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.

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.

The spring-boot-devtools module includes support for automatic application restarts. Whilst not as fast as technologies such as JRebel or Spring Loaded 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.

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.

Spring Loaded goes a little further in that it can reload class definitions with changes in the method signatures. With some customization it can force an ApplicationContext to refresh itself (but there is no general mechanism to ensure that would be safe for a running application anyway, so it would only ever be a development time trick probably).

<plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>springloaded</artifactId>
            <version>1.2.6.RELEASE</version>
        </dependency>
    </dependencies>
</plugin>

84.4.2 Configuring Spring Loaded for use with Gradle and IntelliJ IDEA

You need to jump through a few hoops if you want to use Spring Loaded in combination with Gradle and IntelliJ IDEA. By default, IntelliJ IDEA will compile classes into a different location than Gradle, causing Spring Loaded monitoring to fail.

To configure IntelliJ IDEA correctly you can use the idea Gradle plugin:

buildscript {
    repositories { jcenter() }
    dependencies {
        classpath "org.springframework.boot:spring-boot-gradle-plugin:1.5.16.RELEASE"
        classpath 'org.springframework:springloaded:1.2.6.RELEASE'
    }
}

apply plugin: 'idea'

idea {
    module {
        inheritOutputDirs = false
        outputDir = file("$buildDir/classes/main/")
    }
}

// ...

	Note
	IntelliJ IDEA must be configured to use the same Java version as the command line Gradle task and springloaded must be included as a buildscript dependency.

You can also additionally enable ‘Make Project Automatically’ inside IntelliJ IDEA to automatically compile your code whenever a file is saved.