Springboot 内置了Tomcat的容器，我们今天来说一下Springboot的自启动流程。

一、Spring通过注解导入Bean大体可分为四种方式，我们主要来说以下Import的两种实现方法：

1、通过实现ImportSerlector接口，实现Bean加载：

public class TestServiceImpl { public void testImpl() { System.out.println("我是通过importSelector导入进来的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、 通过实现ImportSerlector接口，实现Bean加载：

public class TestService { public TestService() { System.out.println("我是通过ImportBeanDefinitionRegistrar导入进来的组件"); }}public class TestImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar { @Override public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) { //定义一个BeanDefinition RootBeanDefinition beanDefinition = new RootBeanDefinition(TestService.class); //把自定义的bean定义导入到容器中 registry.registerBeanDefinition("testService",beanDefinition); }}@Configuration@Import(TestImportBeanDefinitionRegistrar.class)public class TestConfig {}

二、 Springboot启动过程中会自动装配

我们从spring-boot-autoconfigure-2.0.6.RELEASE.jar下搜索到Tomcat的相关配置，发现有两个自动装配类，分别包含了三个定制器（面向对象的单一职责原则），还有一个工厂类。

2.1、TomcatWebServerFactoryCustomizer：定制Servlet和Reactive服务器通用的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 将ServerProperties属性应用于Tomcat web服务器。

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 将ServerProperties属性应用于Tomcat web服务器。

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，相当于有了Spring加载的入口

通过AbstractApplicationContext#onReFresh()在IOC 容器中的带动tomcat启动，然后在接着执行 ioc容器的其他步骤。

我们通过断点可以观察Tomcat加载的整个生命周期，以及三个定制器的加载过程。

@Overridepublic 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); //设置是否自动启动 tomcat.getHost().setAutoDeploy(false); //创建Tomcat引擎 configureEngine(tomcat.getEngine()); for (Connector additionalConnector : this.additionalTomcatConnectors) { tomcat.getService().addConnector(additionalConnector); } //刷新上下文 prepareContext(tomcat.getHost(), initializers); //准备启动 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); } }}

备注： 在这个过程中我们需要了解Bean的生命周期，Tomcat的三个定制器均在BeanPostProcessorsRegistrar（Bean后置处理器）过程中加载；

　　构造方法-->Bean后置处理器Before-->InitializingBean-->init-method-->Bean后置处理器After

org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean　　org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#initializeBeanprotected 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) { //构造方法 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后置处理器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后置处理器After wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName); } return wrappedBean;}

总结

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