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來源：http://suo.im/5KUDd2react
前言
不得不說SpringBoot的開發者是在爲大衆程序猿謀福利，把你們都慣成了懶漢，xml不配置了，連tomcat也懶的配置了，典型的一鍵啓動系統，那麼tomcat在springboot是怎麼啓動的呢？web
內置tomcat
開發階段對咱們來講使用內置的tomcat是很是夠用了，固然也可使用jetty。面試
<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("===應用啓動耗時："+(System.currentTimeMillis()-time)+"===");
}
}
這裏是main函數入口，兩句代碼最耀眼，分別是SpringBootApplication註解和SpringApplication.run()方法。spring
發佈生產
發佈的時候，目前大多數的作法仍是排除內置的tomcat，打瓦包（war）而後部署在生產的tomcat中，好吧，那打包的時候應該怎麼處理？後端
<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，主要是繼承SpringBootServletInitializer，並重寫configure()方法。api
@SpringBootApplication
public class MySpringbootTomcatStarter extends SpringBootServletInitializer {
public static void main(String[] args) {
Long time=System.currentTimeMillis();
SpringApplication.run(MySpringbootTomcatStarter.class);
System.out.println("===應用啓動耗時："+(System.currentTimeMillis()-time)+"===");
}
@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
return builder.sources(this.getClass());
}
}
從main函數提及
public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
return run(new Class[]{primarySource}, args);
}
--這裏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，這裏你能夠本身塗鴉一下，換成本身項目的logo
Banner printedBanner = this.printBanner(environment);
//建立應用上下文
context = this.createApplicationContext();
exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
//預處理上下文
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) {
}
}
既然咱們想知道tomcat在SpringBoot中是怎麼啓動的，那麼run方法中，重點關注建立應用上下文（createApplicationContext）和刷新上下文（refreshContext）。tomcat
建立上下文
//建立上下文
protected ConfigurableApplicationContext createApplicationContext() {
Class<?> contextClass = this.applicationContextClass;
if (contextClass == null) {
try {
switch(this.webApplicationType) {
case SERVLET:
//建立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);
}
這裏會建立AnnotationConfigServletWebServerApplicationContext類。而AnnotationConfigServletWebServerApplicationContext類繼承了ServletWebServerApplicationContext，而這個類是最終集成了AbstractApplicationContext。springboot
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) {
}
}
}
//這裏直接調用最終父類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();
//調用各個子類的onRefresh()方法，也就說這裏要回到子類：ServletWebServerApplicationContext，調用該類的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
//在這個方法裏看到了熟悉的面孔，this.createWebServer，神祕的面紗就要揭開了。
protected void onRefresh() {
super.onRefresh();
try {
this.createWebServer();
} catch (Throwable var2) {
}
}
//ServletWebServerApplicationContext.java
//這裏是建立webServer，可是尚未啓動tomcat，這裏是經過ServletWebServerFactory建立，那麼接着看下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);
}
//實現
AbstractServletWebServerFactory
JettyServletWebServerFactory
TomcatServletWebServerFactory
UndertowServletWebServerFactory
這裏ServletWebServerFactory接口有4個實現類
而其中咱們經常使用的有兩個：TomcatServletWebServerFactory和JettyServletWebServerFactory。
//TomcatServletWebServerFactory.java
//這裏咱們使用的tomcat，因此咱們查看TomcatServletWebServerFactory。到這裏總算是看到了tomcat的蹤影。
@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
Tomcat tomcat = new Tomcat();
File baseDir = (this.baseDirectory != null) ? this.baseDirectory : createTempDir("tomcat");
tomcat.setBaseDir(baseDir.getAbsolutePath());
//建立Connector對象
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，看到這裏，若是熟悉tomcat源碼的話，對engine不會感到陌生。
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這個方法建立了Tomcat對象，而且作了兩件重要的事情：把Connector對象添加到tomcat中，configureEngine(tomcat.getEngine());
getWebServer方法返回的是TomcatWebServer。
//TomcatWebServer.java
//這裏調用構造函數實例化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 {
//在控制檯會看到這句日誌
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();
}
});
//===啓動tomcat服務===
this.tomcat.start();
rethrowDeferredStartupExceptions();
try {
ContextBindings.bindClassLoader(context, context.getNamingToken(), getClass().getClassLoader());
}
catch (NamingException ex) {
}
//開啓阻塞非守護進程
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();
}
//這裏server.start又會回到TomcatWebServer的
public void stop() throws LifecycleException {
getServer();
server.stop();
}
//TomcatWebServer.java
//啓動tomcat服務
@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;
//在控制檯打印這句日誌，若是在yml設置了上下文，這裏會打印
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());
}
}
}
//關閉tomcat服務
@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頂層結構圖
tomcat最頂層容器是Server，表明着整個服務器，一個Server包含多個Service。從上圖能夠看除Service主要包括多個Connector和一個Container。Connector用來處理鏈接相關的事情，並提供Socket到Request和Response相關轉化。
Container用於封裝和管理Servlet，以及處理具體的Request請求。那麼上文提到的Engine>Host>Context>Wrapper容器又是怎麼回事呢？咱們來看下圖：
綜上所述，一個tomcat只包含一個Server，一個Server能夠包含多個Service，一個Service只有一個Container，但有多個Connector，這樣一個服務能夠處理多個鏈接。
多個Connector和一個Container就造成了一個Service，有了Service就能夠對外提供服務了，可是Service要提供服務又必須提供一個宿主環境，那就非Server莫屬了，因此整個tomcat的聲明週期都由Server控制。
總結
SpringBoot的啓動主要是經過實例化SpringApplication來啓動的，啓動過程主要作了如下幾件事情：配置屬性、獲取監聽器，發佈應用開始啓動事件初、始化輸入參數、配置環境，輸出banner、建立上下文、預處理上下文、刷新上下文、再刷新上下文、發佈應用已經啓動事件、發佈應用啓動完成事件。
在SpringBoot中啓動tomcat的工做在刷新上下這一步。而tomcat的啓動主要是實例化兩個組件：Connector、Container，一個tomcat實例就是一個Server，一個Server包含多個Service，也就是多個應用程序，每一個Service包含多個Connector和一個Container，而一個Container下又包含多個子容器。
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