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Spring在Web容器中的启动过程

环境

spring-framework：5.1.x

spring-boot: v2.1.2.RELEASE

看一眼历史的感觉

先看一眼我们很久以前用的XML的配置方式，我举得用最原始的方式来学习会相对于简单，因为很多的配置都是显性的。我只截取最核心的部分，大概找一下感觉。

<?xml version="1.0" encoding="UTF-8" ?>  
<web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="  
             http://java.sun.com/xml/ns/j2ee  http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">

    <context-param>
        <param-name>contextConfigLocation</param-name> <!--参数名字不能随意取，约定的。-->
        <param-value>classpath:context.xml</param-value>
    </context-param>

    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <filter>
        <filter-name>encodingFilter</filter-name>
        <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>
    </filter>

    <servlet>
        <servlet-name>dispatcherServlet</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <init-param>
            <param-name>contextConfigLocation</param-name>
            <param-value>classpath:config/spring-mvc.xml</param-value>
        </init-param>
        <load-on-startup>1</load-on-startup>
    </servlet>

</web-app>

上面的配置基本就把一个SpringMVC的项目配置完成了，大家都了解。 web.xml 是一个WEB项目的入口，而这里面就把Spring与Servlet关联起来了。

Loader1： org.springframework.web.context.ContextLoaderListener 父 IOC 容器，管理所有的 Bean 。

Loader2： org.springframework.web.servlet.DispatcherServlet 子 IOC 容器，主要关于与 WEB 相关的一些配置，比如： Controller 、 HandlerMapping 等等。

这里粗略的描述一下WEB项目的一个加载顺序：listener → filter → servlet。

ContextLoaderListener

org.springframework.web.context.ContextLoaderListener  
javax.servlet.ServletContextListener  
javax.servlet.ServletContext  
javax.servlet.ServletContextEvent

ContextLoaderListener（spring中的类）继承ContextLoader（spring中的类），并实现ServletContextListener（servlet中的接口），ServletContextListener监听ServletContext，当容器启动时，会触发ServletContextEvent事件，该事件由ServletContextListener来处理，启动初始化ServletContext时，调用contextInitialized方法。而ContextLoaderListener实现了ServletContextListener，所以，当容器启动时，触发ServletContextEvent事件，让ContextLoaderListener执行实现方法contextInitialized(ServletContextEvent sce);

引自： https://www.jianshu.com/p/c1384f3d5698

@Override
public void contextInitialized(ServletContextEvent event) {  
    initWebApplicationContext(event.getServletContext());
}

我们细看一下我们是如何初始化一个 Context 的：

if (this.context == null) {  
    this.context = createWebApplicationContext(servletContext);
}
// 确定我们容器是哪个Context
protected Class<?> determineContextClass(ServletContext servletContext)  
  // public static final String CONTEXT_CLASS_PARAM = "contextClass";
    String contextClassName = servletContext.getInitParameter(CONTEXT_CLASS_PARAM);
    if (contextClassName != null) {
        try {
            return ClassUtils.forName(contextClassName, ClassUtils.getDefaultClassLoader());
        } catch (ClassNotFoundException ex) {
            throw new ApplicationContextException("Failed to load custom context class [" + contextClassName + "]", ex);
        }
    } else {
        contextClassName = defaultStrategies.getProperty(WebApplicationContext.class.getName());
        try {
            return ClassUtils.forName(contextClassName, ContextLoader.class.getClassLoader());
        }
        catch (ClassNotFoundException ex) {
            throw new ApplicationContextException("Failed to load default context class [" + contextClassName + "]", ex);
        }
    }
}

servletContext.getInitParameter(CONTEXT_CLASS_PARAM); 这句话实际上是优先用用户配置的，否则才会取默认的。如果我们自己配置要在哪儿配置了。对的还是要在我们的web.xml里面。

<context-param>  
  <param-name>contextClass</param-name>   
  <param-value>org.springframework.web.context.support.StaticWebApplicationContext</param-value>   
</context-param>

那我们默认的是哪个呢？

static {  
        // Load default strategy implementations from properties file.
        // This is currently strictly internal and not meant to be customized
        // by application developers.
        try {
            ClassPathResource resource = new ClassPathResource(DEFAULT_STRATEGIES_PATH, ContextLoader.class);
            defaultStrategies = PropertiesLoaderUtils.loadProperties(resource);
        }
        catch (IOException ex) {
            throw new IllegalStateException("Could not load 'ContextLoader.properties': " + ex.getMessage());
        }
    }

private static final String DEFAULT_STRATEGIES_PATH = "ContextLoader.properties";

是滴，有一个properties文件，里面就是默认的Context配置。

org.springframework.web.context.WebApplicationContext=org.springframework.web.context.support.XmlWebApplicationContext

实际上，我们默认的就是 XmlWebApplicationContext 。继续扫读 web.xml 的配置来加载与 Spring 相关的配置。

// public static final String CONFIG_LOCATION_PARAM = "contextConfigLocation";
// 这里就是all.xml/application.xml/context.xml 等的加载地方
String configLocationParam = sc.getInitParameter(CONFIG_LOCATION_PARAM);  
if (configLocationParam != null) {  
    wac.setConfigLocation(configLocationParam);
}

调用refresh开始构建

wac.refresh(); // 然而这里我觉得要单独拿一个篇章来讲Spring是如何来加载Bean。

DispatcherServlet

HttpServlet 及以上部分是 Servlet 标准中提供的接口及类
DispatcherServlet、FrameworkServlet、HttpServletBean 三者是 SpringMVC 提供的类，且后者依次分别是前者的父类。

Spring在Web容器中的启动过程

因为是Servlet，所有会调用init来初始化。

org.springframework.web.servlet.HttpServletBean

public final void init() throws ServletException {  
        // Set bean properties from init parameters.
        PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
        if (!pvs.isEmpty()) {
            try {
                BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
                ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
                bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
                initBeanWrapper(bw);
                bw.setPropertyValues(pvs, true);
            }
            catch (BeansException ex) {
                if (logger.isErrorEnabled()) {
                    logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
                }
                throw ex;
            }
        }

        // Let subclasses do whatever initialization they like.
        initServletBean();
    }

org.springframework.web.servlet.FrameworkServlet

protected final void initServletBean() throws ServletException {  
    getServletContext().log("Initializing Spring " + getClass().getSimpleName() + " '" + getServletName() + "'");
    if (logger.isInfoEnabled()) {
        logger.info("Initializing Servlet '" + getServletName() + "'");
    }
    long startTime = System.currentTimeMillis();

    try {
    // 看到这里，回到之前说初始化ContextLoaderListener的initWebApplicationContext
        this.webApplicationContext = initWebApplicationContext();
        initFrameworkServlet();
    }
    catch (ServletException | RuntimeException ex) {
        logger.error("Context initialization failed", ex);
        throw ex;
    }

    if (logger.isDebugEnabled()) {
        String value = this.enableLoggingRequestDetails ?
                "shown which may lead to unsafe logging of potentially sensitive data" :
                "masked to prevent unsafe logging of potentially sensitive data";
        logger.debug("enableLoggingRequestDetails='" + this.enableLoggingRequestDetails +
                "': request parameters and headers will be " + value);
    }

    if (logger.isInfoEnabled()) {
        logger.info("Completed initialization in " + (System.currentTimeMillis() - startTime) + " ms");
    }
}

org.springframework.web.servlet.FrameworkServlet

protected WebApplicationContext createWebApplicationContext(@Nullable ApplicationContext parent) {  
  //public static final Class<?> DEFAULT_CONTEXT_CLASS = XmlWebApplicationContext.class;
        Class<?> contextClass = getContextClass();
        if (!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
            throw new ApplicationContextException(
                    "Fatal initialization error in servlet with name '" + getServletName() +
                    "': custom WebApplicationContext class [" + contextClass.getName() +
                    "] is not of type ConfigurableWebApplicationContext");
        }
        ConfigurableWebApplicationContext wac =
                (ConfigurableWebApplicationContext) BeanUtils.instantiateClass(contextClass);

        wac.setEnvironment(getEnvironment());
        wac.setParent(parent);
        String configLocation = getContextConfigLocation();
        if (configLocation != null) {
            wac.setConfigLocation(configLocation);
        }
        configureAndRefreshWebApplicationContext(wac); // 这里面又会调用wac.refresh();
        return wac;
    }

看到这里，我们的2个容器都是默认用的 XmlWebApplicationContext 。

那问哪些 HandlerMapping 、 HandlerAdapter 、 ViewResolver 是在哪儿加载进来的？

org.springframework.web.servlet.DispatcherServlet#onRefresh

/**
     * Initialize the strategy objects that this servlet uses.
     * <p>May be overridden in subclasses in order to initialize further strategy objects.
     */
    protected void initStrategies(ApplicationContext context) {
        initMultipartResolver(context);
        initLocaleResolver(context);
        initThemeResolver(context);
        // 初始化HandlerMapping
        initHandlerMappings(context);
    // 初始化HandlerAdapter
        initHandlerAdapters(context);
        initHandlerExceptionResolvers(context);
        initRequestToViewNameTranslator(context);
        initViewResolvers(context);
        initFlashMapManager(context);
    }

org.springframework.web.servlet.handler.AbstractHandlerMethodMapping

protected void detectHandlerMethods(Object handler) {  
        Class<?> handlerType = (handler instanceof String ?
                obtainApplicationContext().getType((String) handler) : handler.getClass());

        if (handlerType != null) {
            Class<?> userType = ClassUtils.getUserClass(handlerType);
            Map<Method, T> methods = MethodIntrospector.selectMethods(userType,
                    (MethodIntrospector.MetadataLookup<T>) method -> {
                        try {
                            return getMappingForMethod(method, userType);
                        }
                        catch (Throwable ex) {
                            throw new IllegalStateException("Invalid mapping on handler class [" +
                                    userType.getName() + "]: " + method, ex);
                        }
                    });
            if (logger.isTraceEnabled()) {
                logger.trace(formatMappings(userType, methods));
            }
            methods.forEach((method, mapping) -> {
                Method invocableMethod = AopUtils.selectInvocableMethod(method, userType);
                registerHandlerMethod(handler, invocableMethod, mapping);
            });
        }
    }

遍历Handler中的所有方法，找出其中被@RequestMapping注解标记的方法。
然后遍历这些方法，生成RequestMappingInfo实例。
将RequestMappingInfo实例以及处理器方法注册到缓存中。

org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping

@Override
@Nullable
protected RequestMappingInfo getMappingForMethod(Method method, Class<?> handlerType) {  
 // RequestMapping requestMapping = AnnotatedElementUtils.findMergedAnnotation(element, RequestMapping.class); 只要打了注解@RequestMapping的方法
    RequestMappingInfo info = createRequestMappingInfo(method);
    if (info != null) {
        RequestMappingInfo typeInfo = createRequestMappingInfo(handlerType);
        if (typeInfo != null) {
            info = typeInfo.combine(info);
        }
        String prefix = getPathPrefix(handlerType);
        if (prefix != null) {
            info = RequestMappingInfo.paths(prefix).build().combine(info);
        }
    }
    return info;
}

public void register(T mapping, Object handler, Method method) {  
            this.readWriteLock.writeLock().lock();
            try {
                HandlerMethod handlerMethod = createHandlerMethod(handler, method);
                assertUniqueMethodMapping(handlerMethod, mapping);
                this.mappingLookup.put(mapping, handlerMethod);

                List<String> directUrls = getDirectUrls(mapping);
                for (String url : directUrls) {
                    this.urlLookup.add(url, mapping);
                }

                String name = null;
                if (getNamingStrategy() != null) {
                    name = getNamingStrategy().getName(handlerMethod, mapping);
                    addMappingName(name, handlerMethod);
                }

                CorsConfiguration corsConfig = initCorsConfiguration(handler, method, mapping);
                if (corsConfig != null) {
                    this.corsLookup.put(handlerMethod, corsConfig);
                }

                this.registry.put(mapping, new MappingRegistration<>(mapping, handlerMethod, directUrls, name));
            }
            finally {
                this.readWriteLock.writeLock().unlock();
            }
        }

把一些请求的映射关系放入到Map中，为后续的路由功能做数据初始化。

private final Map<T, HandlerMethod> mappingLookup = new LinkedHashMap<>();  
private final MultiValueMap<String, T> urlLookup = new LinkedMultiValueMap<>();

对于Request参数的一些封装&映射：

@Override
    public RequestMappingInfo combine(RequestMappingInfo other) {
        String name = combineNames(other);
        PatternsRequestCondition patterns = this.patternsCondition.combine(other.patternsCondition);
        RequestMethodsRequestCondition methods = this.methodsCondition.combine(other.methodsCondition);
        ParamsRequestCondition params = this.paramsCondition.combine(other.paramsCondition);
        HeadersRequestCondition headers = this.headersCondition.combine(other.headersCondition);
        ConsumesRequestCondition consumes = this.consumesCondition.combine(other.consumesCondition);
        ProducesRequestCondition produces = this.producesCondition.combine(other.producesCondition);
        RequestConditionHolder custom = this.customConditionHolder.combine(other.customConditionHolder);

        return new RequestMappingInfo(name, patterns,
                methods, params, headers, consumes, produces, custom.getCondition());
    }

一般我们对关心的是一个url是如何组装的。

public PatternsRequestCondition combine(PatternsRequestCondition other) {  
        Set<String> result = new LinkedHashSet<>();
        if (!this.patterns.isEmpty() && !other.patterns.isEmpty()) {
            for (String pattern1 : this.patterns) {
                for (String pattern2 : other.patterns) {
                    result.add(this.pathMatcher.combine(pattern1, pattern2));
                }
            }
        }
        else if (!this.patterns.isEmpty()) {
            result.addAll(this.patterns);
        }
        else if (!other.patterns.isEmpty()) {
            result.addAll(other.patterns);
        }
        else {
            result.add("");
        }
        return new PatternsRequestCondition(result, this.pathHelper, this.pathMatcher,
                this.useSuffixPatternMatch, this.useTrailingSlashMatch, this.fileExtensions);
    }

这是从注释上 copy 下来的注解，主要有这里的 pathMatcher 来组装。

Spring在Web容器中的启动过程

public String combine(String pattern1, String pattern2) {  
        if (!StringUtils.hasText(pattern1) && !StringUtils.hasText(pattern2)) {
            return "";
        }
        if (!StringUtils.hasText(pattern1)) {
            return pattern2;
        }
        if (!StringUtils.hasText(pattern2)) {
            return pattern1;
        }

        boolean pattern1ContainsUriVar = (pattern1.indexOf('{') != -1);
        if (!pattern1.equals(pattern2) && !pattern1ContainsUriVar && match(pattern1, pattern2)) {
            // /* + /hotel -> /hotel ; "/*.*" + "/*.html" -> /*.html
            // However /user + /user -> /usr/user ; /{foo} + /bar -> /{foo}/bar
            return pattern2;
        }

        // /hotels/* + /booking -> /hotels/booking
        // /hotels/* + booking -> /hotels/booking
        if (pattern1.endsWith(this.pathSeparatorPatternCache.getEndsOnWildCard())) {
            return concat(pattern1.substring(0, pattern1.length() - 2), pattern2);
        }

        // /hotels/** + /booking -> /hotels/**/booking
        // /hotels/** + booking -> /hotels/**/booking
        if (pattern1.endsWith(this.pathSeparatorPatternCache.getEndsOnDoubleWildCard())) {
            return concat(pattern1, pattern2);
        }

        int starDotPos1 = pattern1.indexOf("*.");
        if (pattern1ContainsUriVar || starDotPos1 == -1 || this.pathSeparator.equals(".")) {
            // simply concatenate the two patterns
            return concat(pattern1, pattern2);
        }

        String ext1 = pattern1.substring(starDotPos1 + 1);
        int dotPos2 = pattern2.indexOf('.');
        String file2 = (dotPos2 == -1 ? pattern2 : pattern2.substring(0, dotPos2));
        String ext2 = (dotPos2 == -1 ? "" : pattern2.substring(dotPos2));
        boolean ext1All = (ext1.equals(".*") || ext1.isEmpty());
        boolean ext2All = (ext2.equals(".*") || ext2.isEmpty());
        if (!ext1All && !ext2All) {
            throw new IllegalArgumentException("Cannot combine patterns: " + pattern1 + " vs " + pattern2);
        }
        String ext = (ext1All ? ext2 : ext1);
        return file2 + ext;
    }

还是稍微的有点粗，也只描述了我们最最关心的一些点。后面再继续的对每个细节点做一个总结。