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View Technologies

The rendering of views in Spring WebFlux is pluggable. Whether you decide to use Thymeleaf, FreeMarker, or some other view technology is primarily a matter of a configuration change. This chapter covers the view technologies integrated with Spring WebFlux.

For more context on view rendering, please see View Resolution.

The views of a Spring WebFlux application live within internal trust boundaries of the application. Views have access to beans in the application context, and as such, we do not recommend use the Spring WebFlux template support in applications where the templates are editable by external sources, since this can have security implications.

Thymeleaf

See equivalent in the Servlet stack

Thymeleaf is a 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 (for example, by a designer) without the need for a running server. Thymeleaf offers an extensive set of features, and it is actively developed and maintained. For a more complete introduction, see the Thymeleaf project home page.

The Thymeleaf integration with Spring WebFlux is managed by the Thymeleaf project. The configuration involves a few bean declarations, such as SpringResourceTemplateResolver, SpringWebFluxTemplateEngine, and ThymeleafReactiveViewResolver. For more details, see Thymeleaf+Spring and the WebFlux integration announcement.

FreeMarker

See equivalent in the Servlet stack

Apache FreeMarker is a template engine for generating any kind of text output from HTML to email and others. The Spring Framework has built-in integration for using Spring WebFlux with FreeMarker templates.

View Configuration

See equivalent in the Servlet stack

The following example shows how to configure FreeMarker as a view technology:

Java
Kotlin

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@Override
	public void configureViewResolvers(ViewResolverRegistry registry) {
		registry.freeMarker();
	}

	// Configure FreeMarker...

	@Bean
	public FreeMarkerConfigurer freeMarkerConfigurer() {
		FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
		configurer.setTemplateLoaderPath("classpath:/templates/freemarker");
		return configurer;
	}
}

@Configuration
@EnableWebFlux
class WebConfig : WebFluxConfigurer {

	override fun configureViewResolvers(registry: ViewResolverRegistry) {
		registry.freeMarker()
	}

	// Configure FreeMarker...

	@Bean
	fun freeMarkerConfigurer() = FreeMarkerConfigurer().apply {
		setTemplateLoaderPath("classpath:/templates/freemarker")
	}
}

Your templates need to be stored in the directory specified by the FreeMarkerConfigurer, shown in the preceding example. Given the preceding configuration, if your controller returns the view name, welcome, the resolver looks for the classpath:/templates/freemarker/welcome.ftl template.

FreeMarker Configuration

See equivalent in the Servlet stack

You can pass FreeMarker 'Settings' and 'SharedVariables' directly to the FreeMarker Configuration object (which is 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. The following example shows how to use a FreeMarkerConfigurer:

Java
Kotlin

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	// ...

	@Bean
	public FreeMarkerConfigurer freeMarkerConfigurer() {
		Map<String, Object> variables = new HashMap<>();
		variables.put("xml_escape", new XmlEscape());

		FreeMarkerConfigurer configurer = new FreeMarkerConfigurer();
		configurer.setTemplateLoaderPath("classpath:/templates");
		configurer.setFreemarkerVariables(variables);
		return configurer;
	}
}

@Configuration
@EnableWebFlux
class WebConfig : WebFluxConfigurer {

	// ...

	@Bean
	fun freeMarkerConfigurer() = FreeMarkerConfigurer().apply {
		setTemplateLoaderPath("classpath:/templates")
		setFreemarkerVariables(mapOf("xml_escape" to XmlEscape()))
	}
}

See the FreeMarker documentation for details of settings and variables as they apply to the Configuration object.

Form Handling

See equivalent in the Servlet stack

Spring provides a tag library for use in JSPs that contains, among others, a <spring:bind/> element. This element primarily lets forms display values from form-backing objects and 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

See equivalent in the Servlet stack

A standard set of macros are maintained within the spring-webflux.jar file for FreeMarker, so they are always available to a suitably configured application.

Some of the macros defined in the Spring templating 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 directly call from within your templates. If you wish to view the macro code directly, the file is called spring.ftl and is in the org.springframework.web.reactive.result.view.freemarker package.

For additional details on binding support, see Simple Binding for Spring MVC.

Form Macros

For details on Spring’s form macro support for FreeMarker templates, consult the following sections of the Spring MVC documentation.

Script Views

See equivalent in the Servlet stack

The Spring Framework has a built-in integration for using Spring WebFlux with any templating library that can run on top of the JSR-223 Java scripting engine. The following table shows the templating libraries that we have tested on different script engines:

Scripting Library	Scripting Engine
Handlebars	Nashorn
Mustache	Nashorn
React	Nashorn
EJS	Nashorn
ERB	JRuby
String templates	Jython
Kotlin Script templating	Kotlin

Scripting Library

Scripting Engine

Kotlin Script templating

Kotlin

The basic rule for integrating any other script engine is that it must implement the ScriptEngine and Invocable interfaces.

Requirements

See equivalent in the Servlet stack

You need to have the script engine on your classpath, the details of which vary by script engine:

The 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-util dependency and a META-INF/services/javax.script.ScriptEngineFactory file containing a org.jetbrains.kotlin.script.jsr223.KotlinJsr223JvmLocalScriptEngineFactory line should be added for Kotlin script support. See this example for more detail.

You need to have the script templating library. One way to do that for JavaScript is through WebJars.

Script Templates

See equivalent in the Servlet stack

You can declare a ScriptTemplateConfigurer bean to specify the script engine to use, the script files to load, what function to call to render templates, and so on. The following example uses Mustache templates and the Nashorn JavaScript engine:

Java
Kotlin

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@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;
	}
}

@Configuration
@EnableWebFlux
class WebConfig : WebFluxConfigurer {

	override fun configureViewResolvers(registry: ViewResolverRegistry) {
		registry.scriptTemplate()
	}

	@Bean
	fun configurer() = ScriptTemplateConfigurer().apply {
		engineName = "nashorn"
		setScripts("mustache.js")
		renderObject = "Mustache"
		renderFunction = "render"
	}
}

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 can 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. The following example shows how to set a custom render function:

Java
Kotlin

@Configuration
@EnableWebFlux
public class WebConfig implements WebFluxConfigurer {

	@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;
	}
}

@Configuration
@EnableWebFlux
class WebConfig : WebFluxConfigurer {

	override fun configureViewResolvers(registry: ViewResolverRegistry) {
		registry.scriptTemplate()
	}

	@Bean
	fun configurer() = ScriptTemplateConfigurer().apply {
		engineName = "nashorn"
		setScripts("polyfill.js", "handlebars.js", "render.js")
		renderFunction = "render"
		isSharedEngine = false
	}
}

Setting the sharedEngine property to false is required when using non-thread-safe script engines with templating libraries not designed for concurrency, such as Handlebars or React running on Nashorn. In that case, Java SE 8 update 60 is required, due to this bug, but it is generally recommended to use a recent Java SE patch release in any case.

polyfill.js defines only the window object needed by Handlebars to run properly, as the following snippet shows:

var window = {};

This basic render.js implementation compiles the template before using it. A production ready implementation should also store and reused cached templates or pre-compiled templates. This can be done on the script side, as well as any customization you need (managing template engine configuration for example). The following example shows how compile a template:

function render(template, model) {
	var compiledTemplate = Handlebars.compile(template);
	return compiledTemplate(model);
}

Check out the Spring Framework unit tests, Java, and resources, for more configuration examples.

HTML Fragment

See equivalent in the Servlet stack

HTMX and Hotwire Turbo emphasize an HTML-over-the-wire approach where clients receive server updates in HTML rather than in JSON. This allows the benefits of an SPA (single page app) without having to write much or even any JavaScript. For a good overview and to learn more, please visit their respective websites.

In Spring WebFlux, view rendering typically involves specifying one view and one model. However, in HTML-over-the-wire a common capability is to send multiple HTML fragments that the browser can use to update different parts of the page. For this, controller methods can return Collection<Fragment>. For example:

Java
Kotlin

@GetMapping
List<Fragment> handle() {
	return List.of(Fragment.create("posts"), Fragment.create("comments"));
}

@GetMapping
fun handle(): List<Fragment> {
	return listOf(Fragment.create("posts"), Fragment.create("comments"))
}

The same can be done also by returning the dedicated type FragmentsRendering:

Java
Kotlin

@GetMapping
FragmentsRendering handle() {
	return FragmentsRendering.fragment("posts").fragment("comments").build();
}

@GetMapping
fun handle(): FragmentsRendering {
	return FragmentsRendering.fragment("posts").fragment("comments").build()
}

Each fragment can have an independent model, and that model inherits attributes from the shared model for the request.

HTMX and Hotwire Turbo support streaming updates over SSE (server-sent events). A controller can create FragmentsRendering with a Flux<Fragment>, or with any other reactive producer adaptable to a Reactive Streams Publisher via ReactiveAdapterRegistry. It is also possible to return Flux<Fragment> directly without the FragmentsRendering wrapper.

JSON and XML

See equivalent in the Servlet stack

For Content Negotiation purposes, it is useful to be able to alternate between rendering a model with an HTML template or as other formats (such as JSON or XML), depending on the content type requested by the client. To support doing so, Spring WebFlux provides the HttpMessageWriterView, which you can use to plug in any of the available Codecs from spring-web, such as Jackson2JsonEncoder, Jackson2SmileEncoder, or Jaxb2XmlEncoder.

Unlike other view technologies, HttpMessageWriterView does not require a ViewResolver but is instead configured as a default view. You can configure one or more such default views, wrapping different HttpMessageWriter instances or Encoder instances. The one that matches the requested content type is used at runtime.

In most cases, a model contains multiple attributes. To determine which one to serialize, you can configure HttpMessageWriterView with the name of the model attribute to use for rendering. If the model contains only one attribute, that one is used.