SpringBoot 系列-内嵌 Tomcat 的实现原理解析

<dependencies>
    <dependency>
    	<groupId>org.springframework.boot</groupId>
    	<artifactId>spring-boot-starter</artifactId>
    </dependency>
    <dependency>
    	<groupId>org.springframework.boot</groupId>
    	<artifactId>spring-boot-starter-json</artifactId>
    </dependency>
    <dependency>
    	<groupId>org.springframework.boot</groupId>
    	<artifactId>spring-boot-starter-tomcat</artifactId>
    </dependency>
    <dependency>
    	<groupId>org.hibernate.validator</groupId>
    	<artifactId>hibernate-validator</artifactId>
    </dependency>
    <dependency>
    	<groupId>org.springframework</groupId>
    	<artifactId>spring-web</artifactId>
    </dependency>
    <dependency>
    	<groupId>org.springframework</groupId>
    	<artifactId>spring-webmvc</artifactId>
    </dependency>
</dependencies>

@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

public static class BeanPostProcessorsRegistrar
    implements ImportBeanDefinitionRegistrar, BeanFactoryAware {
    // 省略代码
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,
                                        BeanDefinitionRegistry registry) {
        if (this.beanFactory == null) {
            return;
        }
        // 注册 WebServerFactoryCustomizerBeanPostProcessor
        registerSyntheticBeanIfMissing(registry,
                                       "webServerFactoryCustomizerBeanPostProcessor",
                                       WebServerFactoryCustomizerBeanPostProcessor.class);
        // 注册 errorPageRegistrarBeanPostProcessor
        registerSyntheticBeanIfMissing(registry,
                                       "errorPageRegistrarBeanPostProcessor",
                                       ErrorPageRegistrarBeanPostProcessor.class);
    }
    // 省略代码
}

public class WebServerFactoryCustomizerBeanPostProcessor
		implements BeanPostProcessor, BeanFactoryAware {
    // 省略部分代码
    
    // 在 postProcessBeforeInitialization 方法中，如果当前 bean 是 WebServerFactory，则进行
    // 一些后置处理
    @Override
	public Object postProcessBeforeInitialization(Object bean, String beanName)
			throws BeansException {
		if (bean instanceof WebServerFactory) {
			postProcessBeforeInitialization((WebServerFactory) bean);
		}
		return bean;
	}
    // 这段代码就是拿到所有的 Customizers ，然后遍历调用这些 Customizers 的 customize 方法
    private void postProcessBeforeInitialization(WebServerFactory webServerFactory) {
		LambdaSafe
				.callbacks(WebServerFactoryCustomizer.class, getCustomizers(),
						webServerFactory)
				.withLogger(WebServerFactoryCustomizerBeanPostProcessor.class)
				.invoke((customizer) -> customizer.customize(webServerFactory));
	}
    
    // 省略部分代码
}

@Override
public void customize(ConfigurableServletWebServerFactory factory) {
    PropertyMapper map = PropertyMapper.get().alwaysApplyingWhenNonNull();
    // 端口
    map.from(this.serverProperties::getPort).to(factory::setPort);
    // address
    map.from(this.serverProperties::getAddress).to(factory::setAddress);
    // contextPath
    map.from(this.serverProperties.getServlet()::getContextPath)
        .to(factory::setContextPath);
    // displayName
    map.from(this.serverProperties.getServlet()::getApplicationDisplayName)
        .to(factory::setDisplayName);
    // session 配置
    map.from(this.serverProperties.getServlet()::getSession).to(factory::setSession);
    // ssl
    map.from(this.serverProperties::getSsl).to(factory::setSsl);
    // jsp
    map.from(this.serverProperties.getServlet()::getJsp).to(factory::setJsp);
    // 压缩配置策略实现
    map.from(this.serverProperties::getCompression).to(factory::setCompression);
    // http2 
    map.from(this.serverProperties::getHttp2).to(factory::setHttp2);
    // serverHeader
    map.from(this.serverProperties::getServerHeader).to(factory::setServerHeader);
    // contextParameters
    map.from(this.serverProperties.getServlet()::getContextParameters)
        .to(factory::setInitParameters);
}

@Override
public void customize(TomcatServletWebServerFactory factory) {
    // 拿到 tomcat 相关的配置
    ServerProperties.Tomcat tomcatProperties = this.serverProperties.getTomcat();
    // server.tomcat.additional-tld-skip-patterns
    if (!ObjectUtils.isEmpty(tomcatProperties.getAdditionalTldSkipPatterns())) {
        factory.getTldSkipPatterns()
            .addAll(tomcatProperties.getAdditionalTldSkipPatterns());
    }
    // server.redirectContextRoot
    if (tomcatProperties.getRedirectContextRoot() != null) {
        customizeRedirectContextRoot(factory,
                                     tomcatProperties.getRedirectContextRoot());
    }
    // server.useRelativeRedirects
    if (tomcatProperties.getUseRelativeRedirects() != null) {
        customizeUseRelativeRedirects(factory,
                                      tomcatProperties.getUseRelativeRedirects());
    }
}

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
    // 创建一个 Tomcat 实例
    Tomcat tomcat = new Tomcat();
    // 创建一个 Tomcat 实例工作空间目录
    File baseDir = (this.baseDirectory != null) ? this.baseDirectory
        : createTempDir("tomcat");
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    // 创建连接对象
    Connector connector = new Connector(this.protocol);
    tomcat.getService().addConnector(connector);
    // 1
    customizeConnector(connector);
    tomcat.setConnector(connector);
    tomcat.getHost().setAutoDeploy(false);
    // 配置 Engine，没有什么实质性的操作，可忽略
    configureEngine(tomcat.getEngine());
    // 一些附加链接，默认是 0 个
    for (Connector additionalConnector : this.additionalTomcatConnectors) {
        tomcat.getService().addConnector(additionalConnector);
    }
    // 2
    prepareContext(tomcat.getHost(), initializers);
    // 返回 webServer
    return getTomcatWebServer(tomcat);
}

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    File documentRoot = getValidDocumentRoot();
    // 创建一个 TomcatEmbeddedContext 对象
    TomcatEmbeddedContext context = new TomcatEmbeddedContext();
    if (documentRoot != null) {
        context.setResources(new LoaderHidingResourceRoot(context));
    }
    // 设置描述此容器的名称字符串。在属于特定父项的子容器集内，容器名称必须唯一。
    context.setName(getContextPath());
    // 设置此Web应用程序的显示名称。
    context.setDisplayName(getDisplayName());
    // 设置 webContextPath  默认是   /
    context.setPath(getContextPath());
    File docBase = (documentRoot != null) ? documentRoot
        : createTempDir("tomcat-docbase");
    context.setDocBase(docBase.getAbsolutePath());
    // 注册一个FixContextListener监听，这个监听用于设置context的配置状态以及是否加入登录验证的逻辑
    context.addLifecycleListener(new FixContextListener());
    // 设置 父 ClassLoader
    context.setParentClassLoader(
        (this.resourceLoader != null) ? this.resourceLoader.getClassLoader()
        : ClassUtils.getDefaultClassLoader());
    // 覆盖Tomcat的默认语言环境映射以与其他服务器对齐。
    resetDefaultLocaleMapping(context);
    // 添加区域设置编码映射（请参阅Servlet规范2.4的5.4节）
    addLocaleMappings(context);
    // 设置是否使用相对地址重定向
    context.setUseRelativeRedirects(false);
    try {
        context.setCreateUploadTargets(true);
    }
    catch (NoSuchMethodError ex) {
        // Tomcat is < 8.5.39. Continue.
    }
    configureTldSkipPatterns(context);
    // 设置 WebappLoader ，并且将 父 classLoader 作为构建参数
    WebappLoader loader = new WebappLoader(context.getParentClassLoader());
    // 设置 WebappLoader 的 loaderClass 值
    loader.setLoaderClass(TomcatEmbeddedWebappClassLoader.class.getName());
    // 会将加载类向上委托
    loader.setDelegate(true);
    context.setLoader(loader);
    if (isRegisterDefaultServlet()) {
        addDefaultServlet(context);
    }
    // 是否注册 jspServlet
    if (shouldRegisterJspServlet()) {
        addJspServlet(context);
        addJasperInitializer(context);
    }
    context.addLifecycleListener(new StaticResourceConfigurer(context));
    ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);
    // 在 host 中 加入一个 context 容器
    // add时给context注册了个内存泄漏跟踪的监听MemoryLeakTrackingListener,详见 addChild 方法
    host.addChild(context);
    //对context做了些设置工作，包括TomcatStarter(实例化并set给context),
    // LifecycleListener,contextValue,errorpage,Mime,session超时持久化等以及一些自定义工作
    configureContext(context, initializersToUse);
    // postProcessContext 方法是空的，留给子类重写用的
    postProcessContext(context);
}

// org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory
protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
    // new 一个 TomcatWebServer
    return new TomcatWebServer(tomcat, getPort() >= 0);
}
// org.springframework.boot.web.embedded.tomcat.TomcatWebServer
public TomcatWebServer(Tomcat tomcat, boolean autoStart) {
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    // 初始化
    initialize();
}

private void initialize() throws WebServerException {
    logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));
    synchronized (this.monitor) {
        try {
            // 对全局原子变量 containerCounter+1，由于初始值是-1，
    // 所以 addInstanceIdToEngineName 方法内后续的获取引擎并设置名字的逻辑不会执行
            addInstanceIdToEngineName();
			// 获取 Context 
            Context context = findContext();
            // 给 Context 对象实例生命周期监听器
            context.addLifecycleListener((event) -> {
                if (context.equals(event.getSource())
                    && Lifecycle.START_EVENT.equals(event.getType())) {
                    // 将上面new的connection以service（这里是StandardService[Tomcat]）做key保存到
                    // serviceConnectors中，并将 StandardService 中的connectors 与 service 解绑(connector.setService((Service)null);)，
                    // 解绑后下面利用LifecycleBase启动容器就不会启动到Connector了
                    removeServiceConnectors();
                }
            });
            // 启动服务器以触发初始化监听器
            this.tomcat.start();
            // 这个方法检查初始化过程中的异常，如果有直接在主线程抛出，
            // 检查方法是TomcatStarter中的 startUpException，这个值是在 Context 启动过程中记录的
            rethrowDeferredStartupExceptions();
            try {
                // 绑定命名的上下文和classloader，
                ContextBindings.bindClassLoader(context, context.getNamingToken(),
                                                getClass().getClassLoader());
            }
            catch (NamingException ex) {
                // 设置失败不需要关心
            }

			// ：与Jetty不同，Tomcat所有的线程都是守护线程，所以创建一个非守护线程
            // （例：Thread[container-0,5,main]）来避免服务到这就shutdown了
            startDaemonAwaitThread();
        }
        catch (Exception ex) {
            stopSilently();
            throw new WebServerException("Unable to start embedded Tomcat", ex);
        }
    }
}

private Context findContext() {
    for (Container child : this.tomcat.getHost().findChildren()) {
        if (child instanceof Context) {
            return (Context) child;
        }
    }
    throw new IllegalStateException("The host does not contain a Context");
}

// Start the server to trigger initialization listeners
this.tomcat.start();

public void start() throws LifecycleException {
    // 初始化 server
    getServer();
    // 启动 server
    server.start();
}

public Server getServer() {
	// 如果已经存在的话就直接返回
    if (server != null) {
        return server;
    }
	// 设置系统属性 catalina.useNaming
    System.setProperty("catalina.useNaming", "false");
	// 直接 new 一个 StandardServer
    server = new StandardServer();
	// 初始化 baseDir （catalina.base、catalina.home、 ~/tomcat.{port}）
    initBaseDir();

    // Set configuration source
    ConfigFileLoader.setSource(new CatalinaBaseConfigurationSource(new File(basedir), null));

    server.setPort( -1 );

    Service service = new StandardService();
    service.setName("Tomcat");
    server.addService(service);
    return server;
}