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;
    }


}

@zhugw I'd suggest we still need this option:

spring.thymeleaf.mode=HTML

as the option HTML5 is depreceted in v3.0.0 and will be removed in v3.1.

If you found those default values in org.springframework.boot.autoconfigure.thymeleaf.ThymeleafProperties, which I checked as well in Spring Boot v1.5.4, the mode field is currently set to HTML5, and I assume it came from Thymeleaf 2. If you're not going to customize auto-configuration class ThymeleafAutoConfiguration to change the default value of it to HTML, you cannot but use the application property above to change it manually. Technically the default value HTML5 stands still for the legacy compatibility and is supposed work exactly same to HTML, so keeping it won't cause any problem for now. But it's RECOMMENDED to change it.

Reference

http://www.thymeleaf.org/apidocs/thymeleaf/3.0.7.RELEASE/org/thymeleaf/templatemode/TemplateMode.html

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