SpringBoot內(nèi)置tomcat啟動(dòng)原理

前言

? ? ? ?
?不得不說(shuō)SpringBoot的開(kāi)發(fā)者是在為大眾程序猿謀福利，把大家都慣成了懶漢，xml不配置了，連tomcat也懶的配置了，典型的一鍵啟動(dòng)系統(tǒng)，那么tomcat在springboot是怎么啟動(dòng)的呢？?

內(nèi)置tomcat

? ? ? ? ?開(kāi)發(fā)階段對(duì)我們來(lái)說(shuō)使用內(nèi)置的tomcat是非常夠用了，當(dāng)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("===應(yīng)用啟動(dòng)耗時(shí)："+(System.currentTimeMillis()-time)+"==="); } }
? ? ? ?
這里是main函數(shù)入口，兩句代碼最耀眼，分別是SpringBootApplication注解和SpringApplication.run()方法。

發(fā)布生產(chǎn)

? ? ?發(fā)布的時(shí)候，目前大多數(shù)的做法還是排除內(nèi)置的tomcat，打瓦包（war）然后部署在生產(chǎn)的tomcat中，好吧，那打包的時(shí)候應(yīng)該怎么處理？
<dependency> <groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId> <!-- 移除嵌入式tomcat插件 -->
<exclusions> <exclusion> <groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId> </exclusion> </exclusions>
</dependency> <!--添加servlet-api依賴---> <dependency>
<groupId>javax.servlet</groupId> <artifactId>javax.servlet-api</artifactId>
<version>3.1.0</version> <scope>provided</scope> </dependency>
? ? ? ?更新main函數(shù)，主要是繼承SpringBootServletInitializer，并重寫(xiě)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("===應(yīng)用啟動(dòng)耗時(shí)："+(System.currentTimeMillis()-time)+"==="); }
@Override protected SpringApplicationBuilder configure(SpringApplicationBuilder
builder) { return builder.sources(this.getClass()); } }
從main函數(shù)說(shuō)起
public static ConfigurableApplicationContext run(Class<?> primarySource,
String... args) { return run(new Class[]{primarySource}, args); }
--這里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，這里你可以自己涂鴉一下，換成自己項(xiàng)目的logo
Banner printedBanner = this.printBanner(environment); //創(chuàng)建應(yīng)用上下文 context =
this.createApplicationContext(); exceptionReporters =
this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new
Class[]{ConfigurableApplicationContext.class}, context); //預(yù)處理上下文
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) { } }
? ? ? ? 既然我們想知道tomcat在SpringBoot中是怎么啟動(dòng)的，那么run方法中，重點(diǎn)關(guān)注創(chuàng)建應(yīng)用上下文（
createApplicationContext）和刷新上下文（refreshContext）。??

創(chuàng)建上下文
//創(chuàng)建上下文 protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass; if (contextClass == null)
{ try { switch(this.webApplicationType) { case SERVLET:
//創(chuàng)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); }
這里會(huì)創(chuàng)建AnnotationConfigServletWebServerApplicationContext類。

而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個(gè)類是最終集成了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) { } } }
//這里直接調(diào)用最終父類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();
//調(diào)用各個(gè)子類的onRefresh()方法，也就說(shuō)這里要回到子類：ServletWebServerApplicationContext，調(diào)用該類的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
//在這個(gè)方法里看到了熟悉的面孔，this.createWebServer，神秘的面紗就要揭開(kāi)了。 protected void onRefresh() {
super.onRefresh(); try { this.createWebServer(); } catch (Throwable var2) { } }
//ServletWebServerApplicationContext.java
//這里是創(chuàng)建webServer，但是還沒(méi)有啟動(dòng)tomcat，這里是通過(guò)ServletWebServerFactory創(chuàng)建，那么接著看下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); } //實(shí)現(xiàn) AbstractServletWebServerFactory
JettyServletWebServerFactory TomcatServletWebServerFactory
UndertowServletWebServerFactory
?這里ServletWebServerFactory接口有4個(gè)實(shí)現(xiàn)類

而其中我們常用的有兩個(gè)：TomcatServletWebServerFactory和JettyServletWebServerFactory。
//TomcatServletWebServerFactory.java
//這里我們使用的tomcat，所以我們查看TomcatServletWebServerFactory。到這里總算是看到了tomcat的蹤跡。
@Override public WebServer getWebServer(ServletContextInitializer...
initializers) { Tomcat tomcat = new Tomcat(); File baseDir =
(this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath()); //創(chuàng)建Connector對(duì)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，看到這里，如果熟悉tomcat源碼的話，對(duì)engine不會(huì)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是最高級(jí)別容器，Host是Engine的子容器，Context是Host的子容器，Wrapper是Context的子容器
? ? ? ? ?
?getWebServer這個(gè)方法創(chuàng)建了Tomcat對(duì)象，并且做了兩件重要的事情：把Connector對(duì)象添加到tomcat中，configureEngine(tomcat.getEngine());
? ? ? ? ? ?getWebServer方法返回的是TomcatWebServer。
//TomcatWebServer.java //這里調(diào)用構(gòu)造函數(shù)實(shí)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
{ //在控制臺(tái)會(huì)看到這句日志 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();
} }); //===啟動(dòng)tomcat服務(wù)=== this.tomcat.start();
rethrowDeferredStartupExceptions(); try {
ContextBindings.bindClassLoader(context, context.getNamingToken(),
getClass().getClassLoader()); } catch (NamingException ex) { } //開(kāi)啟阻塞非守護(hù)進(jìn)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(); } //這里server.start又會(huì)回到TomcatWebServer的 public
void stop() throws LifecycleException { getServer(); server.stop(); }
//TomcatWebServer.java //啟動(dòng)tomcat服務(wù) @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; //在控制臺(tái)打印這句日志，如果在yml設(shè)置了上下文，這里會(huì)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()); } } } //關(guān)閉tomcat服務(wù)
@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頂層結(jié)構(gòu)圖


? ? ?
tomcat最頂層容器是Server，代表著整個(gè)服務(wù)器，一個(gè)Server包含多個(gè)Service。從上圖可以看除Service主要包括多個(gè)Connector和一個(gè)Container。Connector用來(lái)處理連接相關(guān)的事情，并提供Socket到Request和Response相關(guān)轉(zhuǎn)化。Container用于封裝和管理Servlet，以及處理具體的Request請(qǐng)求。那么上文提到的Engine>Host>Context>Wrapper容器又是怎么回事呢？
我們來(lái)看下圖：

? ? ?
綜上所述，一個(gè)tomcat只包含一個(gè)Server，一個(gè)Server可以包含多個(gè)Service，一個(gè)Service只有一個(gè)Container，但有多個(gè)Connector，這樣一個(gè)服務(wù)可以處理多個(gè)連接。
? ? ?
多個(gè)Connector和一個(gè)Container就形成了一個(gè)Service，有了Service就可以對(duì)外提供服務(wù)了，但是Service要提供服務(wù)又必須提供一個(gè)宿主環(huán)境，那就非Server莫屬了，所以整個(gè)tomcat的聲明周期都由Server控制。

總結(jié)

?
?SpringBoot的啟動(dòng)主要是通過(guò)實(shí)例化SpringApplication來(lái)啟動(dòng)的，啟動(dòng)過(guò)程主要做了以下幾件事情：配置屬性、獲取監(jiān)聽(tīng)器，發(fā)布應(yīng)用開(kāi)始啟動(dòng)事件初、始化輸入?yún)?shù)、配置環(huán)境，輸出banner、
創(chuàng)建上下文、預(yù)處理上下文、刷新上下文
、再刷新上下文、發(fā)布應(yīng)用已經(jīng)啟動(dòng)事件、發(fā)布應(yīng)用啟動(dòng)完成事件。在SpringBoot中啟動(dòng)tomcat的工作在刷新上下這一步。而tomcat的啟動(dòng)主要是實(shí)例化兩個(gè)組件：Connector、Container，一個(gè)tomcat實(shí)例就是一個(gè)Server，一個(gè)Server包含多個(gè)Service，也就是多個(gè)應(yīng)用程序，每個(gè)Service包含多個(gè)Connector和一個(gè)Container，而一個(gè)Container下又包含多個(gè)子容器。

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