簡述嵌入式tomcat的原理「嵌入式tomcatn優缺點」

準備工作

我們知道SpringBoot的自動裝配的秘密在
org.springframework.boot.autoconfigure包下的spring.factories文件中，而嵌入Tomcat的原理就在這個文件中加載的一個配置類：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);
 }
 }
 }
}

首先看一下上方的幾個註解

1. @AutoConfigureOrder這個註解是決定配置類的加載順序的，當註解里的值越小越先加載，而Ordered.HIGHEST_PRECEDENCE的值是Integer.MIN_VALUE也就是說這個類肯定是最先加載的那一批
2. @ConditionalOnXXX在之前的文章中已經無數次提到了，就不再闡述了
3. @EnableConfigurationProperties開啟ServerProperties類的屬性值配置。而這個類裏面包含的就是Web服務的配置

@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();
}

這個類的代碼太多了，這裡就不一一貼出來了，我們平常在application.properties中配置的server.xxx就是這個類中屬性

1. @Import引入了4個類，看都是什麼吧
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);
 }
 }
 }

這個類註冊了兩個bean：
WebServerFactoryCustomizerBeanPostProcessor和ErrorPageRegistrarBeanPostProcessor關於這兩個bean的作用稍後再詳細介紹

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();
 }
 }

這個類會在存在Tomcat相關jar包時添加一個
TomcatServletWebServerFactorybean

其他兩個相信大家都知道怎麼回事了

1. 除了這些這個類還注入了兩個類ServletWebServerFactoryCustomizer和TomcatServletWebServerFactoryCustomizer

現在前期準備工作已經做好了，看一下這個Tomcat是如何啟動的吧

啟動

啟動入口在
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的啟動就在createWebServer方法裏面了

private void createWebServer() {
 WebServer webServer = this.webServer;
 ServletContext servletContext = getServletContext();
 //第一次訪問的時候兩個對象都為空
 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() {
 // 這裡獲取的beanname就是上方註冊的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);
 }

準備環境里註冊的bean現在出來一個了。注意，上方還註冊了一個後置處理器
EmbeddedServletContainerCustomizerBeanPostProcessor，獲取beantomcatServletWebServerFactory的時候就會執行後置處理器的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();
 }

這個處理器的作用是獲得所有定製器，然後執行定製器的方法

接着往下看

這個時候就可以啟動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);
 }
 }
 }