SpringBoot的兩種啟動(dòng)方式原理，你學(xué)會(huì)了嗎？

作者：Seven97 2025-01-16 08:38:34

關(guān)于Tomcat的屬性都在?org.springframework.boot.autoconfigure.web.ServerProperties?配置類中做了定義，我們只需在application.properties配置屬性做配置即可。通用的Servlet容器配置都以?server?作為前綴。

使用內(nèi)置tomcat啟動(dòng)

配置案例

啟動(dòng)方式

IDEA中main函數(shù)啟動(dòng)
mvn springboot-run
java -jar XXX.jar 使用這種方式時(shí)，為保證服務(wù)在后臺(tái)運(yùn)行，會(huì)使用nohup

nohup java -jar -Xms128m -Xmx128m -Xss256k -XX:+PrintGCDetails -XX:+PrintHeapAtGC -Xloggc:/data/log/web-gc.log web.jar >/data/log/web.log &

使用java -jar默認(rèn)情況下，不會(huì)啟動(dòng)任何嵌入式Application Server，該命令只是啟動(dòng)一個(gè)執(zhí)行jar main的JVM進(jìn)程，當(dāng)spring-boot-starter-web包含嵌入式tomcat服務(wù)器依賴項(xiàng)時(shí)，執(zhí)行java -jar則會(huì)啟動(dòng)Application Server

配置內(nèi)置tomcat屬性

關(guān)于Tomcat的屬性都在 org.springframework.boot.autoconfigure.web.ServerProperties 配置類中做了定義，我們只需在application.properties配置屬性做配置即可。通用的Servlet容器配置都以 server 作為前綴

#配置程序端口，默認(rèn)為8080
server.port= 8080
#用戶會(huì)話session過(guò)期時(shí)間，以秒為單位
server.session.timeout=
#配置默認(rèn)訪問(wèn)路徑，默認(rèn)為/
server.context-path=

而Tomcat特有配置都以 server.tomcat 作為前綴

# 配置Tomcat編碼,默認(rèn)為UTF-8
server.tomcat.uri-encoding=UTF-8
# 配置最大線程數(shù)
server.tomcat.max-threads=1000

注意：使用內(nèi)置tomcat不需要有tomcat-embed-jasper和spring-boot-starter-tomcat依賴，因?yàn)樵趕pring-boot-starter-web依賴中已經(jīng)集成了tomcat

原理

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) {
   thrownew 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()});
 } elseif (servletContext != null) {
try {
   this.getSelfInitializer().onStartup(servletContext);
  } catch (ServletException var4) {

  }
 }

this.initPropertySources();
}

//接口
publicinterface 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，對(duì)應(yīng)著四種容器：

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) {
returnnew 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();
   thrownew 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) {
   thrownew 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) {
   thrownew WebServerException("Unable to stop embedded Tomcat", ex);
  }
finally {
   if (wasStarted) {
    containerCounter.decrementAndGet();
   }
  }
 }
}

使用外置tomcat部署

配置案例

外置Tomcat啟動(dòng)SpringBoot源碼點(diǎn)擊這里

繼承SpringBootServletInitializer

外部容器部署的話，就不能依賴于Application的main函數(shù)了，而是要以類似于web.xml文件配置的方式來(lái)啟動(dòng)Spring應(yīng)用上下文，此時(shí)需要在啟動(dòng)類中繼承SpringBootServletInitializer，并重寫configure方法；還添加 @SpringBootApplication 注解，這是為了能掃描到所有Spring注解的bean

方式一：?jiǎn)?dòng)類繼承SpringBootServletInitializer實(shí)現(xiàn)configure：

@SpringBootApplication
public class SpringBootHelloWorldTomcatApplication extends SpringBootServletInitializer {
 @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(Application.class);
    }
}

這個(gè)類的作用與在web.xml中配置負(fù)責(zé)初始化Spring應(yīng)用上下文的監(jiān)聽(tīng)器作用類似，只不過(guò)在這里不需要編寫額外的XML文件了。

方式二：新增加一個(gè)類繼承SpringBootServletInitializer實(shí)現(xiàn)configure：

public class ServletInitializer extends SpringBootServletInitializer {
    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        //此處的Application.class為帶有@SpringBootApplication注解的啟動(dòng)類
        return builder.sources(Application.class);
    }
}

pom.xml修改tomcat相關(guān)的配置

首先需要將 jar 變成war <packaging>war</packaging>

如果要將最終的打包形式改為war的話，還需要對(duì)pom.xml文件進(jìn)行修改，因?yàn)閟pring-boot-starter-web中包含內(nèi)嵌的tomcat容器，所以直接部署在外部容器會(huì)沖突報(bào)錯(cuò)。因此需要將內(nèi)置tomcat排除

<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>

在這里需要移除對(duì)嵌入式Tomcat的依賴，這樣打出的war包中，在lib目錄下才不會(huì)包含Tomcat相關(guān)的jar包，否則將會(huì)出現(xiàn)啟動(dòng)錯(cuò)誤。

但是移除了tomcat后，原始的sevlet也被移除了，因此還需要額外引入servet的包

<dependency>
      <groupId>javax.servlet</groupId>
      <artifactId>javax.servlet-api</artifactId>
      <version>3.0.1</version>
</dependency>

注意的問(wèn)題

此時(shí)打成的包的名稱應(yīng)該和 application.properties 的 server.context-path=/test 保持一致

<build>
    <finalName>test</finalName>
</build>

如果不一樣發(fā)布到tomcat的webapps下上下文會(huì)變化

原理

tomcat不會(huì)主動(dòng)去啟動(dòng)springboot應(yīng)用，，所以tomcat啟動(dòng)的時(shí)候肯定調(diào)用了SpringBootServletInitializer的SpringApplicationBuilder ，就會(huì)啟動(dòng)springboot。

ServletContainerInitializer的實(shí)現(xiàn)放在jar包的META-INF/services文件夾下，有一個(gè)名為javax.servlet.ServletContainerInitializer的文件，內(nèi)容就是ServletContainerInitializer的實(shí)現(xiàn)類的全類名。當(dāng)servlet容器啟動(dòng)時(shí)候就會(huì)去該文件中找到ServletContainerInitializer的實(shí)現(xiàn)類，從而創(chuàng)建它的實(shí)例調(diào)用onstartUp。這里就是用了SPI機(jī)制

HandlesTypes(WebApplicationInitializer.class)

@HandlesTypes傳入的類為ServletContainerInitializer感興趣的
容器會(huì)自動(dòng)在classpath中找到 WebApplicationInitializer，會(huì)傳入到onStartup方法的webAppInitializerClasses中
Set<Class<?>> webAppInitializerClasses這里面也包括之前定義的TomcatStartSpringBoot

@HandlesTypes(WebApplicationInitializer.class)
public class SpringServletContainerInitializer implements ServletContainerInitializer {

@Override
public void onStartup(@Nullable Set<Class<?>> webAppInitializerClasses, ServletContext servletContext)
      throws ServletException {

   List<WebApplicationInitializer> initializers = new LinkedList<>();

   if (webAppInitializerClasses != null) {
      for (Class<?> waiClass : webAppInitializerClasses) {
        // 如果不是接口 不是抽象 跟WebApplicationInitializer有關(guān)系  就會(huì)實(shí)例化
         if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
               WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
            try {
               initializers.add((WebApplicationInitializer)
                     ReflectionUtils.accessibleConstructor(waiClass).newInstance());
            }
            catch (Throwable ex) {
               thrownew ServletException("Failed to instantiate WebApplicationInitializer class", ex);
            }
         }
      }
   }

   if (initializers.isEmpty()) {
      servletContext.log("No Spring WebApplicationInitializer types detected on classpath");
      return;
   }

   servletContext.log(initializers.size() + " Spring WebApplicationInitializers detected on classpath");
   // 排序
   AnnotationAwareOrderComparator.sort(initializers);
   for (WebApplicationInitializer initializer : initializers) {
      initializer.onStartup(servletContext);
   }
}

@Override
public void onStartup(ServletContext servletContext) throws ServletException {
   // Logger initialization is deferred in case an ordered
   // LogServletContextInitializer is being used
   this.logger = LogFactory.getLog(getClass());
   WebApplicationContext rootApplicationContext = createRootApplicationContext(servletContext);
   if (rootApplicationContext != null) {
      servletContext.addListener(new SpringBootContextLoaderListener(rootApplicationContext, servletContext));
   }
   else {
      this.logger.debug("No ContextLoaderListener registered, as createRootApplicationContext() did not "
            + "return an application context");
   }
}

SpringBootServletInitializer

protected WebApplicationContext createRootApplicationContext(ServletContext servletContext) {
   SpringApplicationBuilder builder = createSpringApplicationBuilder();
   builder.main(getClass());
   ApplicationContext parent = getExistingRootWebApplicationContext(servletContext);
   if (parent != null) {
      this.logger.info("Root context already created (using as parent).");
      servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, null);
      builder.initializers(new ParentContextApplicationContextInitializer(parent));
   }
   builder.initializers(new ServletContextApplicationContextInitializer(servletContext));
   builder.contextClass(AnnotationConfigServletWebServerApplicationContext.class);
   // 調(diào)用configure
   builder = configure(builder); //①
   builder.listeners(new WebEnvironmentPropertySourceInitializer(servletContext));
   SpringApplication application = builder.build();//②
   if (application.getAllSources().isEmpty()
         && MergedAnnotations.from(getClass(), SearchStrategy.TYPE_HIERARCHY).isPresent(Configuration.class)) {
      application.addPrimarySources(Collections.singleton(getClass()));
   }
   Assert.state(!application.getAllSources().isEmpty(),
         "No SpringApplication sources have been defined. Either override the "
               + "configure method or add an @Configuration annotation");
   // Ensure error pages are registered
   if (this.registerErrorPageFilter) {
      application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
   }
   application.setRegisterShutdownHook(false);
   return run(application);//③
}

① 當(dāng)調(diào)用configure就會(huì)來(lái)到TomcatStartSpringBoot .configure，將Springboot啟動(dòng)類傳入到builder.source

@Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
    return builder.sources(Application.class);
}

② 調(diào)用SpringApplication application = builder.build(); 就會(huì)根據(jù)傳入的Springboot啟動(dòng)類來(lái)構(gòu)建一個(gè)SpringApplication

public SpringApplication build(String... args) {
   configureAsChildIfNecessary(args);
   this.application.addPrimarySources(this.sources);
   return this.application;
}

③ 調(diào)用 return run(application); 就會(huì)啟動(dòng)springboot應(yīng)用

protected WebApplicationContext run(SpringApplication application) {
   return (WebApplicationContext) application.run();
}

也就相當(dāng)于Main函數(shù)啟動(dòng)：

public static void main(String[] args) {
    SpringApplication.run(Application.class, args);
}

之后的流程就與上面使用內(nèi)置Tomcat的Main函數(shù)一致了

責(zé)任編輯：武曉燕來(lái)源： Seven97