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hotswap-classloader: Dynamic Hot Reloading

English | 中文

1. Introduction

hotswap-classloader is a dynamic class loader based on the JVM. It utilizes the HotSwapWatcher and HotSwapClassloader technologies to dynamically detect modifications to class files. This project was inspired by the hot loading design of jfinal-undertow. See Reference

Key Features:

  • Achieves rapid application hot reloading, with test results showing hot reloads completed in approximately 1 second.

Loading Speed:

  • Upon integration with spring-boot, directly starting spring-boot in eclipse, and making changes to the controller followed by pressing Ctrl+S to save, the application will automatically restart and parse the class. The entire process completes within 1 second.

Comparable Products:

  • springloaded
  • spring-boot-devtools
  • JRebel

2. Integration and Usage

2.1 Integration with spring-boot

  1. Add Dependency
<dependency>
  <groupId>com.litongjava</groupId>
  <artifactId>hotswap-classloader</artifactId>
  <!--https://central.sonatype.com/artifact/com.litongjava/hotswap-classloader-->
  <version>1.2.2/version>
</dependency>
  1. Add Configuration File
    Create a config.properties file under src/main/resource/ and add the following content:
mode=dev
  1. Modify the Startup Class Code
    Replace SpringApplication.run(Application.class, args); with SpringApplicationWrapper.run(Application.class, args);.

Example:

package com.litongjava.spring.boot.v216;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import com.litongjava.hotswap.wrapper.spring.boot.SpringApplicationWrapper;

@SpringBootApplication
public class Application {
  public static void main(String[] args) {
    SpringApplicationWrapper.run(Application.class, args);
  }
}

Note: SpringApplicationWrapper reads the mode key value from the config.properties file. If the value is dev, it starts the hotswapwather to monitor class changes and enables hot reloading; otherwise, it does not activate.

Upon completing the above steps, you can refer to this project for integration:
View the integrated project

2.2 Integration with other framework

Calls ForkApp.run in its own startup

//params: startup class, startup parameters, hot load, restart class
ForkApp.run(SklearnWebApp.class, args, true, new SelfRestart());

For example

package com.litongjava.tio.boot.djl;

import org.tio.utils.jfinal.P;

import com.litongjava.hotswap.wrapper.forkapp.ForkApp;

public class SklearnWebApp {

  public static void main(String[] args) throws Exception {
    long start = System.currentTimeMillis();
    // Initialize the server and start the server
    P.use("app.properties");
//     Diagnostic.setDebug(true);
//    TioApplicationWrapper.run(SklearnWebApp.class, args);
     ForkApp.run(SklearnWebApp.class, args, true, new SelfRestart());
    long end = System.currentTimeMillis();
    System.out.println("started:" + (end - start) + "(ms)");
  }
}

Write SelfRestart to implement the methods in RestartServer

package com.litongjava.tio.boot.djl;

import com.litongjava.hotswap.debug.Diagnostic;
import com.litongjava.hotswap.kit.HotSwapUtils;
import com.litongjava.hotswap.server.RestartServer;
import com.litongjava.hotswap.wrapper.forkapp.ForkAppBootArgument;
import com.litongjava.tio.boot.TioApplication;
import com.litongjava.tio.boot.context.Context;

import lombok.extern.slf4j.Slf4j;

@Slf4j
public class SelfRestart implements RestartServer {
  public boolean isStarted() {
    return ForkAppBootArgument.getContext().isRunning();
  }

  public void restart() {
    System.err.println("loading");
    long start = System.currentTimeMillis();

    stop();
    // get a new ClassLoader
    ClassLoader hotSwapClassLoader = HotSwapUtils.newClassLoader();
    if (Diagnostic.isDebug()) {
      log.info("new classLoader:{}", hotSwapClassLoader);
    }

    // Set the context loader
    Thread.currentThread().setContextClassLoader(hotSwapClassLoader);

    // get startup class and args
    Class<?> clazz = ForkAppBootArgument.getBootClazz();
    String[] args = ForkAppBootArgument.getArgs();
    // start 
    start(clazz, args);
    long end = System.currentTimeMillis();
    System.err.println("Loading complete in " + (end - start) + " ms (^_^)\n");
  }

  @Override
  public void start(Class<?> primarySource, String[] args) {
    Context context = TioApplication.run(primarySource, args);
    ForkAppBootArgument.setContext(context);
  }

  @Override
  public void stop() {
    ForkAppBootArgument.getContext().close();
  }
}

3.Support for IDE

3.1 Support for IDEA

3.2 Support for IDEA 2021.3.3

3.2.1 Version Information

IDEA version is as follows:

3.2.2 Why Hot Reload Configuration is Needed

HotSwapWatcher mainly listens to modifications of class files under target/classes to trigger hot reloading. However, by default in IDEA, there is no automatic compilation, causing no changes to the files under target/classes. There are two solutions:

  1. Use the Ctrl + F9 shortcut to trigger compilation. (Test failed in IntelliJ IDEA 2019.3.3 (Ultimate Edition))
  2. Configure IDEA to enable automatic compilation, similar to eclipse.

3.2.3 IDEA Hot Reload Settings

  1. Automatically Build Project
    Search for "compiler" in settings, then check "build project automatically".

  2. Allow Automatic Building Even When a Development Application is Running
    Search for "make" in settings, then check "Allow auto-make to start even if developed application is currently running".

  3. Adjust Delay Time
    Use the Ctrl+Shift+Alt+/ shortcut, select "Registry...", then adjust the following configurations:

  • compiler.automake.postpone.when.idle.less.than: Default is 3000, change to 100.
  • compiler.automake.trigger.delay: Default value is 3000, change to 100.
  • compiler.document.save.trigger.delay: Default is 1500, change to 100.
  1. Cancel Automatic Code Saving
    In "File" -> "Settings" -> "Appearance & Behavior" -> "System Settings", uncheck the following options:
  • Older versions: Uncheck "Save files on frame deactivation" and "Synchronize files on frame or editor tab activation".
  • Newer versions: Uncheck "Save files if tab IDE is idle for 10 seconds" and "Save file when switching to a different application or a built-in terminal".
  1. Display "modified" Mark
    After modifying a file, a "star" mark will be displayed in the code editing window's tab area. Navigate to "File" -> "Settings" -> "Editor" -> "General" -> "Editor Tabs", then check "Mark modified(*)".

After completing the above settings, modifying a file and saving it in IDEA will result in the file being automatically compiled, and the application will automatically restart with hot reloading applied.

Note: There might be an issue when a package contains only one .java file. For more details, please check here.

3.3 Support for spring-boot-maven-plugin

If you aim to start the spring-boot project from the command line using mvn spring-boot:run, the default class loader is plexus-classworlds. To use this class loader, you need to follow these steps:

  1. Add the aforementioned dependency.
  2. Modify the startup class.
  3. Add the following configuration to pom.xml to enable plugin support for hot startup:
<plugin>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-maven-plugin</artifactId>
  <configuration>
    <includeSystemScope>true</includeSystemScope>
    <fork>true</fork>
    <mainClass>${start-class}</mainClass>
  </configuration>
</plugin>

1.4 Eclipse

No setup required, natively supported. Modify a Java file and it will be loaded automatically after saving. Development experience is better than IDEA

1.5 Visual Studio Code

No setup required, natively supported. Modify a Java file and it will be loaded automatically after saving. Development experience is better than IDEA

  1. Start the project using:
mvn spring-boot:run

4. Demonstrative Screenshots of Usage

4.1 Eclipse Testing Results

After starting spring-boot, adding a method to the controller, and pressing Ctrl+S to save, the HotSwapClassloader detects file changes and automatically reloads the code. This process is completed in approximately 0.8 seconds.

Eclipse Testing Results

4.2 IDEA Testing Results

After starting spring-boot, adding a method to the controller, and pressing Ctrl+S to save, the HotSwapClassloader detects the file changes and automatically reloads the code. However, in IDEA, due to a compilation delay of about 10 seconds, the entire reloading process takes approximately 10.8 seconds.

IDEA Testing Results

4.3 Command Line Testing Results

When starting the project from the command line using mvn spring-boot:run, you can modify the code in eclipse or IDEA for testing. This test is based on a large project. A regular startup takes 9.5 seconds, while hot reloading takes 3.4 seconds.

Click to view the video demonstration

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