springboot源码解析(三)：springboot内嵌tomcat

​ 在使用springboot搭建一个web应用程序的时候，我们发现不需要自己搭建一个tomcat服务器，只需要引入spring-boot-starter-web,在应用启动时会自动启动嵌入式的tomcat作为服务器，下面来分析下源码的分析流程：

​ 之前我们已经讲过了自动装配的原理，其实tomcat的实现机制也是从自动装配开始的。

1、ServletWebServerFactoryAutoConfiguration类

@Configuration(proxyBeanMethods = false)
@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);
}

@Bean
@ConditionalOnMissingFilterBean(ForwardedHeaderFilter.class)
@ConditionalOnProperty(value = "server.forward-headers-strategy", havingValue = "framework")
public FilterRegistrationBean<ForwardedHeaderFilter> forwardedHeaderFilter() {
ForwardedHeaderFilter filter = new ForwardedHeaderFilter();
FilterRegistrationBean<ForwardedHeaderFilter> registration = new FilterRegistrationBean<>(filter);
registration.setDispatcherTypes(DispatcherType.REQUEST, DispatcherType.ASYNC, DispatcherType.ERROR);
registration.setOrder(Ordered.HIGHEST_PRECEDENCE);
return registration;
}

/**
* 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);
}
}

}

}

从这个类上可以看到，当前配置类主要导入了BeanPostProcessorRegister,该类实现了ImportBeanDefinitionRegister接口，可以用来注册额外的BeanDefinition，同时，该类还导入了EmbeddedTomcat，EmbeddedJetty，EmbeddedUndertow三个类，可以根据用户的需求去选择使用哪一个web服务器，默认情况下使用的是tomcat

2、当自动装配功能完成之后会接着执行onRefresh的方法（ServletWebServerApplicationContext）

@Override
protected void onRefresh() {
//创建主题对象，不用在意
super.onRefresh();
try {
//开始创建web服务
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}

3、创建web服务，默认获取的是tomcat的web容器（ServletWebServerApplicationContext）

private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
//获取servletWebServerFactory,从上下文注册bean中可以找到
ServletWebServerFactory factory = getWebServerFactory();
//获取servletContextInitializer,获取webServer
this.webServer = factory.getWebServer(getSelfInitializer());
}
else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context", ex);
}
}
//替换servlet相关的属性资源
initPropertySources();
}

如何获取tomcat的bean的实例对象呢？从如下代码中可以看出

ServletWebServerApplicationContext

protected ServletWebServerFactory getWebServerFactory() {
// Use bean names so that we don't consider the hierarchy
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);
}

protected ServletWebServerFactory getWebServerFactory() {
// Use bean names so that we don't consider the hierarchy
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);
}

DefaultListableBeanFactoryf

/*
第一个参数type表示要查找的类型
第二个参数表示是否考虑非单例bean
第三个参数表示是否允许提早初始化
*/
@Override
public String[] getBeanNamesForType(@Nullable Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
//配置还未被冻结或者类型为null或者不允许早期初始化
if (!isConfigurationFrozen() || type == null || !allowEagerInit) {
return doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, allowEagerInit);
}
//此处注意isConfigurationFrozen为false的时候表示beanDefinition可能还会发生更改和添加，所以不能进行缓存，如果允许非单例bean，那么从保存所有bean的集合中获取，否则从单例bean中获取
Map<Class<?>, String[]> cache =
(includeNonSingletons ? this.allBeanNamesByType : this.singletonBeanNamesByType);
String[] resolvedBeanNames = cache.get(type);
if (resolvedBeanNames != null) {
return resolvedBeanNames;
}
//如果缓存中没有获取到，那么只能重新获取，获取到之后就存入缓存
resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);
if (ClassUtils.isCacheSafe(type, getBeanClassLoader())) {
cache.put(type, resolvedBeanNames);
}
return resolvedBeanNames;
}

private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) {
List<String> result = new ArrayList<>();

// Check all bean definitions.
for (String beanName : this.beanDefinitionNames) {
// Only consider bean as eligible if the bean name
// is not defined as alias for some other bean.
//如果时别名则跳过（当前集合会保存所有的主beanname，并且不会保存别名，别名由beanfactory中别名map维护）
if (!isAlias(beanName)) {
try {
//获取合并的beandefinition，合并的beandefinition是指spring整合了父beandefinition的属性，将其beandefinition编程了rootBeanDefinition
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// Only check bean definition if it is complete.
//抽象的beandefinition是不做考虑，抽象的就是拿来继承的，如果允许早期初始化，那么直接短路，进入方法体，如果不允许早期初始化，那么需要进一步判断，如果是不允许早期初始化的，并且beanClass已经被加载或者它是可以早期初始化的，那么如果当前bean是工厂bean，并且指定的bean又是工厂那么这个bean就必须被早期初始化，也就是说就不符合我们制定的allowEagerInit为false的情况，直接跳过

if (!mbd.isAbstract() && (allowEagerInit ||
(mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading()) &&
!requiresEagerInitForType(mbd.getFactoryBeanName()))) {
//如果当前bean是工厂bean
boolean isFactoryBean = isFactoryBean(beanName, mbd);
//如果允许早期初始化，那么基本上会调用最后的isTypeMatch方法，这个方法会导致工厂的实例化，但是当前不允许进行早期实例化在不允许早期实例化的情况下，如果当前bean是工厂bean，那么它只能在已经被创建的情况下调用isTypeMatch进行匹配判断否则只能宣告匹配失败，返回false
BeanDefinitionHolder dbd = mbd.getDecoratedDefinition();
boolean matchFound = false;
boolean allowFactoryBeanInit = allowEagerInit || containsSingleton(beanName);
boolean isNonLazyDecorated = dbd != null && !mbd.isLazyInit();
if (!isFactoryBean) {
if (includeNonSingletons || isSingleton(beanName, mbd, dbd)) {
matchFound = isTypeMatch(beanName, type, allowFactoryBeanInit);
}
}
else  {
//如果没有匹配到并且他是个工厂bean，那么加上&前缀，表示要获取factorybean类型的bean
if (includeNonSingletons || isNonLazyDecorated ||
(allowFactoryBeanInit && isSingleton(beanName, mbd, dbd))) {
matchFound = isTypeMatch(beanName, type, allowFactoryBeanInit);
}
if (!matchFound) {
// In case of FactoryBean, try to match FactoryBean instance itself next.
beanName = FACTORY_BEAN_PREFIX + beanName;
matchFound = isTypeMatch(beanName, type, allowFactoryBeanInit);
}
}
//找到便记录到result集合中，等待返回
if (matchFound) {
result.add(beanName);
}
}
}
catch (CannotLoadBeanClassException | BeanDefinitionStoreException ex) {
if (allowEagerInit) {
throw ex;
}
// Probably a placeholder: let's ignore it for type matching purposes.
LogMessage message = (ex instanceof CannotLoadBeanClassException) ?
LogMessage.format("Ignoring bean class loading failure for bean '%s'", beanName) :
LogMessage.format("Ignoring unresolvable metadata in bean definition '%s'", beanName);
logger.trace(message, ex);
onSuppressedException(ex);
}
}
}
// Check manually registered singletons too.
//从单例注册集合中获取，这个单例集合石保存spring内部注入的单例对象，他们的特点就是没有beanDefinition
for (String beanName : this.manualSingletonNames) {
try {
// In case of FactoryBean, match object created by FactoryBean.
//如果是工厂bean,那么调用其getObjectType去匹配是否符合指定类型
if (isFactoryBean(beanName)) {
if ((includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type)) {
result.add(beanName);
// Match found for this bean: do not match FactoryBean itself anymore.
continue;
}
// In case of FactoryBean, try to match FactoryBean itself next.
beanName = FACTORY_BEAN_PREFIX + beanName;
}
// Match raw bean instance (might be raw FactoryBean).
//如果没有匹配成功，那么匹配工厂类
if (isTypeMatch(beanName, type)) {
result.add(beanName);
}
}
catch (NoSuchBeanDefinitionException ex) {
// Shouldn't happen - probably a result of circular reference resolution...
logger.trace(LogMessage.format("Failed to check manually registered singleton with name '%s'", beanName), ex);
}
}

return StringUtils.toStringArray(result);
}

4、tomcat对象的初始化（ServletWebServerApplicationContext）

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
return this::selfInitialize;
}

private void selfInitialize(ServletContext servletContext) throws ServletException {
//使用给定的完全加载的servletContext准备WebApplicationContext
prepareWebApplicationContext(servletContext);
registerApplicationScope(servletContext);
//使用给定的BeanFactory注册特定于web的作用域bean（contextParameters,contextAttributes）
WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
beans.onStartup(servletContext);
}
}

5、完成内嵌tomcat的api调用（TomcatServletWebServerFactory）

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
Registry.disableRegistry();
}
//完成tomcat的api调用
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
Connector connector = new Connector(this.protocol);
connector.setThrowOnFailure(true);
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);
}
//准备tomcatEmbeddedContext并设置到tomcat中
prepareContext(tomcat.getHost(), initializers);
//构建tomcatWebServer
return getTomcatWebServer(tomcat);
}

6、获取tomcat服务（TomcatServletWebServerFactory）

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;
//初始化
initialize();
}

7、完成tomcat的初始化

private void initialize() throws WebServerException {
logger.info("Tomcat initialized with port(s): " + getPortsDescription(false));
synchronized (this.monitor) {
try {
//engineName拼接instanceId
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.
//删除Connectors，以便再启动服务时不发生协议绑定
removeServiceConnectors();
}
});

// Start the server to trigger initialization listeners
//启动服务触发初始化监听器
this.tomcat.start();

// We can re-throw failure exception directly in the main thread
//在主线程中重新抛出失败异常
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
//所有的tomcat线程都是守护线程，我们创建一个阻塞非守护线程来避免立即关闭
startDaemonAwaitThread();
}
catch (Exception ex) {
//异常停止tomcat
stopSilently();
destroySilently();
throw new WebServerException("Unable to start embedded Tomcat", ex);
}
}
}
-----------------------
private void removeServiceConnectors() {
for (Service service : this.tomcat.getServer().findServices()) {
Connector[] connectors = service.findConnectors().clone();
//将将要移除的conntector放到缓存中暂存
this.serviceConnectors.put(service, connectors);
for (Connector connector : connectors) {
//移除connector
service.removeConnector(connector);
}
}
}

8、除了refresh方法之外，在finishRefresh()方法中也对tomcat做了相关的处理（ServletWebServerApplicationContext）

protected void finishRefresh() {
//调用父类的finishRefresh方法
super.finishRefresh();
//启动webServer
WebServer webServer = startWebServer();
if (webServer != null) {
//发布webServer初始化完成事件
publishEvent(new ServletWebServerInitializedEvent(webServer, this));
}
}

ServletWebServerApplicationContext

private WebServer startWebServer() {
WebServer webServer = this.webServer;
if (webServer != null) {
//启动webserver
webServer.start();
}
return webServer;
}

TomcatWebServer

public void start() throws WebServerException {
synchronized (this.monitor) {
if (this.started) {
return;
}
try {
//添加之前移除的connector
addPreviouslyRemovedConnectors();
Connector connector = this.tomcat.getConnector();
if (connector != null && this.autoStart) {
//延迟加载启动
performDeferredLoadOnStartup();
}
//检查connector启动状态是否为失败，失败抛出异常
checkThatConnectorsHaveStarted();
this.started = true;
logger.info("Tomcat started on port(s): " + getPortsDescription(true) + " with context path '"
+ getContextPath() + "'");
}
catch (ConnectorStartFailedException ex) {
//异常停止tomcat
stopSilently();
throw ex;
}
catch (Exception ex) {
if (findBindException(ex) != null) {
throw new PortInUseException(this.tomcat.getConnector().getPort());
}
throw new WebServerException("Unable to start embedded Tomcat server", ex);
}
finally {
Context context = findContext();
//context解绑classload
ContextBindings.unbindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
}
}
}

private void addPreviouslyRemovedConnectors() {
Service[] services = this.tomcat.getServer().findServices();
for (Service service : services) {
//从上面移除connector添加的缓存中取出connector
Connector[] connectors = this.serviceConnectors.get(service);
if (connectors != null) {
for (Connector connector : connectors) {
//添加到tomcat service中
service.addConnector(connector);
if (!this.autoStart) {
//如果不是自动启动，则暂停connector
stopProtocolHandler(connector);
}
}
//添加完成后移除
this.serviceConnectors.remove(service);
}
}
}

private void performDeferredLoadOnStartup() {
try {
for (Container child : this.tomcat.getHost().findChildren()) {
if (child instanceof TomcatEmbeddedContext) {
//延迟加载启动
((TomcatEmbeddedContext) child).deferredLoadOnStartup();
}
}
}
catch (Exception ex) {
if (ex instanceof WebServerException) {
throw (WebServerException) ex;
}
throw new WebServerException("Unable to start embedded Tomcat connectors", ex);
}
}

void deferredLoadOnStartup() throws LifecycleException {
doWithThreadContextClassLoader(getLoader().getClassLoader(),
() -> getLoadOnStartupWrappers(findChildren()).forEach(this::load));
}

9、应用上下文关闭时会调用tomcat的关闭

在refreshContext中注册一个关闭的钩子函数，而钩子函数可以完成关闭的功能

ServletWebServerApplicationContext

@Override
protected void onClose() {
super.onClose();
stopAndReleaseWebServer();
}

private void stopAndReleaseWebServer() {
WebServer webServer = this.webServer;
if (webServer != null) {
try {
webServer.stop();
this.webServer = null;
}
catch (Exception ex) {
throw new IllegalStateException(ex);
}
}
}

TomcatWebServer

@Override
public void stop() throws WebServerException {
synchronized (this.monitor) {
boolean wasStarted = this.started;
try {
this.started = false;
try {
stopTomcat();
this.tomcat.destroy();
}
catch (LifecycleException ex) {
// swallow and continue
}
}
catch (Exception ex) {
throw new WebServerException("Unable to stop embedded Tomcat", ex);
}
finally {
if (wasStarted) {
containerCounter.decrementAndGet();
}
}
}
}