spring boot + thymeleaf 3

application.properties

spring.thymeleaf.cache=false
#idea flags this as invalid..
spring.thymeleaf.mode=HTML

...snip...

BootApplication.java

package com.boot;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.SpringApplication;
//make sure spring boot doesn't attempt 2.1 config
@SpringBootApplication(exclude={org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration.class} )
public class AppApplication {
	public static void main(String[] args) {
		SpringApplication.run(AppSecurityApplication.class, args);
	}
}

pom.xml

<!-- use snapshot build -->

  <repositories>
		<repository>
			<id>sonatype-nexus-snapshots</id>
			<name>Sonatype Nexus Snapshots</name>
			<url>https://oss.sonatype.org/content/repositories/snapshots</url>
			<snapshots>
				<enabled>true</enabled>
			</snapshots>
		</repository>
	</repositories>

	<dependencies>

		<!-- make sure you've removed this.
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-thymeleaf</artifactId>
		</dependency>
		-->

		<dependency>
			<groupId>org.thymeleaf</groupId>
			<artifactId>thymeleaf</artifactId>
			<version>3.0.0-SNAPSHOT</version>
			<scope>compile</scope>
		</dependency>

		<dependency>
			<groupId>org.thymeleaf</groupId>
			<artifactId>thymeleaf-spring4</artifactId>
			<version>3.0.0-SNAPSHOT</version>
		</dependency>

	</dependencies>

spring.factories

#resources/META-INF/spring.factories
#bootstrap in your thymeleaf 3 config
org.springframework.boot.autoconfigure.EnableAutoConfiguration=somewhere.outside.app.namespace.thymeleaf.ThymeleafConfig

ThymeleafConfig.java

package somewhere.outside.app.namespace.thymeleaf;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.AutoConfigureAfter;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.thymeleaf.ThymeleafProperties;
import org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.servlet.ViewResolver;
import org.thymeleaf.TemplateEngine;
import org.thymeleaf.spring4.SpringTemplateEngine;
import org.thymeleaf.spring4.templateresolver.SpringResourceTemplateResolver;
import org.thymeleaf.spring4.view.ThymeleafViewResolver;
import org.thymeleaf.templateresolver.ITemplateResolver;

@Configuration
@ConditionalOnClass({SpringTemplateEngine.class})
@EnableConfigurationProperties({ThymeleafProperties.class})  //no sense rolling our own.
@AutoConfigureAfter({WebMvcAutoConfiguration.class})
public class ThymeleafConfig implements ApplicationContextAware {

    private ApplicationContext applicationContext;

    @Autowired
    private ThymeleafProperties properties;

    public void setApplicationContext(ApplicationContext applicationContext) {
        this.applicationContext = applicationContext;
    }

    @Bean
    public ViewResolver viewResolver() {
        ThymeleafViewResolver resolver = new ThymeleafViewResolver();
        resolver.setOrder(2147483642);
        resolver.setTemplateEngine(templateEngine());
        resolver.setCharacterEncoding("UTF-8");
        return resolver;
    }

    @Bean
    //made this @Bean (vs private in Thymeleaf migration docs ), otherwise MessageSource wasn't autowired.
    public TemplateEngine templateEngine() {
        SpringTemplateEngine engine = new SpringTemplateEngine();
        engine.setTemplateResolver(templateResolver());
        return engine;
    }

    private ITemplateResolver templateResolver() {
        SpringResourceTemplateResolver resolver = new SpringResourceTemplateResolver();
        resolver.setApplicationContext(applicationContext);
        resolver.setPrefix(this.properties.getPrefix());
        resolver.setSuffix(this.properties.getSuffix());
        resolver.setTemplateMode(this.properties.getMode());
        resolver.setCacheable(this.properties.isCache());
        return resolver;
    }


}

I believe it's easier then that options:
https://docs.spring.io/spring-boot/docs/current/reference/htmlsingle/#howto-use-thymeleaf-3