ThymeleafAutoConfiguration fails if thymeleaf-spring5 is not present

[kazuki43zoo]

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Background

Auto-configurations declare conditions to make sure that they back-off when conditions aren't met. For instance, @ConditionalOnClass allows to refer to classes that must be present on the classpath for an auto-configuration to be considered. Spring Boot will parse the bytecode using ASM to identify those classes and skip the auto-configuration altogether if one of them is not present.

Problem

The thymeleaf auto-configuration support relies on the Spring integration module provided by the Thymeleaf project (currently thymeleaf-spring5). Unfortunately, ThymeleafAutoConfiguration only checks for a class in the core thymelaf jar (thymeleaf): if the core jar is present but the integration one isn't, the auto-configuration will be considered and fail:

java.lang.IllegalStateException: Error processing condition on org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration$ThymeleafDefaultConfiguration.templateEngine
	at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:64) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.context.annotation.ConditionEvaluator.shouldSkip(ConditionEvaluator.java:108) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForBeanMethod(ConfigurationClassBeanDefinitionReader.java:181) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitionsForConfigurationClass(ConfigurationClassBeanDefinitionReader.java:141) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassBeanDefinitionReader.loadBeanDefinitions(ConfigurationClassBeanDefinitionReader.java:117) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.processConfigBeanDefinitions(ConfigurationClassPostProcessor.java:327) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.annotation.ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry(ConfigurationClassPostProcessor.java:232) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(PostProcessorRegistrationDelegate.java:275) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:95) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:705) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:531) ~[spring-context-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:775) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:397) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:316) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at com.example.demo.DemoApplicationKt.main(DemoApplication.kt:13) [classes/:na]
Caused by: java.lang.IllegalStateException: @ConditionalOnMissingBean did not specify a bean using type, name or annotation and the attempt to deduce the bean's type failed
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.validate(OnBeanCondition.java:451) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.<init>(OnBeanCondition.java:441) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.<init>(OnBeanCondition.java:416) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition.getMatchOutcome(OnBeanCondition.java:158) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.SpringBootCondition.matches(SpringBootCondition.java:47) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	... 16 common frames omitted
Caused by: org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanTypeDeductionException: Failed to deduce bean type for org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration$ThymeleafDefaultConfiguration.templateEngine
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.addDeducedBeanTypeForBeanMethod(OnBeanCondition.java:496) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.addDeducedBeanType(OnBeanCondition.java:483) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.<init>(OnBeanCondition.java:435) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	... 19 common frames omitted
Caused by: java.lang.ClassNotFoundException: org.thymeleaf.spring5.SpringTemplateEngine
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_171]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_171]
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) ~[na:1.8.0_171]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_171]
	at java.lang.Class.forName0(Native Method) ~[na:1.8.0_171]
	at java.lang.Class.forName(Class.java:348) ~[na:1.8.0_171]
	at org.springframework.util.ClassUtils.forName(ClassUtils.java:275) ~[spring-core-5.1.5.RELEASE.jar:5.1.5.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.getReturnType(OnBeanCondition.java:505) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	at org.springframework.boot.autoconfigure.condition.OnBeanCondition$BeanSearchSpec.addDeducedBeanTypeForBeanMethod(OnBeanCondition.java:491) ~[spring-boot-autoconfigure-2.1.3.RELEASE.jar:2.1.3.RELEASE]
	... 21 common frames omitted

Solution

The conditions on ThymeleafAutoConfiguration should be refined so that we don't process it if thymeleaf-spring5 is not on the classpath. A good solution for this is to extend @ConditionalOnClass to also checks for the presence of SpringTemplateEngine.

Steps to Fix

• Claim this issue with a comment below and ask any clarifying questions you need
• Set up a repository locally following the Contributing Guidelines
• Try to fix the issue following the steps above
• Commit your changes and start a pull request.

[kandebouya]

I will work there

[snicoll]

Great! Thank you. Please let us know if you have any questions.

oh i am late again :( i will be here next time! :)

[snicoll]

@kandebouya checking in to see if there is anything we can do to help. Please let us know.

[snicoll]

@kandebouya we haven't heard back from you and are available if you need help. If we don't hear from you by the end of the week, we'll let someone else have a crack at it. Thanks!

[koziolk]

@snicoll I can work on it if @kandebouya is not available

[snicoll]

Thanks @koziolk, you're third. @oguzyildirim is next if we don't hear from @kandebouya

I will be 4th :)

@snicoll If this issue is done (was trying to see if I can really do it or not and well luckily, I did), should we still wait for the turn or can we submit the PR?