | Explanation |
|---|---|
|
Class that implements |
|
String that is passed to the context instance (specified by |
|
Namespace of the |
|
Whether to throw a By default this is set to "false", in which case the Note that if default servlet handling is also configured, then unresolved requests are always forwarded to the default servlet and a 404 would never be raised. |
1.2.6. Interception
All HandlerMapping implementations supports handler interceptors that are useful when
you want to apply specific functionality to certain requests, for example, checking for
a principal. Interceptors must implement HandlerInterceptor from the
org.springframework.web.servlet package with three methods that should provide enough
flexibility to do all kinds of pre-processing and post-processing:
-
preHandle(..)— before the actual handler is executed -
postHandle(..)— after the handler is executed -
afterCompletion(..)— after the complete request has finished
The preHandle(..) method returns a boolean value. You can use this method to break or
continue the processing of the execution chain. When this method returns true, the
handler execution chain will continue; when it returns false, the DispatcherServlet
assumes the interceptor itself has taken care of requests (and, for example, rendered an
appropriate view) and does not continue executing the other interceptors and the actual
handler in the execution chain.
See Interceptors in the section on MVC configuration for examples of how to
configure interceptors. You can also register them directly via setters on individual
HandlerMapping implementations.
Note that postHandle is less useful with @ResponseBody and ResponseEntity methods for
which the response is written and committed within the HandlerAdapter and before
postHandle. That means its too late to make any changes to the response such as adding
an extra header. For such scenarios you can implement ResponseBodyAdvice and either
declare it as an Controller Advice bean or configure it directly on
RequestMappingHandlerAdapter.
1.2.7. Exceptions
If an exception occurs during request mapping or is thrown from a request handler such as
an @Controller, the DispatcherServlet delegates to a chain of HandlerExceptionResolver
beans to resolve the exception and provide alternative handling, which typically is an
error response.
The table below lists the available HandlerExceptionResolver implementations:
| HandlerExceptionResolver | Description |
|---|---|
|
A mapping between exception class names and error view names. Useful for rendering error pages in a browser application. |
Resolves exceptions raised by Spring MVC and maps them to HTTP status codes.
Also see alternative |
|
|
Resolves exceptions with the |
|
Resolves exceptions by invoking an |
Chain of resolvers
You can form an exception resolver chain simply by declaring multiple HandlerExceptionResolver
beans in your Spring configuration and setting their order properties as needed.
The higher the order property, the later the exception resolver is positioned.
The contract of HandlerExceptionResolver specifies that it can return:
-
ModelAndViewthat points to an error view. -
Empty
ModelAndViewif the exception was handled within the resolver. -
nullif the exception remains unresolved, for subsequent resolvers to try; and if the exception remains at the end, it is allowed to bubble up to the Servlet container.
The MVC Config automatically declares built-in resolvers for default Spring MVC
exceptions, for @ResponseStatus annotated exceptions, and for support of
@ExceptionHandler methods. You can customize that list or replace it.
Container error page
If an exception remains unresolved by any HandlerExceptionResolver and is therefore
left to propagate, or if the response status is set to an error status (i.e. 4xx, 5xx),
Servlet containers may render a default error page in HTML. To customize the default
error page of the container, you can declare an error page mapping in web.xml:
<error-page>
<location>/error</location>
</error-page>Given the above, when an exception bubbles up, or the response has an error status, the
Servlet container makes an ERROR dispatch within the container to the configured URL
(e.g. "/error"). This is then processed by the DispatcherServlet, possibly mapping it
to an @Controller which could be implemented to return an error view name with a model
or to render a JSON response as shown below:
@RestController
public class ErrorController {
@RequestMapping(path = "/error")
public Map<String, Object> handle(HttpServletRequest request) {
Map<String, Object> map = new HashMap<String, Object>();
map.put("status", request.getAttribute("javax.servlet.error.status_code"));
map.put("reason", request.getAttribute("javax.servlet.error.message"));
return map;
}
}|
The Servlet API does not provide a way to create error page mappings in Java. You can
however use both an |
1.2.8. View Resolution
Spring MVC defines the ViewResolver and View interfaces that enable you to render
models in a browser without tying you to a specific view technology. ViewResolver
provides a mapping between view names and actual views. View addresses the preparation
of data before handing over to a specific view technology.
The table below provides more details on the ViewResolver hierarchy:
| ViewResolver | Description |
|---|---|
|
Sub-classes of |
|
Implementation of |
|
Implementation of |
|
Simple implementation of the |
|
Convenient subclass of |
|
Convenient subclass of |
|
Implementation of the |
Handling
You chain view resolvers by declaring more than one resolver beans and, if necessary, by
setting the order property to specify ordering. Remember, the higher the order property,
the later the view resolver is positioned in the chain.
The contract of a ViewResolver specifies that it can return null to indicate the
view could not be found. However in the case of JSPs, and InternalResourceViewResolver,
the only way to figure out if a JSP exists is to perform a dispatch through
RequestDispatcher. Therefore an InternalResourceViewResolver must always be configured
to be last in the overall order of view resolvers.
To configure view resolution is as simple as adding ViewResolver beans to your Spring
configuration. The MVC Config provides provides a dedicated configuration API for
View Resolvers and also for adding logic-less
View Controllers which are useful for HTML template
rendering without controller logic.
Redirecting
The special redirect: prefix in a view name allows you to perform a redirect. The
UrlBasedViewResolver (and sub-classes) recognize this as an instruction that a
redirect is needed. The rest of the view name is the redirect URL.
The net effect is the same as if the controller had returned a RedirectView, but now
the controller itself can simply operate in terms of logical view names. A logical view
name such as redirect:/myapp/some/resource will redirect relative to the current
Servlet context, while a name such as redirect:https://myhost.com/some/arbitrary/path
will redirect to an absolute URL.
Note that if a controller method is annotated with the @ResponseStatus, the annotation
value takes precedence over the response status set by RedirectView.
Forwarding
It is also possible to use a special forward: prefix for view names that are
ultimately resolved by UrlBasedViewResolver and subclasses. This creates an
InternalResourceView which does a RequestDispatcher.forward().
Therefore, this prefix is not useful with InternalResourceViewResolver and
InternalResourceView (for JSPs) but it can be helpful if using another view
technology, but still want to force a forward of a resource to be handled by the
Servlet/JSP engine. Note that you may also chain multiple view resolvers, instead.
Content negotiation
ContentNegotiatingViewResolver
does not resolve views itself but rather delegates
to other view resolvers, and selects the view that resembles the representation requested
by the client. The representation can be determined from the Accept header or from a
query parameter, e.g. "/path?format=pdf".
The ContentNegotiatingViewResolver selects an appropriate View to handle the request
by comparing the request media type(s) with the media type (also known as
Content-Type) supported by the View associated with each of its ViewResolvers. The
first View in the list that has a compatible Content-Type returns the representation
to the client. If a compatible view cannot be supplied by the ViewResolver chain, then
the list of views specified through the DefaultViews property will be consulted. This
latter option is appropriate for singleton Views that can render an appropriate
representation of the current resource regardless of the logical view name. The Accept
header may include wild cards, for example text/*, in which case a View whose
Content-Type was text/xml is a compatible match.
See View Resolvers under MVC Config for configuration details.
1.2.9. Locale
Most parts of Spring’s architecture support internationalization, just as the Spring web
MVC framework does. DispatcherServlet enables you to automatically resolve messages
using the client’s locale. This is done with LocaleResolver objects.
When a request comes in, the DispatcherServlet looks for a locale resolver, and if it
finds one it tries to use it to set the locale. Using the RequestContext.getLocale()
method, you can always retrieve the locale that was resolved by the locale resolver.
In addition to automatic locale resolution, you can also attach an interceptor to the handler mapping (see Interception for more information on handler mapping interceptors) to change the locale under specific circumstances, for example, based on a parameter in the request.
Locale resolvers and interceptors are defined in the
org.springframework.web.servlet.i18n package and are configured in your application
context in the normal way. Here is a selection of the locale resolvers included in
Spring.
TimeZone
In addition to obtaining the client’s locale, it is often useful to know their time zone.
The LocaleContextResolver interface offers an extension to LocaleResolver that allows
resolvers to provide a richer LocaleContext, which may include time zone information.
When available, the user’s TimeZone can be obtained using the
RequestContext.getTimeZone() method. Time zone information will automatically be used
by Date/Time Converter and Formatter objects registered with Spring’s
ConversionService.
Header resolver
This locale resolver inspects the accept-language header in the request that was sent
by the client (e.g., a web browser). Usually this header field contains the locale of
the client’s operating system. Note that this resolver does not support time zone
information.
Cookie resolver
This locale resolver inspects a Cookie that might exist on the client to see if a
Locale or TimeZone is specified. If so, it uses the specified details. Using the
properties of this locale resolver, you can specify the name of the cookie as well as the
maximum age. Find below an example of defining a CookieLocaleResolver.
<bean id="localeResolver" class="org.springframework.web.servlet.i18n.CookieLocaleResolver">
<property name="cookieName" value="clientlanguage"/>
<!-- in seconds. If set to -1, the cookie is not persisted (deleted when browser shuts down) -->
<property name="cookieMaxAge" value="100000"/>
</bean>| Property | Default | Description |
|---|---|---|
cookieName |
classname + LOCALE |
The name of the cookie |
cookieMaxAge |
Servlet container default |
The maximum time a cookie will stay persistent on the client. If -1 is specified, the cookie will not be persisted; it will only be available until the client shuts down their browser. |
cookiePath |
/ |
Limits the visibility of the cookie to a certain part of your site. When cookiePath is specified, the cookie will only be visible to that path and the paths below it. |
Session resolver
The SessionLocaleResolver allows you to retrieve Locale and TimeZone from the
session that might be associated with the user’s request. In contrast to
CookieLocaleResolver, this strategy stores locally chosen locale settings in the
Servlet container’s HttpSession. As a consequence, those settings are just temporary
for each session and therefore lost when each session terminates.
Note that there is no direct relationship with external session management mechanisms
such as the Spring Session project. This SessionLocaleResolver will simply evaluate and
modify corresponding HttpSession attributes against the current HttpServletRequest.
Locale interceptor
You can enable changing of locales by adding the LocaleChangeInterceptor to one of the
HandlerMapping definitions. It detects a parameter in the request and changes the locale
accordingly, calling the setLocale method on the LocaleResolver in the dispatcher’s
application context. The next example shows that calls to all *.view resources
that contain a parameter named siteLanguage now changes the locale. So, for example,
a request for the URL, https://www.sf.net/home.view?siteLanguage=nl, changes the site
language to Dutch. The following example shows how to intercept the locale:
<bean id="localeChangeInterceptor"
class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">
<property name="paramName" value="siteLanguage"/>
</bean>
<bean id="localeResolver"
class="org.springframework.web.servlet.i18n.CookieLocaleResolver"/>
<bean id="urlMapping"
class="org.springframework.web.servlet.handler.SimpleUrlHandlerMapping">
<property name="interceptors">
<list>
<ref bean="localeChangeInterceptor"/>
</list>
</property>
<property name="mappings">
<value>/**/*.view=someController</value>
</property>
</bean>1.2.10. Themes
You can apply Spring Web MVC framework themes to set the overall look-and-feel of your application, thereby enhancing user experience. A theme is a collection of static resources, typically style sheets and images, that affect the visual style of the application.
Define a theme
To use themes in your web application, you must set up an implementation of the
org.springframework.ui.context.ThemeSource interface. The WebApplicationContext
interface extends ThemeSource but delegates its responsibilities to a dedicated
implementation. By default the delegate will be an
org.springframework.ui.context.support.ResourceBundleThemeSource implementation that
loads properties files from the root of the classpath. To use a custom ThemeSource
implementation or to configure the base name prefix of the ResourceBundleThemeSource,
you can register a bean in the application context with the reserved name themeSource.
The web application context automatically detects a bean with that name and uses it.
When using the ResourceBundleThemeSource, a theme is defined in a simple properties
file. The properties file lists the resources that make up the theme. Here is an example:
styleSheet=/themes/cool/style.css background=/themes/cool/img/coolBg.jpg
The keys of the properties are the names that refer to the themed elements from view
code. For a JSP, you typically do this using the spring:theme custom tag, which is
very similar to the spring:message tag. The following JSP fragment uses the theme
defined in the previous example to customize the look and feel:
<%@ taglib prefix="spring" uri="http://www.springframework.org/tags"%>
<html>
<head>
<link rel="stylesheet" href="<spring:theme code='styleSheet'/>" type="text/css"/>
</head>
<body style="background=<spring:theme code='background'/>">
...
</body>
</html>By default, the ResourceBundleThemeSource uses an empty base name prefix. As a result,
the properties files are loaded from the root of the classpath. Thus you would put the
cool.properties theme definition in a directory at the root of the classpath, for
example, in /WEB-INF/classes. The ResourceBundleThemeSource uses the standard Java
resource bundle loading mechanism, allowing for full internationalization of themes. For
example, we could have a /WEB-INF/classes/cool_nl.properties that references a special
background image with Dutch text on it.
Resolve themes
After you define themes, as in the preceding section, you decide which theme to use. The
DispatcherServlet will look for a bean named themeResolver to find out which
ThemeResolver implementation to use. A theme resolver works in much the same way as a
LocaleResolver. It detects the theme to use for a particular request and can also
alter the request’s theme. The following theme resolvers are provided by Spring:
| Class | Description |
|---|---|
|
Selects a fixed theme, set using the |
|
The theme is maintained in the user’s HTTP session. It only needs to be set once for each session, but is not persisted between sessions. |
|
The selected theme is stored in a cookie on the client. |
Spring also provides a ThemeChangeInterceptor that allows theme changes on every
request with a simple request parameter.
1.2.11. Multipart resolver
MultipartResolver from the org.springframework.web.multipart package is a strategy
for parsing multipart requests including file uploads. There is one implementation
based on Commons FileUpload and another
based on Servlet 3.0 multipart request parsing.
To enable multipart handling, you need declare a MultipartResolver bean in your
DispatcherServlet Spring configuration with the name "multipartResolver".
The DispatcherServlet detects it and applies it to incoming request. When a POST with
content-type of "multipart/form-data" is received, the resolver parses the content and
wraps the current HttpServletRequest as MultipartHttpServletRequest in order to
provide access to resolved parts in addition to exposing them as request parameters.
Apache Commons FileUpload
To use Apache Commons FileUpload, simply configure a bean of type
CommonsMultipartResolver with the name multipartResolver. Of course you also need to
have commons-fileupload as a dependency on your classpath.
Servlet 3.0
Servlet 3.0 multipart parsing needs to be enabled through Servlet container configuration:
-
in Java, set a
MultipartConfigElementon the Servlet registration. -
in
web.xml, add a"<multipart-config>"section to the servlet declaration.
public class AppInitializer extends AbstractAnnotationConfigDispatcherServletInitializer {
// ...
@Override
protected void customizeRegistration(ServletRegistration.Dynamic registration) {
// Optionally also set maxFileSize, maxRequestSize, fileSizeThreshold
registration.setMultipartConfig(new MultipartConfigElement("/tmp"));
}
}Once the Servlet 3.0 configuration is in place, simply add a bean of type
StandardServletMultipartResolver with the name multipartResolver.
1.3. Filters
The spring-web module provides some useful filters.
1.3.1. HTTP PUT Form
Browsers can only submit form data via HTTP GET or HTTP POST but non-browser clients can also
use HTTP PUT and PATCH. The Servlet API requires ServletRequest.getParameter*()
methods to support form field access only for HTTP POST.
The spring-web module provides HttpPutFormContentFilter that intercepts HTTP PUT and
PATCH requests with content type application/x-www-form-urlencoded, reads the form data from
the body of the request, and wraps the ServletRequest in order to make the form data
available through the ServletRequest.getParameter*() family of methods.
1.3.2. Forwarded Headers
As a request goes through proxies such as load balancers the host, port, and scheme may change presenting a challenge for applications that need to create links to resources since the links should reflect the host, port, and scheme of the original request as seen from a client perspective.
RFC 7239 defines the "Forwarded" HTTP header for proxies to use to provide information about the original request. There are also other non-standard headers in use such as "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".
ForwardedHeaderFilter detects, extracts, and uses information from the "Forwarded"
header, or from "X-Forwarded-Host", "X-Forwarded-Port", and "X-Forwarded-Proto".
It wraps the request in order to overlay its host, port, and scheme and also "hides"
the forwarded headers for subsequent processing.
Note that there are security considerations when using forwarded headers as explained in Section 8 of RFC 7239. At the application level it is difficult to determine whether forwarded headers can be trusted or not. This is why the network upstream should be configured correctly to filter out untrusted forwarded headers from the outside.
Applications that don’t have a proxy and don’t need to use forwarded headers can
configure the ForwardedHeaderFilter to remove and ignore such headers.
1.3.3. Shallow ETag
The ShallowEtagHeaderFilter filter creates a "shallow" ETag by caching the content
written to the response, and computing an MD5 hash from it. The next time a client sends,
it does the same, but also compares the computed value against the If-None-Match request
header and if the two are equal, it returns a 304 (NOT_MODIFIED).
This strategy saves network bandwidth but not CPU, as the full response must be computed for each request. Other strategies at the controller level, described above, can avoid the computation. See HTTP Caching.
This filter has a writeWeakETag parameter that configures the filter to write Weak ETags,
like this: W/"02a2d595e6ed9a0b24f027f2b63b134d6", as defined in
RFC 7232 Section 2.3.
1.3.4. CORS
Spring MVC provides fine-grained support for CORS configuration through annotations on
controllers. However when used with Spring Security it is advisable to rely on the built-in
CorsFilter that must be ordered ahead of Spring Security’s chain of filters.
See the section on CORS and the CORS Filter for more details.
1.4. Annotated Controllers
Spring MVC provides an annotation-based programming model where @Controller and
@RestController components use annotations to express request mappings, request input,
exception handling, and more. Annotated controllers have flexible method signatures and
do not have to extend base classes nor implement specific interfaces.
@Controller
public class HelloController {
@GetMapping("/hello")
public String handle(Model model) {
model.addAttribute("message", "Hello World!");
return "index";
}
}In this particular example the method accepts a Model and returns a view name as a String
but many other options exist and are explained further below in this chapter.
|
Guides and tutorials on spring.io use the annotation-based programming model described in this section. |
1.4.1. Declaration
You can define controller beans using a standard Spring bean definition in the
Servlet’s WebApplicationContext. The @Controller stereotype allows for auto-detection,
aligned with Spring general support for detecting @Component classes in the classpath
and auto-registering bean definitions for them. It also acts as a stereotype for the
annotated class, indicating its role as a web component.
To enable auto-detection of such @Controller beans, you can add component scanning to
your Java configuration:
@Configuration
@ComponentScan("org.example.web")
public class WebConfig {
// ...
}The XML configuration equivalent:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:p="http://www.springframework.org/schema/p"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
https://www.springframework.org/schema/context/spring-context.xsd">
<context:component-scan base-package="org.example.web"/>
<!-- ... -->
</beans>@RestController is a composed annotation that is
itself meta-annotated with @Controller and @ResponseBody indicating a controller whose
every method inherits the type-level @ResponseBody annotation and therefore writes
directly to the response body vs view resolution and rendering with an HTML template.
AOP proxies
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"/>.
1.4.2. Request Mapping
The @RequestMapping annotation is used to map requests to controllers methods. It has
various attributes to match by URL, HTTP method, request parameters, headers, and media
types. It can be used at the class-level to express shared mappings or at the method level
to narrow down to a specific endpoint mapping.
There are also HTTP method specific shortcut variants of @RequestMapping:
-
@GetMapping -
@PostMapping -
@PutMapping -
@DeleteMapping -
@PatchMapping
The above are Custom Annotations that are provided out of the box
because arguably most controller methods should be mapped to a specific HTTP method vs
using @RequestMapping which by default matches to all HTTP methods. At the same an
@RequestMapping is still needed at the class level to express shared mappings.
Below is an example with type and method level mappings:
@RestController
@RequestMapping("/persons")
class PersonController {
@GetMapping("/{id}")
public Person getPerson(@PathVariable Long id) {
// ...
}
@PostMapping
@ResponseStatus(HttpStatus.CREATED)
public void add(@RequestBody Person person) {
// ...
}
}URI patterns
You can map requests using glob patterns and wildcards:
-
?matches one character -
*matches zero or more characters within a path segment -
**match zero or more path segments
You can also declare URI variables and access their values with @PathVariable:
@GetMapping("/owners/{ownerId}/pets/{petId}")
public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
// ...
}URI variables can be declared at the class and method level:
@Controller
@RequestMapping("/owners/{ownerId}")
public class OwnerController {
@GetMapping("/pets/{petId}")
public Pet findPet(@PathVariable Long ownerId, @PathVariable Long petId) {
// ...
}
}URI variables are automatically converted to the appropriate type or`TypeMismatchException`
is raised. Simple types — int, long, Date, are supported by default and you can
register support for any other data type.
See Type Conversion and DataBinder.
URI variables can be named explicitly — e.g. @PathVariable("customId"), but you can
leave that detail out if the names are the same and your code is compiled with debugging
information or with the -parameters compiler flag on Java 8.
The syntax {varName:regex} declares a URI variable with a regular expressions with the
syntax {varName:regex} — e.g. given URL "/spring-web-3.0.5 .jar", the below method
extracts the name, version, and file extension:
@GetMapping("/{name:[a-z-]+}-{version:\\d\\.\\d\\.\\d}{ext:\\.[a-z]+}")
public void handle(@PathVariable String version, @PathVariable String ext) {
// ...
}URI path patterns can also have embedded ${…} placeholders that are resolved on startup
via PropertyPlaceHolderConfigurer against local, system, environment, and other property
sources. This can be used for example to parameterize a base URL based on some external
configuration.
|
Spring MVC uses the |
Pattern comparison
When multiple patterns match a URL, they must be compared to find the best match. This done
via AntPathMatcher.getPatternComparator(String path) which looks for patterns that more
specific.
A pattern is less specific if it has a lower count of URI variables and single wildcards counted as 1 and double wildcards counted as 2. Given an equal score, the longer pattern is chosen. Given the same score and length, the pattern with more URI variables than wildcards is chosen.
The default mapping pattern /** is excluded from scoring and always
sorted last. Also prefix patterns such as /public/** are considered less
specific than other pattern that don’t have double wildcards.
For the full details see AntPatternComparator in AntPathMatcher and also keep in mind that
the PathMatcher implementation used can be customized. See Path Matching
in the configuration section.
Suffix match
By default Spring MVC performs ".*" suffix pattern matching so that a
controller mapped to /person is also implicitly mapped to /person.*.
The file extension is then used to interpret the requested content type to use for
the response (i.e. instead of the "Accept" header), e.g. /person.pdf,
/person.xml, etc.
Using file extensions like this was necessary when browsers used to send Accept headers that were hard to interpret consistently. At present that is no longer a necessity and using the "Accept" header should be the preferred choice.
Over time the use of file name extensions has proven problematic in a variety of ways. It can cause ambiguity when overlayed with the use of URI variables, path parameters, URI encoding, and it also makes it difficult to reason about URL-based authorization and security (see next section for more details).
To completely disable the use of file extensions, you must set both of these:
-
useSuffixPatternMatching(false), see PathMatchConfigurer -
favorPathExtension(false), see ContentNegotiationConfigurer
URL-based content negotiation can still be useful, for example when typing a URL in a
browser. To enable that we recommend a query parameter based strategy to avoid most of
the issues that come with file extensions. Or if you must use file extensions, consider
restricting them to a list of explicitly registered extensions through the
mediaTypes property of ContentNegotiationConfigurer.
Suffix match and RFD
Reflected file download (RFD) attack is similar to XSS in that it relies on request input, e.g. query parameter, URI variable, being reflected in the response. However instead of inserting JavaScript into HTML, an RFD attack relies on the browser switching to perform a download and treating the response as an executable script when double-clicked later.
In Spring MVC @ResponseBody and ResponseEntity methods are at risk because
they can render different content types which clients can request via URL path extensions.
Disabling suffix pattern matching and the use of path extensions for content negotiation
lower the risk but are not sufficient to prevent RFD attacks.
To prevent RFD attacks, prior to rendering the response body Spring MVC adds a
Content-Disposition:inline;filename=f.txt header to suggest a fixed and safe download
file. This is done only if the URL path contains a file extension that is neither whitelisted
nor explicitly registered for content negotiation purposes. However it may potentially have
side effects when URLs are typed directly into a browser.
Many common path extensions are whitelisted by default. Applications with custom
HttpMessageConverter implementations can explicitly register file extensions for content
negotiation to avoid having a Content-Disposition header added for those extensions.
See Content Types.
Check CVE-2015-5211 for additional recommendations related to RFD.
Consumable media types
You can narrow the request mapping based on the Content-Type of the request:
@PostMapping(path = "/pets", consumes = "application/json")
public void addPet(@RequestBody Pet pet) {
// ...
}The consumes attribute also supports negation expressions — e.g. !text/plain means any
content type other than "text/plain".
You can declare a shared consumes attribute at the class level. Unlike most other request mapping attributes however when used at the class level, a method-level consumes attribute will overrides rather than extend the class level declaration.
|
|
Producible media types
You can narrow the request mapping based on the Accept request header and the list of
content types that a controller method produces:
@GetMapping(path = "/pets/{petId}", produces = "application/json;charset=UTF-8")
@ResponseBody
public Pet getPet(@PathVariable String petId) {
// ...
}The media type can specify a character set. Negated expressions are supported — e.g.
!text/plain means any content type other than "text/plain".
|
For JSON content type, the UTF-8 charset should be specified even if RFC7159 clearly states that "no charset parameter is defined for this registration" because some browsers require it for interpreting correctly UTF-8 special characters. |
You can declare a shared produces attribute at the class level. Unlike most other request mapping attributes however when used at the class level, a method-level produces attribute will overrides rather than extend the class level declaration.
|
|
Parameters, headers
You can narrow request mappings based on request parameter conditions. You can test for the
presence of a request parameter ("myParam"), for the absence ("!myParam"), or for a
specific value ("myParam=myValue"):
@GetMapping(path = "/pets/{petId}", params = "myParam=myValue")
public void findPet(@PathVariable String petId) {
// ...
}You can also use the same with request header conditions:
@GetMapping(path = "/pets", headers = "myHeader=myValue")
public void findPet(@PathVariable String petId) {
// ...
}HTTP HEAD, OPTIONS
@GetMapping — and also @RequestMapping(method=HttpMethod.GET), support HTTP HEAD
transparently for request mapping purposes. Controller methods don’t need to change.
A response wrapper, applied in javax.servlet.http.HttpServlet, ensures a "Content-Length"
header is set to the number of bytes written and without actually writing to the response.
@GetMapping — and also @RequestMapping(method=HttpMethod.GET), are implicitly mapped to
and also support HTTP HEAD. An HTTP HEAD request is processed as if it were HTTP GET except
but instead of writing the body, the number of bytes are counted and the "Content-Length"
header set.
By default HTTP OPTIONS is handled by setting the "Allow" response header to the list of HTTP
methods listed in all @RequestMapping methods with matching URL patterns.
For a @RequestMapping without HTTP method declarations, the "Allow" header is set to
"GET,HEAD,POST,PUT,PATCH,DELETE,OPTIONS". Controller methods should always declare the
supported HTTP methods for example by using the HTTP method specific variants — @GetMapping, @PostMapping, etc.
@RequestMapping method can be explicitly mapped to HTTP HEAD and HTTP OPTIONS, but that
is not necessary in the common case.
Custom Annotations
Spring MVC supports the use of composed annotations
for request mapping. Those are annotations that are themselves meta-annotated with
@RequestMapping and composed to redeclare a subset (or all) of the @RequestMapping
attributes with a narrower, more specific purpose.
@GetMapping, @PostMapping, @PutMapping, @DeleteMapping, and @PatchMapping are
examples of composed annotations. They’re provided out of the box because arguably most
controller methods should be mapped to a specific HTTP method vs using @RequestMapping
which by default matches to all HTTP methods. If you need an example of composed
annotations, look at how those are declared.
Spring MVC also supports custom request mapping attributes with custom request matching
logic. This is a more advanced option that requires sub-classing
RequestMappingHandlerMapping and overriding the getCustomMethodCondition method where
you can check the custom attribute and return your own RequestCondition.
1.4.3. Handler Methods
@RequestMapping handler methods have a flexible signature and can choose from a range of
supported controller method arguments and return values.
Method Arguments
The table below shows supported controller method arguments. Reactive types are not supported for any arguments.
JDK 8’s java.util.Optional is supported as a method argument in combination with
annotations that have a required attribute — e.g. @RequestParam, @RequestHeader,
etc, and is equivalent to required=false.
| Controller method argument | Description |
|---|---|
|
Generic access to request parameters, request & session attributes, without direct use of the Servlet API. |
|
Choose any specific request or response type — e.g. |
|
Enforces the presence of a session. As a consequence, such an argument is never |
|
Servlet 4.0 push builder API for programmatic HTTP/2 resource pushes.
Note that per Servlet spec, the injected |
|
Currently authenticated user; possibly a specific |
|
The HTTP method of the request. |
|
The current request locale, determined by the most specific |
|
The time zone associated with the current request, as determined by a |
|
For access to the raw request body as exposed by the Servlet API. |
|
For access to the raw response body as exposed by the Servlet API. |
|
For access to URI template variables. See URI patterns. |
|
For access to name-value pairs in URI path segments. See Matrix variables. |
|
For access to Servlet request parameters. Parameter values are converted to the declared method argument type. See @RequestParam. Note that use of |
|
For access to request headers. Header values are converted to the declared method argument type. See @RequestHeader. |
|
For access to cookies. Cookies values are converted to the declared method argument type. See @CookieValue. |
|
For access to the HTTP request body. Body content is converted to the declared method
argument type using |
|
For access to request headers and body. The body is converted with |
|
For access to a part in a "multipart/form-data" request. See Multipart. |
|
For access to the model that is used in HTML controllers and exposed to templates as part of view rendering. |
|
Specify attributes to use in case of a redirect — i.e. to be appended to the query string, and/or flash attributes to be stored temporarily until the request after redirect. See Redirect attributes and Flash attributes. |
|
For access to an existing attribute in the model (instantiated if not present) with data binding and validation applied. See @ModelAttribute as well as Model and DataBinder. Note that use of |
|
For access to errors from validation and data binding for a command object
(i.e. |
|
For marking form processing complete which triggers cleanup of session attributes
declared through a class-level |
|
For preparing a URL relative to the current request’s host, port, scheme, context path, and
the literal part of the servlet mapping also taking into account |
|
For access to any session attribute; in contrast to model attributes stored in the session
as a result of a class-level |
|
For access to request attributes. See @RequestAttribute for more details. |
Any other argument |
If a method argument is not matched to any of the above, by default it is resolved as
an |
Return Values
The table below shows supported controller method return values. Reactive types are supported for all return values, see below for more details.
| Controller method return value | Description |
|---|---|
|
The return value is converted through |
|
The return value specifies the full response including HTTP headers and body be converted
through |
|
For returning a response with headers and no body. |
|
A view name to be resolved with |
|
A |
|
Attributes to be added to the implicit model with the view name implicitly determined
through a |
|
An attribute to be added to the model with the view name implicitly determined through
a Note that |
|
The view and model attributes to use, and optionally a response status. |
|
A method with a If none of the above is true, a |
|
Produce any of the above return values asynchronously from any thread — e.g. possibly as a
result of some event or callback. See Async Requests and
|
|
Produce any of the above return values asynchronously in a Spring MVC managed thread.
See Async Requests and |
|
Alternative to |
|
Emit a stream of objects asynchronously to be written to the response with
|
|
Write to the response |
Reactive types — Reactor, RxJava, or others via |
Alternative to For streaming scenarios — e.g. See Async Requests and Reactive types. |
Any other return value |
If a return value is not matched to any of the above, by default it is treated as a view
name, if it is |
Type Conversion
Some annotated controller method arguments that represent String-based request input — e.g.
@RequestParam, @RequestHeader, @PathVariable, @MatrixVariable, and @CookieValue,
may require type conversion if the argument is declared as something other than String.
For such cases type conversion is automatically applied based on the configured converters.
By default simple types such as int, long, Date, etc. are supported. Type conversion
can be customized through a WebDataBinder, see DataBinder, or by registering
Formatters with the FormattingConversionService, see
Spring Field Formatting.
Matrix variables
RFC 3986 discusses name-value pairs in path segments. In Spring MVC we refer to those as "matrix variables" based on an "old post" by Tim Berners-Lee but they can be also be referred to as URI path parameters.
Matrix variables can appear in any path segment, each variable separated by semicolon and
multiple values separated by comma, e.g. "/cars;color=red,green;year=2012". Multiple
values can also be specified through repeated variable names, e.g.
"color=red;color=green;color=blue".
If a URL is expected to contain matrix variables, the request mapping for a controller method must use a URI variable to mask that variable content and ensure the request can be matched successfully independent of matrix variable order and presence. Below is an example:
// GET /pets/42;q=11;r=22
@GetMapping("/pets/{petId}")
public void findPet(@PathVariable String petId, @MatrixVariable int q) {
// petId == 42
// q == 11
}Given that all path segments may contain matrix variables, sometimes you may need to disambiguate which path variable the matrix variable is expected to be in. For example:
// GET /owners/42;q=11/pets/21;q=22
@GetMapping("/owners/{ownerId}/pets/{petId}")
public void findPet(
@MatrixVariable(name="q", pathVar="ownerId") int q1,
@MatrixVariable(name="q", pathVar="petId") int q2) {
// q1 == 11
// q2 == 22
}A matrix variable may be defined as optional and a default value specified:
// GET /pets/42
@GetMapping("/pets/{petId}")
public void findPet(@MatrixVariable(required=false, defaultValue="1") int q) {
// q == 1
}To get all matrix variables, use a MultiValueMap:
// GET /owners/42;q=11;r=12/pets/21;q=22;s=23
@GetMapping("/owners/{ownerId}/pets/{petId}")
public void findPet(
@MatrixVariable MultiValueMap<String, String> matrixVars,
@MatrixVariable(pathVar="petId") MultiValueMap<String, String> petMatrixVars) {
// matrixVars: ["q" : [11,22], "r" : 12, "s" : 23]
// petMatrixVars: ["q" : 22, "s" : 23]
}Note that you need to enable the use of matrix variables. In the MVC Java config you need
to set a UrlPathHelper with removeSemicolonContent=false via
Path Matching. In the MVC XML namespace, use
<mvc:annotation-driven enable-matrix-variables="true"/>.
@RequestParam
Use the @RequestParam annotation to bind Servlet request parameters (i.e. query
parameters or form data) to a method argument in a controller.
The following code snippet shows the usage:
@Controller
@RequestMapping("/pets")
public class EditPetForm {
// ...
@GetMapping
public String setupForm(@RequestParam("petId") int petId, Model model) {
Pet pet = this.clinic.loadPet(petId);
model.addAttribute("pet", pet);
return "petForm";
}
// ...
}Method parameters using this annotation are required by default, but you can specify that
a method parameter is optional by setting @RequestParam's required flag to false
or by declaring the argument with an java.util.Optional wrapper.
Type conversion is applied automatically if the target method parameter type is not
String. See Type Conversion.
When an @RequestParam annotation is declared as Map<String, String> or
MultiValueMap<String, String> argument, the map is populated with all request
parameters.
Note that use of @RequestParam is optional, e.g. to set its attributes.
By default any argument that is a simple value type, as determined by
BeanUtils#isSimpleProperty,
and is not resolved by any other argument resolver, is treated as if it was annotated
with @RequestParam.
@RequestHeader
Use the @RequestHeader annotation to bind a request header to a method argument in a
controller.
Given request with headers:
Host localhost:8080 Accept text/html,application/xhtml+xml,application/xml;q=0.9 Accept-Language fr,en-gb;q=0.7,en;q=0.3 Accept-Encoding gzip,deflate Accept-Charset ISO-8859-1,utf-8;q=0.7,*;q=0.7 Keep-Alive 300
The following gets the value of the Accept-Encoding and Keep-Alive headers:
@GetMapping("/demo")
public void handle(
@RequestHeader("Accept-Encoding") String encoding,
@RequestHeader("Keep-Alive") long keepAlive) {
//...
}Type conversion is applied automatically if the target method parameter type is not
String. See Type Conversion.
When an @RequestHeader annotation is used on a Map<String, String>,
MultiValueMap<String, String>, or HttpHeaders argument, the map is populated
with all header values.
|
Built-in support is available for converting a comma-separated string into an
array/collection of strings or other types known to the type conversion system. For
example a method parameter annotated with |
@CookieValue
Use the @CookieValue annotation to bind the value of an HTTP cookie to a method argument
in a controller.
Given request with the following cookie:
JSESSIONID=415A4AC178C59DACE0B2C9CA727CDD84
The following code sample demonstrates how to get the cookie value:
@GetMapping("/demo")
public void handle(@CookieValue("JSESSIONID") String cookie) {
//...
}Type conversion is applied automatically if the target method parameter type is not
String. See Type Conversion.
@ModelAttribute
Use the @ModelAttribute annotation on a method argument to access an attribute from the
model, or have it instantiated if not present. The model attribute is also overlaid with
values from HTTP Servlet request parameters whose names match to field names. This is
referred to as data binding and it saves you from having to deal with parsing and
converting individual query parameters and form fields. For example:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute Pet pet) { }The Pet instance above is resolved as follows:
-
From the model if already added via Model.
-
From the HTTP session via @SessionAttributes.
-
From a URI path variable passed through a
Converter(example below). -
From the invocation of a default constructor.
-
From the invocation of a "primary constructor" with arguments matching to Servlet request parameters; argument names are determined via JavaBeans
@ConstructorPropertiesor via runtime-retained parameter names in the bytecode.
While it is common to use a Model to populate the model with
attributes, one other alternative is to rely on a Converter<String, T> in combination
with a URI path variable convention. In the example below the model attribute name
"account" matches the URI path variable "account" and the Account is loaded by passing
the String account number through a registered Converter<String, Account>:
@PutMapping("/accounts/{account}")
public String save(@ModelAttribute("account") Account account) {
// ...
}After the model attribute instance is obtained, data binding is applied. The
WebDataBinder class matches Servlet request parameter names (query parameters and form
fields) to field names on the target Object. Matching fields are populated after type
conversion is applied where necessary. For more on data binding (and validation) see
Validation. For more on customizing data binding see
DataBinder.
Data binding may result in errors. By default a BindException is raised but to check
for such errors in the controller method, add a BindingResult argument immediately next
to the @ModelAttribute as shown below:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@ModelAttribute("pet") Pet pet, BindingResult result) {
if (result.hasErrors()) {
return "petForm";
}
// ...
}In some cases you may want access to a model attribute without data binding. For such
cases you can inject the Model into the controller and access it directly or
alternatively set @ModelAttribute(binding=false) as shown below:
@ModelAttribute
public AccountForm setUpForm() {
return new AccountForm();
}
@ModelAttribute
public Account findAccount(@PathVariable String accountId) {
return accountRepository.findOne(accountId);
}
@PostMapping("update")
public String update(@Valid AccountUpdateForm form, BindingResult result,
@ModelAttribute(binding=false) Account account) {
// ...
}Validation can be applied automatically after data binding by adding the
javax.validation.Valid annotation or Spring’s @Validated annotation (also see
Bean validation and
Spring validation). For example:
@PostMapping("/owners/{ownerId}/pets/{petId}/edit")
public String processSubmit(@Valid @ModelAttribute("pet") Pet pet, BindingResult result) {
if (result.hasErrors()) {
return "petForm";
}
// ...
}Note that use of @ModelAttribute is optional, e.g. to set its attributes.
By default any argument that is not a simple value type, as determined by
BeanUtils#isSimpleProperty,
and is not resolved by any other argument resolver, is treated as if it was annotated
with @ModelAttribute.
@SessionAttributes
@SessionAttributes is used to store model attributes in the HTTP Servlet session between
requests. It is a type-level annotation that declares session attributes used by a
specific controller. This will typically list the names of model attributes or types of
model attributes which should be transparently stored in the session for subsequent
requests to access.
For example:
@Controller
@SessionAttributes("pet")
public class EditPetForm {
// ...
}On the first request when a model attribute with the name "pet" is added to the model,
it is automatically promoted to and saved in the HTTP Servlet session. It remains there
until another controller method uses a SessionStatus method argument to clear the
storage:
@Controller
@SessionAttributes("pet")
public class EditPetForm {
// ...
@PostMapping("/pets/{id}")
public String handle(Pet pet, BindingResult errors, SessionStatus status) {
if (errors.hasErrors) {
// ...
}
status.setComplete();
// ...
}
}
}@SessionAttribute
If you need access to pre-existing session attributes that are managed globally,
i.e. outside the controller (e.g. by a filter), and may or may not be present
use the @SessionAttribute annotation on a method parameter:
@RequestMapping("/")
public String handle(@SessionAttribute User user) {
// ...
}For use cases that require adding or removing session attributes consider injecting
org.springframework.web.context.request.WebRequest or
javax.servlet.http.HttpSession into the controller method.
For temporary storage of model attributes in the session as part of a controller
workflow consider using SessionAttributes as described in
@SessionAttributes.
@RequestAttribute
Similar to @SessionAttribute the @RequestAttribute annotation can be used to
access pre-existing request attributes created earlier, e.g. by a Servlet Filter
or HandlerInterceptor:
@GetMapping("/")
public String handle(@RequestAttribute Client client) {
// ...
}Redirect attributes
By default all model attributes are considered to be exposed as URI template variables in the redirect URL. Of the remaining attributes those that are primitive types or collections/arrays of primitive types are automatically appended as query parameters.
Appending primitive type attributes as query parameters may be the desired result if a
model instance was prepared specifically for the redirect. However, in annotated
controllers the model may contain additional attributes added for rendering purposes (e.g.
drop-down field values). To avoid the possibility of having such attributes appear in the
URL, an @RequestMapping method can declare an argument of type RedirectAttributes and
use it to specify the exact attributes to make available to RedirectView. If the method
does redirect, the content of RedirectAttributes is used. Otherwise the content of the
model is used.
The RequestMappingHandlerAdapter provides a flag called
"ignoreDefaultModelOnRedirect" that can be used to indicate the content of the default
Model should never be used if a controller method redirects. Instead the controller
method should declare an attribute of type RedirectAttributes or if it doesn’t do so
no attributes should be passed on to RedirectView. Both the MVC namespace and the MVC
Java config keep this flag set to false in order to maintain backwards compatibility.
However, for new applications we recommend setting it to true
Note that URI template variables from the present request are automatically made
available when expanding a redirect URL and do not need to be added explicitly neither
through Model nor RedirectAttributes. For example:
@PostMapping("/files/{path}")
public String upload(...) {
// ...
return "redirect:files/{path}";
}Another way of passing data to the redirect target is via Flash Attributes. Unlike other redirect attributes, flash attributes are saved in the HTTP session (and hence do not appear in the URL). See Flash attributes for more information.
Flash attributes
Flash attributes provide a way for one request to store attributes intended for use in another. This is most commonly needed when redirecting — for example, the Post/Redirect/Get pattern. Flash attributes are saved temporarily before the redirect (typically in the session) to be made available to the request after the redirect and removed immediately.
Spring MVC has two main abstractions in support of flash attributes. FlashMap is used
to hold flash attributes while FlashMapManager is used to store, retrieve, and manage
FlashMap instances.
Flash attribute support is always “on” and does not need to be enabled explicitly.
However, if not used, it never causes HTTP session creation. On each request, there is an
“input” FlashMap with attributes passed from a previous request (if any) and an
“output” FlashMap with attributes to save for a subsequent request. Both FlashMap
instances are accessible from anywhere in Spring MVC through static methods in
RequestContextUtils.
Annotated controllers typically do not need to work with FlashMap directly. Instead an
@RequestMapping method can accept an argument of type RedirectAttributes and use it
to add flash attributes for a redirect scenario. Flash attributes added via
RedirectAttributes are automatically propagated to the "output" FlashMap. Similarly,
after the redirect, attributes from the "input" FlashMap are automatically added to the
Model of the controller serving the target URL.
Multipart
After a MultipartResolver has been enabled, the content of POST
requests with "multipart/form-data" is parsed and accessible as regular request
parameters. In the example below we access one regular form field and one uploaded
file:
@Controller
public class FileUploadController {
@PostMapping("/form")
public String handleFormUpload(@RequestParam("name") String name,
@RequestParam("file") MultipartFile file) {
if (!file.isEmpty()) {
byte[] bytes = file.getBytes();
// store the bytes somewhere
return "redirect:uploadSuccess";
}
return "redirect:uploadFailure";
}
}|
When using Servlet 3.0 multipart parsing you can also use |
Multipart content can also be used as part of data binding to a command object. For example the above form field and file could have been fields on a form object:
class MyForm {
private String name;
private MultipartFile file;
// ...
}
@Controller
public class FileUploadController {
@PostMapping("/form")
public String handleFormUpload(MyForm form, BindingResult errors) {
if (!form.getFile().isEmpty()) {
byte[] bytes = form.getFile().getBytes();
// store the bytes somewhere
return "redirect:uploadSuccess";
}
return "redirect:uploadFailure";
}
}Multipart requests can also be submitted from non-browser clients in a RESTful service scenario. For example a file along with JSON:
POST /someUrl
Content-Type: multipart/mixed
--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="meta-data"
Content-Type: application/json; charset=UTF-8
Content-Transfer-Encoding: 8bit
{
"name": "value"
}
--edt7Tfrdusa7r3lNQc79vXuhIIMlatb7PQg7Vp
Content-Disposition: form-data; name="file-data"; filename="file.properties"
Content-Type: text/xml
Content-Transfer-Encoding: 8bit
... File Data ...
You can access the "meta-data" part with @RequestParam as a String but you’ll
probably want it deserialized from JSON (similar to @RequestBody). Use the
@RequestPart annotation to access a multipart after converting it with an
HttpMessageConverter:
@PostMapping("/")
public String handle(@RequestPart("meta-data") MetaData metadata,
@RequestPart("file-data") MultipartFile file) {
// ...
}@RequestPart can be used in combination with javax.validation.Valid, or Spring’s
@Validated annotation, which causes Standard Bean Validation to be applied.
By default validation errors cause a MethodArgumentNotValidException which is turned
into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally
within the controller through an Errors or BindingResult argument:
@PostMapping("/")
public String handle(@Valid @RequestPart("meta-data") MetaData metadata,
BindingResult result) {
// ...
}@RequestBody
Use the @RequestBody annotation to have the request body read and deserialized into an
Object through an HttpMessageConverter.
Below is an example with an @RequestBody argument:
@PostMapping("/accounts")
public void handle(@RequestBody Account account) {
// ...
}You can use the Message Converters option of the MVC Config to configure or customize message conversion.
@RequestBody can be used in combination with javax.validation.Valid, or Spring’s
@Validated annotation, which causes Standard Bean Validation to be applied.
By default validation errors cause a MethodArgumentNotValidException which is turned
into a 400 (BAD_REQUEST) response. Alternatively validation errors can be handled locally
within the controller through an Errors or BindingResult argument:
@PostMapping("/accounts")
public void handle(@Valid @RequestBody Account account, BindingResult result) {
// ...
}HttpEntity
HttpEntity is more or less identical to using @RequestBody but based on a
container object that exposes request headers and body. Below is an example:
@PostMapping("/accounts")
public void handle(HttpEntity<Account> entity) {
// ...
}@ResponseBody
Use the @ResponseBody annotation on a method to have the return serialized to the
response body through an
HttpMessageConverter. For example:
@GetMapping("/accounts/{id}")
@ResponseBody
public Account handle() {
// ...
}@ResponseBody is also supported at the class level in which case it is inherited by
all controller methods. This is the effect of @RestController which is nothing more
than a meta-annotation marked with @Controller and @ResponseBody.
@ResponseBody may be used with reactive types.
See Async Requests and Reactive types for more details.
You can use the Message Converters option of the MVC Config to configure or customize message conversion.
@ResponseBody methods can be combined with JSON serialization views.
See Jackson JSON for details.
ResponseEntity
ResponseEntity is like @ResponseBody but with status and headers. For example:
@GetMapping("/something")
public ResponseEntity<String> handle() {
String body = ... ;
String etag = ... ;
return ResponseEntity.ok().eTag(etag).build(body);
}Spring MVC supports using a single value reactive type
to produce the ResponseEntity asynchronously, and/or single and multi-value reactive
types for the body.
Jackson JSON
Jackson serialization views
Spring MVC provides built-in support for
Jackson’s Serialization Views
which allows rendering only a subset of all fields in an Object. To use it with
@ResponseBody or ResponseEntity controller methods, use Jackson’s
@JsonView annotation to activate a serialization view class:
@RestController
public class UserController {
@GetMapping("/user")
@JsonView(User.WithoutPasswordView.class)
public User getUser() {
return new User("eric", "7!jd#h23");
}
}
public class User {
public interface WithoutPasswordView {};
public interface WithPasswordView extends WithoutPasswordView {};
private String username;
private String password;
public User() {
}
public User(String username, String password) {
this.username = username;
this.password = password;
}
@JsonView(WithoutPasswordView.class)
public String getUsername() {
return this.username;
}
@JsonView(WithPasswordView.class)
public String getPassword() {
return this.password;
}
}|
|
If you want to do the above programmatically, instead of declaring an @JsonView annotation,
wrap the return value with MappingJacksonValue and use it to supply the serialization view:
@RestController
public class UserController {
@GetMapping("/user")
public MappingJacksonValue getUser() {
User user = new User("eric", "7!jd#h23");
MappingJacksonValue value = new MappingJacksonValue(user);
value.setSerializationView(User.WithoutPasswordView.class);
return value;
}
}For controllers relying on view resolution, simply add the serialization view class to the model:
@Controller
public class UserController extends AbstractController {
@GetMapping("/user")
public String getUser(Model model) {
model.addAttribute("user", new User("eric", "7!jd#h23"));
model.addAttribute(JsonView.class.getName(), User.WithoutPasswordView.class);
return "userView";
}
}Jackson JSONP
In order to enable JSONP support for @ResponseBody
and ResponseEntity methods, declare a @ControllerAdvice bean that extends
AbstractJsonpResponseBodyAdvice as shown below where the constructor argument indicates
the JSONP query parameter name(s):
@ControllerAdvice
public class JsonpAdvice extends AbstractJsonpResponseBodyAdvice {
public JsonpAdvice() {
super("callback");
}
}For controllers relying on view resolution, JSONP is automatically enabled when the
request has a query parameter named jsonp or callback. Those names can be
customized through jsonpParameterNames property.
|
As of Spring Framework 5.0.7, JSONP support is deprecated and will be removed as of Spring Framework 5.1, CORS should be used instead. |
1.4.4. Model
The @ModelAttribute annotation can be used:
-
On a method argument in
@RequestMappingmethods to create or access an Object from the model, and to bind it to the request through aWebDataBinder. -
As a method-level annotation in
@Controlleror@ControllerAdviceclasses helping to initialize the model prior to any@RequestMappingmethod invocation. -
On a
@RequestMappingmethod to mark its return value is a model attribute.
This section discusses @ModelAttribute methods, or the 2nd from the list above.
A controller can have any number of @ModelAttribute methods. All such methods are
invoked before @RequestMapping methods in the same controller. A @ModelAttribute
method can also be shared across controllers via @ControllerAdvice. See the section on
Controller Advice for more details.
@ModelAttribute methods have flexible method signatures. They support many of the same
arguments as @RequestMapping methods except for @ModelAttribute itself nor anything
related to the request body.
An example @ModelAttribute method:
@ModelAttribute
public void populateModel(@RequestParam String number, Model model) {
model.addAttribute(accountRepository.findAccount(number));
// add more ...
}To add one attribute only:
@ModelAttribute
public Account addAccount(@RequestParam String number) {
return accountRepository.findAccount(number);
}|
When a name is not explicitly specified, a default name is chosen based on the Object
type as explained in the Javadoc for
Conventions.
You can always assign an explicit name by using the overloaded |
@ModelAttribute can also be used as a method-level annotation on @RequestMapping
methods in which case the return value of the @RequestMapping method is interpreted as a
model attribute. This is typically not required, as it is the default behavior in HTML
controllers, unless the return value is a String which would otherwise be interpreted
as a view name (also see [mvc-coc-r2vnt]). @ModelAttribute can also help to customize
the model attribute name:
@GetMapping("/accounts/{id}")
@ModelAttribute("myAccount")
public Account handle() {
// ...
return account;
}1.4.5. DataBinder
@Controller or @ControllerAdvice classes can have @InitBinder methods in order to
initialize instances of WebDataBinder, and those in turn are used to:
-
Bind request parameters (i.e. form data or query) to a model object.
-
Convert String-based request values such as request parameters, path variables, headers, cookies, and others, to the target type of controller method arguments.
-
Format model object values as String values when rendering HTML forms.
@InitBinder methods can register controller-specific java.bean.PropertyEditor, or
Spring Converter and Formatter components. In addition, the
MVC config can be used to register Converter and Formatter
types in a globally shared FormattingConversionService.
@InitBinder methods support many of the same arguments that a @RequestMapping methods
do, except for @ModelAttribute (command object) arguments. Typically they’re are declared
with a WebDataBinder argument, for registrations, and a void return value.
Below is an example:
@Controller
public class FormController {
@InitBinder
public void initBinder(WebDataBinder binder) {
SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");
dateFormat.setLenient(false);
binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, false));
}
// ...
}Alternatively when using a Formatter-based setup through a shared
FormattingConversionService, you could re-use the same approach and register
controller-specific Formatter's:
@Controller
public class FormController {
@InitBinder
protected void initBinder(WebDataBinder binder) {
binder.addCustomFormatter(new DateFormatter("yyyy-MM-dd"));
}
// ...
}1.4.6. Exceptions
@Controller and @ControllerAdvice classes can have
@ExceptionHandler methods to handle exceptions from controller methods. For example:
@Controller
public class SimpleController {
// ...
@ExceptionHandler
public ResponseEntity<String> handle(IOException ex) {
// ...
}
}The exception may match against a top-level exception being propagated (i.e. a direct
IOException thrown), or against the immediate cause within a top-level wrapper exception
(e.g. an IOException wrapped inside an IllegalStateException).
For matching exception types, preferably declare the target exception as a method argument
as shown above. When multiple exception methods match, a root exception match is generally
preferred to a cause exception match. More specifically, the ExceptionDepthComparator is
used to sort exceptions based on their depth from the thrown exception type.
Alternatively, the annotation declaration may narrow the exception types to match:
@ExceptionHandler({FileSystemException.class, RemoteException.class})
public ResponseEntity<String> handle(IOException ex) {
// ...
}Or even a list of specific exception types with a very generic argument signature:
@ExceptionHandler({FileSystemException.class, RemoteException.class})
public ResponseEntity<String> handle(Exception ex) {
// ...
}|
The distinction between root and cause exception matching can be surprising: In the The behavior is even simpler in the |
We generally recommend to be as specific as possible in the argument signature,
reducing the potential for mismatches between root and cause exception types.
Consider breaking a multi-matching method into individual @ExceptionHandler
methods, each matching a single specific exception type through its signature.
In a multi-@ControllerAdvice arrangement, please declare your primary root exception
mappings on a @ControllerAdvice prioritized with a corresponding order. While a root
exception match is preferred to a cause, this is defined among the methods of a given
controller or @ControllerAdvice class. This means a cause match on a higher-priority
@ControllerAdvice bean is preferred to any match (e.g. root) on a lower-priority
@ControllerAdvice bean.
Last but not least, an @ExceptionHandler method implementation may choose to back
out of dealing with a given exception instance by rethrowing it in its original form.
This is useful in scenarios where you are only interested in root-level matches or in
matches within a specific context that cannot be statically determined. A rethrown
exception will be propagated through the remaining resolution chain, just like if
the given @ExceptionHandler method would not have matched in the first place.
Support for @ExceptionHandler methods in Spring MVC is built on the DispatcherServlet
level, HandlerExceptionResolver mechanism.
Method arguments
@ExceptionHandler methods support the following arguments:
| Method argument | Description |
|---|---|
Exception type |
For access to the raised exception. |
|
For access to the controller method that raised the exception. |
|
Generic access to request parameters, request & session attributes, without direct use of the Servlet API. |
|
Choose any specific request or response type — e.g. |
|
Enforces the presence of a session. As a consequence, such an argument is never |
|
Currently authenticated user; possibly a specific |
|
The HTTP method of the request. |
|
The current request locale, determined by the most specific |
|
The time zone associated with the current request, as determined by a |
|
For access to the raw response body as exposed by the Servlet API. |
|
For access to the model for an error response, always empty. |
|
Specify attributes to use in case of a redirect — i.e. to be appended to the query string, and/or flash attributes to be stored temporarily until the request after redirect. See Redirect attributes and Flash attributes. |
|
For access to any session attribute; in contrast to model attributes stored in the session
as a result of a class-level |
|
For access to request attributes. See @RequestAttribute for more details. |
Return Values
@ExceptionHandler methods support the following return values:
| Return value | Description |
|---|---|
|
The return value is converted through |
|
The return value specifies the full response including HTTP headers and body be converted
through |
|
A view name to be resolved with |
|
A |
|
Attributes to be added to the implicit model with the view name implicitly determined
through a |
|
An attribute to be added to the model with the view name implicitly determined through
a Note that |
|
The view and model attributes to use, and optionally a response status. |
|
A method with a If none of the above is true, a |
Any other return value |
If a return value is not matched to any of the above, by default it is treated as a model attribute to be added to the model, unless it is a simple type, as determined by BeanUtils#isSimpleProperty in which case it remains unresolved. |
REST API exceptions
A common requirement for REST services is to include error details in the body of the
response. The Spring Framework does not automatically do this because the representation
of error details in the response body is application specific. However a
@RestController may use @ExceptionHandler methods with a ResponseEntity return
value to set the status and the body of the response. Such methods may also be declared
in @ControllerAdvice classes to apply them globally.
Applications that implement global exception handling with error details in the response
body should consider extending
ResponseEntityExceptionHandler
which provides handling for exceptions that Spring MVC raises along with hooks to
customize the response body. To make use of this, create a subclass of
ResponseEntityExceptionHandler, annotate with @ControllerAdvice, override the
necessary methods, and declare it as a Spring bean.
1.4.7. Controller Advice
Typically @ExceptionHandler, @InitBinder, and @ModelAttribute methods apply within
the @Controller class (or class hierarchy) they are declared in. If you want such
methods to apply more globally, across controllers, you can declare them in a class
marked with @ControllerAdvice or @RestControllerAdvice.
@ControllerAdvice is marked with @Component which means such classes can be registered
as Spring beans via component scanning.
@RestControllerAdvice is also a meta-annotation marked with both @ControllerAdvice and
@ResponseBody which essentially means @ExceptionHandler methods are rendered to the
response body via message conversion (vs view resolution/template rendering).
On startup, the infrastructure classes for @RequestMapping and @ExceptionHandler methods
detect Spring beans of type @ControllerAdvice, and then apply their methods at runtime.
Global @ExceptionHandler methods (from an @ControllerAdvice) are applied after local
ones (from the @Controller). By contrast global @ModelAttribute and @InitBinder
methods are applied before local ones.
By default @ControllerAdvice methods apply to every request, i.e. all controllers, but
you can narrow that down to a subset of controllers via attributes on the annotation:
// Target all Controllers annotated with @RestController
@ControllerAdvice(annotations = RestController.class)
public class ExampleAdvice1 {}
// Target all Controllers within specific packages
@ControllerAdvice("org.example.controllers")
public class ExampleAdvice2 {}
// Target all Controllers assignable to specific classes
@ControllerAdvice(assignableTypes = {ControllerInterface.class, AbstractController.class})
public class ExampleAdvice3 {}Keep in mind the above selectors are evaluated at runtime and may negatively impact performance if used extensively. See the @ControllerAdvice Javadoc for more details.
1.5. URI Links
This section describes various options available in the Spring Framework to work with URI’s.
1.5.1. UriComponents
Spring MVC and Spring WebFlux
UriComponentsBuilder helps to build URI’s from URI templates with variables:
UriComponents uriComponents = UriComponentsBuilder
.fromUriString("https://example.com/hotels/{hotel}") (1)
.queryParam("q", "{q}") (2)
.encode() (3)
.build(); (4)
URI uri = uriComponents.expand("Westin", "123").toUri(); (5)| 1 | Static factory method with a URI template. |
| 2 | Add and/or replace URI components. |
| 3 | Request to have the URI template and URI variables encoded. |
| 4 | Build a UriComponents. |
| 5 | Expand variables, and obtain the URI. |
The above can be consolidated into one chain and shortened with buildAndExpand:
URI uri = UriComponentsBuilder
.fromUriString("https://example.com/hotels/{hotel}")
.queryParam("q", "{q}")
.encode()
.buildAndExpand("Westin", "123")
.toUri();It can be shortened further by going directly to URI (which implies encoding):
URI uri = UriComponentsBuilder
.fromUriString("https://example.com/hotels/{hotel}")
.queryParam("q", "{q}")
.build("Westin", "123");Or shorter further yet, with a full URI template:
URI uri = UriComponentsBuilder
.fromUriString("https://example.com/hotels/{hotel}?q={q}")
.build("Westin", "123");1.5.2. UriBuilder
Spring MVC and Spring WebFlux
UriComponentsBuilder implements UriBuilder. A UriBuilder in turn
can be created with a UriBuilderFactory. Together UriBuilderFactory and UriBuilder
provide a pluggable mechanism to build URIs from URI templates, based on shared
configuration such as a base url, encoding preferences, and others.
The RestTemplate and the WebClient can be configured with a UriBuilderFactory
to customize the preparation of URIs. DefaultUriBuilderFactory is a default
implementation of UriBuilderFactory that uses UriComponentsBuilder internally and
exposes shared configuration options.
RestTemplate example:
// import org.springframework.web.util.DefaultUriBuilderFactory.EncodingMode;
String baseUrl = "https://example.org";
DefaultUriBuilderFactory factory = new DefaultUriBuilderFactory(baseUrl);
factory.setEncodingMode(EncodingMode.TEMPLATE_AND_VARIABLES);
RestTemplate restTemplate = new RestTemplate();
restTemplate.setUriTemplateHandler(factory);WebClient example:
// import org.springframework.web.util.DefaultUriBuilderFactory.EncodingMode;
String baseUrl = "https://example.org";
DefaultUriBuilderFactory factory = new DefaultUriBuilderFactory(baseUrl);
factory.setEncodingMode(EncodingMode.TEMPLATE_AND_VARIABLES);
WebClient client = WebClient.builder().uriBuilderFactory(factory).build();In addition DefaultUriBuilderFactory can also be used directly. It is similar to using
UriComponentsBuilder but instead of static factory methods, it is an actual instance
that holds configuration and preferences:
String baseUrl = "https://example.com";
DefaultUriBuilderFactory uriBuilderFactory = new DefaultUriBuilderFactory(baseUrl);
URI uri = uriBuilderFactory.uriString("/hotels/{hotel}")
.queryParam("q", "{q}")
.build("Westin", "123");1.5.3. URI Encoding
Spring MVC and Spring WebFlux
UriComponentsBuilder exposes encoding options at 2 levels:
-
UriComponentsBuilder#encode() - pre-encodes the URI template first, then strictly encodes URI variables when expanded.
-
UriComponents#encode() - encodes URI components after URI variables are expanded.
Both options replace non-ASCII and illegal characters with escaped octets, however option 1 also replaces characters with reserved meaning that appear in URI variables.
|
Consider ";" which is legal in a path but has reserved meaning. Option 1 replaces ";" with "%3B" in URI variables but not in the URI template. By contrast, option 2 never replaces ";" since it is a legal character in a path. |
For most cases option 1 is likely to give the expected result because it treats URI variables as opaque data to be fully encoded, while option 2 is useful only if URI variables intentionally contain reserved characters.
Example usage using option 1:
URI uri = UriComponentsBuilder.fromPath("/hotel list/{city}")
.queryParam("q", "{q}")
.encode()
.buildAndExpand("New York", "foo+bar")
.toUri();
// Result is "/hotel%20list/New%20York?q=foo%2Bbar"The above can be shortened by going directly to URI (which implies encoding):
URI uri = UriComponentsBuilder.fromPath("/hotel list/{city}")
.queryParam("q", "{q}")
.build("New York", "foo+bar")Or shorter further yet, with a full URI template:
URI uri = UriComponentsBuilder.fromPath("/hotel list/{city}?q={q}")
.build("New York", "foo+bar")The WebClient and the RestTemplate expand and encode URI templates internally through
the UriBuilderFactory strategy. Both can be configured with a custom strategy:
String baseUrl = "https://example.com";
DefaultUriBuilderFactory factory = new DefaultUriBuilderFactory(baseUrl)
factory.setEncodingMode(EncodingMode.TEMPLATE_AND_VALUES);
// Customize the RestTemplate..
RestTemplate restTemplate = new RestTemplate();
restTemplate.setUriTemplateHandler(factory);
// Customize the WebClient..
WebClient client = WebClient.builder().uriBuilderFactory(factory).build();The DefaultUriBuilderFactory implementation uses UriComponentsBuilder internally to
expand and encode URI templates. As a factory it provides a single place to configure
the approach to encoding based on one of the below encoding modes:
-
TEMPLATE_AND_VALUES— usesUriComponentsBuilder#encode(), corresponding to option 1 above, to pre-encode the URI template and strictly encode URI variables when expanded. -
VALUES_ONLY— does not encode the URI template and instead applies strict encoding to URI variables viaUriUtils#encodeUriUriVariablesprior to expanding them into the template. -
URI_COMPONENTS— usesUriComponents#encode(), corresponding to option 2 above, to encode URI component value after URI variables are expanded. -
NONE— no encoding is applied.
Out of the box the RestTemplate is set to EncodingMode.URI_COMPONENTS for historic
reasons and for backwards compatibility. The WebClient relies on the default value
in DefaultUriBuilderFactory which was changed from EncodingMode.URI_COMPONENTS in
5.0.x to EncodingMode.TEMPLATE_AND_VALUES in 5.1.
1.5.4. Servlet request relative
You can use ServletUriComponentsBuilder to create URIs relative to the current request:
HttpServletRequest request = ...
// Re-uses host, scheme, port, path and query string...
ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromRequest(request)
.replaceQueryParam("accountId", "{id}").build()
.expand("123")
.encode();You can create URIs relative to the context path:
// Re-uses host, port and context path...
ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromContextPath(request)
.path("/accounts").build()You can create URIs relative to a Servlet (e.g. /main/*):
// Re-uses host, port, context path, and Servlet prefix...
ServletUriComponentsBuilder ucb = ServletUriComponentsBuilder.fromServletMapping(request)
.path("/accounts").build()|
|
1.5.5. Links to controllers
Spring MVC provides a mechanism to prepare links to controller methods. For example, the following MVC controller easily allows for link creation:
@Controller
@RequestMapping("/hotels/{hotel}")
public class BookingController {
@GetMapping("/bookings/{booking}")
public ModelAndView getBooking(@PathVariable Long booking) {
// ...
}
}You can prepare a link by referring to the method by name:
UriComponents uriComponents = MvcUriComponentsBuilder
.fromMethodName(BookingController.class, "getBooking", 21).buildAndExpand(42);
URI uri = uriComponents.encode().toUri();In the above example we provided actual method argument values, in this case the long value 21,
to be used as a path variable and inserted into the URL. Furthermore, we provided the
value 42 in order to fill in any remaining URI variables such as the "hotel" variable inherited
from the type-level request mapping. If the method had more arguments you can supply null for
arguments not needed for the URL. In general only @PathVariable and @RequestParam arguments
are relevant for constructing the URL.
There are additional ways to use MvcUriComponentsBuilder. For example you can use a technique
akin to mock testing through proxies to avoid referring to the controller method by name
(the example assumes static import of MvcUriComponentsBuilder.on):
UriComponents uriComponents = MvcUriComponentsBuilder
.fromMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);
URI uri = uriComponents.encode().toUri();|
Controller method signatures are limited in their design when supposed to be usable for
link creation with |
The above examples use static methods in MvcUriComponentsBuilder. Internally they rely
on ServletUriComponentsBuilder to prepare a base URL from the scheme, host, port,
context path and servlet path of the current request. This works well in most cases,
however sometimes it may be insufficient. For example you may be outside the context of
a request (e.g. a batch process that prepares links) or perhaps you need to insert a path
prefix (e.g. a locale prefix that was removed from the request path and needs to be
re-inserted into links).
For such cases you can use the static "fromXxx" overloaded methods that accept a
UriComponentsBuilder to use base URL. Or you can create an instance of MvcUriComponentsBuilder
with a base URL and then use the instance-based "withXxx" methods. For example:
UriComponentsBuilder base = ServletUriComponentsBuilder.fromCurrentContextPath().path("/en");
MvcUriComponentsBuilder builder = MvcUriComponentsBuilder.relativeTo(base);
builder.withMethodCall(on(BookingController.class).getBooking(21)).buildAndExpand(42);
URI uri = uriComponents.encode().toUri();|
|
1.5.6. Links in views
You can also build links to annotated controllers from views such as JSP, Thymeleaf,
FreeMarker. This can be done using the fromMappingName method in MvcUriComponentsBuilder
which refers to mappings by name.
Every @RequestMapping is assigned a default name based on the capital letters of the
class and the full method name. For example, the method getFoo in class FooController
is assigned the name "FC#getFoo". This strategy can be replaced or customized by creating
an instance of HandlerMethodMappingNamingStrategy and plugging it into your
RequestMappingHandlerMapping. The default strategy implementation also looks at the
name attribute on @RequestMapping and uses that if present. That means if the default
mapping name assigned conflicts with another (e.g. overloaded methods) you can assign
a name explicitly on the @RequestMapping.
|
The assigned request mapping names are logged at TRACE level on startup. |
The Spring JSP tag library provides a function called mvcUrl that can be used to
prepare links to controller methods based on this mechanism.
For example given:
@RequestMapping("/people/{id}/addresses")
public class PersonAddressController {
@RequestMapping("/{country}")
public HttpEntity getAddress(@PathVariable String country) { ... }
}You can prepare a link from a JSP as follows:
<%@ taglib uri="http://www.springframework.org/tags" prefix="s" %>
...
<a href="${s:mvcUrl('PAC#getAddress').arg(0,'US').buildAndExpand('123')}">Get Address</a>The above example relies on the mvcUrl JSP function declared in the Spring tag library
(i.e. META-INF/spring.tld). For more advanced cases (e.g. a custom base URL as explained
in the previous section), it is easy to define your own function, or use a custom tag file,
in order to use a specific instance of MvcUriComponentsBuilder with a custom base URL.
1.6. Async Requests
Spring MVC has an extensive integration with Servlet 3.0 asynchronous request processing:
-
DeferredResultandCallablereturn values in controller method provide basic support for a single asynchronous return value. -
Controllers can stream multiple values including SSE and raw data.
-
Controllers can use reactive clients and return reactive types for response handling.
1.6.1. DeferredResult
Once the asynchronous request processing feature is
enabled in the Servlet container, controller methods can
wrap any supported controller method return value with DeferredResult:
@GetMapping("/quotes")
@ResponseBody
public DeferredResult<String> quotes() {
DeferredResult<String> deferredResult = new DeferredResult<String>();
// Save the deferredResult somewhere..
return deferredResult;
}
// From some other thread...
deferredResult.setResult(data);The controller can produce the return value asynchronously, from a different thread, for example in response to an external event (JMS message), a scheduled task, or other.
1.6.2. Callable
A controller may also wrap any supported return value with java.util.concurrent.Callable:
@PostMapping
public Callable<String> processUpload(final MultipartFile file) {
return new Callable<String>() {
public String call() throws Exception {
// ...
return "someView";
}
};
}The return value will then be obtained by executing the the given task through the
configured TaskExecutor.
1.6.3. Processing
Here is a very concise overview of Servlet asynchronous request processing:
-
A
ServletRequestcan be put in asynchronous mode by callingrequest.startAsync(). The main effect of doing so is that the Servlet, as well as any Filters, can exit but the response will remain open to allow processing to complete later. -
The call to
request.startAsync()returnsAsyncContextwhich can be used for further control over async processing. For example it provides the methoddispatch, that is similar to a forward from the Servlet API except it allows an application to resume request processing on a Servlet container thread. -
The
ServletRequestprovides access to the currentDispatcherTypethat can be used to distinguish between processing the initial request, an async dispatch, a forward, and other dispatcher types.
DeferredResult processing:
-
Controller returns a
DeferredResultand saves it in some in-memory queue or list where it can be accessed. -
Spring MVC calls
request.startAsync(). -
Meanwhile the
DispatcherServletand all configured Filter’s exit the request processing thread but the response remains open. -
The application sets the
DeferredResultfrom some thread and Spring MVC dispatches the request back to the Servlet container. -
The
DispatcherServletis invoked again and processing resumes with the asynchronously produced return value.
Callable processing:
-
Controller returns a
Callable. -
Spring MVC calls
request.startAsync()and submits theCallableto aTaskExecutorfor processing in a separate thread. -
Meanwhile the
DispatcherServletand all Filter’s exit the Servlet container thread but the response remains open. -
Eventually the
Callableproduces a result and Spring MVC dispatches the request back to the Servlet container to complete processing. -
The
DispatcherServletis invoked again and processing resumes with the asynchronously produced return value from theCallable.
For further background and context you can also read the blog posts that introduced asynchronous request processing support in Spring MVC 3.2.
Exception handling
When using a DeferredResult you can choose whether to call setResult or
setErrorResult with an exception. In both cases Spring MVC dispatches the request back
to the Servlet container to complete processing. It is then treated either as if the
controller method returned the given value, or as if it produced the given exception.
The exception then goes through the regular exception handling mechanism, e.g. invoking
@ExceptionHandler methods.
When using Callable, similar processing logic follows. The main difference being that
the result is returned from the Callable or an exception is raised by it.
Interception
HandlerInterceptor's can also be AsyncHandlerInterceptor in order to receive the
afterConcurrentHandlingStarted callback on the initial request that starts asynchronous
processing instead of postHandle and afterCompletion.
HandlerInterceptor's can also register a CallableProcessingInterceptor
or a DeferredResultProcessingInterceptor in order to integrate more deeply with the
lifecycle of an asynchronous request for example to handle a timeout event. See
AsyncHandlerInterceptor
for more details.
DeferredResult provides onTimeout(Runnable) and onCompletion(Runnable) callbacks.
See the Javadoc of DeferredResult for more details. Callable can be substituted for
WebAsyncTask that exposes additional methods for timeout and completion callbacks.
Compared to WebFlux
The Servlet API was originally built for making a single pass through the Filter-Servlet
chain. Asynchronous request processing, added in Servlet 3.0, allows applications to exit
the Filter-Servlet chain but leave the response open for further processing. The Spring MVC
async support is built around that mechanism. When a controller returns a DeferredResult,
the Filter-Servlet chain is exited and the Servlet container thread is released. Later when
the DeferredResult is set, an ASYNC dispatch (to the same URL) is made during which the
controller is mapped again but rather than invoking it, the DeferredResult value is used
(as if the controller returned it) to resume processing.
By contrast Spring WebFlux is neither built on the Servlet API, nor does it need such an asynchronous request processing feature because it is asynchronous by design. Asynchronous handling is built into all framework contracts and is intrinsically supported through :: stages of request processing.
From a programming model perspective, both Spring MVC and Spring WebFlux support asynchronous and Reactive types as return values in controller methods. Spring MVC even supports streaming, including reactive back pressure. However individual writes to the response remain blocking (and performed on a separate thread) unlike WebFlux that relies on non-blocking I/O and does not need an extra thread for each write.
Another fundamental difference is that Spring MVC does not support asynchronous or
reactive types in controller method arguments, e.g. @RequestBody, @RequestPart, and
others, nor does it have any explicit support for asynchronous and reactive types as
model attributes. Spring WebFlux does support all that.
1.6.4. HTTP Streaming
DeferredResult and Callable can be used for a single asynchronous return value.
What if you want to produce multiple asynchronous values and have those written to the
response?
Objects
The ResponseBodyEmitter return value can be used to produce a stream of Objects, where
each Object sent is serialized with an
HttpMessageConverter and written to the
response. For example:
@GetMapping("/events")
public ResponseBodyEmitter handle() {
ResponseBodyEmitter emitter = new ResponseBodyEmitter();
// Save the emitter somewhere..
return emitter;
}
// In some other thread
emitter.send("Hello once");
// and again later on
emitter.send("Hello again");
// and done at some point
emitter.complete();ResponseBodyEmitter can also be used as the body in a ResponseEntity allowing you to
customize the status and headers of the response.
When an emitter throws an IOException (e.g. if the remote client went away) applications
are not responsible for cleaning up the connection, and should not invoke emitter.complete
or emitter.completeWithError. Instead the servlet container automatically initiates an
AsyncListener error notification in which Spring MVC makes a completeWithError call,
which in turn performs one a final ASYNC dispatch to the application during which Spring MVC
invokes the configured exception resolvers and completes the request.
SSE
SseEmitter is a sub-class of ResponseBodyEmitter that provides support for
Server-Sent Events where events sent from the server
are formatted according to the W3C SSE specification. In order to produce an SSE
stream from a controller simply return SseEmitter:
@GetMapping(path="/events", produces=MediaType.TEXT_EVENT_STREAM_VALUE)
public SseEmitter handle() {
SseEmitter emitter = new SseEmitter();
// Save the emitter somewhere..
return emitter;
}
// In some other thread
emitter.send("Hello once");
// and again later on
emitter.send("Hello again");
// and done at some point
emitter.complete();While SSE is the main option for streaming into browsers, note that Internet Explorer does not support Server-Sent Events. Consider using Spring’s WebSocket messaging with SockJS fallback transports (including SSE) that target a wide range of browsers.
Also see previous section for notes on exception handling.
Raw data
Sometimes it is useful to bypass message conversion and stream directly to the response
OutputStream for example for a file download. Use the of the StreamingResponseBody
return value type to do that:
@GetMapping("/download")
public StreamingResponseBody handle() {
return new StreamingResponseBody() {
@Override
public void writeTo(OutputStream outputStream) throws IOException {
// write...
}
};
}StreamingResponseBody can be used as the body in a ResponseEntity allowing you to
customize the status and headers of the response.
1.6.5. Reactive types
Spring MVC supports use of reactive client libraries in a controller (also read
Reactive Libraries in the WebFlux section).
This includes the WebClient from spring-webflux and others, such as Spring Data
reactive data repositories. In such scenarios, it is convenient to be able to return
reactive types from the controller method.
Reactive return values are handled as follows:
-
A single-value promise is adapted to, and similar to using
DeferredResult. Examples includeMono(Reactor) orSingle(RxJava). -
A multi-value stream, with a streaming media type such as
"application/stream+json"or"text/event-stream", is adapted to, and similar to usingResponseBodyEmitterorSseEmitter. Examples includeFlux(Reactor) orObservable(RxJava). Applications can also returnFlux<ServerSentEvent>orObservable<ServerSentEvent>. -
A multi-value stream, with any other media type (e.g. "application/json"), is adapted to, and similar to using
DeferredResult<List<?>>.
|
Spring MVC supports Reactor and RxJava through the
ReactiveAdapterRegistry from
|
For streaming to the response, reactive back pressure is supported, but writes to the
response are still blocking, and are executed on a separate thread through the
configured TaskExecutor in order to avoid
blocking the upstream source (e.g. a Flux returned from the WebClient).
By default SimpleAsyncTaskExecutor is used for the blocking writes but that is not
suitable under load. If you plan to stream with a reactive type, please use the
MVC config to configure a task executor.
1.6.6. Disconnects
The Servlet API does not provide any notification when a remote client goes away. Therefore while streaming to the response, whether via SseEmitter or <<mvc-ann-async-reactive-types,reactive types>, it is important to send data periodically, since the write would fail if the client has disconnected. The send could take the form of an empty (comment-only) SSE event, or any other data that the other side would have to to interpret as a heartbeat and ignore.
Alternatively consider using web messaging solutions such as STOMP over WebSocket or WebSocket with SockJS that have a built-in heartbeat mechanism.
1.6.7. Configuration
The async request processing feature must be enabled at the Servlet container level. The MVC config also exposes several options for asynchronous requests.
Servlet container
Filter and Servlet declarations have an asyncSupported that needs to be set to true
in order enable asynchronous request processing. In addition, Filter mappings should be
declared to handle the ASYNC javax.servlet.DispatchType.
In Java configuration, when you use AbstractAnnotationConfigDispatcherServletInitializer
to initialize the Servlet container, this is done automatically.
In web.xml configuration, add <async-supported>true</async-supported> to the
DispatcherServlet and to Filter declarations, and also add
<dispatcher>ASYNC</dispatcher> to filter mappings.
Spring MVC
The MVC config exposes options related to async request processing:
-
Java config — use the
configureAsyncSupportcallback onWebMvcConfigurer. -
XML namespace — use the
<async-support>element under<mvc:annotation-driven>.
You can configure the following:
-
Default timeout value for async requests, which if not set, depends on the underlying Servlet container (e.g. 10 seconds on Tomcat).
-
AsyncTaskExecutorto use for blocking writes when streaming with Reactive types, and also for executingCallable's returned from controller methods. It is highly recommended to configure this property if you’re streaming with reactive types or have controller methods that returnCallablesince by default it is aSimpleAsyncTaskExecutor. -
DeferredResultProcessingInterceptor's andCallableProcessingInterceptor's.
Note that the default timeout value can also be set on a DeferredResult,
ResponseBodyEmitter and SseEmitter. For a Callable, use WebAsyncTask to provide
a timeout value.
1.7. CORS
1.7.1. Introduction
For security reasons browsers prohibit AJAX calls to resources outside the current origin. For example you could have your bank account in one tab and evil.com in another. Scripts from evil.com should not be able to make AJAX requests to your bank API with your credentials, e.g. withdrawing money from your account!
Cross-Origin Resource Sharing (CORS) is a W3C specification implemented by most browsers that allows you to specify what kind of cross domain requests are authorized rather than using less secure and less powerful workarounds based on IFRAME or JSONP.
1.7.2. Processing
The CORS specification distinguishes between preflight, simple, and actual requests. To learn how CORS works, you can read this article, among many others, or refer to the specification for more details.
Spring MVC HandlerMapping's provide built-in support for CORS. After successfully
mapping a request to a handler, HandlerMapping's check the CORS configuration for the
given request and handler and take further actions. Preflight requests are handled
directly while simple and actual CORS requests are intercepted, validated, and have
required CORS response headers set.
In order to enable cross-origin requests (i.e. the Origin header is present and
differs from the host of the request) you need to have some explicitly declared CORS
configuration. If no matching CORS configuration is found, preflight requests are
rejected. No CORS headers are added to the responses of simple and actual CORS requests
and consequently browsers reject them.
Each HandlerMapping can be
configured
individually with URL pattern based CorsConfiguration mappings. In most cases applications
will use the MVC Java config or the XML namespace to declare such mappings, which results
in a single, global map passed to all HadlerMappping's.
Global CORS configuration at the HandlerMapping level can be combined with more
fine-grained, handler-level CORS configuration. For example annotated controllers can use
class or method-level @CrossOrigin annotations (other handlers can implement
CorsConfigurationSource).
The rules for combining global and local configuration are generally additive — e.g.
all global and all local origins. For those attributes where only a single value can be
accepted such as allowCredentials and maxAge, the local overrides the global value. See
CorsConfiguration#combine(CorsConfiguration)
for more details.
|
To learn more from the source or make advanced customizations, check:
|
1.7.3. @CrossOrigin
The @CrossOrigin
annotation enables cross-origin requests on annotated controller methods:
@RestController
@RequestMapping("/account")
public class AccountController {
@CrossOrigin
@GetMapping("/{id}")
public Account retrieve(@PathVariable Long id) {
// ...
}
@DeleteMapping("/{id}")
public void remove(@PathVariable Long id) {
// ...
}
}By default @CrossOrigin allows:
-
All origins.
-
All headers.
-
All HTTP methods to which the controller method is mapped.
-
allowedCredentialsis not enabled by default since that establishes a trust level that exposes sensitive user-specific information such as cookies and CSRF tokens, and should only be used where appropriate. -
maxAgeis set to 30 minutes.
@CrossOrigin is supported at the class level too and inherited by all methods:
@CrossOrigin(origins = "https://domain2.com", maxAge = 3600)
@RestController
@RequestMapping("/account")
public class AccountController {
@GetMapping("/{id}")
public Account retrieve(@PathVariable Long id) {
// ...
}
@DeleteMapping("/{id}")
public void remove(@PathVariable Long id) {
// ...
}
}CrossOrigin can be used at both class and method-level:
@CrossOrigin(maxAge = 3600)
@RestController
@RequestMapping("/account")
public class AccountController {
@CrossOrigin("https://domain2.com")
@GetMapping("/{id}")
public Account retrieve(@PathVariable Long id) {
// ...
}
@DeleteMapping("/{id}")
public void remove(@PathVariable Long id) {
// ...
}
}1.7.4. Global Config
In addition to fine-grained, controller method level configuration you’ll probably want to
define some global CORS configuration too. You can set URL-based CorsConfiguration
mappings individually on any HandlerMapping. Most applications however will use the
MVC Java config or the MVC XNM namespace to do that.
By default global configuration enables the following:
-
All origins.
-
All headers.
-
GET,HEAD, andPOSTmethods. -
allowedCredentialsis not enabled by default since that establishes a trust level that exposes sensitive user-specific information such as cookies and CSRF tokens, and should only be used where appropriate. -
maxAgeis set to 30 minutes.
Java Config
To enable CORS in the MVC Java config, use the CorsRegistry callback:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void addCorsMappings(CorsRegistry registry) {
registry.addMapping("/api/**")
.allowedOrigins("https://domain2.com")
.allowedMethods("PUT", "DELETE")
.allowedHeaders("header1", "header2", "header3")
.exposedHeaders("header1", "header2")
.allowCredentials(true).maxAge(3600);
// Add more mappings...
}
}XML Config
To enable CORS in the XML namespace, use the <mvc:cors> element:
<mvc:cors>
<mvc:mapping path="/api/**"
allowed-origins="https://domain1.com, https://domain2.com"
allowed-methods="GET, PUT"
allowed-headers="header1, header2, header3"
exposed-headers="header1, header2" allow-credentials="true"
max-age="123" />
<mvc:mapping path="/resources/**"
allowed-origins="https://domain1.com" />
</mvc:cors>1.7.5. CORS Filter
You can apply CORS support through the built-in
CorsFilter.
|
If you’re trying to use the |
To configure the filter pass a
CorsConfigurationSource to its constructor:
CorsConfiguration config = new CorsConfiguration();
// Possibly...
// config.applyPermitDefaultValues()
config.setAllowCredentials(true);
config.addAllowedOrigin("https://domain1.com");
config.addAllowedHeader("*");
config.addAllowedMethod("*");
UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
source.registerCorsConfiguration("/**", config);
CorsFilter filter = new CorsFilter(source);1.8. Web Security
The Spring Security project provides support for protecting web applications from malicious exploits. Check out the Spring Security reference documentation including:
HDIV is another web security framework that integrates with Spring MVC.
1.9. HTTP Caching
HTTP caching can significantly improve the performance of a web application. HTTP caching
revolves around the "Cache-Control" response header and subsequently conditional request
headers such as "Last-Modified" and "ETag". "Cache-Control" advises private (e.g. browser)
and public (e.g. proxy) caches how to cache and re-use responses. An "ETag" header is used
to make a conditional request that may result in a 304 (NOT_MODIFIED) without a body,
if the content has not changed. "ETag" can be seen as a more sophisticated successor to
the Last-Modified header.
This section describes HTTP caching related options available in Spring Web MVC.
1.9.1. CacheControl
CacheControl provides support for
configuring settings related to the "Cache-Control" header and is accepted as an argument
in a number of places:
While RFC 7234 describes all possible
directives for the "Cache-Control" response header, the CacheControl type takes a
use case oriented approach focusing on the common scenarios:
// Cache for an hour - "Cache-Control: max-age=3600"
CacheControl ccCacheOneHour = CacheControl.maxAge(1, TimeUnit.HOURS);
// Prevent caching - "Cache-Control: no-store"
CacheControl ccNoStore = CacheControl.noStore();
// Cache for ten days in public and private caches,
// public caches should not transform the response
// "Cache-Control: max-age=864000, public, no-transform"
CacheControl ccCustom = CacheControl.maxAge(10, TimeUnit.DAYS).noTransform().cachePublic();WebContentGenerator also accept a simpler cachePeriod property, in seconds, that
works as follows:
-
A
-1value won’t generate a "Cache-Control" response header. -
A
0value will prevent caching using the'Cache-Control: no-store'directive. -
An
n > 0value will cache the given response fornseconds using the'Cache-Control: max-age=n'directive.
1.9.2. Controllers
Controllers can add explicit support for HTTP caching. This is recommended since the
lastModified or ETag value for a resource needs to be calculated before it can be compared
against conditional request headers. A controller can add an ETag and "Cache-Control"
settings to a ResponseEntity:
@GetMapping("/book/{id}")
public ResponseEntity<Book> showBook(@PathVariable Long id) {
Book book = findBook(id);
String version = book.getVersion();
return ResponseEntity
.ok()
.cacheControl(CacheControl.maxAge(30, TimeUnit.DAYS))
.eTag(version) // lastModified is also available
.body(book);
}This will send an 304 (NOT_MODIFIED) response with an empty body, if the comparison to the conditional request headers indicates the content has not changed. Otherwise the "ETag" and "Cache-Control" headers will be added to the response.
The check against conditional request headers can also be made in the controller:
@RequestMapping
public String myHandleMethod(WebRequest request, Model model) {
long eTag = ... (1)
if (request.checkNotModified(eTag)) {
return null; (2)
}
model.addAttribute(...); (3)
return "myViewName";
}| 1 | Application-specific calculation. |
| 2 | Response has been set to 304 (NOT_MODIFIED), no further processing. |
| 3 | Continue with request processing. |
There are 3 variants for checking conditional requests against eTag values, lastModified values, or both. For conditional "GET" and "HEAD" requests, the response may be set to 304 (NOT_MODIFIED). For conditional "POST", "PUT", and "DELETE", the response would be set to 409 (PRECONDITION_FAILED) instead to prevent concurrent modification.
1.9.3. Static resources
Static resources should be served with a "Cache-Control" and conditional response headers for optimal performance. See section on configuring Static Resources.
1.9.4. ETag Filter
The ShallowEtagHeaderFilter can be used to add "shallow" eTag values, computed from the
response content and thus saving bandwidth but not CPU time. See Shallow ETag.
1.10. View Technologies
The use of view technologies in Spring MVC is pluggable, whether you decide to use Thymeleaf, Groovy Markup Templates, JSPs, or other, is primarily a matter of a configuration change. This chapter covers view technologies integrated with Spring MVC. We assume you are already familiar with View Resolution.
1.10.1. Thymeleaf
Thymeleaf is modern server-side Java template engine that emphasizes natural HTML templates that can be previewed in a browser by double-clicking, which is very helpful for independent work on UI templates, e.g. by designer, without the need for a running server. If you’re looking to replace JSPs, Thymeleaf offers one of the most extensive set of features that will make such a transition easier. Thymeleaf is actively developed and maintained. For a more complete introduction see the Thymeleaf project home page.
The Thymeleaf integration with Spring MVC is managed by the Thymeleaf project.
The configuration involves a few bean declarations such as
ServletContextTemplateResolver, SpringTemplateEngine, and ThymeleafViewResolver.
See Thymeleaf+Spring for more details.
1.10.2. FreeMarker
Apache FreeMarker is a template engine for generating any kind of text output from HTML to email, and others. The Spring Framework has a built-in integration for using Spring MVC with FreeMarker templates.
View config
To configure FreeMarker as a view technology:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.freemarker();
}
// Configure FreeMarker...
@Bean
public FreeMarkerConfigurer freeMarkerConfigurer() {
FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
configurer.setTemplateLoaderPath("/WEB-INF/freemarker");
return configurer;
}
}To configure the same in XML:
<mvc:annotation-driven/>
<mvc:view-resolvers>
<mvc:freemarker/>
</mvc:view-resolvers>
<!-- Configure FreeMarker... -->
<mvc:freemarker-configurer>
<mvc:template-loader-path location="/WEB-INF/freemarker"/>
</mvc:freemarker-configurer>Or you can also declare the FreeMarkerConfigurer bean for full control over all
properties:
<bean id="freemarkerConfig" class="org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer">
<property name="templateLoaderPath" value="/WEB-INF/freemarker/"/>
</bean>Your templates need to be stored in the directory specified by the FreeMarkerConfigurer
shown above. Given the above configuration if your controller returns the view name
"welcome" then the resolver will look for the /WEB-INF/freemarker/welcome.ftl template.
FreeMarker config
FreeMarker 'Settings' and 'SharedVariables' can be passed directly to the FreeMarker
Configuration object managed by Spring by setting the appropriate bean properties on
the FreeMarkerConfigurer bean. The freemarkerSettings property requires a
java.util.Properties object and the freemarkerVariables property requires a
java.util.Map.
<bean id="freemarkerConfig" class="org.springframework.web.servlet.view.freemarker.FreeMarkerConfigurer">
<property name="templateLoaderPath" value="/WEB-INF/freemarker/"/>
<property name="freemarkerVariables">
<map>
<entry key="xml_escape" value-ref="fmXmlEscape"/>
</map>
</property>
</bean>
<bean id="fmXmlEscape" class="freemarker.template.utility.XmlEscape"/>See the FreeMarker documentation for details of settings and variables as they apply to
the Configuration object.
Form handling
Spring provides a tag library for use in JSP’s that contains, amongst others, a
<spring:bind/> tag. This tag primarily enables forms to display values from form
backing objects and to show the results of failed validations from a Validator in the
web or business tier. Spring also has support for the same functionality in FreeMarker,
with additional convenience macros for generating form input elements themselves.
The bind macros
A standard set of macros are maintained within the spring-webmvc.jar file for both
languages, so they are always available to a suitably configured application.
Some of the macros defined in the Spring libraries are considered internal (private) but
no such scoping exists in the macro definitions making all macros visible to calling
code and user templates. The following sections concentrate only on the macros you need
to be directly calling from within your templates. If you wish to view the macro code
directly, the file is called spring.ftl in the package
org.springframework.web.servlet.view.freemarker.
Simple binding
In your HTML forms (vm / ftl templates) which act as a form view for a Spring MVC
controller, you can use code similar to the following to bind to field values and
display error messages for each input field in similar fashion to the JSP equivalent.
Example code is shown below for the personForm view configured earlier:
<!-- freemarker macros have to be imported into a namespace. We strongly
recommend sticking to 'spring' -->
<#import "/spring.ftl" as spring/>
<html>
...
<form action="" method="POST">
Name:
<@spring.bind "myModelObject.name"/>
<input type="text"
name="${spring.status.expression}"
value="${spring.status.value?html}"/><br>
<#list spring.status.errorMessages as error> <b>${error}</b> <br> </#list>
<br>
...
<input type="submit" value="submit"/>
</form>
...
</html><@spring.bind> requires a 'path' argument which consists of the name of your command
object (it will be 'command' unless you changed it in your FormController properties)
followed by a period and the name of the field on the command object you wish to bind to.
Nested fields can be used too such as "command.address.street". The bind macro assumes
the default HTML escaping behavior specified by the ServletContext parameter
defaultHtmlEscape in web.xml.
The optional form of the macro called <@spring.bindEscaped> takes a second argument
and explicitly specifies whether HTML escaping should be used in the status error
messages or values. Set to true or false as required. Additional form handling macros
simplify the use of HTML escaping and these macros should be used wherever possible.
They are explained in the next section.
Input macros
Additional convenience macros for both languages simplify both binding and form generation (including validation error display). It is never necessary to use these macros to generate form input fields, and they can be mixed and matched with simple HTML or calls direct to the spring bind macros highlighted previously.
The following table of available macros show the FTL definitions and the parameter list that each takes.
| macro | FTL definition |
|---|---|
message (output a string from a resource bundle based on the code parameter) |
<@spring.message code/> |
messageText (output a string from a resource bundle based on the code parameter, falling back to the value of the default parameter) |
<@spring.messageText code, text/> |
url (prefix a relative URL with the application’s context root) |
<@spring.url relativeUrl/> |
formInput (standard input field for gathering user input) |
<@spring.formInput path, attributes, fieldType/> |
formHiddenInput * (hidden input field for submitting non-user input) |
<@spring.formHiddenInput path, attributes/> |
formPasswordInput * (standard input field for gathering passwords. Note that no value will ever be populated in fields of this type) |
<@spring.formPasswordInput path, attributes/> |
formTextarea (large text field for gathering long, freeform text input) |
<@spring.formTextarea path, attributes/> |
formSingleSelect (drop down box of options allowing a single required value to be selected) |
<@spring.formSingleSelect path, options, attributes/> |
formMultiSelect (a list box of options allowing the user to select 0 or more values) |
<@spring.formMultiSelect path, options, attributes/> |
formRadioButtons (a set of radio buttons allowing a single selection to be made from the available choices) |
<@spring.formRadioButtons path, options separator, attributes/> |
formCheckboxes (a set of checkboxes allowing 0 or more values to be selected) |
<@spring.formCheckboxes path, options, separator, attributes/> |
formCheckbox (a single checkbox) |
<@spring.formCheckbox path, attributes/> |
showErrors (simplify display of validation errors for the bound field) |
<@spring.showErrors separator, classOrStyle/> |
-
In FTL (FreeMarker),
formHiddenInputandformPasswordInputare not actually required as you can use the normalformInputmacro, specifyinghiddenorpasswordas the value for thefieldTypeparameter.
The parameters to any of the above macros have consistent meanings:
-
path: the name of the field to bind to (ie "command.name")
-
options: a Map of all the available values that can be selected from in the input field. The keys to the map represent the values that will be POSTed back from the form and bound to the command object. Map objects stored against the keys are the labels displayed on the form to the user and may be different from the corresponding values posted back by the form. Usually, such a map is supplied as reference data by the controller. You can use any
Mapimplementation, depending on required behavior. For strictly sorted maps, you can use aSortedMap(such as aTreeMap) with a suitableComparatorand, for arbitrary Maps that should return values in insertion order, use aLinkedHashMapor aLinkedMapfromcommons-collections. -
separator: Where multiple options are available as discreet elements (radio buttons or checkboxes), the sequence of characters used to separate each one in the list (such as<br>). -
attributes: An additional string of arbitrary tags or text to be included within the HTML tag itself. This string is echoed literally by the macro. For example, in atextareafield, you may supply attributes (such as 'rows="5" cols="60"'), or you could pass style information such as 'style="border:1px solid silver"'. -
classOrStyle: For theshowErrorsmacro, the name of the CSS class that thespanelement that wraps each error uses. If no information is supplied (or the value is empty), the errors are wrapped in<b></b>tags.
Examples of the macros are outlined below some in FTL and some in VTL. Where usage differences exist between the two languages, they are explained in the notes.
The formInput macro takes the path parameter (command.name) and an additional attributes parameter which is empty in the example above. The macro, along with all other form generation macros, performs an implicit spring bind on the path parameter. The binding remains valid until a new bind occurs so the showErrors macro doesn’t need to pass the path parameter again - it simply operates on whichever field a bind was last created for.
The showErrors macro takes a separator parameter (the characters that will be used to separate multiple errors on a given field) and also accepts a second parameter, this time a class name or style attribute. Note that FreeMarker is able to specify default values for the attributes parameter.
<@spring.formInput "command.name"/>
<@spring.showErrors "<br>"/>Output is shown below of the form fragment generating the name field, and displaying a validation error after the form was submitted with no value in the field. Validation occurs through Spring’s Validation framework.
The generated HTML looks like this:
Name:
<input type="text" name="name" value="">
<br>
<b>required</b>
<br>
<br>The formTextarea macro works the same way as the formInput macro and accepts the same parameter list. Commonly, the second parameter (attributes) will be used to pass style information or rows and cols attributes for the textarea.
Four selection field macros can be used to generate common UI value selection inputs in your HTML forms.
-
formSingleSelect
-
formMultiSelect
-
formRadioButtons
-
formCheckboxes
Each of the four macros accepts a Map of options containing the value for the form field, and the label corresponding to that value. The value and the label can be the same.
An example of radio buttons in FTL is below. The form backing object specifies a default value of 'London' for this field and so no validation is necessary. When the form is rendered, the entire list of cities to choose from is supplied as reference data in the model under the name 'cityMap'.
...
Town:
<@spring.formRadioButtons "command.address.town", cityMap, ""/><br><br>This renders a line of radio buttons, one for each value in cityMap using the
separator "". No additional attributes are supplied (the last parameter to the macro is
missing). The cityMap uses the same String for each key-value pair in the map. The map’s
keys are what the form actually submits as POSTed request parameters, map values are the
labels that the user sees. In the example above, given a list of three well known cities
and a default value in the form backing object, the HTML would be
Town:
<input type="radio" name="address.town" value="London">London</input>
<input type="radio" name="address.town" value="Paris" checked="checked">Paris</input>
<input type="radio" name="address.town" value="New York">New York</input>If your application expects to handle cities by internal codes for example, the map of codes would be created with suitable keys like the example below.
protected Map<String, String> referenceData(HttpServletRequest request) throws Exception {
Map<String, String> cityMap = new LinkedHashMap<>();
cityMap.put("LDN", "London");
cityMap.put("PRS", "Paris");
cityMap.put("NYC", "New York");
Map<String, String> model = new HashMap<>();
model.put("cityMap", cityMap);
return model;
}The code would now produce output where the radio values are the relevant codes but the user still sees the more user friendly city names.
Town:
<input type="radio" name="address.town" value="LDN">London</input>
<input type="radio" name="address.town" value="PRS" checked="checked">Paris</input>
<input type="radio" name="address.town" value="NYC">New York</input>HTML escaping
Default usage of the form macros above will result in HTML tags that are HTML 4.01 compliant and that use the default value for HTML escaping defined in your web.xml as used by Spring’s bind support. In order to make the tags XHTML compliant or to override the default HTML escaping value, you can specify two variables in your template (or in your model where they will be visible to your templates). The advantage of specifying them in the templates is that they can be changed to different values later in the template processing to provide different behavior for different fields in your form.
To switch to XHTML compliance for your tags, specify a value of true for a
model/context variable named xhtmlCompliant:
<#-- for FreeMarker -->
<#assign xhtmlCompliant = true>Any tags generated by the Spring macros will now be XHTML compliant after processing this directive.
In similar fashion, HTML escaping can be specified per field:
<#-- until this point, default HTML escaping is used -->
<#assign htmlEscape = true>
<#-- next field will use HTML escaping -->
<@spring.formInput "command.name"/>
<#assign htmlEscape = false in spring>
<#-- all future fields will be bound with HTML escaping off -->1.10.3. Groovy Markup
Groovy Markup Template Engine is primarily aimed at generating XML-like markup (XML, XHTML, HTML5, etc) but that can be used to generate any text based content. The Spring Framework has a built-in integration for using Spring MVC with Groovy Markup.
|
The Groovy Markup Template engine requires Groovy 2.3.1+. |
Configuration
To configure the Groovy Markup Template Engine:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.groovy();
}
// Configure the Groovy Markup Template Engine...
@Bean
public GroovyMarkupConfigurer groovyMarkupConfigurer() {
GroovyMarkupConfigurer configurer = new GroovyMarkupConfigurer();
configurer.setResourceLoaderPath("/WEB-INF/");
return configurer;
}
}To configure the same in XML:
<mvc:annotation-driven/>
<mvc:view-resolvers>
<mvc:groovy/>
</mvc:view-resolvers>
<!-- Configure the Groovy Markup Template Engine... -->
<mvc:groovy-configurer resource-loader-path="/WEB-INF/"/>Example
Unlike traditional template engines, Groovy Markup relies on a DSL that uses a builder syntax. Here is a sample template for an HTML page:
yieldUnescaped '<!DOCTYPE html>'
html(lang:'en') {
head {
meta('http-equiv':'"Content-Type" content="text/html; charset=utf-8"')
title('My page')
}
body {
p('This is an example of HTML contents')
}
}1.10.4. Script Views
The Spring Framework has a built-in integration for using Spring MVC with any templating library that can run on top of the JSR-223 Java scripting engine. Below is a list of templating libraries we’ve tested on different script engines:
|
The basic rule for integrating any other script engine is that it must implement the
|
Requirements
You need to have the script engine on your classpath:
-
Nashorn JavaScript engine is provided with Java 8+. Using the latest update release available is highly recommended.
-
JRuby should be added as a dependency for Ruby support.
-
Jython should be added as a dependency for Python support.
-
org.jetbrains.kotlin:kotlin-script-utildependency and aMETA-INF/services/javax.script.ScriptEngineFactoryfile containing aorg.jetbrains.kotlin.script.jsr223.KotlinJsr223JvmLocalScriptEngineFactoryline should be added for Kotlin script support, see this example for more details.
You need to have the script templating library. One way to do that for Javascript is through WebJars.
Script templates
Declare a ScriptTemplateConfigurer bean in order to specify the script engine to use,
the script files to load, what function to call to render templates, and so on.
Below is an example with Mustache templates and the Nashorn JavaScript engine:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.scriptTemplate();
}
@Bean
public ScriptTemplateConfigurer configurer() {
ScriptTemplateConfigurer configurer = new ScriptTemplateConfigurer();
configurer.setEngineName("nashorn");
configurer.setScripts("mustache.js");
configurer.setRenderObject("Mustache");
configurer.setRenderFunction("render");
return configurer;
}
}The same in XML:
<mvc:annotation-driven/>
<mvc:view-resolvers>
<mvc:script-template/>
</mvc:view-resolvers>
<mvc:script-template-configurer engine-name="nashorn" render-object="Mustache" render-function="render">
<mvc:script location="mustache.js"/>
</mvc:script-template-configurer>The controller would look no different:
@Controller
public class SampleController {
@GetMapping("/sample")
public String test(Model model) {
model.addObject("title", "Sample title");
model.addObject("body", "Sample body");
return "template";
}
}And the Mustache template is:
<html>
<head>
<title>{{title}}</title>
</head>
<body>
<p>{{body}}</p>
</body>
</html>The render function is called with the following parameters:
-
String template: the template content -
Map model: the view model -
RenderingContext renderingContext: the RenderingContext that gives access to the application context, the locale, the template loader and the url (since 5.0)
Mustache.render() is natively compatible with this signature, so you can call it directly.
If your templating technology requires some customization, you may provide a script that implements a custom render function. For example, Handlerbars needs to compile templates before using them, and requires a polyfill in order to emulate some browser facilities not available in the server-side script engine.
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.scriptTemplate();
}
@Bean
public ScriptTemplateConfigurer configurer() {
ScriptTemplateConfigurer configurer = new ScriptTemplateConfigurer();
configurer.setEngineName("nashorn");
configurer.setScripts("polyfill.js", "handlebars.js", "render.js");
configurer.setRenderFunction("render");
configurer.setSharedEngine(false);
return configurer;
}
}|
Setting the |
polyfill.js only defines the window object needed by Handlebars to run properly:
var window = {};This basic render.js implementation compiles the template before using it. A production
ready implementation should also store and reused cached templates / pre-compiled templates.
This can be done on the script side, as well as any customization you need (managing
template engine configuration for example).
function render(template, model) {
var compiledTemplate = Handlebars.compile(template);
return compiledTemplate(model);
}1.10.5. JSP & JSTL
The Spring Framework has a built-in integration for using Spring MVC with JSP and JSTL.
View resolvers
When developing with JSPs, you can declare a InternalResourceViewResolver or a
ResourceBundleViewResolver bean.
ResourceBundleViewResolver relies on a properties file to define the view names
mapped to a class and a URL. With a ResourceBundleViewResolver, you can mix
different types of views by using only one resolver, as the following example shows:
<!-- the ResourceBundleViewResolver -->
<bean id="viewResolver" class="org.springframework.web.servlet.view.ResourceBundleViewResolver">
<property name="basename" value="views"/>
</bean>
# And a sample properties file is used (views.properties in WEB-INF/classes):
welcome.(class)=org.springframework.web.servlet.view.JstlView
welcome.url=/WEB-INF/jsp/welcome.jsp
productList.(class)=org.springframework.web.servlet.view.JstlView
productList.url=/WEB-INF/jsp/productlist.jspInternalResourceViewResolver can also be used for JSPs. As a best practice, we strongly
encourage placing your JSP files in a directory under the 'WEB-INF' directory so there
can be no direct access by clients.
<bean id="viewResolver" class="org.springframework.web.servlet.view.InternalResourceViewResolver">
<property name="viewClass" value="org.springframework.web.servlet.view.JstlView"/>
<property name="prefix" value="/WEB-INF/jsp/"/>
<property name="suffix" value=".jsp"/>
</bean>JSPs versus JSTL
When using the JSP Standard Tag Library (JSTL) you must use a special view class, the
JstlView, as JSTL needs some preparation before things such as the I18N features can
work.
Spring’s JSP tag library
Spring provides data binding of request parameters to command objects as described in earlier chapters. To facilitate the development of JSP pages in combination with those data binding features, Spring provides a few tags that make things even easier. All Spring tags haveHTML escaping features to enable or disable escaping of characters.
The spring.tld tag library descriptor (TLD) is included in the spring-webmvc.jar.
For a comprehensive reference on individual tags, browse the
API reference
or see the tag library description.
Spring’s form tag library
As of version 2.0, Spring provides a comprehensive set of data binding-aware tags for handling form elements when using JSP and Spring Web MVC. Each tag provides support for the set of attributes of its corresponding HTML tag counterpart, making the tags familiar and intuitive to use. The tag-generated HTML is HTML 4.01/XHTML 1.0 compliant.
Unlike other form/input tag libraries, Spring’s form tag library is integrated with Spring Web MVC, giving the tags access to the command object and reference data your controller deals with. As you will see in the following examples, the form tags make JSPs easier to develop, read and maintain.
Let’s go through the form tags and look at an example of how each tag is used. We have included generated HTML snippets where certain tags require further commentary.
Configuration
The form tag library comes bundled in spring-webmvc.jar. The library descriptor is
called spring-form.tld.
To use the tags from this library, add the following directive to the top of your JSP page:
<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form" %>where form is the tag name prefix you want to use for the tags from this library.
The form tag
This tag renders an HTML 'form' tag and exposes a binding path to inner tags for
binding. It puts the command object in the PageContext so that the command object can
be accessed by inner tags. All the other tags in this library are nested tags of the
form tag.
Let’s assume we have a domain object called User. It is a JavaBean with properties
such as firstName and lastName. We will use it as the form backing object of our
form controller which returns form.jsp. Below is an example of what form.jsp would
look like:
<form:form>
<table>
<tr>
<td>First Name:</td>
<td><form:input path="firstName"/></td>
</tr>
<tr>
<td>Last Name:</td>
<td><form:input path="lastName"/></td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form:form>The firstName and lastName values are retrieved from the command object placed in
the PageContext by the page controller. Keep reading to see more complex examples of
how inner tags are used with the form tag.
The generated HTML looks like a standard form:
<form method="POST">
<table>
<tr>
<td>First Name:</td>
<td><input name="firstName" type="text" value="Harry"/></td>
</tr>
<tr>
<td>Last Name:</td>
<td><input name="lastName" type="text" value="Potter"/></td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form>The preceding JSP assumes that the variable name of the form backing object is
'command'. If you have put the form backing object into the model under another name
(definitely a best practice), then you can bind the form to the named variable like so:
<form:form modelAttribute="user">
<table>
<tr>
<td>First Name:</td>
<td><form:input path="firstName"/></td>
</tr>
<tr>
<td>Last Name:</td>
<td><form:input path="lastName"/></td>
</tr>
<tr>
<td colspan="2">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form:form>The input tag
This tag renders an HTML 'input' tag using the bound value and type='text' by default. For an example of this tag, see The form tag. You may also use HTML5-specific types like 'email', 'tel', 'date', and others.
The checkbox tag
This tag renders an HTML 'input' tag with type 'checkbox'.
Let’s assume our User has preferences such as newsletter subscription and a list of
hobbies. Below is an example of the Preferences class:
public class Preferences {
private boolean receiveNewsletter;
private String[] interests;
private String favouriteWord;
public boolean isReceiveNewsletter() {
return receiveNewsletter;
}
public void setReceiveNewsletter(boolean receiveNewsletter) {
this.receiveNewsletter = receiveNewsletter;
}
public String[] getInterests() {
return interests;
}
public void setInterests(String[] interests) {
this.interests = interests;
}
public String getFavouriteWord() {
return favouriteWord;
}
public void setFavouriteWord(String favouriteWord) {
this.favouriteWord = favouriteWord;
}
}The form.jsp would look like:
<form:form>
<table>
<tr>
<td>Subscribe to newsletter?:</td>
<%-- Approach 1: Property is of type java.lang.Boolean --%>
<td><form:checkbox path="preferences.receiveNewsletter"/></td>
</tr>
<tr>
<td>Interests:</td>
<%-- Approach 2: Property is of an array or of type java.util.Collection --%>
<td>
Quidditch: <form:checkbox path="preferences.interests" value="Quidditch"/>
Herbology: <form:checkbox path="preferences.interests" value="Herbology"/>
Defence Against the Dark Arts: <form:checkbox path="preferences.interests" value="Defence Against the Dark Arts"/>
</td>
</tr>
<tr>
<td>Favourite Word:</td>
<%-- Approach 3: Property is of type java.lang.Object --%>
<td>
Magic: <form:checkbox path="preferences.favouriteWord" value="Magic"/>
</td>
</tr>
</table>
</form:form>There are 3 approaches to the checkbox tag which should meet all your checkbox needs.
-
Approach One - When the bound value is of type
java.lang.Boolean, theinput(checkbox)is marked as 'checked' if the bound value istrue. Thevalueattribute corresponds to the resolved value of thesetValue(Object)value property. -
Approach Two - When the bound value is of type
arrayorjava.util.Collection, theinput(checkbox)is marked as 'checked' if the configuredsetValue(Object)value is present in the boundCollection. -
Approach Three - For any other bound value type, the
input(checkbox)is marked as 'checked' if the configuredsetValue(Object)is equal to the bound value.
Note that regardless of the approach, the same HTML structure is generated. Below is an HTML snippet of some checkboxes:
<tr>
<td>Interests:</td>
<td>
Quidditch: <input name="preferences.interests" type="checkbox" value="Quidditch"/>
<input type="hidden" value="1" name="_preferences.interests"/>
Herbology: <input name="preferences.interests" type="checkbox" value="Herbology"/>
<input type="hidden" value="1" name="_preferences.interests"/>
Defence Against the Dark Arts: <input name="preferences.interests" type="checkbox" value="Defence Against the Dark Arts"/>
<input type="hidden" value="1" name="_preferences.interests"/>
</td>
</tr>What you might not expect to see is the additional hidden field after each checkbox.
When a checkbox in an HTML page is not checked, its value will not be sent to the
server as part of the HTTP request parameters once the form is submitted, so we need a
workaround for this quirk in HTML in order for Spring form data binding to work. The
checkbox tag follows the existing Spring convention of including a hidden parameter
prefixed by an underscore ("_") for each checkbox. By doing this, you are effectively
telling Spring that "the checkbox was visible in the form and I want my object to
which the form data will be bound to reflect the state of the checkbox no matter what".
The checkboxes tag
This tag renders multiple HTML 'input' tags with type 'checkbox'.
Building on the example from the previous checkbox tag section. Sometimes you prefer
not to have to list all the possible hobbies in your JSP page. You would rather provide
a list at runtime of the available options and pass that in to the tag. That is the
purpose of the checkboxes tag. You pass in an Array, a List or a Map containing
the available options in the "items" property. Typically the bound property is a
collection so it can hold multiple values selected by the user. Below is an example of
the JSP using this tag:
<form:form>
<table>
<tr>
<td>Interests:</td>
<td>
<%-- Property is of an array or of type java.util.Collection --%>
<form:checkboxes path="preferences.interests" items="${interestList}"/>
</td>
</tr>
</table>
</form:form>This example assumes that the "interestList" is a List available as a model attribute
containing strings of the values to be selected from. In the case where you use a Map,
the map entry key will be used as the value and the map entry’s value will be used as
the label to be displayed. You can also use a custom object where you can provide the
property names for the value using "itemValue" and the label using "itemLabel".
The radiobutton tag
This tag renders an HTML 'input' tag with type 'radio'.
A typical usage pattern will involve multiple tag instances bound to the same property but with different values.
<tr>
<td>Sex:</td>
<td>
Male: <form:radiobutton path="sex" value="M"/> <br/>
Female: <form:radiobutton path="sex" value="F"/>
</td>
</tr>The radiobuttons tag
This tag renders multiple HTML 'input' tags with type 'radio'.
Just like the checkboxes tag above, you might want to pass in the available options as
a runtime variable. For this usage you would use the radiobuttons tag. You pass in an
Array, a List or a Map containing the available options in the "items" property.
In the case where you use a Map, the map entry key will be used as the value and the map
entry’s value will be used as the label to be displayed. You can also use a custom
object where you can provide the property names for the value using "itemValue" and the
label using "itemLabel".
<tr>
<td>Sex:</td>
<td><form:radiobuttons path="sex" items="${sexOptions}"/></td>
</tr>The password tag
This tag renders an HTML 'input' tag with type 'password' using the bound value.
<tr>
<td>Password:</td>
<td>
<form:password path="password"/>
</td>
</tr>Please note that by default, the password value is not shown. If you do want the
password value to be shown, then set the value of the 'showPassword' attribute to
true, like so.
<tr>
<td>Password:</td>
<td>
<form:password path="password" value="^76525bvHGq" showPassword="true"/>
</td>
</tr>The select tag
This tag renders an HTML 'select' element. It supports data binding to the selected
option as well as the use of nested option and options tags.
Let’s assume a User has a list of skills.
<tr>
<td>Skills:</td>
<td><form:select path="skills" items="${skills}"/></td>
</tr>If the User’s skill were in Herbology, the HTML source of the 'Skills' row would look
like:
<tr>
<td>Skills:</td>
<td>
<select name="skills" multiple="true">
<option value="Potions">Potions</option>
<option value="Herbology" selected="selected">Herbology</option>
<option value="Quidditch">Quidditch</option>
</select>
</td>
</tr>The option tag
This tag renders an HTML 'option'. It sets 'selected' as appropriate based on the bound value.
<tr>
<td>House:</td>
<td>
<form:select path="house">
<form:option value="Gryffindor"/>
<form:option value="Hufflepuff"/>
<form:option value="Ravenclaw"/>
<form:option value="Slytherin"/>
</form:select>
</td>
</tr>If the User’s house was in Gryffindor, the HTML source of the 'House' row would look
like:
<tr>
<td>House:</td>
<td>
<select name="house">
<option value="Gryffindor" selected="selected">Gryffindor</option>
<option value="Hufflepuff">Hufflepuff</option>
<option value="Ravenclaw">Ravenclaw</option>
<option value="Slytherin">Slytherin</option>
</select>
</td>
</tr>The options tag
This tag renders a list of HTML 'option' tags. It sets the 'selected' attribute as appropriate based on the bound value.
<tr>
<td>Country:</td>
<td>
<form:select path="country">
<form:option value="-" label="--Please Select"/>
<form:options items="${countryList}" itemValue="code" itemLabel="name"/>
</form:select>
</td>
</tr>If the User lived in the UK, the HTML source of the 'Country' row would look like:
<tr>
<td>Country:</td>
<td>
<select name="country">
<option value="-">--Please Select</option>
<option value="AT">Austria</option>
<option value="UK" selected="selected">United Kingdom</option>
<option value="US">United States</option>
</select>
</td>
</tr>As the example shows, the combined usage of an option tag with the options tag
generates the same standard HTML, but allows you to explicitly specify a value in the
JSP that is for display only (where it belongs) such as the default string in the
example: "-- Please Select".
The items attribute is typically populated with a collection or array of item objects.
itemValue and itemLabel simply refer to bean properties of those item objects, if
specified; otherwise, the item objects themselves will be stringified. Alternatively,
you may specify a Map of items, in which case the map keys are interpreted as option
values and the map values correspond to option labels. If itemValue and/or itemLabel
happen to be specified as well, the item value property will apply to the map key and
the item label property will apply to the map value.
The textarea tag
This tag renders an HTML 'textarea'.
<tr>
<td>Notes:</td>
<td><form:textarea path="notes" rows="3" cols="20"/></td>
<td><form:errors path="notes"/></td>
</tr>The hidden tag
This tag renders an HTML 'input' tag with type 'hidden' using the bound value. To submit
an unbound hidden value, use the HTML input tag with type 'hidden'.
<form:hidden path="house"/>If we choose to submit the 'house' value as a hidden one, the HTML would look like:
<input name="house" type="hidden" value="Gryffindor"/>The errors tag
This tag renders field errors in an HTML 'span' tag. It provides access to the errors created in your controller or those that were created by any validators associated with your controller.
Let’s assume we want to display all error messages for the firstName and lastName
fields once we submit the form. We have a validator for instances of the User class
called UserValidator.
public class UserValidator implements Validator {
public boolean supports(Class candidate) {
return User.class.isAssignableFrom(candidate);
}
public void validate(Object obj, Errors errors) {
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "required", "Field is required.");
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "lastName", "required", "Field is required.");
}
}The form.jsp would look like:
<form:form>
<table>
<tr>
<td>First Name:</td>
<td><form:input path="firstName"/></td>
<%-- Show errors for firstName field --%>
<td><form:errors path="firstName"/></td>
</tr>
<tr>
<td>Last Name:</td>
<td><form:input path="lastName"/></td>
<%-- Show errors for lastName field --%>
<td><form:errors path="lastName"/></td>
</tr>
<tr>
<td colspan="3">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form:form>If we submit a form with empty values in the firstName and lastName fields, this is
what the HTML would look like:
<form method="POST">
<table>
<tr>
<td>First Name:</td>
<td><input name="firstName" type="text" value=""/></td>
<%-- Associated errors to firstName field displayed --%>
<td><span name="firstName.errors">Field is required.</span></td>
</tr>
<tr>
<td>Last Name:</td>
<td><input name="lastName" type="text" value=""/></td>
<%-- Associated errors to lastName field displayed --%>
<td><span name="lastName.errors">Field is required.</span></td>
</tr>
<tr>
<td colspan="3">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form>What if we want to display the entire list of errors for a given page? The example below
shows that the errors tag also supports some basic wildcarding functionality.
-
path="*"- displays all errors -
path="lastName"- displays all errors associated with thelastNamefield -
if
pathis omitted - object errors only are displayed
The example below will display a list of errors at the top of the page, followed by field-specific errors next to the fields:
<form:form>
<form:errors path="*" cssClass="errorBox"/>
<table>
<tr>
<td>First Name:</td>
<td><form:input path="firstName"/></td>
<td><form:errors path="firstName"/></td>
</tr>
<tr>
<td>Last Name:</td>
<td><form:input path="lastName"/></td>
<td><form:errors path="lastName"/></td>
</tr>
<tr>
<td colspan="3">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form:form>The HTML would look like:
<form method="POST">
<span name="*.errors" class="errorBox">Field is required.<br/>Field is required.</span>
<table>
<tr>
<td>First Name:</td>
<td><input name="firstName" type="text" value=""/></td>
<td><span name="firstName.errors">Field is required.</span></td>
</tr>
<tr>
<td>Last Name:</td>
<td><input name="lastName" type="text" value=""/></td>
<td><span name="lastName.errors">Field is required.</span></td>
</tr>
<tr>
<td colspan="3">
<input type="submit" value="Save Changes"/>
</td>
</tr>
</table>
</form>The spring-form.tld tag library descriptor (TLD) is included in the spring-webmvc.jar.
For a comprehensive reference on individual tags, browse the
API reference
or see the tag library description.
HTTP method conversion
A key principle of REST is the use of the Uniform Interface. This means that all
resources (URLs) can be manipulated using the same four HTTP methods: GET, PUT, POST,
and DELETE. For each method, the HTTP specification defines the exact semantics. For
instance, a GET should always be a safe operation, meaning that it has no side effects,
and a PUT or DELETE should be idempotent, meaning that you can repeat these operations
over and over again, but the end result should be the same. While HTTP defines these
four methods, HTML only supports two: GET and POST. Fortunately, there are two possible
workarounds: you can either use JavaScript to do your PUT or DELETE, or simply do a POST
with the 'real' method as an additional parameter (modeled as a hidden input field in an
HTML form). This latter trick is what Spring’s HiddenHttpMethodFilter does. This
filter is a plain Servlet Filter and therefore it can be used in combination with any
web framework (not just Spring MVC). Simply add this filter to your web.xml, and a POST
with a hidden _method parameter will be converted into the corresponding HTTP method
request.
To support HTTP method conversion, the Spring MVC form tag was updated to support setting the HTTP method. For example, the following snippet comes from the Pet Clinic sample:
<form:form method="delete">
<p class="submit"><input type="submit" value="Delete Pet"/></p>
</form:form>The preceding example performs an HTTP POST, with the 'real' DELETE method hidden behind
a request parameter. It is picked up by the HiddenHttpMethodFilter, which is defined in
web.xml, as the following example shows:
<filter>
<filter-name>httpMethodFilter</filter-name>
<filter-class>org.springframework.web.filter.HiddenHttpMethodFilter</filter-class>
</filter>
<filter-mapping>
<filter-name>httpMethodFilter</filter-name>
<servlet-name>petclinic</servlet-name>
</filter-mapping>The corresponding @Controller method is shown below:
@RequestMapping(method = RequestMethod.DELETE)
public String deletePet(@PathVariable int ownerId, @PathVariable int petId) {
this.clinic.deletePet(petId);
return "redirect:/owners/" + ownerId;
}HTML5 tags
The Spring form tag library allows entering dynamic attributes, which means you can enter any HTML5 specific attributes.
The form input tag supports entering a type attribute other than 'text'. This is intended to allow rendering new HTML5 specific input types such as 'email', 'date', 'range', and others. Note that entering type='text' is not required since 'text' is the default type.
1.10.6. Tiles
It is possible to integrate Tiles - just as any other view technology - in web applications using Spring. The following describes in a broad way how to do this.
|
This section focuses on Spring’s support for Tiles v3 in the
|
Dependencies
To be able to use Tiles, you have to add a dependency on Tiles version 3.0.1 or higher and its transitive dependencies to your project.
Configuration
To be able to use Tiles, you have to configure it using files containing definitions
(for basic information on definitions and other Tiles concepts, please have a look at
https://tiles.apache.org). In Spring this is done using the TilesConfigurer. Have a
look at the following piece of example ApplicationContext configuration:
<bean id="tilesConfigurer" class="org.springframework.web.servlet.view.tiles3.TilesConfigurer">
<property name="definitions">
<list>
<value>/WEB-INF/defs/general.xml</value>
<value>/WEB-INF/defs/widgets.xml</value>
<value>/WEB-INF/defs/administrator.xml</value>
<value>/WEB-INF/defs/customer.xml</value>
<value>/WEB-INF/defs/templates.xml</value>
</list>
</property>
</bean>As you can see, there are five files containing definitions, which are all located in
the 'WEB-INF/defs' directory. At initialization of the WebApplicationContext, the
files will be loaded and the definitions factory will be initialized. After that has
been done, the Tiles includes in the definition files can be used as views within your
Spring web application. To be able to use the views you have to have a ViewResolver
just as with any other view technology used with Spring. Below you can find two
possibilities, the UrlBasedViewResolver and the ResourceBundleViewResolver.
You can specify locale specific Tiles definitions by adding an underscore and then the locale. For example:
<bean id="tilesConfigurer" class="org.springframework.web.servlet.view.tiles3.TilesConfigurer">
<property name="definitions">
<list>
<value>/WEB-INF/defs/tiles.xml</value>
<value>/WEB-INF/defs/tiles_fr_FR.xml</value>
</list>
</property>
</bean>With this configuration, tiles_fr_FR.xml will be used for requests with the fr_FR locale,
and tiles.xml will be used by default.
|
Since underscores are used to indicate locales, it is recommended to avoid using them otherwise in the file names for Tiles definitions. |
UrlBasedViewResolver
The UrlBasedViewResolver instantiates the given viewClass for each view it has to
resolve.
<bean id="viewResolver" class="org.springframework.web.servlet.view.UrlBasedViewResolver">
<property name="viewClass" value="org.springframework.web.servlet.view.tiles3.TilesView"/>
</bean>ResourceBundleViewResolver
The ResourceBundleViewResolver has to be provided with a property file containing
view names and view classes the resolver can use:
<bean id="viewResolver" class="org.springframework.web.servlet.view.ResourceBundleViewResolver">
<property name="basename" value="views"/>
</bean>...
welcomeView.(class)=org.springframework.web.servlet.view.tiles3.TilesView
welcomeView.url=welcome (this is the name of a Tiles definition)
vetsView.(class)=org.springframework.web.servlet.view.tiles3.TilesView
vetsView.url=vetsView (again, this is the name of a Tiles definition)
findOwnersForm.(class)=org.springframework.web.servlet.view.JstlView
findOwnersForm.url=/WEB-INF/jsp/findOwners.jsp
...As you can see, when using the ResourceBundleViewResolver, you can easily mix
different view technologies.
Note that the TilesView class supports JSTL (the JSP Standard Tag Library) out of the
box.
SimpleSpringPreparerFactory and SpringBeanPreparerFactory
As an advanced feature, Spring also supports two special Tiles PreparerFactory
implementations. Check out the Tiles documentation for details on how to use
ViewPreparer references in your Tiles definition files.
Specify SimpleSpringPreparerFactory to autowire ViewPreparer instances based on
specified preparer classes, applying Spring’s container callbacks as well as applying
configured Spring BeanPostProcessors. If Spring’s context-wide annotation configuration has
been activated, annotations in ViewPreparer classes are automatically detected and
applied. Note that this expects preparer classes in the Tiles definition files, as
the default PreparerFactory does.
Specify SpringBeanPreparerFactory to operate on specified preparer names instead
of classes, obtaining the corresponding Spring bean from the DispatcherServlet’s
application context. The full bean creation process will be in the control of the Spring
application context in this case, allowing for the use of explicit dependency injection
configuration, scoped beans, and so on. Note that you need to define one Spring bean definition
for each preparer name (as used in your Tiles definitions). The following example shows
how to define a SpringBeanPreparerFactory property on a TilesConfigurer bean:
<bean id="tilesConfigurer" class="org.springframework.web.servlet.view.tiles3.TilesConfigurer">
<property name="definitions">
<list>
<value>/WEB-INF/defs/general.xml</value>
<value>/WEB-INF/defs/widgets.xml</value>
<value>/WEB-INF/defs/administrator.xml</value>
<value>/WEB-INF/defs/customer.xml</value>
<value>/WEB-INF/defs/templates.xml</value>
</list>
</property>
<!-- resolving preparer names as Spring bean definition names -->
<property name="preparerFactoryClass"
value="org.springframework.web.servlet.view.tiles3.SpringBeanPreparerFactory"/>
</bean>1.10.7. RSS, Atom
Both AbstractAtomFeedView and AbstractRssFeedView inherit from the base class
AbstractFeedView and are used to provide Atom and RSS Feed views respectfully. They
are based on java.net’s ROME project and are located in the
package org.springframework.web.servlet.view.feed.
AbstractAtomFeedView requires you to implement the buildFeedEntries() method and
optionally override the buildFeedMetadata() method (the default implementation is
empty), as shown below.
public class SampleContentAtomView extends AbstractAtomFeedView {
@Override
protected void buildFeedMetadata(Map<String, Object> model,
Feed feed, HttpServletRequest request) {
// implementation omitted
}
@Override
protected List<Entry> buildFeedEntries(Map<String, Object> model,
HttpServletRequest request, HttpServletResponse response) throws Exception {
// implementation omitted
}
}Similar requirements apply for implementing AbstractRssFeedView, as shown below.
public class SampleContentRssView extends AbstractRssFeedView {
@Override
protected void buildFeedMetadata(Map<String, Object> model,
Channel feed, HttpServletRequest request) {
// implementation omitted
}
@Override
protected List<Item> buildFeedItems(Map<String, Object> model,
HttpServletRequest request, HttpServletResponse response) throws Exception {
// implementation omitted
}
}The buildFeedItems() and buildFeedEntires() methods pass in the HTTP request in case
you need to access the Locale. The HTTP response is passed in only for the setting of
cookies or other HTTP headers. The feed will automatically be written to the response
object after the method returns.
For an example of creating an Atom view please refer to Alef Arendsen’s Spring Team Blog entry.
1.10.8. PDF, Excel
Introduction to document views
Returning an HTML page isn’t always the best way for the user to view the model output, and Spring makes it simple to generate a PDF document or an Excel spreadsheet dynamically from the model data. The document is the view and will be streamed from the server with the correct content type to (hopefully) enable the client PC to run their spreadsheet or PDF viewer application in response.
In order to use Excel views, you need to add the Apache POI library to your classpath. For PDF generation, you need to add (preferably) the OpenPDF library.
| You should use the latest versions of the underlying document-generation libraries, if possible. In particular, we strongly recommend OpenPDF (for example, OpenPDF 1.0.5) instead of the outdated original iText 2.1.7, since OpenPDF is actively maintained and fixes an important vulnerability for untrusted PDF content. |
PDF views
A simple PDF view for a word list could extend
org.springframework.web.servlet.view.document.AbstractPdfView and implement the
buildPdfDocument() method as follows:
public class PdfWordList extends AbstractPdfView {
protected void buildPdfDocument(Map<String, Object> model, Document doc, PdfWriter writer,
HttpServletRequest request, HttpServletResponse response) throws Exception {
List<String> words = (List<String>) model.get("wordList");
for (String word : words) {
doc.add(new Paragraph(word));
}
}
}A controller can return such a view either from an external view definition
(referencing it by name) or as a View instance from the handler method.
Excel views
Since Spring Framework 4.2,
org.springframework.web.servlet.view.document.AbstractXlsView is provided as a base
class for Excel views. It is based on Apache POI, with specialized subclasses (AbstractXlsxView
and AbstractXlsxStreamingView) that supersede the outdated AbstractExcelView class.
The programming model is similar to AbstractPdfView, with buildExcelDocument()
as the central template method and controllers being able to return such a view from
an external definition (by name) or as a View instance from the handler method.
1.10.9. Jackson
Jackson-based JSON views
The MappingJackson2JsonView uses the Jackson library’s ObjectMapper to render the response
content as JSON. By default, the entire contents of the model map (with the exception of
framework-specific classes) are encoded as JSON. For cases where the contents of the
map need to be filtered, you can specify a specific set of model attributes to encode
by using the modelKeys property. You can also use the extractValueFromSingleKeyModel
property to have the value in single-key models extracted and serialized directly rather
than as a map of model attributes.
JSON mapping can be customized as needed through the use of Jackson’s provided
annotations. When further control is needed, a custom ObjectMapper can be injected
through the ObjectMapper property for cases where custom JSON
serializers/deserializers need to be provided for specific types.
Jackson-based XML views
MappingJackson2XmlView uses the
Jackson XML extension’s XmlMapper
to render the response content as XML. If the model contains multiple entries, you should
explicitly set the object to be serialized by using the modelKey bean property. If the
model contains a single entry, it is serialized automatically.
XML mapping can be customized as needed through the use of JAXB or Jackson’s provided
annotations. When further control is needed, a custom XmlMapper can be injected
through the ObjectMapper property for cases where custom XML
serializers/deserializers need to be provided for specific types.
1.10.10. XML marshalling
The MarshallingView uses an XML Marshaller (defined in the org.springframework.oxm
package) to render the response content as XML. You can explicitly set the object to be
marshalled by using a MarshallingView instance’s modelKey bean property. Alternatively,
the view iterates over all model properties and marshals the first type that is supported
by the Marshaller. For more information on the functionality in the
org.springframework.oxm package, see Marshalling XML using O/X Mappers.
1.10.11. XSLT views
XSLT is a transformation language for XML and is popular as a view technology within web applications. XSLT can be a good choice as a view technology if your application naturally deals with XML, or if your model can easily be converted to XML. The following section shows how to produce an XML document as model data and have it transformed with XSLT in a Spring Web MVC application.
This example is a trivial Spring application that creates a list of words in the
Controller and adds them to the model map. The map is returned along with the view
name of our XSLT view. See Annotated Controllers for details of Spring Web MVC’s
Controller interface. The XSLT Controller will turn the list of words into a simple XML
document ready for transformation.
Beans
Configuration is standard for a simple Spring web application: The MVC configuration
has to define an XsltViewResolver bean and regular MVC annotation configuration.
@EnableWebMvc
@ComponentScan
@Configuration
public class WebConfig implements WebMvcConfigurer {
@Bean
public XsltViewResolver xsltViewResolver() {
XsltViewResolver viewResolver = new XsltViewResolver();
viewResolver.setPrefix("/WEB-INF/xsl/");
viewResolver.setSuffix(".xslt");
return viewResolver;
}
}And we need a Controller that encapsulates our word generation logic.
Controller
The controller logic is encapsulated in a @Controller class, with the
handler method being defined as follows:
@Controller
public class XsltController {
@RequestMapping("/")
public String home(Model model) throws Exception {
Document document = DocumentBuilderFactory.newInstance().newDocumentBuilder().newDocument();
Element root = document.createElement("wordList");
List<String> words = Arrays.asList("Hello", "Spring", "Framework");
for (String word : words) {
Element wordNode = document.createElement("word");
Text textNode = document.createTextNode(word);
wordNode.appendChild(textNode);
root.appendChild(wordNode);
}
model.addAttribute("wordList", root);
return "home";
}
}So far we’ve only created a DOM document and added it to the Model map. Note that you
can also load an XML file as a Resource and use it instead of a custom DOM document.
Of course, there are software packages available that will automatically 'domify' an object graph, but within Spring, you have complete flexibility to create the DOM from your model in any way you choose. This prevents the transformation of XML playing too great a part in the structure of your model data which is a danger when using tools to manage the domification process.
Next, XsltViewResolver will resolve the "home" XSLT template file and merge the
DOM document into it to generate our view.
Transformation
Finally, the XsltViewResolver will resolve the "home" XSLT template file and merge the
DOM document into it to generate our view. As shown in the XsltViewResolver
configuration, XSLT templates live in the war file in the 'WEB-INF/xsl' directory
and end with a "xslt" file extension.
<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:output method="html" omit-xml-declaration="yes"/>
<xsl:template match="/">
<html>
<head><title>Hello!</title></head>
<body>
<h1>My First Words</h1>
<ul>
<xsl:apply-templates/>
</ul>
</body>
</html>
</xsl:template>
<xsl:template match="word">
<li><xsl:value-of select="."/></li>
</xsl:template>
</xsl:stylesheet>This is rendered as:
<html>
<head>
<META http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Hello!</title>
</head>
<body>
<h1>My First Words</h1>
<ul>
<li>Hello</li>
<li>Spring</li>
<li>Framework</li>
</ul>
</body>
</html>1.11. MVC Config
The MVC Java config and the MVC XML namespace provide default configuration suitable for most applications along with a configuration API to customize it.
For more advanced customizations, not available in the configuration API, see Advanced Java Config and Advanced XML Config.
You do not need to understand the underlying beans created by the MVC Java config and the MVC namespace but if you want to learn more, see Special Bean Types and Web MVC Config.
1.11.1. Enable MVC Config
In Java config use the @EnableWebMvc annotation:
@Configuration
@EnableWebMvc
public class WebConfig {
}In XML use the <mvc:annotation-driven> element:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/mvc
https://www.springframework.org/schema/mvc/spring-mvc.xsd">
<mvc:annotation-driven/>
</beans>The above registers a number of Spring MVC infrastructure beans also adapting to dependencies available on the classpath: e.g. payload converters for JSON, XML, etc.
1.11.2. MVC Config API
In Java config implement WebMvcConfigurer interface:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
// Implement configuration methods...
}In XML check attributes and sub-elements of <mvc:annotation-driven/>. You can
view the Spring MVC XML schema or use
the code completion feature of your IDE to discover what attributes and
sub-elements are available.
1.11.3. Type conversion
By default formatters for Number and Date types are installed, including support for
the @NumberFormat and @DateTimeFormat annotations. Full support for the Joda-Time
formatting library is also installed if Joda-Time is present on the classpath.
In Java config, register custom formatters and converters:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void addFormatters(FormatterRegistry registry) {
// ...
}
}In XML, the same:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/mvc
https://www.springframework.org/schema/mvc/spring-mvc.xsd">
<mvc:annotation-driven conversion-service="conversionService"/>
<bean id="conversionService"
class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
<property name="converters">
<set>
<bean class="org.example.MyConverter"/>
</set>
</property>
<property name="formatters">
<set>
<bean class="org.example.MyFormatter"/>
<bean class="org.example.MyAnnotationFormatterFactory"/>
</set>
</property>
<property name="formatterRegistrars">
<set>
<bean class="org.example.MyFormatterRegistrar"/>
</set>
</property>
</bean>
</beans>|
See FormatterRegistrar SPI
and the |
1.11.4. Validation
By default if Bean Validation is present
on the classpath — e.g. Hibernate Validator, the LocalValidatorFactoryBean is registered
as a global Validator for use with @Valid and Validated on
controller method arguments.
In Java config, you can customize the global Validator instance:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public Validator getValidator(); {
// ...
}
}In XML, the same:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/mvc
https://www.springframework.org/schema/mvc/spring-mvc.xsd">
<mvc:annotation-driven validator="globalValidator"/>
</beans>Note that you can also register Validator's locally:
@Controller
public class MyController {
@InitBinder
protected void initBinder(WebDataBinder binder) {
binder.addValidators(new FooValidator());
}
}|
If you need to have a |
1.11.5. Interceptors
In Java config, register interceptors to apply to incoming requests:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void addInterceptors(InterceptorRegistry registry) {
registry.addInterceptor(new LocaleChangeInterceptor());
registry.addInterceptor(new ThemeChangeInterceptor()).addPathPatterns("/**").excludePathPatterns("/admin/**");
registry.addInterceptor(new SecurityInterceptor()).addPathPatterns("/secure/*");
}
}In XML, the same:
<mvc:interceptors>
<bean class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor"/>
<mvc:interceptor>
<mvc:mapping path="/**"/>
<mvc:exclude-mapping path="/admin/**"/>
<bean class="org.springframework.web.servlet.theme.ThemeChangeInterceptor"/>
</mvc:interceptor>
<mvc:interceptor>
<mvc:mapping path="/secure/*"/>
<bean class="org.example.SecurityInterceptor"/>
</mvc:interceptor>
</mvc:interceptors>1.11.6. Content Types
You can configure how Spring MVC determines the requested media types from the request — e.g. Accept header, URL path extension, query parameter, etc.
By default the URL path extension is checked first — with json, xml, rss, and atom
registered as known extensions depending on classpath dependencies, and the "Accept" header
is checked second.
Consider changing those defaults to Accept header only and if you must use URL-based
content type resolution consider the query parameter strategy over the path extensions. See
Suffix match and Suffix match and RFD for
more details.
In Java config, customize requested content type resolution:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
configurer.mediaType("json", MediaType.APPLICATION_JSON);
configurer.mediaType("xml", MediaType.APPLICATION_XML);
}
}In XML, the same:
<mvc:annotation-driven content-negotiation-manager="contentNegotiationManager"/>
<bean id="contentNegotiationManager" class="org.springframework.web.accept.ContentNegotiationManagerFactoryBean">
<property name="mediaTypes">
<value>
json=application/json
xml=application/xml
</value>
</property>
</bean>1.11.7. Message Converters
Customization of HttpMessageConverter can be achieved in Java config by overriding
configureMessageConverters()
if you want to replace the default converters created by Spring MVC, or by overriding
extendMessageConverters()
if you just want to customize them or add additional converters to the default ones.
Below is an example that adds Jackson JSON and XML converters with a customized
ObjectMapper instead of default ones:
@Configuration
@EnableWebMvc
public class WebConfiguration implements WebMvcConfigurer {
@Override
public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder()
.indentOutput(true)
.dateFormat(new SimpleDateFormat("yyyy-MM-dd"))
.modulesToInstall(new ParameterNamesModule());
converters.add(new MappingJackson2HttpMessageConverter(builder.build()));
converters.add(new MappingJackson2XmlHttpMessageConverter(builder.createXmlMapper(true).build()));
}
}In this example,
Jackson2ObjectMapperBuilder
is used to create a common configuration for both MappingJackson2HttpMessageConverter and
MappingJackson2XmlHttpMessageConverter with indentation enabled, a customized date format
and the registration of
jackson-module-parameter-names
that adds support for accessing parameter names (feature added in Java 8).
This builder customizes Jackson’s default properties with the following ones:
-
DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIESis disabled. -
MapperFeature.DEFAULT_VIEW_INCLUSIONis disabled.
It also automatically registers the following well-known modules if they are detected on the classpath:
-
jackson-datatype-jdk7: support for Java 7 types like
java.nio.file.Path. -
jackson-datatype-joda: support for Joda-Time types.
-
jackson-datatype-jsr310: support for Java 8 Date & Time API types.
-
jackson-datatype-jdk8: support for other Java 8 types like
Optional.
|
Enabling indentation with Jackson XML support requires
|
Other interesting Jackson modules are available:
-
jackson-datatype-money: support for
javax.moneytypes (unofficial module) -
jackson-datatype-hibernate: support for Hibernate specific types and properties (including lazy-loading aspects)
It is also possible to do the same in XML:
<mvc:annotation-driven>
<mvc:message-converters>
<bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
<property name="objectMapper" ref="objectMapper"/>
</bean>
<bean class="org.springframework.http.converter.xml.MappingJackson2XmlHttpMessageConverter">
<property name="objectMapper" ref="xmlMapper"/>
</bean>
</mvc:message-converters>
</mvc:annotation-driven>
<bean id="objectMapper" class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean"
p:indentOutput="true"
p:simpleDateFormat="yyyy-MM-dd"
p:modulesToInstall="com.fasterxml.jackson.module.paramnames.ParameterNamesModule"/>
<bean id="xmlMapper" parent="objectMapper" p:createXmlMapper="true"/>1.11.8. View Controllers
This is a shortcut for defining a ParameterizableViewController that immediately
forwards to a view when invoked. Use it in static cases when there is no Java controller
logic to execute before the view generates the response.
An example of forwarding a request for "/" to a view called "home" in Java:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void addViewControllers(ViewControllerRegistry registry) {
registry.addViewController("/").setViewName("home");
}
}And the same in XML use the <mvc:view-controller> element:
<mvc:view-controller path="/" view-name="home"/>1.11.9. View Resolvers
The MVC config simplifies the registration of view resolvers.
The following is a Java config example that configures content negotiation view
resolution using JSP and Jackson as a default View for JSON rendering:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.enableContentNegotiation(new MappingJackson2JsonView());
registry.jsp();
}
}And the same in XML:
<mvc:view-resolvers>
<mvc:content-negotiation>
<mvc:default-views>
<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
</mvc:default-views>
</mvc:content-negotiation>
<mvc:jsp/>
</mvc:view-resolvers>Note however that FreeMarker, Tiles, Groovy Markup and script templates also require configuration of the underlying view technology.
The MVC namespace provides dedicated elements. For example with FreeMarker:
<mvc:view-resolvers>
<mvc:content-negotiation>
<mvc:default-views>
<bean class="org.springframework.web.servlet.view.json.MappingJackson2JsonView"/>
</mvc:default-views>
</mvc:content-negotiation>
<mvc:freemarker cache="false"/>
</mvc:view-resolvers>
<mvc:freemarker-configurer>
<mvc:template-loader-path location="/freemarker"/>
</mvc:freemarker-configurer>In Java config simply add the respective "Configurer" bean:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureViewResolvers(ViewResolverRegistry registry) {
registry.enableContentNegotiation(new MappingJackson2JsonView());
registry.freeMarker().cache(false);
}
@Bean
public FreeMarkerConfigurer freeMarkerConfigurer() {
FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
configurer.setTemplateLoaderPath("/freemarker");
return configurer;
}
}1.11.10. Static Resources
This option provides a convenient way to serve static resources from a list of Resource-based locations.
In the example below, given a request that starts with "/resources", the relative path is
used to find and serve static resources relative to "/public" under the web application
root or on the classpath under "/static". The resources are served with a 1-year future
expiration to ensure maximum use of the browser cache and a reduction in HTTP requests
made by the browser. The Last-Modified header is also evaluated and if present a 304
status code is returned.
In Java config:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
registry.addResourceHandler("/resources/**")
.addResourceLocations("/public", "classpath:/static/")
.setCachePeriod(31556926);
}
}In XML:
<mvc:resources mapping="/resources/**"
location="/public, classpath:/static/"
cache-period="31556926" />The resource handler also supports a chain of ResourceResolvers and ResourceTransformers. which can be used to create a toolchain for working with optimized resources.
The VersionResourceResolver can be used for versioned resource URLs based on an MD5 hash
computed from the content, a fixed application version, or other. A
ContentVersionStrategy (MD5 hash) is a good choice with some notable exceptions such as
JavaScript resources used with a module loader.
For example in Java config;
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void addResourceHandlers(ResourceHandlerRegistry registry) {
registry.addResourceHandler("/resources/**")
.addResourceLocations("/public/")
.resourceChain(true)
.addResolver(new VersionResourceResolver().addContentVersionStrategy("/**"));
}
}In XML, the same:
<mvc:resources mapping="/resources/**" location="/public/">
<mvc:resource-chain>
<mvc:resource-cache/>
<mvc:resolvers>
<mvc:version-resolver>
<mvc:content-version-strategy patterns="/**"/>
</mvc:version-resolver>
</mvc:resolvers>
</mvc:resource-chain>
</mvc:resources>You can use ResourceUrlProvider to rewrite URLs and apply the full chain of resolvers and
transformers — e.g. to insert versions. The MVC config provides a ResourceUrlProvider
bean so it can be injected into others. You can also make the rewrite transparent with the
ResourceUrlEncodingFilter for Thymeleaf, JSPs, FreeMarker, and others with URL tags that
rely on HttpServletResponse#encodeURL.
WebJars are also supported through the
WebJarsResourceResolver which is automatically registered when the
org.webjars:webjars-locator-core library is present on the classpath. The resolver can
re-write URLs to include the version of the jar and can also match against incoming URLs
without versions — for example, from /jquery/jquery.min.js to
/jquery/1.2.0/jquery.min.js.
1.11.11. Default Servlet
This allows for mapping the DispatcherServlet to "/" (thus overriding the mapping
of the container’s default Servlet), while still allowing static resource requests to be
handled by the container’s default Servlet. It configures a
DefaultServletHttpRequestHandler with a URL mapping of "/**" and the lowest priority
relative to other URL mappings.
This handler will forward all requests to the default Servlet. Therefore it is important
that it remains last in the order of all other URL HandlerMappings. That will be the
case if you use <mvc:annotation-driven> or alternatively if you are setting up your
own customized HandlerMapping instance be sure to set its order property to a value
lower than that of the DefaultServletHttpRequestHandler, which is Integer.MAX_VALUE.
To enable the feature using the default setup use:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
configurer.enable();
}
}Or in XML:
<mvc:default-servlet-handler/>The caveat to overriding the "/" Servlet mapping is that the RequestDispatcher for the
default Servlet must be retrieved by name rather than by path. The
DefaultServletHttpRequestHandler will attempt to auto-detect the default Servlet for
the container at startup time, using a list of known names for most of the major Servlet
containers (including Tomcat, Jetty, GlassFish, JBoss, Resin, WebLogic, and WebSphere).
If the default Servlet has been custom configured with a different name, or if a
different Servlet container is being used where the default Servlet name is unknown,
then the default Servlet’s name must be explicitly provided as in the following example:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
configurer.enable("myCustomDefaultServlet");
}
}Or in XML:
<mvc:default-servlet-handler default-servlet-name="myCustomDefaultServlet"/>1.11.12. Path Matching
This allows customizing options related to URL matching and treatment of the URL. For details on the individual options check out the PathMatchConfigurer API.
Example in Java config:
@Configuration
@EnableWebMvc
public class WebConfig implements WebMvcConfigurer {
@Override
public void configurePathMatch(PathMatchConfigurer configurer) {
configurer
.setUseSuffixPatternMatch(true)
.setUseTrailingSlashMatch(false)
.setUseRegisteredSuffixPatternMatch(true)
.setPathMatcher(antPathMatcher())
.setUrlPathHelper(urlPathHelper());
}
@Bean
public UrlPathHelper urlPathHelper() {
//...
}
@Bean
public PathMatcher antPathMatcher() {
//...
}
}In XML, the same:
<mvc:annotation-driven>
<mvc:path-matching
suffix-pattern="true"
trailing-slash="false"
registered-suffixes-only="true"
path-helper="pathHelper"
path-matcher="pathMatcher"/>
</mvc:annotation-driven>
<bean id="pathHelper" class="org.example.app.MyPathHelper"/>
<bean id="pathMatcher" class="org.example.app.MyPathMatcher"/>1.11.13. Advanced Java Config
@EnableWebMvc imports DelegatingWebMvcConfiguration that (1) provides default Spring
configuration for Spring MVC applications and (2) detects and delegates to
WebMvcConfigurer's to customize that configuration.
For advanced mode, remove @EnableWebMvc and extend directly from
DelegatingWebMvcConfiguration instead of implementing WebMvcConfigurer:
@Configuration
public class WebConfig extends DelegatingWebMvcConfiguration {
// ...
}You can keep existing methods in WebConfig but you can now also override bean declarations
from the base class and you can still have any number of other WebMvcConfigurer's on
the classpath.
1.11.14. Advanced XML Config
The MVC namespace does not have an advanced mode. If you need to customize a property on
a bean that you can’t change otherwise, you can use the BeanPostProcessor lifecycle
hook of the Spring ApplicationContext:
@Component
public class MyPostProcessor implements BeanPostProcessor {
public Object postProcessBeforeInitialization(Object bean, String name) throws BeansException {
// ...
}
}Note that MyPostProcessor needs to be declared as a bean either explicitly in XML or
detected through a <component-scan/> declaration.
1.12. HTTP/2
Servlet 4 containers are required to support HTTP/2 and Spring Framework 5 is compatible with Servlet API 4. From a programming model perspective there is nothing specific that applications need to do. However there are considerations related to server configuration. For more details please check out the HTTP/2 wiki page.
The Servlet API does expose one construct related to HTTP/2. The
javax.servlet.http.PushBuilder can used to proactively push resources to clients and it
is supported as a method argument to @RequestMapping methods.
2. REST Clients
This section describes options for client-side access to REST endpoints.
2.1. RestTemplate
RestTemplate is a synchronous client to perform HTTP requests. It is the original
Spring REST client, exposing a simple, template method API over underlying HTTP client
libraries.
|
As of 5.0 the |
See RestTemplate for details.
2.2. WebClient
WebClient is a non-blocking, reactive client to perform HTTP requests. It was
introduced in 5.0 and offers a modern alternative to the RestTemplate with efficient
support for both synchronous and asynchronous, as well as streaming scenarios.
See WebClient for more details.
3. Testing
This section summarizes the options available in spring-test for Spring MVC applications.
Servlet API Mocks
Mock implementations of Servlet API contracts for unit testing controllers, filters, and other web components. See Servlet API mock objects for more details.
TestContext Framework
Support for loading Spring configuration in JUnit and TestNG tests including efficient
caching of the loaded configuration across test methods and support for loading a
WebApplicationContext with a MockServletContext.
See TestContext Framework for more details.
Spring MVC Test
A framework, also known as MockMvc, for testing annotated controllers through the
DispatcherServlet, i.e. supporting annotations and complete with Spring MVC
infrastructure, but without an HTTP server. See
Spring MVC Test for more details.
Client-side REST
spring-test provides a MockRestServiceServer that can be used as a mock server for
testing client-side code that internally uses the RestTemplate.
See Client REST Tests for more details.
WebTestClient
WebTestClient was built for testing WebFlux applications but it can also be used for
end-to-end integration testing, to any server, over an HTTP connection. It is a
non-blocking, reactive client and well suited for testing asynchronous and streaming
scenarios.
4. WebSockets
This part of the reference documentation covers support for Servlet stack, WebSocket messaging that includes raw WebSocket interactions, WebSocket emulation via SockJS, and pub-sub messaging via STOMP as a sub-protocol over WebSocket.
4.1. Introduction
The WebSocket protocol RFC 6455 provides a standardized way to establish a full-duplex, two-way communication channel between client and server over a single TCP connection. It is a different TCP protocol from HTTP but is designed to work over HTTP, using ports 80 and 443 and allowing re-use of existing firewall rules.
A WebSocket interaction begins with an HTTP request that uses the HTTP "Upgrade" header
to upgrade, or in this case to switch, to the WebSocket protocol:
GET /spring-websocket-portfolio/portfolio HTTP/1.1 Host: localhost:8080 Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: Uc9l9TMkWGbHFD2qnFHltg== Sec-WebSocket-Protocol: v10.stomp, v11.stomp Sec-WebSocket-Version: 13 Origin: http://localhost:8080
Instead of the usual 200 status code, a server with WebSocket support returns:
HTTP/1.1 101 Switching Protocols Upgrade: websocket Connection: Upgrade Sec-WebSocket-Accept: 1qVdfYHU9hPOl4JYYNXF623Gzn0= Sec-WebSocket-Protocol: v10.stomp
After a successful handshake the TCP socket underlying the HTTP upgrade request remains open for both client and server to continue to send and receive messages.
A complete introduction of how WebSockets work is beyond the scope of this document. Please read RFC 6455, the WebSocket chapter of HTML5, or one of many introductions and tutorials on the Web.
Note that if a WebSocket server is running behind a web server (e.g. nginx) you will likely need to configure it to pass WebSocket upgrade requests on to the WebSocket server. Likewise if the application runs in a cloud environment, check the instructions of the cloud provider related to WebSocket support.
4.1.1. HTTP vs WebSocket
Even though WebSocket is designed to be HTTP compatible and starts with an HTTP request, it is important to understand that the two protocols lead to very different architectures and application programming models.
In HTTP and REST, an application is modeled as many URLs. To interact with the application clients access those URLs, request-response style. Servers route requests to the appropriate handler based on the HTTP URL, method, and headers.
By contrast in WebSockets there is usually just one URL for the initial connect and subsequently all application messages flow on that same TCP connection. This points to an entirely different asynchronous, event-driven, messaging architecture.
WebSocket is also a low-level transport protocol which unlike HTTP does not prescribe any semantics to the content of messages. That means there is no way to route or process a message unless client and server agree on message semantics.
WebSocket clients and servers can negotiate the use of a higher-level, messaging protocol
(e.g. STOMP), via the "Sec-WebSocket-Protocol" header on the HTTP handshake request,
or in the absence of that they need to come up with their own conventions.
4.1.2. When to use it?
WebSockets can make a web page dynamic and interactive. However in many cases a combination of Ajax and HTTP streaming and/or long polling could provide a simple and effective solution.
For example news, mail, and social feeds need to update dynamically but it may be perfectly okay to do so every few minutes. Collaboration, games, and financial apps on the other hand need to be much closer to real time.
Latency alone is not a deciding factor. If the volume of messages is relatively low (e.g. monitoring network failures) HTTP streaming or polling may provide an effective solution. It is the combination of low latency, high frequency and high volume that make the best case for the use WebSocket.
Keep in mind also that over the Internet, restrictive proxies outside your control,
may preclude WebSocket interactions either because they are not configured to pass on the
Upgrade header or because they close long lived connections that appear idle? This
means that the use of WebSocket for internal applications within the firewall is a more
straight-forward decision than it is for public facing applications.
4.2. WebSocket API
The Spring Framework provides a WebSocket API that can be used to write client and server side applications that handle WebSocket messages.
4.2.1. 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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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.
4.2.2. 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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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 Deployment for more on this subject).
Both the Java-config and XML namespace make it possible to configure a custom
HandshakeHandler.
|
Spring provides a |
4.2.3. Deployment
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
server-specific RequestUpgradeStrategy's even when running in a JSR-356 runtime.
Such strategies currently exist for Tomcat, Jetty, GlassFish, WebLogic, WebSphere, and
Undertow (and WildFly).
|
A request to overcome the above limitation in the Java WebSocket API has been created and can be followed at WEBSOCKET_SPEC-211. Tomcat, Undertow and WebSphere provide their own API alternatives that makes it possible to this, and it’s also possible with Jetty. We are hopeful that more servers will follow do the same. |
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
https://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
https://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
version="3.0">
<absolute-ordering>
<name>spring_web</name>
</absolute-ordering>
</web-app>4.2.4. Server config
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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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>|
For client side WebSocket configuration, you should use |
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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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>4.2.5. 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-Optionsis set toSAMEORIGIN, 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://orhttps://. 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("https://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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://www.springframework.org/schema/websocket/spring-websocket.xsd">
<websocket:handlers allowed-origins="https://mydomain.com">
<websocket:mapping path="/myHandler" handler="myHandler" />
</websocket:handlers>
<bean id="myHandler" class="org.springframework.samples.MyHandler"/>
</beans>4.3. SockJS Fallback
Over the public Internet, restrictive proxies outside your control may preclude WebSocket
interactions either because they are not configured to pass on the Upgrade header or
because they close long lived connections that appear idle.
The solution to this problem is WebSocket emulation, i.e. attempting to use WebSocket first and then falling back on HTTP-based techniques that emulate a WebSocket interaction and expose the same application-level API.
On the Servlet stack the Spring Framework provides both server (and also client) support for the SockJS protocol.
4.3.1. Overview
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-websocketmodule. -
As of 4.1
spring-websocketalso 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.
4.3.2. 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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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.
4.3.3. IE 8, 9
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.
|
Spring Security 3.2+ provides support for setting See Section 7.1. "Default Security Headers"
of the Spring Security documentation for details on how to configure the
setting of the |
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");
}
// ...
}|
During initial development, do enable the SockJS client |
4.3.4. Heartbeats
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.
|
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.
4.3.5. Client disconnects
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.
|
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 |
4.3.6. SockJS and CORS
If you allow cross-origin requests (see 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 totrue. -
"Access-Control-Request-Headers"- initialized from values from the equivalent request header. -
"Access-Control-Allow-Methods"- the HTTP methods a transport supports (seeTransportTypeenum). -
"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.
4.3.7. SockJsClient
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:
-
StandardWebSocketClientin a JSR-356 runtime -
JettyWebSocketClientusing 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:
-
RestTemplateXhrTransportuses Spring’sRestTemplatefor HTTP requests. -
JettyXhrTransportuses Jetty’sHttpClientfor 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");|
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
|
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);
}
// ...
}4.4. STOMP
The WebSocket protocol defines two types of messages, text and binary, but their content is undefined. The defines a mechanism for client and server to negotiate a sub-protocol — i.e. a higher level messaging protocol, to use on top of WebSocket to define what kind of messages each can send, what is the format and content for each message, and so on. The use of a sub-protocol is optional but either way client and server will need to agree on some protocol that defines message content.
4.4.1. Overview
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 minimal 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, message payloads 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.
4.4.2. Benefits
Use of STOMP as a sub-protocol enables the Spring Framework and Spring Security to provide a richer programming model vs using raw WebSockets. The same point can be made about how HTTP vs raw TCP and how it enables Spring MVC and other web frameworks to provide rich functionality. The following is a list of benefits:
-
No need to invent a custom messaging protocol and message format.
-
STOMP clients are available including a Java client in the Spring Framework.
-
Message brokers such as RabbitMQ, ActiveMQ, and others can be used (optionally) to manage subscriptions and broadcast messages.
-
Application logic can be organized in any number of
@Controller's and messages routed to them based on the STOMP destination header vs handling raw WebSocket messages with a singleWebSocketHandlerfor a given connection. -
Use Spring Security to secure messages based on STOMP destinations and message types.
4.4.3. Enable STOMP
STOMP over WebSocket support is available in the spring-messaging and the
spring-websocket modules. Once you have those dependencies, you can expose a STOMP
endpoints, over WebSocket with SockJS Fallback, as shown below:
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(); (1)
}
@Override
public void configureMessageBroker(MessageBrokerRegistry config) {
config.setApplicationDestinationPrefixes("/app"); (2)
config.enableSimpleBroker("/topic", "/queue"); (3)
}
}| 1 | "/portfolio" is the HTTP URL for the endpoint to which a WebSocket (or SockJS)
client will need to connect to for the WebSocket handshake. |
| 2 | STOMP messages whose destination header begins with "/app" are routed to
@MessageMapping methods in @Controller classes. |
| 3 | Use the built-in, message broker for subscriptions and broadcasting; Route messages whose destination header begins with "/topic" or "/queue" to the broker. |
The same configuration 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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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>|
For the built-in, simple broker the "/topic" and "/queue" prefixes do not have any special meaning. They’re merely a convention to differentiate between pub-sub vs point-to-point messaging (i.e. many subscribers vs one consumer). When using an external broker, please check the STOMP page of the broker to understand what kind of STOMP destinations and prefixes it supports. |
To connect from a browser, for SockJS you can use the sockjs-client. For STOMP many applications have used the jmesnil/stomp-websocket library (also known as stomp.js) which is feature complete and has been used in production for years but is no longer maintained. At present the JSteunou/webstomp-client is the most actively maintained and evolving successor of that library and the example code below is based on it:
var socket = new SockJS("/spring-websocket-portfolio/portfolio");
var stompClient = webstomp.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 Connect to Broker and
Authentication for more information on authentication.
For a more example code see:
-
Using WebSocket to build an interactive web application getting started guide.
-
Stock Portfolio sample application.
4.4.4. Flow of Messages
Once a STOMP endpoint is exposed, the Spring application becomes a STOMP broker for connected clients. This section describes the flow of messages on the server side.
The spring-messaging module contains foundational support for messaging applications
that originated in Spring Integration and was
later extracted and incorporated into the Spring Framework for broader use across many
Spring projects and application scenarios.
Below is a list of a few of the available messaging abstractions:
-
Message — simple representation for a message including headers and payload.
-
MessageHandler — contract for handling a message.
-
MessageChannel — contract for sending a message that enables loose coupling between producers and consumers.
-
SubscribableChannel —
MessageChannelwithMessageHandlersubscribers. -
ExecutorSubscribableChannel —
SubscribableChannelthat uses anExecutorfor delivering messages.
Both the Java config (i.e. @EnableWebSocketMessageBroker) and the XML namespace config
(i.e. <websocket:message-broker>) use the above components to assemble a message
workflow. The diagram below shows the components used when the simple, built-in message
broker is enabled:
There are 3 message channels in the above diagram:
-
"clientInboundChannel"— for passing messages received from WebSocket clients. -
"clientOutboundChannel"— for sending server messages to WebSocket clients. -
"brokerChannel"— for sending messages to the message broker from within server-side, application code.
The next diagram shows the components used when an external broker (e.g. RabbitMQ) is configured for managing subscriptions and broadcasting messages:
The main difference in the above diagram is the use of the "broker relay" for passing messages up to the external STOMP broker over TCP, and for passing messages down from the broker to subscribed clients.
When messages are received from a WebSocket connectin, they’re decoded to STOMP frames,
then turned into a Spring Message representation, and sent to the
"clientInboundChannel" for further processing. For example STOMP messages whose
destination header starts with "/app" may be routed to @MessageMapping methods in
annotated controllers, while "/topic" and "/queue" messages may be routed directly
to the message broker.
An annotated @Controller handling a STOMP message from a client may send a message to
the message broker through the "brokerChannel", and the broker will broadcast the
message to matching subscribers through the "clientOutboundChannel". The same
controller can also do the same in response to HTTP requests, so a client may perform an
HTTP POST and then an @PostMapping method can send a message to the message broker
to broadcast to subscribed clients.
Let’s trace the flow through a simple example. Given the following server setup:
@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;
}
}-
Client connects to
"http://localhost:8080/portfolio"and once a WebSocket connection is established, STOMP frames begin to flow on it. -
Client sends SUBSCRIBE frame with destination header
"/topic/greeting". Once received and decoded, the message is sent to the"clientInboundChannel", then routed to the message broker which stores the client subscription. -
Client sends SEND frame to
"/app/greeting". The"/app"prefix helps to route it to annotated controllers. After the"/app"prefix is stripped, the remaining"/greeting"part of the destination is mapped to the@MessageMappingmethod inGreetingController. -
The value returned from
GreetingControlleris turned into a SpringMessagewith a payload based on the return value and a default destination header of"/topic/greeting"(derived from the input destination with"/app"replaced by"/topic"). The resulting message is sent to the "brokerChannel" and handled by the message broker. -
The message broker finds all matching subscribers, and sends a MESSAGE frame to each through the
"clientOutboundChannel"from where messages are encoded as STOMP frames and sent on the WebSocket connection.
The next section provides more details on annotated methods including the kinds of arguments and return values supported.
4.4.5. Annotated Controllers
Applications can use annotated @Controller classes to handle messages from clients.
Such classes can declare @MessageMapping, @SubscribeMapping, and @ExceptionHandler
methods as described next.
@MessageMapping
The @MessageMapping annotation can be used on methods to route messages based on their
destination. It is supported at the method level as well as at the type level. At type
level @MessageMapping is used to express shared mappings across all methods in a
controller.
By default destination mappings are expected to be Ant-style, path patterns, e.g. "/foo*",
"/foo/**". The patterns include support for template variables, e.g. "/foo/{id}", that can
be referenced with @DestinationVariable method arguments.
|
Applications can choose to switch to a dot-separated destination convention. See Dot as Separator. |
@MessageMapping methods can have flexible signatures with the following arguments:
| Method argument | Description |
|---|---|
|
For access to the complete message. |
|
For access to the headers within the |
|
For access to the headers via typed accessor methods. |
|
For access to the payload of the message, converted (e.g. from JSON) via a configured
The presence of this annotation is not required since it is assumed by default if no other argument is matched. Payload arguments may be annotated with |
|
For access to a specific header value along with type conversion using an
|
|
For access to all headers in the message. This argument must be assignable to
|
|
For access to template variables extracted from the message destination. Values will be converted to the declared method argument type as necessary. |
|
Reflects the user logged in at the time of the WebSocket HTTP handshake. |
When an @MessageMapping method returns a value, by default the value is serialized to
a payload through a configured MessageConverter, and then sent as a Message to the
"brokerChannel" from where it is broadcast to subscribers. The destination of the
outbound message is the same as that of the inbound message but prefixed with "/topic".
You can use the @SendTo method annotation to customize the destination to send
the payload to. @SendTo can also be used at the class level to share a default target
destination to send messages to. @SendToUser is an variant for sending messages only to
the user associated with a message. See User Destinations for details.
The return value from an @MessageMapping method may be wrapped with ListenableFuture,
CompletableFuture, or CompletionStage in order to produce the payload asynchronously.
As an alternative to returning a payload from an @MessageMapping method you can also
send messages using the SimpMessagingTemplate, which is also how return values are
handled under the covers. See Send Messages.
@SubscribeMapping
@SubscribeMapping is similar to @MessageMapping but narrows the mapping to
subscription messages only. It supports the same
method arguments as @MessageMapping does. However
for the return value, by default a message is sent directly to the client via
"clientOutboundChannel" in response to the subscription, and not to the broker via
"brokerChannel" as a broadcast to matching subscriptions. Adding @SendTo or
@SendToUser overrides this behavior and sends to the broker instead.
When is this useful? Let’s assume the broker is mapped to "/topic" and "/queue" while application controllers are mapped to "/app". In this setup, the broker stores all subscriptions to "/topic" and "/queue" that are intended for repeated broadcasts, and there is no need for the application to get involved. A client could also also subscribe to some "/app" destination and a controller could return a value in response to that subscription without involving the broker, effectively a one-off, request-reply exchange, without storing or using the subscription again. One case for this is populating a UI with initial data on startup.
When is this not useful? Do not try to map broker and controllers to the same destination prefix unless you want both to process messages, including subscriptions, independently for some reason. Inbound messages are handled in parallel. There are no guarantees whether broker or controller will process a given message first. If the goal is to be notified when a subscription is stored and ready for broadcasts, then a client should ask for a receipt if the server supports it (simple broker does not). For example with the Java STOMP Client:
@Autowired
private TaskScheduler messageBrokerTaskScheduler;
// During initialization..
stompClient.setTaskScheduler(this.messageBrokerTaskScheduler);
// When subscribing..
StompHeaders headers = new StompHeaders();
headers.setDestination("/topic/...");
headers.setReceipt("r1");
FrameHandler handler = ...;
stompSession.subscribe(headers, handler).addReceiptTask(() -> {
// Subscription ready...
});A server side option is to register an
ExecutorChannelInterceptor on the brokerChannel and implement the afterMessageHandled
method that is invoked after messages, including subscriptions, have been handled.
@MessageExceptionHandler
An application can use @MessageExceptionHandler methods to handle exceptions from
@MessageMapping methods. Exceptions of interest can be declared in the annotation
itself, or through a method argument if you want to get access to the exception instance:
@Controller
public class MyController {
// ...
@MessageExceptionHandler
public ApplicationError handleException(MyException exception) {
// ...
return appError;
}
}@MessageExceptionHandler methods support flexible method signatures and support the same
method argument types and return values as @MessageMapping methods.
Typically @MessageExceptionHandler methods apply within the @Controller class (or
class hierarchy) they are declared in. If you want such methods to apply more globally,
across controllers, you can declare them in a class marked with @ControllerAdvice.
This is comparable to similar support in Spring MVC.
4.4.6. Send 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.
4.4.7. 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.
|
Applications can also use dot-separated destinations (vs slash). See Dot as Separator. |
4.4.8. External 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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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.
|
Please add |
Furthermore, application components (e.g. HTTP request handling methods, business services, etc.) can also send messages to the broker relay, as described in Send Messages, in order to broadcast messages to subscribed WebSocket clients.
In effect, the broker relay enables robust and scalable message broadcasting.
4.4.9. Connect to 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.
|
The STOMP broker relay always sets the |
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.
Any 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.
By default, the STOMP broker relay always connects, and reconnects as needed if connectivity is lost, to the same host and port. If you wish to supply multiple addresses, on each attempt to connect, you can configure a supplier of addresses, instead of a fixed host and port. For example:
@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig extends AbstractWebSocketMessageBrokerConfigurer {
// ...
@Override
public void configureMessageBroker(MessageBrokerRegistry registry) {
registry.enableStompBrokerRelay("/queue/", "/topic/").setTcpClient(createTcpClient());
registry.setApplicationDestinationPrefixes("/app");
}
private ReactorNettyTcpClient<byte[]> createTcpClient() {
Consumer<ClientOptions.Builder<?>> builderConsumer = builder -> {
builder.connectAddress(()-> {
// Select address to connect to ...
});
};
return new ReactorNettyTcpClient<>(builderConsumer, new StompReactorNettyCodec());
}
}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.
4.4.10. Dot as Separator
When messages are routed to @MessageMapping methods, they’re matched with
AntPathMatcher and by default patterns are expected to use slash "/" as separator.
This is a good convention in a web applications and similar to HTTP URLs. However if
you are more used to messaging conventions, you can switch to using dot "." as separator.
In Java config:
@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {
// ...
@Override
public void configureMessageBroker(MessageBrokerRegistry registry) {
registry.setPathMatcher(new AntPathMatcher("."));
registry.enableStompBrokerRelay("/queue", "/topic");
registry.setApplicationDestinationPrefixes("/app");
}
}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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://www.springframework.org/schema/websocket/spring-websocket.xsd">
<websocket:message-broker application-destination-prefix="/app" path-matcher="pathMatcher">
<websocket:stomp-endpoint path="/stomp"/>
<websocket:stomp-broker-relay prefix="/topic,/queue" />
</websocket:message-broker>
<bean id="pathMatcher" class="org.springframework.util.AntPathMatcher">
<constructor-arg index="0" value="."/>
</bean>
</beans>After that a controller may use dot "." as separator in @MessageMapping methods:
@Controller
@MessageMapping("foo")
public class FooController {
@MessageMapping("bar.{baz}")
public void handleBaz(@DestinationVariable String baz) {
// ...
}
}The client can now send a message to "/app/foo.bar.baz123".
In the example above we did not change the prefixes on the "broker relay" because those depend entirely on the external message broker. Check the STOMP documentation pages of the broker you’re using to see what conventions it supports for the destination header.
The "simple broker" on the other hand does rely on the configured PathMatcher so if
you switch the separator that will also apply to the broker and the way matches
destinations from a message to patterns in subscriptions.
4.4.11. Authentication
Every STOMP over WebSocket messaging session begins with an HTTP request — that can be a request to upgrade to WebSockets (i.e. a WebSocket handshake) or in the case of SockJS fallbacks a series of SockJS HTTP transport requests.
Web applications already have authentication and authorization in place to secure HTTP requests. Typically a user is authenticated via Spring Security using some mechanism such as a login page, HTTP basic authentication, or other. The security context for the authenticated user is saved in the HTTP session and is associated with subsequent requests in the same cookie-based session.
Therefore for a WebSocket handshake, or for SockJS HTTP transport requests,
typically there will already be an authenticated user accessible via
HttpServletRequest#getUserPrincipal(). Spring automatically associates that user
with a WebSocket or SockJS session created for them and subsequently with all
STOMP messages transported over that session through a user header.
In short there is nothing special a typical web application needs to do above
and beyond what it already does for security. The user is authenticated at
the HTTP request level with a security context maintained through a cookie-based
HTTP session which is then associated with WebSocket or SockJS sessions created
for that user and results in a user header stamped on every Message flowing
through the application.
Note that the STOMP protocol does have a "login" and "passcode" headers
on the CONNECT frame. Those were originally designed for and are still needed
for example for STOMP over TCP. However for STOMP over WebSocket by default
Spring ignores authorization headers at the STOMP protocol level and assumes
the user is already authenticated at the HTTP transport level and expects that
the WebSocket or SockJS session contain the authenticated user.
|
Spring Security provides
WebSocket sub-protocol authorization
that uses a |
4.4.12. Token Authentication
Spring Security OAuth provides support for token based security including JSON Web Token (JWT). This can be used as the authentication mechanism in Web applications including STOMP over WebSocket interactions just as described in the previous section, i.e. maintaining identity through a cookie-based session.
At the same time cookie-based sessions are not always the best fit for example in applications that don’t wish to maintain a server-side session at all or in mobile applications where it’s common to use headers for authentication.
The WebSocket protocol RFC 6455 "doesn’t prescribe any particular way that servers can authenticate clients during the WebSocket handshake." In practice however browser clients can only use standard authentication headers (i.e. basic HTTP authentication) or cookies and cannot for example provide custom headers. Likewise the SockJS JavaScript client does not provide a way to send HTTP headers with SockJS transport requests, see sockjs-client issue 196. Instead it does allow sending query parameters that can be used to send a token but that has its own drawbacks, for example as the token may be inadvertently logged with the URL in server logs.
|
The above limitations are for browser-based clients and do not apply to the Spring Java-based STOMP client which does support sending headers with both WebSocket and SockJS requests. |
Therefore applications that wish to avoid the use of cookies may not have any good alternatives for authentication at the HTTP protocol level. Instead of using cookies they may prefer to authenticate with headers at the STOMP messaging protocol level There are 2 simple steps to doing that:
-
Use the STOMP client to pass authentication header(s) at connect time.
-
Process the authentication header(s) with a
ChannelInterceptor.
Below is the example server-side configuration to register a custom authentication
interceptor. Note that an interceptor only needs to authenticate and set
the user header on the CONNECT Message. Spring will note and save the authenticated
user and associate it with subsequent STOMP messages on the same session:
@Configuration
@EnableWebSocketMessageBroker
public class MyConfig implements WebSocketMessageBrokerConfigurer {
@Override
public void configureClientInboundChannel(ChannelRegistration registration) {
registration.interceptors(new ChannelInterceptor() {
@Override
public Message<?> preSend(Message<?> message, MessageChannel channel) {
StompHeaderAccessor accessor =
MessageHeaderAccessor.getAccessor(message, StompHeaderAccessor.class);
if (StompCommand.CONNECT.equals(accessor.getCommand())) {
Authentication user = ... ; // access authentication header(s)
accessor.setUser(user);
}
return message;
}
});
}
}Also note that when using Spring Security’s authorization for messages, at present
you will need to ensure that the authentication ChannelInterceptor config is ordered
ahead of Spring Security’s. This is best done by declaring the custom interceptor in
its own implementation of WebSocketMessageBrokerConfigurer marked with
@Order(Ordered.HIGHEST_PRECEDENCE + 99).
4.4.13. 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;
}
}|
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 |
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());
}
}|
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 |
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.
4.4.14. Events
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 theSimpMessagingTemplateshould subscribe to this event and avoid sending messages at times when the broker is not available. In any case they should be prepared to handleMessageDeliveryExceptionwhen 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 usingSimpMessageHeaderAccessororStompMessageHeaderAccessor. -
SessionConnectedEvent— published shortly after aSessionConnectEventwhen 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.
|
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. |
4.4.15. Interception
Events provide notifications for the lifecycle
of a STOMP connection and not for every client message. Applications can also register a
ChannelInterceptor to intercept any message, and in any part of the processing chain.
For example to intercept inbound messages from clients:
@Configuration
@EnableWebSocketMessageBroker
public class WebSocketConfig implements WebSocketMessageBrokerConfigurer {
@Override
public void configureClientInboundChannel(ChannelRegistration registration) {
registration.interceptors(new MyChannelInterceptor());
}
}A custom ChannelInterceptor can use StompHeaderAccessor or SimpMessageHeaderAccessor
to access information about the message.
public class MyChannelInterceptor implements ChannelInterceptor {
@Override
public Message<?> preSend(Message<?> message, MessageChannel channel) {
StompHeaderAccessor accessor = StompHeaderAccessor.wrap(message);
StompCommand command = accessor.getStompCommand();
// ...
return message;
}
}Applications may also implement ExecutorChannelInterceptor which is a sub-interface
of ChannelInterceptor with callbacks in the thread in which the messages are handled.
While a ChannelInterceptor is invoked once for per message sent to a channel, the
ExecutorChannelInterceptor provides hooks in the thread of each MessageHandler
subscribed to messages from the channel.
Note that just like with the SesionDisconnectEvent above, a DISCONNECT message
may have been sent from the client, or it may also be automatically generated when
the WebSocket session is closed. In some cases an interceptor may intercept this
message more than once per session. Components should be idempotent with regard to
multiple disconnect events.
4.4.16. 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 heartbeatsIn 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
SockJsClient.
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.
|
When using |
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.
4.4.17. 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.
4.4.18. 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 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.
|
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 |
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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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 the JavaScript webstomp-client and others 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
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/websocket
https://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.
4.4.19. 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.
4.4.20. Testing
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:
-
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.
-
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.
5. Other Web Frameworks
5.1. Introduction
This chapter details Spring’s integration with third party web frameworks.
One of the core value propositions of the Spring Framework is that of enabling choice. In a general sense, Spring does not force one to use or buy into any particular architecture, technology, or methodology (although it certainly recommends some over others). This freedom to pick and choose the architecture, technology, or methodology that is most relevant to a developer and their development team is arguably most evident in the web area, where Spring provides its own web framework (Spring MVC), while at the same time providing integration with a number of popular third party web frameworks.
5.2. Common config
Before diving into the integration specifics of each supported web framework, let us first take a look at the Spring configuration that is not specific to any one web framework. (This section is equally applicable to Spring’s own web framework, Spring MVC.)
One of the concepts (for want of a better word) espoused by (Spring’s) lightweight
application model is that of a layered architecture. Remember that in a 'classic'
layered architecture, the web layer is but one of many layers; it serves as one of the
entry points into a server side application and it delegates to service objects
(facades) defined in a service layer to satisfy business specific (and
presentation-technology agnostic) use cases. In Spring, these service objects, any other
business-specific objects, data access objects, etc. exist in a distinct 'business
context', which contains no web or presentation layer objects (presentation objects
such as Spring MVC controllers are typically configured in a distinct 'presentation
context'). This section details how one configures a Spring container (a
WebApplicationContext) that contains all of the 'business beans' in one’s application.
On to specifics: all that one need do is to declare a
ContextLoaderListener
in the standard Java EE servlet web.xml file of one’s web application, and add a
contextConfigLocation<context-param/> section (in the same file) that defines which
set of Spring XML configuration files to load.
Find below the <listener/> configuration:
<listener>
<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>Find below the <context-param/> configuration:
<context-param>
<param-name>contextConfigLocation</param-name>
<param-value>/WEB-INF/applicationContext*.xml</param-value>
</context-param>If you don’t specify the contextConfigLocation context parameter, the
ContextLoaderListener will look for a file called /WEB-INF/applicationContext.xml to
load. Once the context files are loaded, Spring creates a
WebApplicationContext
object based on the bean definitions and stores it in the ServletContext of the web
application.
All Java web frameworks are built on top of the Servlet API, and so one can use the
following code snippet to get access to this 'business context' ApplicationContext
created by the ContextLoaderListener.
WebApplicationContext ctx = WebApplicationContextUtils.getWebApplicationContext(servletContext);The
WebApplicationContextUtils
class is for convenience, so you don’t have to remember the name of the ServletContext
attribute. Its getWebApplicationContext() method will return null if an object
doesn’t exist under the WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE
key. Rather than risk getting NullPointerExceptions in your application, it’s better
to use the getRequiredWebApplicationContext() method. This method throws an exception
when the ApplicationContext is missing.
Once you have a reference to the WebApplicationContext, you can retrieve beans by
their name or type. Most developers retrieve beans by name and then cast them to one of
their implemented interfaces.
Fortunately, most of the frameworks in this section have simpler ways of looking up beans. Not only do they make it easy to get beans from a Spring container, but they also allow you to use dependency injection on their controllers. Each web framework section has more detail on its specific integration strategies.
5.3. JSF
JavaServer Faces (JSF) is the JCP’s standard component-based, event-driven web user interface framework. As of Java EE 5, it is an official part of the Java EE umbrella.
For a popular JSF runtime as well as for popular JSF component libraries, check out the Apache MyFaces project. The MyFaces project also provides common JSF extensions such as MyFaces Orchestra: a Spring-based JSF extension that provides rich conversation scope support.
|
Spring Web Flow 2.0 provides rich JSF support through its newly established Spring Faces module, both for JSF-centric usage (as described in this section) and for Spring-centric usage (using JSF views within a Spring MVC dispatcher). Check out the Spring Web Flow website for details! |
The key element in Spring’s JSF integration is the JSF ELResolver mechanism.
5.3.1. Spring Bean Resolver
SpringBeanFacesELResolver is a JSF 1.2+ compliant ELResolver implementation,
integrating with the standard Unified EL as used by JSF 1.2 and JSP 2.1. Like
SpringBeanVariableResolver, it delegates to the Spring’s 'business context'
WebApplicationContext first, then to the default resolver of the underlying JSF
implementation.
Configuration-wise, simply define SpringBeanFacesELResolver in your JSF
faces-context.xml file:
<faces-config>
<application>
<el-resolver>org.springframework.web.jsf.el.SpringBeanFacesELResolver</el-resolver>
...
</application>
</faces-config>5.3.2. FacesContextUtils
A custom VariableResolver works well when mapping one’s properties to beans
in faces-config.xml, but at times one may need to grab a bean explicitly. The
FacesContextUtils
class makes this easy. It is similar to WebApplicationContextUtils, except that it
takes a FacesContext parameter rather than a ServletContext parameter.
ApplicationContext ctx = FacesContextUtils.getWebApplicationContext(FacesContext.getCurrentInstance());5.4. Apache Struts 2.x
Invented by Craig McClanahan, Struts is an open source project hosted by the Apache Software Foundation. At the time, it greatly simplified the JSP/Servlet programming paradigm and won over many developers who were using proprietary frameworks. It simplified the programming model, it was open source (and thus free as in beer), and it had a large community, which allowed the project to grow and become popular among Java web developers.
Check out the Struts Spring Plugin for the built-in Spring integration shipped with Struts.
5.5. Tapestry 5.x
From the Tapestry homepage:
Tapestry is a "Component oriented framework for creating dynamic, robust, highly scalable web applications in Java."
While Spring has its own powerful web layer, there are a number of unique advantages to building an enterprise Java application using a combination of Tapestry for the web user interface and the Spring container for the lower layers.
For more information, check out Tapestry’s dedicated integration module for Spring.