1.3.5 Getting to know Spring Boot DevTools

As its name suggests, DevTools provides Spring developers with some handy development-time tools. Among those are the following:

• Automatic application restart when code changes
• Automatic browser refresh when browser-destined resources (such as templates, JavaScript, stylesheets, and so on) change
• Automatic disabling of template caches
• Built in H2 Console, if the H2 database is in use

It’s important to understand that DevTools isn’t an IDE plugin, nor does it require that you use a specific IDE. It works equally well in Spring Tool Suite, IntelliJ IDEA, and NetBeans. Furthermore, because it’s intended only for development purposes, it’s smart enough to disable itself when deploying in a production setting. We’ll discuss how it does this when you get around to deploying your application in chapter 18. For now, let’s focus on the most useful features of Spring Boot DevTools, starting with automatic application restart.

AUTOMATIC APPLICATION RESTART

With DevTools as part of your project, you’ll be able to make changes to Java code and properties files in the project and see those changes applied after a brief moment. DevTools monitors for changes, and when it sees something has changed, it automatically restarts the application.

More precisely, when DevTools is active, the application is loaded into two separate class loaders in the Java virtual machine (JVM). One class loader is loaded with your Java code, property files, and pretty much anything that’s in the src/main/ path of the project. These are items that are likely to change frequently. The other class loader is loaded with dependency libraries, which aren’t likely to change as often.

When a change is detected, DevTools reloads only the class loader containing your project code and restarts the Spring application context but leaves the other class loader and the JVM intact. Although subtle, this strategy affords a small reduction in the time it takes to start the application.

The downside of this strategy is that changes to dependencies won’t be available in automatic restarts. That’s because the class loader containing dependency libraries isn’t automatically reloaded. Any time you add, change, or remove a dependency in your build specification, you’ll need to do a hard restart of the application for those changes to take effect.

AUTOMATIC BROWSER REFRESH AND TEMPLATE CACHE DISABLE

By default, template options such as Thymeleaf and FreeMarker are configured to cache the results of template parsing so that templates don’t need to be reparsed with every request they serve. This is great in production, because it buys a bit of a performance benefit.

Cached templates, however, are not so great at development time. They make it impossible to make changes to the templates while the application is running and see the results after refreshing the browser. Even if you’ve made changes, the cached template will still be in use until you restart the application.

DevTools addresses this issue by automatically disabling all template caching. Make as many changes as you want to your templates and know that you’re only a browser refresh away from seeing the results.

But if you’re like me, you don’t even want to be burdened with the effort of clicking the browser’s refresh button. It’d be much nicer if you could make the changes and witness the results in the browser immediately. Fortunately, DevTools has something special for those of us who are too lazy to click a refresh button.

DevTools automatically enables a LiveReload server （http://livereload.com/）along with your application. By itself, the LiveReload server isn’t very useful. But when coupled with a corresponding LiveReload browser plugin, it causes your browser to automatically refresh when changes are made to templates, images, stylesheets, JavaScript, and so on—in fact, almost anything that ends up being served to your browser.

LiveReload has browser plugins for Google Chrome, Safari, and Firefox browsers. (Sorry, Internet Explorer and Edge fans.) Visit http://livereload.com/extensions/ to find information on how to install LiveReload for your browser.

BUILT-IN H2 CONSOLE

Although your project doesn’t yet use a database, that will change in chapter 3. If you choose to use the H2 database for development, DevTools will also automatically enable an H2 console that you can access from your web browser. You only need to point your web browser to http://localhost:8080/h2-console to gain insight into the data your application is working with.

At this point, you’ve written a complete, albeit simple, Spring application. You’ll expand on it throughout the course of the book. But now is a good time to step back and review what you’ve accomplished and how Spring played a part.