Spring Boot supports hot swapping. This section answers questions about how it works.
86.1 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 (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
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.
86.2 Reload Templates without Restarting the Container
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
for you at development time.
86.2.1 Thymeleaf Templates
If you use Thymeleaf, set
for other Thymeleaf customization options.
86.2.2 FreeMarker Templates
If you use FreeMarker, set
for other FreeMarker customization options.
86.3 Fast Application Restarts
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.
86.4 Reload Java Classes without Restarting the Container
Many modern IDEs (Eclipse, IDEA, and others) support hot swapping of bytecode.
Consequently, if you make a change that does not affect class or method signatures, it
should reload cleanly with no side effects.