Spring Boot DevTools

Learn how Spring Boot DevTools speeds up development with automatic restarts and LiveReload, and how to enable it in your project.

Introduction

Without any help, the normal Java development loop looks like this: change a line of code, stop the running application, rebuild it, and start it again, just to see one small change take effect. Spring Boot DevTools exists specifically to shrink that loop down to almost nothing.

Adding DevTools

DevTools is added like any other starter dependency. It is marked `optional` so that it never gets bundled into a production build by accident.

pom.xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>

It Disables Itself in Production

DevTools automatically detects when an application is running from a packaged JAR (as opposed to being launched from an IDE or build tool) and disables all of its development-time features, so it is safe to leave in the dependency list.

Automatic Restart

With DevTools on the classpath, Spring Boot loads your application classes using two separate classloaders: one for libraries that rarely change, and one for your own application code. When you recompile a class (for example, by saving a file in an IDE with "build automatically" enabled), DevTools detects the change and restarts only the application-code classloader.

This makes restarts noticeably faster than a full cold start, because none of the third-party libraries need to be reloaded.

Console Output After Saving a File

Click Run to see what this code prints.

LiveReload

DevTools also embeds a LiveReload server. When paired with a LiveReload browser extension, your browser automatically refreshes the page the moment the application finishes restarting — so you can edit a template or a controller, save, and see the result in the browser without manually hitting refresh.

Common Mistakes

Avoid These Mistakes

Expecting DevTools to detect changes without a recompile — most IDEs need "build automatically" or a manual rebuild to trigger a restart.
Being surprised that DevTools does nothing in a packaged, deployed JAR — this is intentional, not a bug.
Forgetting to install a LiveReload browser extension and assuming automatic browser refresh should already be working.

Best Practices

Always mark spring-boot-devtools as optional (Maven) or use developmentOnly (Gradle) so it never ships in production.
Enable "build automatically" in your IDE to get the fastest possible restart feedback loop.
Pair DevTools with the LiveReload browser extension when working on server-rendered HTML views.

Frequently Asked Questions

Does DevTools slow down my production application?

No. DevTools detects when the app is running from a packaged, production-style JAR and automatically disables all of its restart and LiveReload behavior in that case.

Is DevTools a hot-swap tool like JRebel?

Not exactly. DevTools performs a fast application restart rather than true hot code swapping, but because it reuses an unchanged classloader for libraries, the restart is fast enough to feel nearly instant for most projects.

Do I need a special browser to use LiveReload?

No, but you do need a LiveReload-compatible browser extension installed, which is available for all major browsers, to receive the automatic refresh signal from the embedded LiveReload server.

Key Takeaways

DevTools automatically restarts the application when application code changes, using a fast, split-classloader restart.
It embeds a LiveReload server that can automatically refresh the browser after a restart.
DevTools disables itself automatically when running from a production-style packaged JAR.
It should always be marked optional/developmentOnly so it never ships to production.

Summary

Spring Boot DevTools removes the friction of the manual stop-rebuild-start cycle, giving you fast automatic restarts and, with a browser extension, automatic page refreshes too. Next, in the final lesson of this batch, you will put everything together and build your first real REST controller.

Next Lesson →