Spring Boot与Spring MVC集成启动过程源码分析

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从一个最简单的Spring Boot Web项目聊起

我们知道，用spring-boot写一个web项目非常容易，pom继承spring-boot-parent然后引入依赖spring-boot-starter-web，再写一个这样的主启动类，然后就可以去写Controller了，十分简单，就像这样：

@SpringBootApplication
public class SampleApplication {
    public static void main(String[] args) {
        SpringApplication.run(SampleApplication.class, args);
    }
}
// 然后再写一个Controller声明一个Rest服务
@RestController
@RequestMapping("/perf")
public class PerfController {
    @RequestMapping("/trace")
    public Object trace() {
        Object result = yourLogic();
        return result;
    }
}

聊聊SpringApplication.run

可是我们思考过，这背后spring-boot到底做了什么使我们的工作如此简单，它如何将spring、spring-mvc、tomcat整合到一起的呢？接下来我们以项目启动角度来分析整个初始化过程。

PS：下面代码分析过程中，着重于流程的串接，调用到某个变量时，作者会直接给出这个变量的具体实现，读者也许会产生困惑，但是不要停下来，先想当然的按照作者的思路把流程捋完，后面会针对各个主要的变量初始化及选择实现的过程进行逐个解释。

从SpringApplication.run说起：
方法定义如下

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
    return new SpringApplication(primarySources).run(args);
}
public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
    configureHeadlessProperty();
    SpringApplicationRunListeners listeners = getRunListeners(args);
    listeners.starting();
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
        configureIgnoreBeanInfo(environment);
        Banner printedBanner = printBanner(environment);
        context = createApplicationContext();//1)
        exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
                new Class[] { ConfigurableApplicationContext.class }, context);
        prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        refreshContext(context);//2)
        afterRefresh(context, applicationArguments);
        stopWatch.stop();
        if (this.logStartupInfo) {
            new StartupInfoLogger(this.mainApplicationClass).logStarted(getApplicationLog(), stopWatch);
        }
        listeners.started(context);
        callRunners(context, applicationArguments);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, listeners);
        throw new IllegalStateException(ex);
    }

    try {
        listeners.running(context);
    }
    catch (Throwable ex) {
        handleRunFailure(context, ex, exceptionReporters, null);
        throw new IllegalStateException(ex);
    }
    return context;
}

我们来分解下这个run方法
先看1)context = createApplicationContext()
负责创建spring主容器，这个方法内部是根据具体项目运行时依赖的类来动态选择实现的，如果是web项目则会选择AnnotationConfigServletWebServerApplicationContext，至于选择的规则及原因，这里先忽略，后面会专门介绍（时空门：ServletWebServerApplicationContext）。
接下来我们重点看2)refreshContext(context)方法
其方法内部最终调用了((AbstractApplicationContext) applicationContext).refresh()方法，我们把这个方法展开

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        prepareRefresh();
        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
        prepareBeanFactory(beanFactory);
        try {
            postProcessBeanFactory(beanFactory);
            invokeBeanFactoryPostProcessors(beanFactory);
            registerBeanPostProcessors(beanFactory);
            initMessageSource();
            initApplicationEventMulticaster();
            onRefresh();//3)
            registerListeners();
            finishBeanFactoryInitialization(beanFactory);
            finishRefresh();
        }
        catch (BeansException ex) {
            if (logger.isWarnEnabled()) {
                logger.warn("Exception encountered during context initialization - " +
                        "cancelling refresh attempt: " + ex);
            }
            destroyBeans();
            cancelRefresh(ex);
            throw ex;
        }
        finally {
            resetCommonCaches();
        }
    }
}

实际上，这里我们的调用已经到了spring-context包，其实跟spring-boot已经没啥关系了，这其实就是一个标准的SpringApplicationContext的标准启动过程中refresh()部分，我们不是对spring启动过程分解，所以我们只关注与tomcat，spring-mvc结合的部分。
直接看3)onRefresh()方法，因为AnnotationConfigServletWebServerApplicationContext是ServletWebServerApplicationContext的子类，所以流程进入ServletWebServerApplicationContext的onRefresh()方法

@Override
protected void onRefresh() {
    super.onRefresh();
    try {
        createWebServer();//4)
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start web server", ex);
    }
}

可以看到这个4)createWebServer()，是我们的关键

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = getServletContext();
    if (webServer == null && servletContext == null) {
        ServletWebServerFactory factory = getWebServerFactory();//5)
        this.webServer = factory.getWebServer(getSelfInitializer());//6)
    }
    else if (servletContext != null) {
        try {
            getSelfInitializer().onStartup(servletContext);
        }
        catch (ServletException ex) {
            throw new ApplicationContextException("Cannot initialize servlet context", ex);
        }
    }
    initPropertySources();
}

其中：
5)ServletWebServerFactory factory = getWebServerFactory();
上面这句获取到的具体实现是TomcatServletWebServerFactory（时空门：TomcatServletWebServerFactory）
6)this.webServer = factory.getWebServer(getSelfInitializer());
先看6)中的getSelfInitializer()方法：

private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return this::selfInitialize;
}
private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareWebApplicationContext(servletContext);
    registerApplicationScope(servletContext);
    WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}

这块有点意思，返回的是一个this::selfInitialize，方法定义是返回org.springframework.boot.web.servlet.ServletContextInitializer，我们看下它是什么定义

@FunctionalInterface
public interface ServletContextInitializer {
    void onStartup(ServletContext servletContext) throws ServletException;
}

@FunctionalInterface是java8中lambda支持的一种函数式接口selfInitialize这段逻辑在后面过程当中会被调用。
继续看6)中this.webServer = factory.getWebServer(...)，我们看下实现：

@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);
    tomcat.getHost().setAutoDeploy(false);
    configureEngine(tomcat.getEngine());
    for (Connector additionalConnector : this.additionalTomcatConnectors) {
        tomcat.getService().addConnector(additionalConnector);
    }
    prepareContext(tomcat.getHost(), initializers);//7)
    return getTomcatWebServer(tomcat);
}

可以看到在里面创建了Tomcat实例作为webServer的内部实现，然后向Tomcat的Service容器注入Connector，然后设置默认Host容器的AutoDeploy属性及其他的Tomcat初始化工作，最重要的一行是7)
我们来看一下：

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    File documentRoot = getValidDocumentRoot();
    TomcatEmbeddedContext context = new TomcatEmbeddedContext();
    if (documentRoot != null) {
        context.setResources(new LoaderHidingResourceRoot(context));
    }
    ...//省略我们不关注的部分代码
    ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);//8)
    host.addChild(context);//将context加入host作为host的子容器
    configureContext(context, initializersToUse);//9)
    postProcessContext(context);
}

我们可以看到其调用host.addChild(context)将context加入host作为host的子容器，然后
其中8)查找所有ServletContextInitializer实现并合并为一个数组，然后调用9)configureContext方法，我们来看一下：

protected void configureContext(Context context, ServletContextInitializer[] initializers) {
    TomcatStarter starter = new TomcatStarter(initializers);//10)
    if (context instanceof TomcatEmbeddedContext) {
        TomcatEmbeddedContext embeddedContext = (TomcatEmbeddedContext) context;
        embeddedContext.setStarter(starter);
        embeddedContext.setFailCtxIfServletStartFails(true);
    }
    context.addServletContainerInitializer(starter, NO_CLASSES);//11)
    ...//忽略
}

10)创建了TomcatStarter对象，并将starter加入context的conainerInitializer列表，见11)，这样在tomcat的容器启动过程中就会调用到这个TomcatStarter实例。
我们来看下TomcatStarter做了什么

class TomcatStarter implements ServletContainerInitializer {
    ...
    private final ServletContextInitializer[] initializers;
    ...
    TomcatStarter(ServletContextInitializer[] initializers) {
        this.initializers = initializers;
    }
    ...
    @Override
    public void onStartup(Set<Class<?>> classes, ServletContext servletContext) throws ServletException {
        try {
            for (ServletContextInitializer initializer : this.initializers) {
                initializer.onStartup(servletContext);
            }
        }
        catch (Exception ex) {
            this.startUpException = ex;
            if (logger.isErrorEnabled()) {
                logger.error("Error starting Tomcat context. Exception: " + ex.getClass().getName() + ". Message: "
                        + ex.getMessage());
            }
        }
    }
    ...
}

可以看到TomcatStarter相当于hook了context启动的事件，然后调用所有注入的initializers的onStartup方法，似曾相识是吗？这就是前面说的@FunctionalInterface函数接口，接下来我们就深入看下前面提到的那个initializer的onStartup的具体内容

//ServletWebServerApplicationContext类当中
private org.springframework.boot.web.servlet.ServletContextInitializer getSelfInitializer() {
    return this::selfInitialize;
}
private void selfInitialize(ServletContext servletContext) throws ServletException {
    prepareWebApplicationContext(servletContext);
    registerApplicationScope(servletContext);
    WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}

可以看到其对getServletContextInitializerBeans()的每个ServletContextInitializer均调用了onStartup方法

protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    return new ServletContextInitializerBeans(getBeanFactory());
}

看看new ServletContextInitializerBeans(getBeanFactory())做了什么

@SafeVarargs
public ServletContextInitializerBeans(ListableBeanFactory beanFactory,
        Class<? extends ServletContextInitializer>... initializerTypes) {
    this.initializers = new LinkedMultiValueMap<>();
    this.initializerTypes = (initializerTypes.length != 0) ? Arrays.asList(initializerTypes)
            : Collections.singletonList(ServletContextInitializer.class);
    addServletContextInitializerBeans(beanFactory);
    addAdaptableBeans(beanFactory);
    List<ServletContextInitializer> sortedInitializers = this.initializers.values().stream()
            .flatMap((value) -> value.stream().sorted(AnnotationAwareOrderComparator.INSTANCE))
            .collect(Collectors.toList());
    this.sortedList = Collections.unmodifiableList(sortedInitializers);
    logMappings(this.initializers);
}

可以看到其从beanFactory中获取spring容器中所有的ServletContextInitializer实现，这里关于集成的部分在ServletRegistrationBean中，ServletRegistrationBean的注入过程参考：时空门：Dispatcherservletregistrationbean

private void addServletContextInitializerBeans(ListableBeanFactory beanFactory) {
    for (Class<? extends ServletContextInitializer> initializerType : this.initializerTypes) {
        for (Entry<String, ? extends ServletContextInitializer> initializerBean : getOrderedBeansOfType(beanFactory,
                initializerType)) {
            addServletContextInitializerBean(initializerBean.getKey(), initializerBean.getValue(), beanFactory);
        }
    }
}
private void addServletContextInitializerBean(String beanName, ServletContextInitializer initializer,
        ListableBeanFactory beanFactory) {
    if (initializer instanceof ServletRegistrationBean) {
        Servlet source = ((ServletRegistrationBean<?>) initializer).getServlet();
        addServletContextInitializerBean(Servlet.class, beanName, initializer, beanFactory, source);
    }
    else if (initializer instanceof FilterRegistrationBean) {
        Filter source = ((FilterRegistrationBean<?>) initializer).getFilter();
        addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);
    }
    else if (initializer instanceof DelegatingFilterProxyRegistrationBean) {
        String source = ((DelegatingFilterProxyRegistrationBean) initializer).getTargetBeanName();
        addServletContextInitializerBean(Filter.class, beanName, initializer, beanFactory, source);
    }
    else if (initializer instanceof ServletListenerRegistrationBean) {
        EventListener source = ((ServletListenerRegistrationBean<?>) initializer).getListener();
        addServletContextInitializerBean(EventListener.class, beanName, initializer, beanFactory, source);
    }
    else {
        addServletContextInitializerBean(ServletContextInitializer.class, beanName, initializer, beanFactory,
                initializer);
    }
}

然后流程就顺了，我们会调用到ServletRegistrationBean的onStartup方法，最终会调用到servletContext.addServlet的Servlet3.0的标准将DispatchServlet注入到servlet容器中拦截所有的请求。
见下面代码：

//RegistrationBean
@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
    String description = getDescription();
    if (!isEnabled()) {
        logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
        return;
    }
    register(description, servletContext);
}
//DynamicRegistrationBean
@Override
protected final void register(String description, ServletContext servletContext) {
    D registration = addRegistration(description, servletContext);
    if (registration == null) {
        logger.info(
                StringUtils.capitalize(description) + " was not registered " + "(possibly already registered?)");
        return;
    }
    configure(registration);
}
//ServletRegistrationBean
@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
    String name = getServletName();
    return servletContext.addServlet(name, this.servlet);
}

至此所有集成完毕，启动过程交给tomcat完成。

没讲完的故事：各个依赖的组件是如何初始化的

TomcatServletWebServerFactory

spring-boot-autoconfigure/META-INF/spring.factories中有一段配置：

...
# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
...

然后我们来看下ServletWebServerFactoryAutoConfiguration类

@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
        ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
        ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
        ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {
    ...
}

其中@Import部分引入了ServletWebServerFactoryConfiguration.EmbeddedTomcat.class，深入看一下

@Configuration
class ServletWebServerFactoryConfiguration {
    @Configuration
    @ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
    @ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
    public static class EmbeddedTomcat {
        @Bean
        public TomcatServletWebServerFactory tomcatServletWebServerFactory() {
            return new TomcatServletWebServerFactory();
        }
    }
    ...
}

这块Spring Boot根据@ConditionalOnClass判断当前运行时环境是否符合条件，即包含了tomcat的jar包，如果满足则创建TomcatServletWebServerFactory的Bean实例加入spring容器管理，后面有用。

ServletWebServerApplicationContext

实际启动时，启动的是其子类AnnotationConfigServletWebServerApplicationContext，我们来看下SpringApplication类，实际上SpringApplication在运行时根据情况决定使用哪种ApplicationContext

查看createApplicationContext()方法

那么这个this.webApplicationType又是哪来的值呢？
我们看下这个构造方法

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}

WebApplicationType.deduceFromClasspath()用来自动识别这个值，看下实现：

static WebApplicationType deduceFromClasspath() {
    if (ClassUtils.isPresent(WEBFLUX_INDICATOR_CLASS, null) && !ClassUtils.isPresent(WEBMVC_INDICATOR_CLASS, null)
            && !ClassUtils.isPresent(JERSEY_INDICATOR_CLASS, null)) {
        return WebApplicationType.REACTIVE;
    }
    for (String className : SERVLET_INDICATOR_CLASSES) {
        if (!ClassUtils.isPresent(className, null)) {
            return WebApplicationType.NONE;
        }
    }
    return WebApplicationType.SERVLET;
}

可以看到它是通过判断classloader中是否有Servlet相关的class来判断的，所以是运行时判断的。

DispatcherServletRegistrationBean

DispatcherServletRegistrationBean是保证我们的DispatcherServlet被注入到Servlet容器并生效的关键，我们来看下它是如何初始化的
spring-boot-autoconfigure/META-INF/spring.factories中有一段配置：

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\

看看实现

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@Configuration
@ConditionalOnWebApplication(type = Type.SERVLET)
@ConditionalOnClass(DispatcherServlet.class)
@AutoConfigureAfter(ServletWebServerFactoryAutoConfiguration.class)
public class DispatcherServletAutoConfiguration {
    @Configuration
    @Conditional(DispatcherServletRegistrationCondition.class)
    @ConditionalOnClass(ServletRegistration.class)
    @EnableConfigurationProperties(WebMvcProperties.class)
    @Import(DispatcherServletConfiguration.class)
    protected static class DispatcherServletRegistrationConfiguration {

        private final WebMvcProperties webMvcProperties;

        private final MultipartConfigElement multipartConfig;

        public DispatcherServletRegistrationConfiguration(WebMvcProperties webMvcProperties,
                ObjectProvider<MultipartConfigElement> multipartConfigProvider) {
            this.webMvcProperties = webMvcProperties;
            this.multipartConfig = multipartConfigProvider.getIfAvailable();
        }

        @Bean(name = DEFAULT_DISPATCHER_SERVLET_REGISTRATION_BEAN_NAME)
        @ConditionalOnBean(value = DispatcherServlet.class, name = DEFAULT_DISPATCHER_SERVLET_BEAN_NAME)
        public DispatcherServletRegistrationBean dispatcherServletRegistration(DispatcherServlet dispatcherServlet) {
            DispatcherServletRegistrationBean registration = new DispatcherServletRegistrationBean(dispatcherServlet,
                    this.webMvcProperties.getServlet().getPath());
            registration.setName(DEFAULT_DISPATCHER_SERVLET_BEAN_NAME);
            registration.setLoadOnStartup(this.webMvcProperties.getServlet().getLoadOnStartup());
            if (this.multipartConfig != null) {
                registration.setMultipartConfig(this.multipartConfig);
            }
            return registration;
        }

    }
}

可以看到，其像spring容器注册了DispatcherServletRegistrationBean的Bean实例，看一下它的继承关系：

其父类ServletRegistrationBean类有如下方法：

@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
    String name = getServletName();
    return servletContext.addServlet(name, this.servlet);
}

其调用了ServletContext.addServlet方法将DispatchServlet加入到Servlet容器，这是Servlet3.0中注册servlet的方法。
那么你也许会问，addRegistration又是什么时机调用的呢？
根据继承关系，查看其父类的父类RegistrationBean，其有一个

@Override
public final void onStartup(ServletContext servletContext) throws ServletException {
    String description = getDescription();
    if (!isEnabled()) {
        logger.info(StringUtils.capitalize(description) + " was not registered (disabled)");
        return;
    }
    register(description, servletContext);
}

register方法是一个模板方法，调用子类DynamicRegistrationBean的实现

@Override
protected final void register(String description, ServletContext servletContext) {
    D registration = addRegistration(description, servletContext);
    if (registration == null) {
        logger.info(StringUtils.capitalize(description) + " was not registered " + "(possibly already registered?)");
        return;
    }
    configure(registration);
}

addRegistration方法又是一个模板方法，实现就是前面ServletRegistrationBean的addRegistration实现，而onStartup方法会在SpringApplication.run()方法的流程中被调用到，讲主流程的时候已经讲到，这里不再赘述
这样就将DispatchServlet与Tomcat进行了集成，DispatchServlet使用模板方法设计模式，将具体的请求分配给不同的handler处理，这个后面会讲到，本篇就主要专注在Spring Boot与Spring MVC及Tomcat的集成原理部分。