Thymeleaf auto-configuration in a reactive application can fail due to duplicate templateEngine beans

[wilkinsona]

ThymeleafAutoConfiguration has two inner-classes that define beans named templateEngine:

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/thymeleaf/ThymeleafAutoConfiguration.java

Lines 133 to 148 in e927cd7

	@Configuration(proxyBeanMethods = false)
	protected static class ThymeleafDefaultConfiguration {
	@Bean
	@ConditionalOnMissingBean(ISpringTemplateEngine.class)
	SpringTemplateEngine templateEngine(ThymeleafProperties properties,
	ObjectProvider<ITemplateResolver> templateResolvers, ObjectProvider<IDialect> dialects) {
	SpringTemplateEngine engine = new SpringTemplateEngine();
	engine.setEnableSpringELCompiler(properties.isEnableSpringElCompiler());
	engine.setRenderHiddenMarkersBeforeCheckboxes(properties.isRenderHiddenMarkersBeforeCheckboxes());
	templateResolvers.orderedStream().forEach(engine::addTemplateResolver);
	dialects.orderedStream().forEach(engine::addDialect);
	return engine;
	}
	}

spring-boot/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/thymeleaf/ThymeleafAutoConfiguration.java

Lines 202 to 219 in e927cd7

	@Configuration(proxyBeanMethods = false)
	@ConditionalOnWebApplication(type = Type.REACTIVE)
	@ConditionalOnProperty(name = "spring.thymeleaf.enabled", matchIfMissing = true)
	static class ThymeleafReactiveConfiguration {
	@Bean
	@ConditionalOnMissingBean(ISpringWebFluxTemplateEngine.class)
	SpringWebFluxTemplateEngine templateEngine(ThymeleafProperties properties,
	ObjectProvider<ITemplateResolver> templateResolvers, ObjectProvider<IDialect> dialects) {
	SpringWebFluxTemplateEngine engine = new SpringWebFluxTemplateEngine();
	engine.setEnableSpringELCompiler(properties.isEnableSpringElCompiler());
	engine.setRenderHiddenMarkersBeforeCheckboxes(properties.isRenderHiddenMarkersBeforeCheckboxes());
	templateResolvers.orderedStream().forEach(engine::addTemplateResolver);
	dialects.orderedStream().forEach(engine::addDialect);
	return engine;
	}
	}

If ThymeleafDefaultConfiguration is processed first, it'll define the templateEngine bean. When ThymeleafReactiveConfiguration is processed, it too will try to define a bean named templateEngine as the existing templateEngine bean is an ISpringTemplateEngine rather than an ISpringWebFluxTemplateEngine.

We need to move the configurations for the template engine out into a separate class and then import them such that ThymeleafReactiveConfiguration is guaranteed to be processed before ThymeleafDefaultConfiguration. This was already fixed on main as part of 26fecbe. We need to back port those changes to 2.5.x and then merge forwards.