SpringBoot嵌入式Tomcat的自動配置原理

在讀本篇文章之前如果你讀過這篇文章 "SpringBoot自動裝配原理解析" 應該會更加輕鬆準備工作我們知道SpringBoot的自動裝配的秘密在包下的文件中，而嵌入Tomcat的原理就在這個文件中載入的一個配置類：首先看一下上方的幾個註解 1. 這個註解是決定配置類的載入順序的，當註解里 ...

在讀本篇文章之前如果你讀過這篇文章SpringBoot自動裝配原理解析應該會更加輕鬆

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載入的一個配置類：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

@AutoConfigureOrder這個註解是決定配置類的載入順序的，當註解里的值越小越先載入，而Ordered.HIGHEST_PRECEDENCE的值是Integer.MIN_VALUE也就是說這個類肯定是最先載入的那一批
@ConditionalOnXXX在之前的文章中已經無數次提到了，就不再闡述了
@EnableConfigurationProperties開啟ServerProperties類的屬性值配置。而這個類裡面包含的就是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

@Import引入了4個類，看都是什麼吧
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

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

除了這些這個類還註入了兩個類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方法裡面了

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。註意，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

接著往下看

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);
            }
        }
    }