Spring Boot與Spring MVC集成啓動過程源碼分析

時間  2019-11-07
標籤 spring boot mvc 集成 啓動 過程 源碼 分析 欄目 Spring 简体版
原文   原文鏈接

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Pepper Metrics是我與同事開發的一個開源工具(github.com/zrbcool/pep…)，其經過收集jedis/mybatis/httpservlet/dubbo/motan的運行性能統計，並暴露成prometheus等主流時序數據庫兼容數據，經過grafana展現趨勢。其插件化的架構也很是方便使用者擴展並集成其餘開源組件。
請你們給個star，同時歡迎你們成爲開發者提交PR一塊兒完善項目。java

從一個最簡單的Spring Boot Web項目聊起

咱們知道，用spring-boot寫一個web項目很是容易，pom繼承spring-boot-parent而後引入依賴spring-boot-starter-web，再寫一個這樣的主啓動類，而後就能夠去寫Controller了，十分簡單，就像這樣：git

@SpringBootApplication
public class SampleApplication {
    public static void main(String[] args) {
        SpringApplication.run(SampleApplication.class, args);
    }
}
// 而後再寫一個Controller聲明一個Rest服務
@RestController
@RequestMapping("/perf")
public class PerfController {
    @RequestMapping("/trace")
    public Object trace() {
        Object result = yourLogic();
        return result;
    }
}
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聊聊SpringApplication.run

但是咱們思考過，這背後spring-boot到底作了什麼使咱們的工做如此簡單，它如何將spring、spring-mvc、tomcat整合到一塊兒的呢？接下來咱們以項目啓動角度來分析整個初始化過程。github

PS：下面代碼分析過程當中，着重於流程的串接，調用到某個變量時，做者會直接給出這個變量的具體實現，讀者也許會產生困惑，可是不要停下來，先想固然的按照做者的思路把流程捋完，後面會針對各個主要的變量初始化及選擇實現的過程進行逐個解釋。web

從SpringApplication.run提及： 方法定義以下spring

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;
}
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咱們來分解下這個run方法
先看1)context = createApplicationContext()
負責建立spring主容器，這個方法內部是根據具體項目運行時依賴的類來動態選擇實現的，若是是web項目則會選擇AnnotationConfigServletWebServerApplicationContext，至於選擇的規則及緣由，這裏先忽略，後面會專門介紹（時空門：ServletWebServerApplicationContext）。
接下來咱們重點看2)refreshContext(context)方法
其方法內部最終調用了((AbstractApplicationContext) applicationContext).refresh()方法，咱們把這個方法展開數據庫

@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();
        }
    }
}
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實際上，這裏咱們的調用已經到了spring-context包，其實跟spring-boot已經沒啥關係了，這其實就是一個標準的SpringApplicationContext的標準啓動過程當中refresh()部分，咱們不是對spring啓動過程分解，因此咱們只關注與tomcat，spring-mvc結合的部分。
直接看3)onRefresh()方法，由於AnnotationConfigServletWebServerApplicationContext是ServletWebServerApplicationContext的子類，因此流程進入ServletWebServerApplicationContext的onRefresh()方法設計模式

@Override
protected void onRefresh() {
    super.onRefresh();
    try {
        createWebServer();//4)
    }
    catch (Throwable ex) {
        throw new ApplicationContextException("Unable to start web server", ex);
    }
}
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能夠看到這個4)createWebServer()，是咱們的關鍵api

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();
}
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其中：
5)ServletWebServerFactory factory = getWebServerFactory();
上面這句獲取到的具體實現是TomcatServletWebServerFactory（時空門：TomcatServletWebServerFactory） 6)this.webServer = factory.getWebServer(getSelfInitializer());
先看6)中的getSelfInitializer()方法：數組

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);
    }
}
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這塊有點意思，返回的是一個this::selfInitialize，方法定義是返回org.springframework.boot.web.servlet.ServletContextInitializer，咱們看下它是什麼定義spring-mvc

@FunctionalInterface
public interface ServletContextInitializer {
	void onStartup(ServletContext servletContext) throws ServletException;
}
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@FunctionalInterface是java8中lambda支持的一種函數式接口selfInitialize這段邏輯在後面過程中會被調用。
繼續看6)中this.webServer = factory.getWebServer(...)，咱們看下實現：

@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);
}
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能夠看到在裏面建立了Tomcat實例做爲webServer的內部實現，而後向Tomcat的Service容器注入Connector，而後設置默認Host容器的AutoDeploy屬性及其餘的Tomcat初始化工做，最重要的一行是7)
咱們來看一下：

protected void prepareContext(Host host, ServletContextInitializer[] initializers) {
    File documentRoot = getValidDocumentRoot();
    TomcatEmbeddedContext context = new TomcatEmbeddedContext();
    if (documentRoot != null) {
        context.setResources(new LoaderHidingResourceRoot(context));
    }
    ...//省略咱們不關注的部分代碼
    ServletContextInitializer[] initializersToUse = mergeInitializers(initializers);//8)
    host.addChild(context);//將context加入host做爲host的子容器
    configureContext(context, initializersToUse);//9)
    postProcessContext(context);
}
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咱們能夠看到其調用host.addChild(context)將context加入host做爲host的子容器，而後 其中8)查找全部ServletContextInitializer實現併合併爲一個數組，而後調用9)configureContext方法，咱們來看一下：

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
}
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10)建立了TomcatStarter對象，並將starter加入context的conainerInitializer列表，見11)，這樣在tomcat的容器啓動過程當中就會調用到這個TomcatStarter實例。
咱們來看下TomcatStarter作了什麼

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());
			}
		}
	}
    ...
}
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能夠看到TomcatStarter至關於hook了context啓動的事件，而後調用全部注入的initializers的onStartup方法，似曾相識是嗎？這就是前面說的@FunctionalInterface函數接口，接下來咱們就深刻看下前面提到的那個initializer的onStartup的具體內容

//ServletWebServerApplicationContext類當中
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);
    }
}
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能夠看到其對getServletContextInitializerBeans()的每一個ServletContextInitializer均調用了onStartup方法

protected Collection<ServletContextInitializer> getServletContextInitializerBeans() {
    return new ServletContextInitializerBeans(getBeanFactory());
}
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看看new ServletContextInitializerBeans(getBeanFactory())作了什麼

@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);
}
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能夠看到其從beanFactory中獲取spring容器中全部的ServletContextInitializer實現，這裏關於集成的部分在ServletRegistrationBean中，ServletRegistrationBean的注入過程參考：時空門：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);
    }
}
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而後流程就順了，咱們會調用到ServletRegistrationBean的onStartup方法，最終會調用到servletContext.addServlet的Servlet3.0的標準將DispatchServlet注入到servlet容器中攔截全部的請求。
見下面代碼：

//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);
}
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至此全部集成完畢，啓動過程交給tomcat完成。

沒講完的故事：各個依賴的組件是如何初始化的

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,\
...
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而後咱們來看下ServletWebServerFactoryAutoConfiguration類

@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 {
    ...
}
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其中@Import部分引入了ServletWebServerFactoryConfiguration.EmbeddedTomcat.class，深刻看一下

@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();
		}
	}
	...
}
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這塊Spring Boot根據@ConditionalOnClass判斷當前運行時環境是否符合條件，即包含了tomcat的jar包，若是知足則建立TomcatServletWebServerFactory的Bean實例加入spring容器管理，後面有用。

ServletWebServerApplicationContext

實際啓動時，啓動的是其子類AnnotationConfigServletWebServerApplicationContext，咱們來看下SpringApplication類，實際上SpringApplication在運行時根據狀況決定使用哪一種ApplicationContext

查看createApplicationContext()方法
那麼這個this.webApplicationType又是哪來的值呢？ 咱們看下這個構造方法 

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();
}
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WebApplicationType.deduceFromClasspath()用來自動識別這個值，看下實現：

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;
}
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能夠看到它是經過判斷classloader中是否有Servlet相關的class來判斷的，因此是運行時判斷的。

DispatcherServletRegistrationBean

DispatcherServletRegistrationBean是保證咱們的DispatcherServlet被注入到Servlet容器並生效的關鍵，咱們來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,\
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看看實現

@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;
		}

	}
}
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能夠看到，其像spring容器註冊了DispatcherServletRegistrationBean的Bean實例，看一下它的繼承關係：


其父類ServletRegistrationBean類有以下方法： 

@Override
protected ServletRegistration.Dynamic addRegistration(String description, ServletContext servletContext) {
    String name = getServletName();
    return servletContext.addServlet(name, this.servlet);
}
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其調用了ServletContext.addServlet方法將DispatchServlet加入到Servlet容器，這是Servlet3.0中註冊servlet的方法。
那麼你也許會問，addRegistration又是什麼時機調用的呢？ 根據繼承關係，查看其父類的父類RegistrationBean，其有一個

@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);
}
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register方法是一個模板方法，調用子類DynamicRegistrationBean的實現

@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);
}
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addRegistration方法又是一個模板方法，實現就是前面ServletRegistrationBean的addRegistration實現，而onStartup方法會在SpringApplication.run()方法的流程中被調用到，講主流程的時候已經講到，這裏再也不贅述 這樣就將DispatchServlet與Tomcat進行了集成，DispatchServlet使用模板方法設計模式，將具體的請求分配給不一樣的handler處理，這個後面會講到，本篇就主要專一在Spring Boot與Spring MVC及Tomcat的集成原理部分。

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