Springboot 內建了Tomcat的容器，我們今天來說一下Springboot的自啟動流程。

一、Spring通過註解匯入Bean大體可分為四種方式，我們主要來說以下Import的兩種實現方法：

1、通過實現ImportSerlector介面，實現Bean載入：

public class TestServiceImpl {
 public void testImpl() {
 System.out.println("我是通過importSelector匯入進來的service");
 }
}
public class TestService implements ImportSelector {
 @Override
 public String[] selectImports(AnnotationMetadata annotationMetadata) {
 return new String[]{"com.ycdhz.service.TestServiceImpl"};
 }
}
@Configuration
@Import(value = {TestService.class})
public class TestConfig {
}
public class TestController {
 @Autowired
 private TestServiceImpl testServiceImpl;
 
 @RequestMapping("testImpl")
 public String testTuling() {
 testServiceImpl.testImpl();
 return "Ok";
 }
}

2、 通過實現ImportSerlector介面，實現Bean載入：

public class TestService {
 public TestService() {
 System.out.println("我是通過ImportBeanDefinitionRegistrar匯入進來的元件");
 }
}
public class TestImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
 @Override
 public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata,BeanDefinitionRegistry registry) {
 //定義一個BeanDefinition
 RootBeanDefinition beanDefinition = new RootBeanDefinition(TestService.class);
 //把自定義的bean定義匯入到容器中
 registry.registerBeanDefinition("testService",beanDefinition);
 }
}
@Configuration
@Import(TestImportBeanDefinitionRegistrar.class)
public class TestConfig {
}

二、 Springboot啟動過程中會自動裝配

我們從spring-boot-autoconfigure-2.0.6.RELEASE.jar下搜尋到Tomcat的相關配置，發現有兩個自動裝配類，分別包含了三個定製器（面向物件的單一職責原則），還有一個工廠類。

2.1、TomcatWebServerFactoryCustomizer：定製Servlet和Reactive伺服器通用的Tomcat特定功能。

public class TomcatWebServerFactoryCustomizer implements
 WebServerFactoryCustomizer<ConfigurableTomcatWebServerFactory>,Ordered {
 @Override
 public void customize(ConfigurableTomcatWebServerFactory factory) {
 ServerProperties properties = this.serverProperties;
 ServerProperties.Tomcat tomcatProperties = properties.getTomcat();
 PropertyMapper propertyMapper = PropertyMapper.get();
 propertyMapper.from(tomcatProperties::getBasedir).whenNonNull()
 .to(factory::setBaseDirectory);
 propertyMapper.from(tomcatProperties::getBackgroundProcessorDelay).whenNonNull()
 .as(Duration::getSeconds).as(Long::intValue)
 .to(factory::setBackgroundProcessorDelay);
 customizeRemoteIpValve(factory);
 propertyMapper.from(tomcatProperties::getMaxThreads).when(this::isPositive)
 .to((maxThreads) -> customizeMaxThreads(factory,tomcatProperties.getMaxThreads()));
 propertyMapper.from(tomcatProperties::getMinSpareThreads).when(this::isPositive)
 .to((minSpareThreads) -> customizeMinThreads(factory,minSpareThreads));
 propertyMapper.from(() -> determineMaxHttpHeaderSize()).when(this::isPositive)
 .to((maxHttpHeaderSize) -> customizeMaxHttpHeaderSize(factory,maxHttpHeaderSize));
 propertyMapper.from(tomcatProperties::getMaxHttpPostSize)
 .when((maxHttpPostSize) -> maxHttpPostSize != 0)
 .to((maxHttpPostSize) -> customizeMaxHttpPostSize(factory,maxHttpPostSize));
 propertyMapper.from(tomcatProperties::getAccesslog)
 .when(ServerProperties.Tomcat.Accesslog::isEnabled)
 .to((enabled) -> customizeAccessLog(factory));
 propertyMapper.from(tomcatProperties::getUriEncoding).whenNonNull()
 .to(factory::setUriEncoding);
 propertyMapper.from(properties::getConnectionTimeout).whenNonNull()
 .to((connectionTimeout) -> customizeConnectionTimeout(factory,connectionTimeout));
 propertyMapper.from(tomcatProperties::getMaxConnections).when(this::isPositive)
 .to((maxConnections) -> customizeMaxConnections(factory,maxConnections));
 propertyMapper.from(tomcatProperties::getAcceptCount).when(this::isPositive)
 .to((acceptCount) -> customizeAcceptCount(factory,acceptCount));
 customizeStaticResources(factory);
 customizeErrorReportValve(properties.getError(),factory);
 }
}

2.2、ServletWebServerFactoryCustomizer：WebServerFactoryCustomizer 將ServerProperties屬性應用於Tomcat web伺服器。

public class ServletWebServerFactoryCustomizer implements
 WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>,Ordered {
 private final ServerProperties serverProperties;
 public ServletWebServerFactoryCustomizer(ServerProperties serverProperties) {
 this.serverProperties = serverProperties;
 }
 @Override
 public int getOrder() {
 return 0;
 }
 @Override
 public void customize(ConfigurableServletWebServerFactory factory) {
 PropertyMapper map = PropertyMapper.get().alwaysApplyingWhenNonNull();
 map.from(this.serverProperties::getPort).to(factory::setPort);
 map.from(this.serverProperties::getAddress).to(factory::setAddress);
 map.from(this.serverProperties.getServlet()::getContextPath)
 .to(factory::setContextPath);
 map.from(this.serverProperties.getServlet()::getApplicationDisplayName)
 .to(factory::setDisplayName);
 map.from(this.serverProperties.getServlet()::getSession).to(factory::setSession);
 map.from(this.serverProperties::getSsl).to(factory::setSsl);
 map.from(this.serverProperties.getServlet()::getJsp).to(factory::setJsp);
 map.from(this.serverProperties::getCompression).to(factory::setCompression);
 map.from(this.serverProperties::getHttp2).to(factory::setHttp2);
 map.from(this.serverProperties::getServerHeader).to(factory::setServerHeader);
 map.from(this.serverProperties.getServlet()::getContextParameters)
 .to(factory::setInitParameters);
 }
}

2.3、ServletWebServerFactoryCustomizer ：WebServerFactoryCustomizer 將ServerProperties屬性應用於Tomcat web伺服器。

public class TomcatServletWebServerFactoryCustomizer
 implements WebServerFactoryCustomizer<TomcatServletWebServerFactory>,Ordered {
 private final ServerProperties serverProperties;
 public TomcatServletWebServerFactoryCustomizer(ServerProperties serverProperties) {
 this.serverProperties = serverProperties;
 }
 @Override
 public void customize(TomcatServletWebServerFactory factory) {
 ServerProperties.Tomcat tomcatProperties = this.serverProperties.getTomcat();
 if (!ObjectUtils.isEmpty(tomcatProperties.getAdditionalTldSkipPatterns())) {
 factory.getTldSkipPatterns()
  .addAll(tomcatProperties.getAdditionalTldSkipPatterns());
 }
 if (tomcatProperties.getRedirectContextRoot() != null) {
 customizeRedirectContextRoot(factory,tomcatProperties.getRedirectContextRoot());
 }
 if (tomcatProperties.getUseRelativeRedirects() != null) {
 customizeUseRelativeRedirects(factory,tomcatProperties.getUseRelativeRedirects());
 }
 }
}

三、有了TomcatServletWebServerFactory，相當於有了Spring載入的入口

通過AbstractApplicationContext#onReFresh()在IOC 容器中的帶動tomcat啟動，然後在接著執行 ioc容器的其他步驟。

我們通過斷點可以觀察Tomcat載入的整個生命週期，以及三個定製器的載入過程。

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
 Tomcat tomcat = new Tomcat();
 File baseDir = (this.baseDirectory != null) ? this.baseDirectory
 : createTempDir("tomcat");
 tomcat.setBaseDir(baseDir.getAbsolutePath());
 Connector connector = new Connector(this.protocol);
 tomcat.getService().addConnector(connector);
 customizeConnector(connector);
 tomcat.setConnector(connector);
 //設定是否自動啟動
 tomcat.getHost().setAutoDeploy(false);
 //建立Tomcat引擎
 configureEngine(tomcat.getEngine());
 for (Connector additionalConnector : this.additionalTomcatConnectors) {
 tomcat.getService().addConnector(additionalConnector);
 }
 //重新整理上下文
 prepareContext(tomcat.getHost(),initializers);
 //準備啟動
 return getTomcatWebServer(tomcat);
}

private void initialize() throws WebServerException {
 TomcatWebServer.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())) {
  // Remove service connectors so that protocol binding doesn't
  // happen when the service is started.
  removeServiceConnectors();
 }
 });
 // Start the server to trigger initialization listeners
 this.tomcat.start();
 // We can re-throw failure exception directly in the main thread
 rethrowDeferredStartupExceptions();
 try {
 ContextBindings.bindClassLoader(context,context.getNamingToken(),getClass().getClassLoader());
 }
 catch (NamingException ex) {
 // Naming is not enabled. Continue
 }
 // Unlike Jetty,all Tomcat threads are daemon threads. We create a
 // blocking non-daemon to stop immediate shutdown
 startDaemonAwaitThread();
 }
 catch (Exception ex) {
 stopSilently();
 throw new WebServerException("Unable to start embedded Tomcat",ex);
 }
 }
}

備註： 在這個過程中我們需要了解Bean的生命週期，Tomcat的三個定製器均在BeanPostProcessorsRegistrar（Bean後置處理器）過程中載入；

　　構造方法-->Bean後置處理器Before-->InitializingBean-->init-method-->Bean後置處理器After

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
　　org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBean

protected Object doCreateBean(final String beanName,final RootBeanDefinition mbd,final @Nullable Object[] args)
 throws BeanCreationException {
 // Instantiate the bean.
 BeanWrapper instanceWrapper = null;
 if (mbd.isSingleton()) {
 instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
 }
 if (instanceWrapper == null) {
 //構造方法
 instanceWrapper = createBeanInstance(beanName,mbd,args);
 }
 final Object bean = instanceWrapper.getWrappedInstance();
 Class<?> beanType = instanceWrapper.getWrappedClass();
 if (beanType != NullBean.class) {
 mbd.resolvedTargetType = beanType;
 }
 // Initialize the bean instance.
 ......
 return exposedObject;
}
protected Object initializeBean(final String beanName,final Object bean,@Nullable RootBeanDefinition mbd) {
 if (System.getSecurityManager() != null) {
 AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
 invokeAwareMethods(beanName,bean);
 return null;
 },getAccessControlContext());
 }
 else {
 invokeAwareMethods(beanName,bean);
 }
 Object wrappedBean = bean;
 if (mbd == null || !mbd.isSynthetic()) {
 //Bean後置處理器Before
 wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean,beanName);
 }
 try {
 invokeInitMethods(beanName,wrappedBean,mbd);
 }
 catch (Throwable ex) {
 throw new BeanCreationException(
 (mbd != null ? mbd.getResourceDescription() : null),beanName,"Invocation of init method failed",ex);
 }
 if (mbd == null || !mbd.isSynthetic()) {
 //Bean後置處理器After
 wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean,beanName);
 }
 return wrappedBean;
}

總結

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