84. Hot swapping

84.1 Reload static content

There are several options for hot reloading. The recommended approach is to use spring-boot-devtools as it provides additional development-time features such as support for fast application restarts and LiveReload as well as sensible development-time configuration (e.g. template caching). Devtools works by monitoring the classpath for changes. This means that static resource changes must be "built" for the change to take affect. By default, this happens automatically in Eclipse when you save your changes. In IntelliJ IDEA, Make Project will trigger the necessary build. Due to the default restart exclusions, changes to static resources will not trigger a restart of your application. They will, however, trigger a live reload.

Alternatively, running in an IDE (especially with debugging on) is a good way to do development (all modern IDEs allow reloading of static resources and usually also hot-swapping of Java class changes).

Finally, the Maven and Gradle plugins can be configured (see the addResources property) to support running from the command line with reloading of static files directly from source. You can use that with an external css/js compiler process if you are writing that code with higher level tools.

84.2 Reload templates without restarting the container

Most of the templating technologies supported by Spring Boot include a configuration option to disable caching (see below for details). If you’re using the spring-boot-devtools module these properties will be automatically configured for you at development time.

84.2.1 Thymeleaf templates

If you are using Thymeleaf, then set spring.thymeleaf.cache to false. See ThymeleafAutoConfiguration for other Thymeleaf customization options.

84.2.2 FreeMarker templates

If you are using FreeMarker, then set spring.freemarker.cache to false. See FreeMarkerAutoConfiguration for other FreeMarker customization options.

84.2.3 Groovy templates

If you are using Groovy templates, then set spring.groovy.template.cache to false. See GroovyTemplateAutoConfiguration for other Groovy customization options.

84.3 Fast application restarts

The spring-boot-devtools module includes support for automatic application restarts. Whilst not as fast as technologies such as JRebel it’s usually significantly faster than a “cold start”. You should probably give it a try before investigating some of the more complex reload options discussed below.

For more details see the Chapter 20, Developer tools section.

84.4 Reload Java classes without restarting the container

Modern IDEs (Eclipse, IDEA, etc.) all support hot swapping of bytecode, so if you make a change that doesn’t affect class or method signatures it should reload cleanly with no side effects.