SpringBoot内置tomcat启动原理 - 好文

前言

不得不说SpringBoot的开发者是在为大众程序猿谋福利，把大家都惯成了懒汉，xml不配置了，连tomcat也懒的配置了，典型的一键启动系统，那么tomcat在springboot是怎么启动的呢？

内置tomcat

开发阶段对我们来说使用内置的tomcat是非常够用了，当然也可以使用jetty。
<dependency> <groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
<version>2.1.6.RELEASE</version> </dependency> @SpringBootApplication public
class MySpringbootTomcatStarter{ public static void main(String[] args) { Long
time=System.currentTimeMillis();
SpringApplication.run(MySpringbootTomcatStarter.class);
System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"==="); } }

这里是main函数入口，两句代码最耀眼，分别是SpringBootApplication注解和SpringApplication.run()方法。

发布生产

发布的时候，目前大多数的做法还是排除内置的tomcat，打瓦包（war）然后部署在生产的tomcat中，好吧，那打包的时候应该怎么处理？
<dependency> <groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId> 
<exclusions> <exclusion> <groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId> </exclusion> </exclusions>
</dependency>  <dependency>
<groupId>javax.servlet</groupId> <artifactId>javax.servlet-api</artifactId>
<version>3.1.0</version> <scope>provided</scope> </dependency>
更新main函数，主要是继承SpringBootServletInitializer，并重写configure()方法。
@SpringBootApplication public class MySpringbootTomcatStarter extends
SpringBootServletInitializer { public static void main(String[] args) { Long
time=System.currentTimeMillis();
SpringApplication.run(MySpringbootTomcatStarter.class);
System.out.println("===应用启动耗时："+(System.currentTimeMillis()-time)+"==="); }
@Override protected SpringApplicationBuilder configure(SpringApplicationBuilder
builder) { return builder.sources(this.getClass()); } }
从main函数说起
public static ConfigurableApplicationContext run(Class<?> primarySource,
String... args) { return run(new Class[]{primarySource}, args); }
--这里run方法返回的是ConfigurableApplicationContext public static
ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
return (new SpringApplication(primarySources)).run(args); } public
ConfigurableApplicationContext run(String... args) {
ConfigurableApplicationContext context = null;
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
this.configureHeadlessProperty(); SpringApplicationRunListeners listeners =
this.getRunListeners(args); listeners.starting(); Collection
exceptionReporters; try { ApplicationArguments applicationArguments = new
DefaultApplicationArguments(args); ConfigurableEnvironment environment =
this.prepareEnvironment(listeners, applicationArguments);
this.configureIgnoreBeanInfo(environment); //打印banner，这里你可以自己涂鸦一下，换成自己项目的logo
Banner printedBanner = this.printBanner(environment); //创建应用上下文 context =
this.createApplicationContext(); exceptionReporters =
this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new
Class[]{ConfigurableApplicationContext.class}, context); //预处理上下文
this.prepareContext(context, environment, listeners, applicationArguments,
printedBanner); //刷新上下文 this.refreshContext(context); //再刷新上下文
this.afterRefresh(context, applicationArguments); listeners.started(context);
this.callRunners(context, applicationArguments); } catch (Throwable var10) { }
try { listeners.running(context); return context; } catch (Throwable var9) { } }
既然我们想知道tomcat在SpringBoot中是怎么启动的，那么run方法中，重点关注创建应用上下文（
createApplicationContext）和刷新上下文（refreshContext）。

创建上下文
//创建上下文 protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass; if (contextClass == null)
{ try { switch(this.webApplicationType) { case SERVLET:
//创建AnnotationConfigServletWebServerApplicationContext contextClass =
Class.forName("org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext");
break; case REACTIVE: contextClass =
Class.forName("org.springframework.boot.web.reactive.context.AnnotationConfigReactiveWebServerApplicationContext");
break; default: contextClass =
Class.forName("org.springframework.context.annotation.AnnotationConfigApplicationContext");
} } catch (ClassNotFoundException var3) { throw new
IllegalStateException("Unable create a default ApplicationContext, please
specify an ApplicationContextClass", var3); } } return
(ConfigurableApplicationContext)BeanUtils.instantiateClass(contextClass); }
这里会创建AnnotationConfigServletWebServerApplicationContext类。

而AnnotationConfigServletWebServerApplicationContext类继承了ServletWebServerApplicationContext，而这个类是最终集成了AbstractApplicationContext。

刷新上下文
//SpringApplication.java //刷新上下文 private void
refreshContext(ConfigurableApplicationContext context) { this.refresh(context);
if (this.registerShutdownHook) { try { context.registerShutdownHook(); } catch
(AccessControlException var3) { } } }
//这里直接调用最终父类AbstractApplicationContext.refresh()方法 protected void
refresh(ApplicationContext applicationContext) {
((AbstractApplicationContext)applicationContext).refresh(); }
//AbstractApplicationContext.java public void refresh() throws BeansException,
IllegalStateException { synchronized(this.startupShutdownMonitor) {
this.prepareRefresh(); ConfigurableListableBeanFactory beanFactory =
this.obtainFreshBeanFactory(); this.prepareBeanFactory(beanFactory); try {
this.postProcessBeanFactory(beanFactory);
this.invokeBeanFactoryPostProcessors(beanFactory);
this.registerBeanPostProcessors(beanFactory); this.initMessageSource();
this.initApplicationEventMulticaster();
//调用各个子类的onRefresh()方法，也就说这里要回到子类：ServletWebServerApplicationContext，调用该类的onRefresh()方法
this.onRefresh(); this.registerListeners();
this.finishBeanFactoryInitialization(beanFactory); this.finishRefresh(); }
catch (BeansException var9) { this.destroyBeans(); this.cancelRefresh(var9);
throw var9; } finally { this.resetCommonCaches(); } } }
//ServletWebServerApplicationContext.java
//在这个方法里看到了熟悉的面孔，this.createWebServer，神秘的面纱就要揭开了。 protected void onRefresh() {
super.onRefresh(); try { this.createWebServer(); } catch (Throwable var2) { } }
//ServletWebServerApplicationContext.java
//这里是创建webServer，但是还没有启动tomcat，这里是通过ServletWebServerFactory创建，那么接着看下ServletWebServerFactory
private void createWebServer() { WebServer webServer = this.webServer;
ServletContext servletContext = this.getServletContext(); if (webServer == null
&& servletContext == null) { ServletWebServerFactory factory =
this.getWebServerFactory(); this.webServer = factory.getWebServer(new
ServletContextInitializer[]{this.getSelfInitializer()}); } else if
(servletContext != null) { try {
this.getSelfInitializer().onStartup(servletContext); } catch (ServletException
var4) { } } this.initPropertySources(); } //接口 public interface
ServletWebServerFactory { WebServer getWebServer(ServletContextInitializer...
initializers); } //实现 AbstractServletWebServerFactory
JettyServletWebServerFactory TomcatServletWebServerFactory
UndertowServletWebServerFactory
这里ServletWebServerFactory接口有4个实现类

而其中我们常用的有两个：TomcatServletWebServerFactory和JettyServletWebServerFactory。
//TomcatServletWebServerFactory.java
//这里我们使用的tomcat，所以我们查看TomcatServletWebServerFactory。到这里总算是看到了tomcat的踪迹。
@Override public WebServer getWebServer(ServletContextInitializer...
initializers) { Tomcat tomcat = new Tomcat(); File baseDir =
(this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath()); //创建Connector对象 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); } //Tomcat.java
//返回Engine容器，看到这里，如果熟悉tomcat源码的话，对engine不会感到陌生。 public Engine getEngine() {
Service service = getServer().findServices()[0]; if (service.getContainer() !=
null) { return service.getContainer(); } Engine engine = new StandardEngine();
engine.setName( "Tomcat" ); engine.setDefaultHost(hostname);
engine.setRealm(createDefaultRealm()); service.setContainer(engine); return
engine; } //Engine是最高级别容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器

getWebServer这个方法创建了Tomcat对象，并且做了两件重要的事情：把Connector对象添加到tomcat中，configureEngine(tomcat.getEngine());
getWebServer方法返回的是TomcatWebServer。
//TomcatWebServer.java //这里调用构造函数实例化TomcatWebServer 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
{ //在控制台会看到这句日志 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())) { removeServiceConnectors();
} }); //===启动tomcat服务=== this.tomcat.start();
rethrowDeferredStartupExceptions(); try {
ContextBindings.bindClassLoader(context, context.getNamingToken(),
getClass().getClassLoader()); } catch (NamingException ex) { } //开启阻塞非守护进程
startDaemonAwaitThread(); } catch (Exception ex) { stopSilently();
destroySilently(); throw new WebServerException("Unable to start embedded
Tomcat", ex); } } } //Tomcat.java public void start() throws LifecycleException
{ getServer(); server.start(); } //这里server.start又会回到TomcatWebServer的 public
void stop() throws LifecycleException { getServer(); server.stop(); }
//TomcatWebServer.java //启动tomcat服务 @Override public void start() throws
WebServerException { synchronized (this.monitor) { if (this.started) { return;
} try { addPreviouslyRemovedConnectors(); Connector connector =
this.tomcat.getConnector(); if (connector != null && this.autoStart) {
performDeferredLoadOnStartup(); } checkThatConnectorsHaveStarted();
this.started = true; //在控制台打印这句日志，如果在yml设置了上下文，这里会打印 logger.info("Tomcat
started on port(s): " + getPortsDescription(true) + " with context path '" +
getContextPath() + "'"); } catch (ConnectorStartFailedException ex) {
stopSilently(); throw ex; } catch (Exception ex) { throw new
WebServerException("Unable to start embedded Tomcat server", ex); } finally {
Context context = findContext(); ContextBindings.unbindClassLoader(context,
context.getNamingToken(), getClass().getClassLoader()); } } } //关闭tomcat服务
@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) { }
} catch (Exception ex) { throw new WebServerException("Unable to stop embedded
Tomcat", ex); } finally { if (wasStarted) { containerCounter.decrementAndGet();
} } } }

附：tomcat顶层结构图

tomcat最顶层容器是Server，代表着整个服务器，一个Server包含多个Service。从上图可以看除Service主要包括多个Connector和一个Container。Connector用来处理连接相关的事情，并提供Socket到Request和Response相关转化。Container用于封装和管理Servlet，以及处理具体的Request请求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？
我们来看下图：

综上所述，一个tomcat只包含一个Server，一个Server可以包含多个Service，一个Service只有一个Container，但有多个Connector，这样一个服务可以处理多个连接。

多个Connector和一个Container就形成了一个Service，有了Service就可以对外提供服务了，但是Service要提供服务又必须提供一个宿主环境，那就非Server莫属了，所以整个tomcat的声明周期都由Server控制。

总结

SpringBoot的启动主要是通过实例化SpringApplication来启动的，启动过程主要做了以下几件事情：配置属性、获取监听器，发布应用开始启动事件初、始化输入参数、配置环境，输出banner、
创建上下文、预处理上下文、刷新上下文
、再刷新上下文、发布应用已经启动事件、发布应用启动完成事件。在SpringBoot中启动tomcat的工作在刷新上下这一步。而tomcat的启动主要是实例化两个组件：Connector、Container，一个tomcat实例就是一个Server，一个Server包含多个Service，也就是多个应用程序，每个Service包含多个Connector和一个Container，而一个Container下又包含多个子容器。

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