花了一个礼拜的时间，大致走读了一下springboot内嵌tomcat的代码，为下一步自己实现一个web容器做知识储备。这里对走读的代码做一个大致的记录。
springboot内嵌tomcat
我们知道，springboot自动装配的相关功能封装在spring-boot-autoconfigure包中。这里通过走读spring-boot的2.4.0-M版本的代码来大致看一下tomcat的启动过程。
首先，写一个简单的demo，调用springboot的run方法，一步一步来看tomcat如何启动的。
@SpringBootApplication
public class TestApplication {
public static void main(String[] args) {
SpringApplication.run(TestApplication.class);
}
}
查看run方法里做了什么。
public ConfigurableApplicationContext run(String... args) {
StopWatch stopWatch = new StopWatch();
stopWatch.start();
ConfigurableApplicationContext context = null;
Collection exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
SpringApplicationRunListeners listeners = getRunListeners(args);
listeners.starting();
try {
//封装参数
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
//准备springboot运行相关的环境
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
configureIgnoreBeanInfo(environment);
Banner printedBanner = printBanner(environment);
//确定spring的上下文信息，这里是webservlet类型，通过反射得到AnnotationConfigServletWebServerApplicationContext
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
//这里真正调到spring里，进行bean的初始化
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
if (this.logStartupInfo) {
new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
}
listeners.started(context);
callRunners(context, applicationArguments);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, listeners);
throw new IllegalStateException(ex);
}
try {
listeners.running(context);
}
catch (Throwable ex) {
handleRunFailure(context, ex, exceptionReporters, null);
throw new IllegalStateException(ex);
}
return context;
}
这里，初始化的context是一个AnnotationConfigServletWebServerApplicationContext。我们看一下该类的继承关系。
AnnotationConfigServletWebServerApplicationContext.png
本篇日记重点看的就是refreshContext(context)，这个真正的进入spring bean的处理。也在这个方法里，进行了tomcat的初始化及启动。继续走读这个方法的代码。一步一步跟下去，就进入了AbstractApplicationContext的refresh方法中。方法的代码如下：
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
通过spring的注释，可以大致猜测，关于tomcat的初始化，应该在方法onRefresh中实现。当然，这里是通过debug源码，看调用栈信息能确切的知道这个方法就是tomcat初始化的入口。这个方法实现在子类里。上面在创建spring的context时，我们看到，这里创建的是一个ServletWebServerApplicationContext，onRefresh就在该类中实现。进入该方法，看到如下代码：
@Override
protected void onRefresh() {
super.onRefresh();
try {
createWebServer();
}
catch (Throwable ex) {
throw new ApplicationContextException("Unable to start web server", ex);
}
}
createWebServer就是创建webserver的代码。代码：
private void createWebServer() {
WebServer webServer = this.webServer;
ServletContext servletContext = getServletContext();
if (webServer == null && servletContext == null) {
ServletWebServerFactory factory = getWebServerFactory();
this.webServer = factory.getWebServer(getSelfInitializer());
getBeanFactory().registerSingleton("webServerGracefulShutdown",
new WebServerGracefulShutdownLifecycle(this.webServer));
getBeanFactory().registerSingleton("webServerStartStop",
new WebServerStartStopLifecycle(this, this.webServer));
}
else if (servletContext != null) {
try {
getSelfInitializer().onStartup(servletContext);
}
catch (ServletException ex) {
throw new ApplicationContextException("Cannot initialize servlet context", ex);
}
}
initPropertySources();
}
ServletWebServerFactory为自动装配进来，此时在BeanFactory中已经存在了。具体代码配置在spring-boot-autoconfigure包的META-INF/spring.factories中。
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
......
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
ServletWebServerFactoryAutoConfiguration中import了一些类，其中impot了EmbeddedTomcat。
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
static class EmbeddedTomcat {
@Bean
TomcatServletWebServerFactory tomcatServletWebServerFactory(
ObjectProvider connectorCustomizers,
ObjectProvider contextCustomizers,
ObjectProvider> protocolHandlerCustomizers) {
TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
factory.getTomcatConnectorCustomizers()
.addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatContextCustomizers()
.addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatProtocolHandlerCustomizers()
.addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
在这里生成了一个TomcatServletWebServerFactory的bean。
再次回到createWebServer中，this.webServer = factory.getWebServer(getSelfInitializer());从factory中生产出一个webserver。我们看这个webServer是如何生产出来的。
public WebServer getWebServer(ServletContextInitializer... initializers) {
if (this.disableMBeanRegistry) {
Registry.disableRegistry();
}
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);
}
prepareContext(tomcat.getHost(), initializers);
return getTomcatWebServer(tomcat);
}
上面这段代码，就正式进入到了tomcat的领域了。为了能更好的阅读代码，我们需要了解tomcat的体系结构。这里通过代码的阅读和分析，总结了一张结构图。
tomcat结构.png
tomcat的初始化，无非就是将图中的各个部分初始化，塞到对应的位置。有了这张图，代码读起来就比较轻松了，代码也相对简单，可以自己按照图中的结构，进行走读。这里不再记录。
getWebServer代码走完，tomcat就初始化完成了，那么是不是就可以接收请求了呢。当然不行。socket还没起来，tomcat的工作线程还没起来。这部分功能不在上面的onRefresh中，而在接下来的finishRefresh中，只有所有的准备工作都已经完成了，请求才能接进来。这部分功能将在下一篇日记中走读。
至此，springboot的内嵌tomcat初始化完成。这里只记录了主流程的初始化，其中一些配置类信息如
ServerProperties等的初始化并没有在这篇日记中体现。