Hot Reload Spring Boot Apps Without Restart: DevTools, IDEA HotSwap & JRebel

Background

During development, modifying Java code usually requires restarting the Spring Boot application. Small projects restart quickly, but large projects can take 1–2 minutes, slowing development and extending work hours.

The goal is to see code changes instantly without a full restart, which can be achieved through hot deployment techniques.

Method 1: spring-boot-devtools

spring-boot-devtools

is a Spring Boot developer tool that monitors changes in the classpath and triggers an automatic restart. It does not perform true hot swapping; it uses a fast restart mechanism based on two class loaders: a base loader for unchanged classes (e.g., third‑party jars) and a restart loader for frequently changed classes.

When the application restarts, the restart class loader is discarded and recreated, making the restart much faster than a cold start.

To enable it, add the following dependency to pom.xml:

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

Demo

Run or debug the Spring Boot application.

Open a browser and request http://localhost:8081/HotSwap/01; the response is “No.1”.

Modify the HotSwapController#hotSwap01 method to return “No.2”.

Recompile the project (IDEA: Build → Build Project or shortcut Command + F9 on macOS, Ctrl + F9 on Windows). The application restarts automatically and the endpoint now returns “No.2”.

Enabling “Build project automatically” in IDEA’s Preferences → Compiler allows the compilation to happen when the IDE loses focus, further reducing manual steps.

Method 2: IntelliJ IDEA HotSwap plugin

The HotSwap plugin provides true hot deployment by swapping the changed class bytecode into the running JVM.

Demo

Run or debug the Spring Boot application and access http://localhost:8081/HotSwap/01 (returns “No.1”).

Change the HotSwapController#hotSwap01 method to return “No.2”.

Compile the project manually ( Build → Build Project) or enable “Build project automatically” so that compilation occurs when the IDE loses focus.

After compilation, IDEA shows a “class reloaded” notification, and the endpoint now returns “No.2”.

Method 3: JRebel

Install JRebel from the IDE plugin marketplace and activate it (trial or license). Launch the application with JRebel instead of the standard IDEA run configuration; any code change triggers hot deployment instantly.