Java基于自定义类加载器实现热部署过程解析

package Dynamic;
 
import java.io.*;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.concurrent.TimeUnit;
 
public class ClassLoadStudy {
  public static void main(String[] args) throws Exception {
    HotDeploy hot = new HotDeploy("Dynamic.Task");
    hot.monitor();
    while (true) {
      TimeUnit.SECONDS.sleep(2);
      hot.getTask().run();
    }
  }
}
 
// 热部署
 
class HotDeploy {
  private static volatile Runnable instance;
  private final String FILE_NAME;
  private final String CLASS_NAME;
 
  public HotDeploy(String name) {
    CLASS_NAME = name; // 类的完全限定名
    name = name.replaceAll("\\.", "/") + ".class";
    FILE_NAME = (getClass().getResource("/") + name).substring(6); // 判断class文件修改时间使用，substring(6)去掉开头的file:/
  }
 
  // 获取一个任务
  public Runnable getTask() {
    if (instance == null) { // 双重检查锁，单例，线程安全
      synchronized (HotDeploy.class) {
        if (instance == null) {
          try {
            instance = createTask();
          } catch (Exception e) {
            e.printStackTrace();
          }
        }
      }
    }
    return instance;
  }
 
  // 创建一个任务，重新加载 class 文件
  private Runnable createTask() {
    try {
      Class clazz = MyClassLoader.getLoader().loadClass(CLASS_NAME);
      if (clazz != null)
        return (Runnable)clazz.newInstance();
    } catch (Exception e) {
      e.printStackTrace();
    }
    return null;
  }
 
 
  // 监视器，监视class文件是否被修改过，如果是的话，则重新加载
  public void monitor() throws IOException {
    Thread t = new Thread(()->{
      try {
        long lastModified = Files.getLastModifiedTime(Path.of(FILE_NAME)).toMillis();
        while(true) {
          Thread.sleep(500);
          long now = Files.getLastModifiedTime(Path.of(FILE_NAME)).toMillis();
          if(now != lastModified) { // 如果class文件被修改过了
            lastModified = now;
            instance = createTask(); // 重新加载
          }
        }
      } catch (InterruptedException | IOException e) {
        e.printStackTrace();
      }
    });
    t.setDaemon(true); // 守护线程
    t.start();
  }
}
 
// 自定义的类加载器
class MyClassLoader extends ClassLoader {
  @Override
  public Class<?> findClass(String name) throws ClassNotFoundException {
    try {
      String fileName = "/" + name.replaceAll("\\.", "/") + ".class";
      InputStream is = getClass().getResourceAsStream(fileName);
      byte[] b = is.readAllBytes();
      return defineClass(name, b, 0, b.length);
    } catch (IOException e) {
      throw new ClassNotFoundException(name);
    }
  }
  public static MyClassLoader getLoader() {
    return new MyClassLoader();
  }
}