Spring Boot supports hot swapping. This section answers questions about how it works.
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 (such as 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, the Make Project
command triggers the necessary build. Due to the
default restart
exclusions, changes to static resources do not trigger a restart of your application.
They do, 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 allow 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.
Most of the templating technologies supported by Spring Boot include a configuration
option to disable caching (described later in this document). If you use the
spring-boot-devtools module, these properties are
automatically configured
for you at development time.
If you use Thymeleaf, set spring.thymeleaf.cache to false. See
ThymeleafAutoConfiguration
for other Thymeleaf customization options.
If you use FreeMarker, set spring.freemarker.cache to false. See
FreeMarkerAutoConfiguration
for other FreeMarker customization options.
If you use Groovy templates, set spring.groovy.template.cache to false. See
GroovyTemplateAutoConfiguration
for other Groovy customization options.
The spring-boot-devtools module includes support for automatic application restarts.
While not as fast as technologies such as
JRebel it is usually significantly faster than
a “cold start”. You should probably give it a try before investigating some of the more
complex reload options discussed later in this document.
For more details, see the Chapter 20, Developer Tools section.