Careeroza

Spring Boot DevTools

What DevTools Gives You

  • Automatic restart: when a class changes on the classpath (you save/build in the IDE), the app restarts in about a second.
  • LiveReload: refreshes the browser when static files or templates change (with a browser extension).
  • Development defaults: disables template caching, enables more detailed web error logging.
  • Automatically disabled in production: it turns itself off when running from a packaged jar with java -jar.
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>

Restart in Action

Example

Change the text returned by a controller, save (VS Code) or press Ctrl+F9 / Build Project (IntelliJ):

# edit HelloController.java and save

Example output

... INFO 18244 --- [shopease] [  restartedMain] com.shopease.ShopeaseApplication : Started ShopeaseApplication in 0.612 seconds (process running for 95.31)
... INFO 18244 --- [shopease] [  restartedMain] .ConditionEvaluationDeltaLoggingListener : Condition evaluation unchanged

Tip: In IntelliJ, enable automatic restart on save: Settings → Build, Execution, Deployment → Compiler → Build project automatically, and Advanced Settings → Allow auto-make to start even if developed application is currently running.

Why Restart Is Fast

DevTools uses two class loaders. Library jars (Spring, Tomcat, Jackson) are loaded once in a base class loader. Your own classes are loaded in a restart class loader. On a change, only the restart class loader is thrown away and recreated, which is much faster than a cold start.

Note: A restart creates a fresh application context, so in-memory data (like a List of products added at run time) is lost. That is one reason real apps keep data in a database.