How does Spring boot put Thymeleaf view resolver into its bean container

Question

It is a spring boot project and the web page is rendered by Thymeleaf. When I put spring-boot-starter-thymeleaf in the pom.xml and start the applicaiton, it is trying to find all the beans which implements ViewResolver in its container. And you see here it find thymeleafViewResolver.

I am just curious that when and how does Spring boot put this ThymeleafViewResolver class into its bean container?

Ken Chan Ken Chan · Accepted Answer · 2019-06-10T06:31:30

It is due to SpringBoot auto-configuration feature which will automatically create a bean dynamically based on different conditions such as if a library can be found from the class paths, or if developers already define a bean of certain type etc...

If you turn on the debug mode by putting debug=true in the application.properties , it will print out a report during application startup saying which beans are auto created due to which conditions.

In the example of the spring-boot-starter-thymeleaf , you can find the following from the report :

ThymeleafAutoConfiguration.ThymeleafWebMvcConfiguration.ThymeleafViewResolverConfiguration#thymeleafViewResolver matched:
                  - @ConditionalOnMissingBean (names: thymeleafViewResolver; SearchStrategy: all) did not find any beans (OnBeanCondition)

And by tracing the source codes of ThymeleafViewResolverConfiguration :

@Bean
@ConditionalOnMissingBean(name = "thymeleafViewResolver")
public ThymeleafViewResolver thymeleafViewResolver() {
    ThymeleafViewResolver resolver = new ThymeleafViewResolver();
    resolver.setTemplateEngine(this.templateEngine);
    resolver.setCharacterEncoding(this.properties.getEncoding().name());
    //.......   
    return resolver;
}

You could find out thymeleafViewResolver is in a type of ThymeleafViewResolver and @ConditionalOnMissingBean here means that this bean will only be created if there is no bean of the ThymeleafViewResolver type is defined yet.

How does Spring boot put Thymeleaf view resolver into its bean container

1 Answers