springboot 基于Tomcat的自启动流程

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ImportBeanDefinitionRegistrar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。

@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);
    //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#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) {
        //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;
}

ImportBeanDefinitionRegistrar