SpringBoot嵌入式Tomcat的自动配置原理

  • 2019 年 10 月 8 日
  • 筆記

在读本篇文章之前如果你读过这篇文章SpringBoot自动装配原理解析应该会更加轻松

准备工作

我们知道SpringBoot的自动装配的秘密在org.springframework.boot.autoconfigure包下的spring.factories文件中，而嵌入Tomcat的原理就在这个文件中加载的一个配置类：org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration

@Configuration  @AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)  @ConditionalOnClass(ServletRequest.class)  @ConditionalOnWebApplication(type = Type.SERVLET)  @EnableConfigurationProperties(ServerProperties.class)  @Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,  		ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,  		ServletWebServerFactoryConfiguration.EmbeddedJetty.class,  		ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })  public class ServletWebServerFactoryAutoConfiguration {    	@Bean  	public ServletWebServerFactoryCustomizer servletWebServerFactoryCustomizer(  			ServerProperties serverProperties) {  		return new ServletWebServerFactoryCustomizer(serverProperties);  	}    	@Bean  	@ConditionalOnClass(name = "org.apache.catalina.startup.Tomcat")  	public TomcatServletWebServerFactoryCustomizer tomcatServletWebServerFactoryCustomizer(  			ServerProperties serverProperties) {  		return new TomcatServletWebServerFactoryCustomizer(serverProperties);  	}    	/**  	 * Registers a {@link WebServerFactoryCustomizerBeanPostProcessor}. Registered via  	 * {@link ImportBeanDefinitionRegistrar} for early registration.  	 */  	public static class BeanPostProcessorsRegistrar  			implements ImportBeanDefinitionRegistrar, BeanFactoryAware {    		private ConfigurableListableBeanFactory beanFactory;    		@Override  		public void setBeanFactory(BeanFactory beanFactory) throws BeansException {  			if (beanFactory instanceof ConfigurableListableBeanFactory) {  				this.beanFactory = (ConfigurableListableBeanFactory) beanFactory;  			}  		}    		@Override  		public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,  				BeanDefinitionRegistry registry) {  			if (this.beanFactory == null) {  				return;  			}  			registerSyntheticBeanIfMissing(registry,  					"webServerFactoryCustomizerBeanPostProcessor",  					WebServerFactoryCustomizerBeanPostProcessor.class);  			registerSyntheticBeanIfMissing(registry,  					"errorPageRegistrarBeanPostProcessor",  					ErrorPageRegistrarBeanPostProcessor.class);  		}    		private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,  				String name, Class<?> beanClass) {  			if (ObjectUtils.isEmpty(  					this.beanFactory.getBeanNamesForType(beanClass, true, false))) {  				RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);  				beanDefinition.setSynthetic(true);  				registry.registerBeanDefinition(name, beanDefinition);  			}  		}    	}    }

首先看一下上方的几个注解

  1. @AutoConfigureOrder这个注解是决定配置类的加载顺序的，当注解里的值越小越先加载，而Ordered.HIGHEST_PRECEDENCE的值是Integer.MIN_VALUE也就是说这个类肯定是最先加载的那一批
  2. @ConditionalOnXXX在之前的文章中已经无数次提到了，就不再阐述了
  3. @EnableConfigurationProperties开启ServerProperties类的属性值配置。而这个类里面包含的就是Web服务的配置
@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)  public class ServerProperties {    	private Integer port;    	private InetAddress address;    	@NestedConfigurationProperty  	private final ErrorProperties error = new ErrorProperties();    	private Boolean useForwardHeaders;    	private String serverHeader;    	private int maxHttpHeaderSize = 0; // bytes    	private Duration connectionTimeout;    	@NestedConfigurationProperty  	private Ssl ssl;    	@NestedConfigurationProperty  	private final Compression compression = new Compression();    	@NestedConfigurationProperty  	private final Http2 http2 = new Http2();    	private final Servlet servlet = new Servlet();    	private final Tomcat tomcat = new Tomcat();    	private final Jetty jetty = new Jetty();    	private final Undertow undertow = new Undertow();  }

这个类的代码太多了，这里就不一一贴出来了，我们平常在application.properties中配置的server.xxx就是这个类中属性

  1. @Import引入了4个类，看都是什么吧
  2. BeanPostProcessorsRegistrar
public static class BeanPostProcessorsRegistrar  			implements ImportBeanDefinitionRegistrar, BeanFactoryAware {  		@Override  		public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,  				BeanDefinitionRegistry registry) {  			if (this.beanFactory == null) {  				return;  			}  			registerSyntheticBeanIfMissing(registry,  					"webServerFactoryCustomizerBeanPostProcessor",  					WebServerFactoryCustomizerBeanPostProcessor.class);  			registerSyntheticBeanIfMissing(registry,  					"errorPageRegistrarBeanPostProcessor",  					ErrorPageRegistrarBeanPostProcessor.class);  		}    		private void registerSyntheticBeanIfMissing(BeanDefinitionRegistry registry,  				String name, Class<?> beanClass) {  			if (ObjectUtils.isEmpty(  					this.beanFactory.getBeanNamesForType(beanClass, true, false))) {  				RootBeanDefinition beanDefinition = new RootBeanDefinition(beanClass);  				beanDefinition.setSynthetic(true);  				registry.registerBeanDefinition(name, beanDefinition);  			}  		}    	}

这个类注册了两个bean：WebServerFactoryCustomizerBeanPostProcessor和ErrorPageRegistrarBeanPostProcessor关于这两个bean的作用稍后再详细介绍

  1. EmbeddedTomcat
@Configuration  	@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })  	@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)  	public static class EmbeddedTomcat {    		@Bean  		public TomcatServletWebServerFactory tomcatServletWebServerFactory() {  			return new TomcatServletWebServerFactory();  		}    	}

这个类会在存在Tomcat相关jar包时添加一个TomcatServletWebServerFactorybean

其他两个相信大家都知道怎么回事了

  1. 除了这些这个类还注入了两个类ServletWebServerFactoryCustomizer和TomcatServletWebServerFactoryCustomizer

现在前期准备工作已经做好了，看一下这个Tomcat是如何启动的吧

启动

启动入口在ServletWebServerApplicationContext中的onRefresh方法

protected void onRefresh() {  		super.onRefresh();  		try {  			createWebServer();  		}  		catch (Throwable ex) {  			throw new ApplicationContextException("Unable to start web server", ex);  		}  	}

Tomcat的启动就在createWebServer方法里面了

private void createWebServer() {  		WebServer webServer = this.webServer;  		ServletContext servletContext = getServletContext();    		//第一次访问的时候两个对象都为空  		if (webServer == null && servletContext == null) {  			ServletWebServerFactory factory = getWebServerFactory();  			this.webServer = factory.getWebServer(getSelfInitializer());  		}  		else if (servletContext != null) {  			try {  				getSelfInitializer().onStartup(servletContext);  			}  			catch (ServletException ex) {  				throw new ApplicationContextException("Cannot initialize servlet context",  						ex);  			}  		}  		initPropertySources();  	}

首先看一下getWebServerFactory

protected ServletWebServerFactory getWebServerFactory() {  		// 这里获取的beanname就是上方注册的tomcatServletWebServerFactory了  		String[] beanNames = getBeanFactory()  				.getBeanNamesForType(ServletWebServerFactory.class);  		if (beanNames.length == 0) {  			throw new ApplicationContextException(  					"Unable to start ServletWebServerApplicationContext due to missing "  							+ "ServletWebServerFactory bean.");  		}  		if (beanNames.length > 1) {  			throw new ApplicationContextException(  					"Unable to start ServletWebServerApplicationContext due to multiple "  							+ "ServletWebServerFactory beans : "  							+ StringUtils.arrayToCommaDelimitedString(beanNames));  		}  		return getBeanFactory().getBean(beanNames[0], ServletWebServerFactory.class);  	}

准备环境里注册的bean现在出来一个了。注意，上方还注册了一个后置处理器EmbeddedServletContainerCustomizerBeanPostProcessor，获取beantomcatServletWebServerFactory的时候就会执行后置处理器的postProcessBeforeInitialization方法

public Object postProcessBeforeInitialization(Object bean, String beanName)  			throws BeansException {  	if (bean instanceof WebServerFactory) {  		postProcessBeforeInitialization((WebServerFactory) bean);  	}  	return bean;  }  private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {  		LambdaSafe  				.callbacks(WebServerFactoryCustomizer.class, getCustomizers(),  						webServerFactory)  				.withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)  				.invoke((customizer) -> customizer.customize(webServerFactory));  	}    	private Collection<WebServerFactoryCustomizer<?>> getCustomizers() {  		if (this.customizers == null) {  			// Look up does not include the parent context  			this.customizers = new ArrayList<>(getWebServerFactoryCustomizerBeans());  			this.customizers.sort(AnnotationAwareOrderComparator.INSTANCE);  			this.customizers = Collections.unmodifiableList(this.customizers);  		}  		return this.customizers;  	}    	@SuppressWarnings({ "unchecked", "rawtypes" })  	private Collection<WebServerFactoryCustomizer<?>> getWebServerFactoryCustomizerBeans() {  		return (Collection) this.beanFactory  				.getBeansOfType(WebServerFactoryCustomizer.class, false, false).values();  	}

这个处理器的作用是获得所有定制器，然后执行定制器的方法

接着往下看

这个时候就可以启动Tomcat了

public WebServer getWebServer(ServletContextInitializer... initializers) {  		Tomcat tomcat = new Tomcat();  		File baseDir = (this.baseDirectory != null ? this.baseDirectory  				: createTempDir("tomcat"));  		tomcat.setBaseDir(baseDir.getAbsolutePath());  		Connector connector = new Connector(this.protocol);  		tomcat.getService().addConnector(connector);  		customizeConnector(connector);  		tomcat.setConnector(connector);  		tomcat.getHost().setAutoDeploy(false);  		configureEngine(tomcat.getEngine());  		for (Connector additionalConnector : this.additionalTomcatConnectors) {  			tomcat.getService().addConnector(additionalConnector);  		}  		prepareContext(tomcat.getHost(), initializers);  		return getTomcatWebServer(tomcat);  	}  protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {  		return new TomcatWebServer(tomcat, getPort() >= 0);  	}  public TomcatWebServer(Tomcat tomcat, boolean autoStart) {  		Assert.notNull(tomcat, "Tomcat Server must not be null");  		this.tomcat = tomcat;  		this.autoStart = autoStart;  		initialize();  	}  private void initialize() throws WebServerException {  		TomcatWebServer.logger  				.info("Tomcat initialized with port(s): " + getPortsDescription(false));  		synchronized (this.monitor) {  			try {  				addInstanceIdToEngineName();    				Context context = findContext();  				context.addLifecycleListener((event) -> {  					if (context.equals(event.getSource())  							&& Lifecycle.START_EVENT.equals(event.getType())) {  						// Remove service connectors so that protocol binding doesn't  						// happen when the service is started.  						removeServiceConnectors();  					}  				});    				// Start the server to trigger initialization listeners  				this.tomcat.start();    				// We can re-throw failure exception directly in the main thread  				rethrowDeferredStartupExceptions();    				try {  					ContextBindings.bindClassLoader(context, context.getNamingToken(),  							getClass().getClassLoader());  				}  				catch (NamingException ex) {  					// Naming is not enabled. Continue  				}    				// Unlike Jetty, all Tomcat threads are daemon threads. We create a  				// blocking non-daemon to stop immediate shutdown  				startDaemonAwaitThread();  			}  			catch (Exception ex) {  				throw new WebServerException("Unable to start embedded Tomcat", ex);  			}  		}  	}

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