無聊在家看源碼，雖然沒看懂多少亡嫌，但是還是想紀(jì)錄一下嚎于，埋下一顆種子掘而。

有時(shí)候，一些問題于购，我總是想要追根尋底袍睡，一直搞的Web開發(fā)，但是卻每次只能在Controller層去編寫代碼肋僧，調(diào)試代碼斑胜，感覺心里總有些(其實(shí)是很)不爽，所以擅自超出自己的能力范圍嫌吠，去看了看Tomcat和Servlet和Spring的源碼止潘，果然如我所料，的確是非常難居兆，所以覆山，只能記下一些關(guān)鍵點(diǎn)，為以后留下點(diǎn)東西泥栖。

看了好久簇宽，直到最后，才發(fā)現(xiàn)這個(gè)關(guān)鍵點(diǎn)吧享，如下圖所示魏割。

Paste_Image.png

發(fā)現(xiàn)沒有，這里是三個(gè)包之間的交互點(diǎn)钢颂，從這三條堆棧的運(yùn)行來看钞它，就可以看到這三個(gè)關(guān)鍵點(diǎn)之間是如何交互的。

那么一個(gè)一個(gè)函數(shù)來看殊鞭。

    private synchronized void initServlet(Servlet servlet) throws ServletException {
        if(!this.instanceInitialized || this.singleThreadModel) {
            try {
                this.instanceSupport.fireInstanceEvent("beforeInit", servlet);
                if(Globals.IS_SECURITY_ENABLED) {
                    boolean f = false;

                    try {
                        Object[] args = new Object[]{this.facade};
                        SecurityUtil.doAsPrivilege("init", servlet, classType, args);
                        f = true;
                    } finally {
                        if(!f) {
                            SecurityUtil.remove(servlet);
                        }

                    }
                } else {
                    servlet.init(this.facade);
                }

                this.instanceInitialized = true;
                this.instanceSupport.fireInstanceEvent("afterInit", servlet);
            } catch (UnavailableException var10) {
                this.instanceSupport.fireInstanceEvent("afterInit", servlet, var10);
                this.unavailable(var10);
                throw var10;
            } catch (ServletException var11) {
                this.instanceSupport.fireInstanceEvent("afterInit", servlet, var11);
                throw var11;
            } catch (Throwable var12) {
                ExceptionUtils.handleThrowable(var12);
                this.getServletContext().log("StandardWrapper.Throwable", var12);
                this.instanceSupport.fireInstanceEvent("afterInit", servlet, var12);
                throw new ServletException(sm.getString("standardWrapper.initException", new Object[]{this.getName()}), var12);
            }
        }
    }

這個(gè)就是Tomcat的那條堆棧的函數(shù)遭垛。看到很多blog都很空泛的說操灿，Web容器會(huì)加載web.xml的內(nèi)容锯仪，然后加載里面定義的Servlet,之后會(huì)調(diào)用init()的內(nèi)容，但是都沒有代碼級別的示例趾盐，這讓我著實(shí)心癢庶喜。所以想盡辦法來窺的一絲一毫。

這里函數(shù)里面其實(shí)已經(jīng)加載好了Servlet,如果在Idea里面把鼠標(biāo)移動(dòng)到Servlet上面可以看到類型就是我在web.xml里面配置的DispatherServlet的實(shí)例救鲤。

關(guān)鍵代碼就是Servlet.init(this.facade)久窟。也就是這里Tomcat調(diào)用Servlet的init()函數(shù)。這邊也應(yīng)該要注意到的是參數(shù)this.facade本缠，沒錯(cuò),這個(gè)就是Servlet接口中的init(ServletConfig config)的config參數(shù)斥扛，里面最重要的就是給了Servlet一個(gè)ServletContext的引用，這樣每個(gè)Servlet就可以訪問到唯一的一個(gè)ServletContext丹锹。

下面的是Servlet的接口源代碼犹赖。

package javax.servlet;

import java.io.IOException;
import java.io.Serializable;
import java.util.Enumeration;
import javax.servlet.Servlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;

public abstract class GenericServlet implements Servlet, ServletConfig, Serializable {
    private static final long serialVersionUID = 1L;
    private transient ServletConfig config;

    public GenericServlet() {
    }

    public void destroy() {
    }

    public ServletConfig getServletConfig() {
        return this.config;
    }

    public ServletContext getServletContext() {
        return this.getServletConfig().getServletContext();
    }

    public String getServletInfo() {
        return "";
    }

    public void init(ServletConfig config) throws ServletException {
        this.config = config;
        this.init();
    }

    public void init() throws ServletException {
    }
    
    public abstract void service(ServletRequest var1, ServletResponse var2) throws ServletException, IOException;

}

從接口中可以更加明顯的看到config賦值行為队他。之后又接著調(diào)用了沒有參數(shù)的init()函數(shù)。之后就是Spring的有個(gè)HttpServletBean的類進(jìn)行了init()函數(shù)的實(shí)現(xiàn)峻村。具體源代碼如下。

public final void init() throws ServletException {
    if(this.logger.isDebugEnabled()) {
        this.logger.debug("Initializing servlet \'" + this.getServletName() + "\'");
    }

    try {
        HttpServletBean.ServletConfigPropertyValues ex = new HttpServletBean.ServletConfigPropertyValues(this.getServletConfig(), this.requiredProperties);
        BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
        ServletContextResourceLoader resourceLoader = new ServletContextResourceLoader(this.getServletContext());
        bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, this.getEnvironment()));
        this.initBeanWrapper(bw);
        bw.setPropertyValues(ex, true);
    } catch (BeansException var4) {
        this.logger.error("Failed to set bean properties on servlet \'" + this.getServletName() + "\'", var4);
        throw var4;
    }

    this.initServletBean();
    if(this.logger.isDebugEnabled()) {
        this.logger.debug("Servlet \'" + this.getServletName() + "\' configured successfully");
    }

}

從這個(gè)函數(shù)開始就沒有Tomcat的什么事情了锡凝，就開始進(jìn)入到Spring的管理中來了粘昨。

好的，下面雖然看的不是很懂了窜锯，但是能走幾步就走幾步张肾。

按照調(diào)用棧的過程，可以看到之后是FrameworkServlet的initServletBean的調(diào)用锚扎。這回先上代碼吞瞪。

protected final void initServletBean() throws ServletException {
    this.getServletContext().log("Initializing Spring FrameworkServlet \'" + this.getServletName() + "\'");
    if(this.logger.isInfoEnabled()) {
        this.logger.info("FrameworkServlet \'" + this.getServletName() + "\': initialization started");
    }

    long startTime = System.currentTimeMillis();

    try {
        //開始初始化前端控制器自己的Bean容器
        this.webApplicationContext = this.initWebApplicationContext();
        this.initFrameworkServlet();
    } catch (ServletException var5) {
        this.logger.error("Context initialization failed", var5);
        throw var5;
    } catch (RuntimeException var6) {
        this.logger.error("Context initialization failed", var6);
        throw var6;
    }

    if(this.logger.isInfoEnabled()) {
        long elapsedTime = System.currentTimeMillis() - startTime;
        this.logger.info("FrameworkServlet \'" + this.getServletName() + "\': initialization completed in " + elapsedTime + " ms");
    }

}

從代碼中可以看到最先做的事情是初始化了DispatherServlet自己的Bean容器，好驾孔，在進(jìn)入這個(gè)初始的代碼看看芍秆，又做了一些什么事情呢。

protected WebApplicationContext initWebApplicationContext() {
    //先嘗試從ServletContext中獲得全局的Spring容器
    //全局的Spring容器是通過監(jiān)聽器獲得的翠勉，但是我這里沒有配置
    //所以返回的是個(gè)null妖啥，rootContext=null
    WebApplicationContext rootContext = WebApplicationContextUtils.getWebApplicationContext(this.getServletContext());
    WebApplicationContext wac = null;
    //分為兩種情況，一種是Bean容器已存在(什么情況下會(huì)這樣呢?)
    if(this.webApplicationContext != null) {
        wac = this.webApplicationContext;
        if(wac instanceof ConfigurableWebApplicationContext) {
            ConfigurableWebApplicationContext attrName = (ConfigurableWebApplicationContext)wac;
            if(!attrName.isActive()) {
                if(attrName.getParent() == null) {
                    attrName.setParent(rootContext);
                }
                //最后調(diào)用這個(gè)配置和刷新Bean容器
                //如果是下面一個(gè)創(chuàng)建的分支的話对碌，最后也是會(huì)執(zhí)行這個(gè)函數(shù)的
                this.configureAndRefreshWebApplicationContext(attrName);
            }
        }
    }
    //如果沒有初始化荆虱，第一次進(jìn)來的時(shí)候一定是沒有初始化的
    //找一下?怎么找?
    if(wac == null) {
        wac = this.findWebApplicationContext();
    }
    //找不到就造一個(gè)，實(shí)例化一個(gè)
    if(wac == null) {
        wac = this.createWebApplicationContext(rootContext);
    }

    if(!this.refreshEventReceived) {
        this.onRefresh(wac);
    }

    if(this.publishContext) {
        String attrName1 = this.getServletContextAttributeName();
        this.getServletContext().setAttribute(attrName1, wac);
        if(this.logger.isDebugEnabled()) {
            this.logger.debug("Published WebApplicationContext of servlet \'" + this.getServletName() + "\' as ServletContext attribute with name [" + attrName1 + "]");
        }
    }

    return wac;
}

在走兩步到FrameServlet的createWebApplicationContext這里朽们。有個(gè)關(guān)鍵一個(gè)是把從ServletContext中獲得原始的Bean容器作為了DispatherServlet自己的父容器了怀读。

protected WebApplicationContext createWebApplicationContext(ApplicationContext parent) {
    Class contextClass = this.getContextClass();
    if(this.logger.isDebugEnabled()) {
        this.logger.debug("Servlet with name \'" + this.getServletName() + "\' will try to create custom WebApplicationContext context of class \'" + contextClass.getName() + "\'" + ", using parent context [" + parent + "]");
    }

    if(!ConfigurableWebApplicationContext.class.isAssignableFrom(contextClass)) {
        throw new ApplicationContextException("Fatal initialization error in servlet with name \'" + this.getServletName() + "\': custom WebApplicationContext class [" + contextClass.getName() + "] is not of type ConfigurableWebApplicationContext");
    } else {
        ConfigurableWebApplicationContext wac = (ConfigurableWebApplicationContext)BeanUtils.instantiateClass(contextClass);
        wac.setEnvironment(this.getEnvironment());
        //將從監(jiān)聽器獲得的Spring容器作為父容器
        wac.setParent(parent);
        wac.setConfigLocation(this.getContextConfigLocation());
        //調(diào)用這個(gè)配置和刷新Web應(yīng)用上下文的函數(shù)  
        this.configureAndRefreshWebApplicationContext(wac);
        return wac;
    }
}

好，接著看骑脱。

protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
    if(ObjectUtils.identityToString(wac).equals(wac.getId())) {
        if(this.contextId != null) {
            wac.setId(this.contextId);
        } else {
            wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX + ObjectUtils.getDisplayString(this.getServletContext().getContextPath()) + "/" + this.getServletName());
        }
    }

    wac.setServletContext(this.getServletContext());
    wac.setServletConfig(this.getServletConfig());
    wac.setNamespace(this.getNamespace());
    wac.addApplicationListener(new SourceFilteringListener(wac, new FrameworkServlet.ContextRefreshListener(null)));
    ConfigurableEnvironment env = wac.getEnvironment();
    if(env instanceof ConfigurableWebEnvironment) {
        ((ConfigurableWebEnvironment)env).initPropertySources(this.getServletContext(), this.getServletConfig());
    }

    this.postProcessWebApplicationContext(wac);
    this.applyInitializers(wac);
    //上面都是把一些ServletContext和ServletConfig之類的設(shè)置進(jìn)入Web應(yīng)用上下菜枷，下面是refresh()
    wac.refresh();
}

再進(jìn)入到refresh()函數(shù)中，md惜姐，完全沒看懂犁跪，逃...

public void refresh() throws BeansException, IllegalStateException {
    Object var1 = this.startupShutdownMonitor;
    synchronized(this.startupShutdownMonitor) {
        this.prepareRefresh();
        ConfigurableListableBeanFactory beanFactory = this.obtainFreshBeanFactory();
        this.prepareBeanFactory(beanFactory);

        try {
            this.postProcessBeanFactory(beanFactory);
            this.invokeBeanFactoryPostProcessors(beanFactory);
            this.registerBeanPostProcessors(beanFactory);
            this.initMessageSource();
            this.initApplicationEventMulticaster();
            this.onRefresh();
            this.registerListeners();
            this.finishBeanFactoryInitialization(beanFactory);
           //以上都不知道在干啥了 
           this.finishRefresh();
        } catch (BeansException var5) {
            this.destroyBeans();
            this.cancelRefresh(var5);
            throw var5;
        }

    }
}

中間跳過無數(shù)看不懂的東西，好像是一些事件傳遞歹袁，監(jiān)聽坷衍，委托之類的，最后終于調(diào)用了DispatherServlet的onRefresh函數(shù)和initStrategies函數(shù)条舔。但是其實(shí)在DispatherServlet被Tomcat初始化的時(shí)候會(huì)先調(diào)用DiapatherServlet的靜態(tài)區(qū)塊枫耳，之后才是無參構(gòu)造函數(shù)。

protected void onRefresh(ApplicationContext context) {
    this.initStrategies(context);
}

protected void initStrategies(ApplicationContext context) {
    this.initMultipartResolver(context);
    this.initLocaleResolver(context);
    this.initThemeResolver(context);
    this.initHandlerMappings(context);
    this.initHandlerAdapters(context);
    this.initHandlerExceptionResolvers(context);
    this.initRequestToViewNameTranslator(context);
    this.initViewResolvers(context);
    this.initFlashMapManager(context);
}

再補(bǔ)上我這個(gè)Debug項(xiàng)目的web.xml文件孟抗。

<?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">

    <servlet>
        <servlet-name>spitter</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <!--<init-param>-->
            <!--<param-name>spring</param-name>-->
            <!--<param-value>classpath:/spring/demo_produce.xml</param-value>-->
        <!--</init-param>-->
        <load-on-startup>1</load-on-startup>
    </servlet>
    <servlet-mapping>
        <servlet-name>spitter</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>

    <servlet-mapping>
        <servlet-name>spitter</servlet-name>
        <url-pattern>*.service</url-pattern>
    </servlet-mapping>

</web-app>

以及本次的Debug的環(huán)境是Tomcat8.0和Spring4.0.6版本迁杨，不同的版本之間應(yīng)該會(huì)有一些出入钻心，所以紀(jì)錄一下版本，想要調(diào)試的源代碼版本是Github上面的AllDemo的demo_producer的612dd1316d4d762c166901698ba1818f1b18bfca版本铅协。

還得補(bǔ)一張完整堆棧圖捷沸。

Paste_Image.png

看了那么多，小結(jié)一下狐史，DispatcherServlet繼承了FrameWorkServlet痒给，這個(gè)類里面其實(shí)幫助初始化了Web上下文，最后才是核心控制器的東西骏全。