核心流程

1.spring.factories

定义了很多需要被初始化的类，程序启动的时候会扫描spring.factories并注册事件监听者RestartApplicationListener

# Application Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.devtools.restart.RestartScopeInitializer

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.devtools.restart.RestartApplicationListener,\
org.springframework.boot.devtools.logger.DevToolsLogFactory.Listener

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.devtools.autoconfigure.DevToolsDataSourceAutoConfiguration,\
org.springframework.boot.devtools.autoconfigure.LocalDevToolsAutoConfiguration,\
org.springframework.boot.devtools.autoconfigure.RemoteDevToolsAutoConfiguration

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.devtools.env.DevToolsHomePropertiesPostProcessor,\
org.springframework.boot.devtools.env.DevToolsPropertyDefaultsPostProcessor

2.`RestartApplicationListener`

监听到ApplicationStartingEvent事件以后，另外启动一个线程重新启动main函数使用RestartClassLoader来加载类，将原来的主线程hold住

onApplicationStartingEvent(ApplicationStartingEvent event)

Restarter#initialize

Restarter#immediateRestart

private void immediateRestart() {
	try {
	    // 新启动一个线程执行runnable, 执行完成以后会join()，hold住主线程
		getLeakSafeThread().callAndWait(() -> {
		    // start-》doStart-》relaunch，RestartLauncher新启动一个线程执行SpringApplication类的main函数
			start(FailureHandler.NONE);
			// 当Spring初始化成功以后，先清理相关缓存
			cleanupCaches();
			return null;
		});
	}
	catch (Exception ex) {
		this.logger.warn("Unable to initialize restarter", ex);
	}
	// 这里是在程序结束以后才会执行到，抛出异常，终止主线程
	SilentExitExceptionHandler.exitCurrentThread();
}

public class RestartLauncher extends Thread {
 	    // ....省略
    	@Override
  	public void run() {
  		try {
  			Class<?> mainClass = getContextClassLoader().loadClass(this.mainClassName);
  			Method mainMethod = mainClass.getDeclaredMethod("main", String[].class);
  			mainMethod.invoke(null, new Object[] { this.args });
  		}
  		catch (Throwable ex) {
  			this.error = ex;
  			getUncaughtExceptionHandler().uncaughtException(this, ex);
  		}
  	}
 }

3.监听文件变化后进行重启

ClassPathFileSystemWatcher在初始化的时候调用了fileSystemWatcher.addListener创建了一个监听者，并且启动了文件夹监控线程

public class ClassPathFileSystemWatcher implements InitializingBean, DisposableBean, ApplicationContextAware {
	......
	@Override
	public void afterPropertiesSet() throws Exception {
		if (this.restartStrategy != null) {
			FileSystemWatcher watcherToStop = null;
			if (this.stopWatcherOnRestart) {
				watcherToStop = this.fileSystemWatcher;
			}
			this.fileSystemWatcher.addListener(
					new ClassPathFileChangeListener(this.applicationContext, this.restartStrategy, watcherToStop));
		}
		this.fileSystemWatcher.start();
	}
	......
}

文件监控FileSystemWatcher线程类，就是将要监控的目录加入到this.folders, 启动线程不断的扫描文件夹的diff, 如果有ChangedFiles，通知监听者fireListeners

private void scan() throws InterruptedException {
			Thread.sleep(this.pollInterval - this.quietPeriod);
			Map<File, FolderSnapshot> previous;
			Map<File, FolderSnapshot> current = this.folders;
			do {
				previous = current;
				current = getCurrentSnapshots();
				Thread.sleep(this.quietPeriod);
			}
			while (isDifferent(previous, current));
			if (isDifferent(this.folders, current)) {
			    // 得到changeSet，然后fireListeners通知监听者
				updateSnapshots(current.values());
			}
}

ClassPathFileChangeListener.onChange fireListeners时被调用

publishEvent(new ClassPathChangedEvent(this, changeSet, restart))

LocalDevToolsAutoConfiguration.RestartConfiguration监听到ClassPathChangedEvent事件然后调用Restarter.getInstance().restart重启

static class RestartConfiguration implements ApplicationListener<ClassPathChangedEvent> {
   .......
	@Override
	public void onApplicationEvent(ClassPathChangedEvent event) {
		if (event.isRestartRequired()) {
			Restarter.getInstance().restart(new FileWatchingFailureHandler(fileSystemWatcherFactory()));
		}
	}
    ......
}

 public void restart(FailureHandler failureHandler) {
 	getLeakSafeThread().call(() -> {
 	    // 调用rootContexts.close()销毁所有的bean，清理缓存，gc
 		Restarter.this.stop();
 		// 再重启
 		Restarter.this.start(failureHandler);
 		return null;
 	});
 }

4.RestartClassLoader

优先从updatedFiles中取更新过的类进行加载ClassLoaderFiles#getFile

public class RestartClassLoader extends URLClassLoader implements SmartClassLoader {
    ......
	private final ClassLoaderFileRepository updatedFiles;
	public RestartClassLoader(ClassLoader parent, URL[] urls, ClassLoaderFileRepository updatedFiles, Log logger) {
		super(urls, parent);
		this.updatedFiles = updatedFiles;
		this.logger = logger;
	}

	@Override
	public Enumeration<URL> getResources(String name) throws IOException {
		// 父类classLoader加载资源
		Enumeration<URL> resources = getParent().getResources(name);
		// 变更类的资源
		ClassLoaderFile file = this.updatedFiles.getFile(name);
		if (file != null) {
			// Assume that we're replacing just the first item
			if (resources.hasMoreElements()) {
				resources.nextElement();
			}
			if (file.getKind() != Kind.DELETED) {
			    // 将更新过的ClassLoaderFile类放在URL的前面，优先加载变更类的URL
				return new CompoundEnumeration<>(createFileUrl(name, file), resources);
			}
		}
		return resources;
	}

	@Override
	public URL getResource(String name) {
	    // 先加载变更类的URL
		ClassLoaderFile file = this.updatedFiles.getFile(name);
		if (file != null && file.getKind() == Kind.DELETED) {
			return null;
		}
		URL resource = findResource(name);
		if (resource != null) {
			return resource;
		}
		return getParent().getResource(name);
	}

	@Override
	public URL findResource(String name) {
		final ClassLoaderFile file = this.updatedFiles.getFile(name);
		if (file == null) {
			return super.findResource(name);
		}
		if (file.getKind() == Kind.DELETED) {
			return null;
		}
		return AccessController.doPrivileged((PrivilegedAction<URL>) () -> createFileUrl(name, file));
	}

	@Override
	public Class<?> loadClass(String name, boolean resolve) throws ClassNotFoundException {
		String path = name.replace('.', '/').concat(".class");
		ClassLoaderFile file = this.updatedFiles.getFile(path);
		if (file != null && file.getKind() == Kind.DELETED) {
			throw new ClassNotFoundException(name);
		}
		synchronized (getClassLoadingLock(name)) {
			Class<?> loadedClass = findLoadedClass(name);
			if (loadedClass == null) {
				try {
					loadedClass = findClass(name);
				}
				catch (ClassNotFoundException ex) {
					loadedClass = getParent().loadClass(name);
				}
			}
			if (resolve) {
				resolveClass(loadedClass);
			}
			return loadedClass;
		}
	}

	@Override
	protected Class<?> findClass(String name) throws ClassNotFoundException {
		String path = name.replace('.', '/').concat(".class");
		final ClassLoaderFile file = this.updatedFiles.getFile(path);
		if (file == null) {
			return super.findClass(name);
		}
		if (file.getKind() == Kind.DELETED) {
			throw new ClassNotFoundException(name);
		}
		return AccessController.doPrivileged((PrivilegedAction<Class<?>>) () -> {
			byte[] bytes = file.getContents();
			return defineClass(name, bytes, 0, bytes.length);
		});
	}

	private URL createFileUrl(String name, ClassLoaderFile file) {
		try {
			return new URL("reloaded", null, -1, "/" + name, new ClassLoaderFileURLStreamHandler(file));
		}
		catch (MalformedURLException ex) {
			throw new IllegalStateException(ex);
		}
	}
	......
}

5.清理缓存

Restarter#cleanupCaches 和其他热更新类似，清理缓存

private void cleanupCaches() throws Exception {
		Introspector.flushCaches();
		cleanupKnownCaches();
	}

	private void cleanupKnownCaches() throws Exception {
		ResolvableType.clearCache();
		cleanCachedIntrospectionResultsCache();
		ReflectionUtils.clearCache();
		clearAnnotationUtilsCache();
		if (!JavaVersion.getJavaVersion().isEqualOrNewerThan(JavaVersion.NINE)) {
			clear("com.sun.naming.internal.ResourceManager", "propertiesCache");
		}
	}

	private void cleanCachedIntrospectionResultsCache() throws Exception {
		clear(CachedIntrospectionResults.class, "acceptedClassLoaders");
		clear(CachedIntrospectionResults.class, "strongClassCache");
		clear(CachedIntrospectionResults.class, "softClassCache");
	}

	private void clearAnnotationUtilsCache() throws Exception {
		try {
			AnnotationUtils.clearCache();
		}
		catch (Throwable ex) {
			clear(AnnotationUtils.class, "findAnnotationCache");
			clear(AnnotationUtils.class, "annotatedInterfaceCache");
		}
	}

	private void clear(String className, String fieldName) {
		try {
			clear(Class.forName(className), fieldName);
		}
		catch (Exception ex) {
			if (this.logger.isDebugEnabled()) {
				this.logger.debug("Unable to clear field " + className + " " + fieldName, ex);
			}
		}
	}

	private void clear(Class<?> type, String fieldName) throws Exception {
		try {
			Field field = type.getDeclaredField(fieldName);
			field.setAccessible(true);
			Object instance = field.get(null);
			if (instance instanceof Set) {
				((Set<?>) instance).clear();
			}
			if (instance instanceof Map) {
				((Map<?, ?>) instance).keySet().removeIf(this::isFromRestartClassLoader);
			}
		}
		catch (Exception ex) {
			if (this.logger.isDebugEnabled()) {
				this.logger.debug("Unable to clear field " + type + " " + fieldName, ex);
			}
		}
	}

远程更新

服务端

RemoteDevToolsAutoConfiguration 配置，只有设置了spring.devtools.remote.secret才初始化

增加了GET /接口进行健康检查
增加了POST /restart接口进行远程热更新，将body体反序列化得到变更的类资源ClassLoaderFiles，然后调用RestartServer#restart进行重启
增加了AccessManager验证接口的请求头X-AUTH-TOKEN传递的secret

@Configuration
// 只有设置了spring.devtools.remote.secret才初始化
@ConditionalOnProperty(prefix = "spring.devtools.remote", name = "secret")
public class RemoteDevToolsAutoConfiguration {
    ......
	@Bean
	@ConditionalOnMissingBean
	public AccessManager remoteDevToolsAccessManager() {
	    // 权限空值，根据请求头的X-AUTH-TOKEN来验证密钥是否一致
		RemoteDevToolsProperties remoteProperties = this.properties.getRemote();
		return new HttpHeaderAccessManager(remoteProperties.getSecretHeaderName(), remoteProperties.getSecret());
	}

    // 增加一个根路径接口GET /来进行监控检查，使用HttpStatusHandler返回一个HttpStatus.OK来判定服务已经启动成功
	@Bean
	public HandlerMapper remoteDevToolsHealthCheckHandlerMapper() {
		Handler handler = new HttpStatusHandler();
		Servlet servlet = this.serverProperties.getServlet();
		String servletContextPath = (servlet.getContextPath() != null) ? servlet.getContextPath() : "";
		return new UrlHandlerMapper(servletContextPath + this.properties.getRemote().getContextPath(), handler);
	}

	@Bean
	@ConditionalOnMissingBean
	public DispatcherFilter remoteDevToolsDispatcherFilter(AccessManager accessManager,
			Collection<HandlerMapper> mappers) {
		// 上面定义HandlerMapper使用AccessManager进行拦截权限
		Dispatcher dispatcher = new Dispatcher(accessManager, mappers);
		return new DispatcherFilter(dispatcher);
	}

	@Configuration
	@ConditionalOnProperty(prefix = "spring.devtools.remote.restart", name = "enabled", matchIfMissing = true)
	static class RemoteRestartConfiguration {
	    ......
	    // 增加一个接口POST /restart进行资源更新和重启，
		@Bean
		@ConditionalOnMissingBean(name = "remoteRestartHandlerMapper")
		public UrlHandlerMapper remoteRestartHandlerMapper(HttpRestartServer server) {
			Servlet servlet = this.serverProperties.getServlet();
			RemoteDevToolsProperties remote = this.properties.getRemote();
			String servletContextPath = (servlet.getContextPath() != null) ? servlet.getContextPath() : "";
			String url = servletContextPath + remote.getContextPath() + "/restart";
			logger.warn("Listening for remote restart updates on " + url);
			Handler handler = new HttpRestartServerHandler(server);
			return new UrlHandlerMapper(url, handler);
		}
	}
}

RestartServer#restart

	protected void restart(Set<URL> urls, ClassLoaderFiles files) {
		Restarter restarter = Restarter.getInstance();
		restarter.addUrls(urls);
		// 优先从变更的类中加载类
		restarter.addClassLoaderFiles(files);
		restarter.restart();
	}

客户端

RemoteClientConfiguration配置初始化

RemoteRestartClientConfiguration
- ClassPathFileSystemWatcher 目录监控初始化
- ClassPathChangeUploader 监听ClassPathChangedEvent事件，调用远程服务http://remoteUrl/restart接口上传更新的classLoaderFiles序列化字节数据
监听ClassPathChangedEvent事件，当文件变更以后，休眠shutdownTime时间，循环调用http://remoteUrl/接口，判断远端服务是否重启成功，启动成功以后this.liveReloadServer.triggerReload()自动更新浏览器