Spring Boot DevTools for Auto-Restart | Generated by AI

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When you run a Spring Boot application using mvn spring-boot:run, file watchers and auto-restart or hot-reload capabilities are not enabled by default. However, you can enable them by adding the spring-boot-devtools dependency to your project. This module provides features like automatic restarts and LiveReload for faster development.

Details

  1. Default Behavior Without DevTools:
    • Running mvn spring-boot:run without spring-boot-devtools does not include file watchers or auto-restart. You need to manually stop and restart the application to apply changes to Java classes, static resources, or templates.
    • Static resources (e.g., HTML, CSS, JS) may require a full rebuild or restart unless configured otherwise.
  2. With spring-boot-devtools:
    • File Watchers: DevTools monitors the classpath for changes to Java files, properties, and certain resources (e.g., /resources, /static, /templates).
    • Auto-Restart: When a file on the classpath changes (e.g., a Java class or properties file), DevTools triggers an automatic restart of the application. This is faster than a cold start because it uses two classloaders: one for unchanged third-party libraries (base classloader) and another for your application code (restart classloader).
    • LiveReload: Changes to static resources (e.g., HTML, CSS, JS in /static, /public, or /templates) or templates (e.g., Thymeleaf) trigger a browser refresh instead of aBelow is an example of how to configure spring-boot-devtools for file watching, auto-restart, and hot-reloading in your Spring Boot application using a application.yml file. This configuration is tailored to your blog-server project, based on the logs you provided, which show DevTools is active and monitoring target/classes.

application.yml Configuration

spring:
  devtools:
    restart:
      # Enable auto-restart (default: true)
      enabled: true
      # Additional directories to monitor for restarts (e.g., custom config folder)
      additional-paths:
        - /home/lzw/Projects/blog-server/config
      # Files/directories to exclude from triggering restarts (default exclusions kept)
      exclude: static/**,public/**,templates/**,logs/**,generated/**
      # Poll interval for file changes (in milliseconds, default: 1000)
      poll-interval: 1000
      # Quiet period after file changes before restarting (in milliseconds, default: 400)
      quiet-period: 400
      # Optional: File to manually trigger restarts
      trigger-file: .restart
    livereload:
      # Enable LiveReload for browser refresh on static resource changes (default: true)
      enabled: true

Explanation of Settings

Steps to Apply

  1. Create or update src/main/resources/application.yml with the above configuration.
  2. Ensure spring-boot-devtools is in your pom.xml: 
    <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>
  3. Run mvn spring-boot:run. DevTools will monitor target/classes and /home/lzw/Projects/blog-server/config (if added), restarting the app on Java or properties file changes and refreshing the browser on static resource changes.
  4. For LiveReload, install a browser extension (e.g., from http://livereload.com/extensions/) or integrate a LiveReload client in your front-end.

Notes

This YAML configuration should work seamlessly with your setup, enhancing the file-watching and auto-restart features you’re already seeing. full restart, provided you have a LiveReload browser extension installed (supported for Chrome, Firefox, Safari). try (BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()))) { String line; while ((line = reader.readLine()) != null) { output.append(line).append(System.lineSeparator()); } }

        // Capture error output
        try (BufferedReader errorReader = new BufferedReader(new InputStreamReader(process.getErrorStream()))) {
            String errorLine;
            while ((errorLine = errorReader.readLine()) != null) {
                errorOutput.append(errorLine).append(System.lineSeparator());
            }
        }    - **Exclusions**: By default, resources in `/META-INF/maven`, `/META-INF/resources`, `/resources`, `/static`, `/public`, and `/templates` don’t trigger a restart but do trigger a LiveReload. You can customize this with `spring.devtools.restart.exclude`.[](https://docs.spring.io/spring-boot/reference/using/devtools.html)
  1. Setup for DevTools: Add the following dependency to your pom.xml: 
    <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>
    • The <optional>true</optional> ensures DevTools is not included in production builds.
    • Run the application with mvn spring-boot:run. DevTools will automatically enable file watching and auto-restart.
  2. Behavior in IDEs:
    • Eclipse: Saving changes (Ctrl+S) automatically triggers a build, which DevTools detects and restarts the application.
    • IntelliJ IDEA: You need to manually trigger a build (Ctrl+F9 or “Make Project”) for DevTools to detect changes, unless you configure auto-build. Alternatively, enable “Build project automatically” in IntelliJ settings for seamless restarts.
    • For LiveReload, install the browser extension from http://livereload.com/extensions/ and enable it.
  3. Alternative: Spring Loaded:
    • Instead of DevTools, you can use Spring Loaded for more advanced hot-swapping (e.g., method signature changes). Add it to the spring-boot-maven-plugin: 
      <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>springloaded</artifactId>
            <version>1.2.8.RELEASE</version>
        </dependency>
    </dependencies>
</plugin>
    • Spring Loaded is less recommended than DevTools, as it’s not as actively maintained and may not support all frameworks.
  4. Hot-Reloading Static Resources:
    • Without DevTools, you can enable hot-reloading of static resources by setting spring-boot-maven-plugin’s addResources property: 
      <plugin>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-maven-plugin</artifactId>
    <configuration>
        <addResources>true</addResources>
    </configuration>
</plugin>
    • This adds src/main/resources to the classpath, allowing in-place editing of static files, but it’s less comprehensive than DevTools.
  5. Caveats:
    • DevTools may cause classloading issues in multi-module projects. If this happens, try disabling restart with spring.devtools.restart.enabled=false or use JRebel for advanced reloading.
    • For non-classpath files, use spring.devtools.restart.additional-paths to monitor additional directories.
    • LiveReload requires a browser extension and may not work for all front-end setups (e.g., React with Webpack).
    • If restarts are slow, adjust spring.devtools.restart.poll-interval and spring.devtools.restart.quiet-period to optimize file watching.

Steps for a Simple App

  1. Create a basic Spring Boot app (e.g., using Spring Initializr with spring-boot-starter-web).
  2. Add the spring-boot-devtools dependency to pom.xml.
  3. Run mvn spring-boot:run.
  4. Modify a Java file, properties file, or static resource (e.g., HTML in src/main/resources/static).
  5. Observe the auto-restart (for Java/properties) or browser refresh (for static resources with LiveReload enabled).

Example

For a simple app with a REST controller:

@RestController
public class HelloController {
    @GetMapping("/hello")
    public String hello() {
        return "Hello, World!";
    }
}

Conclusion

For a simple Spring Boot app, adding spring-boot-devtools is the easiest way to enable file watchers, auto-restart, and hot-reloading. Use mvn spring-boot:run with DevTools for a seamless development experience. If you need more advanced hot-swapping, consider Spring Loaded or JRebel, but DevTools is sufficient for most cases.

Below is an example of how to configure spring-boot-devtools for file watching, auto-restart, and hot-reloading in your Spring Boot application using a application.yml file. This configuration is tailored to your blog-server project, based on the logs you provided, which show DevTools is active and monitoring target/classes.

application.yml Configuration

spring:
  devtools:
    restart:
      # Enable auto-restart (default: true)
      enabled: true
      # Additional directories to monitor for restarts (e.g., custom config folder)
      additional-paths:
        - /home/lzw/Projects/blog-server/config
      # Files/directories to exclude from triggering restarts (default exclusions kept)
      exclude: static/**,public/**,templates/**,logs/**,generated/**
      # Poll interval for file changes (in milliseconds, default: 1000)
      poll-interval: 1000
      # Quiet period after file changes before restarting (in milliseconds, default: 400)
      quiet-period: 400
      # Optional: File to manually trigger restarts
      trigger-file: .restart
    livereload:
      # Enable LiveReload for browser refresh on static resource changes (default: true)
      enabled: true

Explanation of Settings

Steps to Apply

  1. Create or update src/main/resources/application.yml with the above configuration.
  2. Ensure spring-boot-devtools is in your pom.xml: 
    <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-devtools</artifactId>
    <scope>runtime</scope>
    <optional>true</optional>
</dependency>
  3. Run mvn spring-boot:run. DevTools will monitor target/classes and /home/lzw/Projects/blog-server/config (if added), restarting the app on Java or properties file changes and refreshing the browser on static resource changes.
  4. For LiveReload, install a browser extension (e.g., from http://livereload.com/extensions/) or integrate a LiveReload client in your front-end.

Notes

This YAML configuration should work seamlessly with your setup, enhancing the file-watching and auto-restart features you’re already seeing.

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