基于Spring MVC框架的Http流程分析

Java框架

浏览数:94

2019-5-6


一、问题提出

我们可以方便的利用Spring MVC进行业务开发,请求的大部分工作都被框架和容器封装,使得我们只需要做很少量的工作。但是整个http请求流程是怎么样的?Spring MVC框架在其中起到什么作用?它是怎么和Web容器进行交互的?Controller中的一个方法怎么被暴露出来提供http请求服务的?本着这些想法,我们对整个http请求过程进行讨索。全文以spring-mvc-demo为例

二、整体处理流程概述

整个过程包括三部分:应用启动、请求路由与处理、请求返回。

应用启动:web容器初始化(context建立等)、应用初始化(初始化handlerMap)。

请求路由与处理:请求路由(根据url找到Context、根据context找到dispatcherServlet、根据url找到handler、根据url找到handler的方法)、method反射调用获取ModelAndView。

请求返回:逻辑视图到物理视图的转换、物理视图的渲染、视图返回。

具体流程如下:

系统启动:

1、web容器自己去将contextPath、docBase设置到一个context里面,这里面的一个context就是对应一个web应用。

2、web容器会根据docBase的值去获取web.xml,并解析它来获取servlet信息,并设置web容器启动完毕的监听器。

3、web容器启动后,会触发spring mvc容器的启动,spring mvc容器启动时,会解析controller,并将@RequestMapping、@GetMapping、@PostMapping的值设置到handlerMap中,方便后续请求路由。

请求发送:

1、外部发送请求(http://localhost:8080/spring-mvc-demo/user/register)时,请求会被转发到web容器(这里以tomcat为例),实际上就是tomcat与客户端建立了socket链接。

2、根据url,tomcat会对应的host,host找到context,context找到对应的servlet(这里为dispatcherServlet)。

3、dispatcherServlet会根据url,在handlerMap中去查到到对应的handler,然后将handler转化为handlerAdapter。

4、AnnotationMethodHandlerAdapter会调用ServletHandlerMethodInvoker.invokeHandlerMethod方法,ServletHandlerMethodInvoker会通过反射的方式去调用controller的对应方法。

请求返回:

1、根据controller的返回,获取对应的ModelAndView。

2、DispatcherServlet的resolveViewName方法会将逻辑视图转换为物理视图。

3、org.springframework.web.servlet.view.AbstractView#render方法会进行视图渲染工作,具体的渲染视图为org.springframework.web.servlet.view.JstlView

4、jsp文件会被编译成一个servlet,然后,jspServlet会调用service方法,最后会将视图写到客户端。

三、系统启动

1、context设置

我们通过shell脚本调用gradle的tomcatRun方法来启动应用,然后在本地debug的方式来获取运行参数。在org.apache.catalina.startup.Tomcat#addWebapp(org.apache.catalina.Host, java.lang.String, java.lang.String)的方法上打断点,获取信息如下:

这里的listener为ContextConfig,它会监听容器相关事件,其中一项工作就是监听tomcat启动后去解析web.xml。也可以看出contextPath、docBase的值。

被调用的addWebapp方法就是初始化context,并将context添加到host中。具体如下:

    public Context addWebapp(Host host, String contextPath, String docBase,
            LifecycleListener config) {

        silence(host, contextPath);

        Context ctx = createContext(host, contextPath);
        ctx.setPath(contextPath);
        ctx.setDocBase(docBase);
        ctx.addLifecycleListener(new DefaultWebXmlListener());
        ctx.setConfigFile(getWebappConfigFile(docBase, contextPath));

        ctx.addLifecycleListener(config);

        if (config instanceof ContextConfig) {
            // prevent it from looking ( if it finds one - it'll have dup error )
            ((ContextConfig) config).setDefaultWebXml(noDefaultWebXmlPath());
        }

        if (host == null) {
            getHost().addChild(ctx);
        } else {
            host.addChild(ctx);
        }

        return ctx;
    }

2、context中servlet设置

通过在ContextConfig的lifecycleEvent方法是监听系统事件的入口:

    public void lifecycleEvent(LifecycleEvent event) {

        // Identify the context we are associated with
        try {
            context = (Context) event.getLifecycle();
        } catch (ClassCastException e) {
            log.error(sm.getString("contextConfig.cce", event.getLifecycle()), e);
            return;
        }

        // Process the event that has occurred
        if (event.getType().equals(Lifecycle.CONFIGURE_START_EVENT)) {
            configureStart();
        } else if (event.getType().equals(Lifecycle.BEFORE_START_EVENT)) {
            beforeStart();
        } else if (event.getType().equals(Lifecycle.AFTER_START_EVENT)) {
            // Restore docBase for management tools
            if (originalDocBase != null) {
                context.setDocBase(originalDocBase);
            }
        } else if (event.getType().equals(Lifecycle.CONFIGURE_STOP_EVENT)) {
            configureStop();
        } else if (event.getType().equals(Lifecycle.AFTER_INIT_EVENT)) {
            init();
        } else if (event.getType().equals(Lifecycle.AFTER_DESTROY_EVENT)) {
            destroy();
        }

    }

通过在这个方法上打断点,在监听到after_init事件后,我们可以看到context的servletMappings的值如下:

对照web.xml的配置:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
	http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath:/applicationContext.xml</param-value>
    </context-param>
    <listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>

    <servlet>
        <servlet-name>smart</servlet-name>
        <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
        <load-on-startup>1</load-on-startup>
    </servlet>

    <servlet-mapping>
        <servlet-name>smart</servlet-name>
        <url-pattern>/</url-pattern>
    </servlet-mapping>
    
    <welcome-file-list>
        <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>

</web-app>

可以看到,DispatcherServlet被加载到context中,因此,该context中的“/”请求会被分配给DispatcherServlet处理。

3、handlerMap初始化

在org.springframework.web.servlet.handler.AbstractDetectingUrlHandlerMapping#detectHandlers上打断点,我们可以看见org.springframework.web.servlet.handler.BeanNameUrlHandlerMapping和org.springframework.web.servlet.mvc.annotation.DefaultAnnotationHandlerMapping会被用来检测handler。

其中BeanNameUrlHandlerMapping的检测方式如下:

	protected String[] determineUrlsForHandler(String beanName) {
		List<String> urls = new ArrayList<String>();
		if (beanName.startsWith("/")) {
			urls.add(beanName);
		}
		String[] aliases = getApplicationContext().getAliases(beanName);
		for (String alias : aliases) {
			if (alias.startsWith("/")) {
				urls.add(alias);
			}
		}
		return StringUtils.toStringArray(urls);
	}

它会检测到如下类型的handler

@Controller("/person")
public class PersonController{}

DefaultAnnotationHandlerMapping的检测方式如下:

@Override
	protected String[] determineUrlsForHandler(String beanName) {
		ApplicationContext context = getApplicationContext();
		Class<?> handlerType = context.getType(beanName);
		RequestMapping mapping = context.findAnnotationOnBean(beanName, RequestMapping.class);
		if (mapping != null) {
			// @RequestMapping found at type level
			this.cachedMappings.put(handlerType, mapping);
			Set<String> urls = new LinkedHashSet<String>();
			String[] typeLevelPatterns = mapping.value();
			if (typeLevelPatterns.length > 0) {
				// @RequestMapping specifies paths at type level
				String[] methodLevelPatterns = determineUrlsForHandlerMethods(handlerType, true);
				for (String typeLevelPattern : typeLevelPatterns) {
					if (!typeLevelPattern.startsWith("/")) {
						typeLevelPattern = "/" + typeLevelPattern;
					}
					boolean hasEmptyMethodLevelMappings = false;
					for (String methodLevelPattern : methodLevelPatterns) {
						if (methodLevelPattern == null) {
							hasEmptyMethodLevelMappings = true;
						}
						else {
							String combinedPattern = getPathMatcher().combine(typeLevelPattern, methodLevelPattern);
							addUrlsForPath(urls, combinedPattern);
						}
					}
					if (hasEmptyMethodLevelMappings ||
							org.springframework.web.servlet.mvc.Controller.class.isAssignableFrom(handlerType)) {
						addUrlsForPath(urls, typeLevelPattern);
					}
				}
				return StringUtils.toStringArray(urls);
			}
			else {
				// actual paths specified by @RequestMapping at method level
				return determineUrlsForHandlerMethods(handlerType, false);
			}
		}
		else if (AnnotationUtils.findAnnotation(handlerType, Controller.class) != null) {
			// @RequestMapping to be introspected at method level
			return determineUrlsForHandlerMethods(handlerType, false);
		}
		else {
			return null;
		}
	}

即根据@RequestMapping来检测url,检测到url后,会将url为key,对应的controller为value放到handlerMap中。

四、请求发送

1、请求context获取

在org.apache.catalina.mapper.Mapper#internalMap方法中,会根据url去查找host和context。

这里的host为localhost,根据这个去hosts列表中查找对应的host。

再在查找到的host的contextlist中去查找context。找到后,会将context的信息设置到mappingData

2、servlet获取

获取到context后,在根据请求url以及context中的servletMapping就可以得到对应的servlet,之后就会调用对应的servlet的service方法。以请求http://localhost:8080/spring-mvc-demo/user/register(get方法)为例,会调用org.springframework.web.servlet.FrameworkServlet#doGet方法,顺着流程,就会走到DispatcherServlet的doDispatch方法了。

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

		if (logger.isDebugEnabled()) {
			String requestUri = urlPathHelper.getRequestUri(request);
			String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
			logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
					" processing " + request.getMethod() + " request for [" + requestUri + "]");
		}

		// Keep a snapshot of the request attributes in case of an include,
		// to be able to restore the original attributes after the include.
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
			logger.debug("Taking snapshot of request attributes before include");
			attributesSnapshot = new HashMap<String, Object>();
			Enumeration<?> attrNames = request.getAttributeNames();
			while (attrNames.hasMoreElements()) {
				String attrName = (String) attrNames.nextElement();
				if (this.cleanupAfterInclude || attrName.startsWith("org.springframework.web.servlet")) {
					attributesSnapshot.put(attrName, request.getAttribute(attrName));
				}
			}
		}

		// Make framework objects available to handlers and view objects.
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());

		FlashMap inputFlashMap = this.flashMapManager.retrieveAndUpdate(request, response);
		if (inputFlashMap != null) {
			request.setAttribute(INPUT_FLASH_MAP_ATTRIBUTE, Collections.unmodifiableMap(inputFlashMap));
		}
		request.setAttribute(OUTPUT_FLASH_MAP_ATTRIBUTE, new FlashMap());
		request.setAttribute(FLASH_MAP_MANAGER_ATTRIBUTE, this.flashMapManager);

		try {
			doDispatch(request, response);
		}
		finally {
			if (WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
				return;
			}
			// Restore the original attribute snapshot, in case of an include.
			if (attributesSnapshot != null) {
				restoreAttributesAfterInclude(request, attributesSnapshot);
			}
		}
	}

3、handler获取

在前文说过,handler会被放到handlerMap中,key为请求的url。

请求处理已经在《Spring MVC请求处理流程分析》说过,就不再详述了。

五、请求返回

视图渲染在方法:org.springframework.web.servlet.DispatcherServlet#render中进行,具体如下:

我们配置的视图为:org.springframework.web.servlet.view.JstlView,它会将视图渲染后,然后,通过JspServlet的service方法将视图通过writer.out输出到客户端。

我们打开register_jsp.java文件,其所在目录如下:

其service方法内容如下:

public void _jspService(final javax.servlet.http.HttpServletRequest request, final javax.servlet.http.HttpServletResponse response)
        throws java.io.IOException, javax.servlet.ServletException {

final java.lang.String _jspx_method = request.getMethod();
if (!"GET".equals(_jspx_method) && !"POST".equals(_jspx_method) && !"HEAD".equals(_jspx_method) && !javax.servlet.DispatcherType.ERROR.equals(request.getDispatcherType())) {
response.sendError(HttpServletResponse.SC_METHOD_NOT_ALLOWED, "JSPs only permit GET POST or HEAD");
return;
}

    final javax.servlet.jsp.PageContext pageContext;
    javax.servlet.http.HttpSession session = null;
    final javax.servlet.ServletContext application;
    final javax.servlet.ServletConfig config;
    javax.servlet.jsp.JspWriter out = null;
    final java.lang.Object page = this;
    javax.servlet.jsp.JspWriter _jspx_out = null;
    javax.servlet.jsp.PageContext _jspx_page_context = null;


    try {
      response.setContentType("text/html; charset=UTF-8");
      pageContext = _jspxFactory.getPageContext(this, request, response,
      			null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;

      out.write("\n");
      out.write("\n");
      out.write("\n");
      out.write("<html>\n");
      out.write("<head>\n");
      out.write("    <title>新增用户</title>\n");
      out.write("</head>\n");
      out.write("<body>\n");
      out.write("<form method=\"post\" action=\"");
      if (_jspx_meth_c_005furl_005f0(_jspx_page_context))
        return;
      out.write("\">\n");
      out.write("    <table>\n");
      out.write("        <tr>\n");
      out.write("            <td>用户名:</td>\n");
      out.write("            <td><input type=\"text\" name=\"userName\"  value=\"");
      out.write((java.lang.String) org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate("${user.userName}", java.lang.String.class, (javax.servlet.jsp.PageContext)_jspx_page_context, null));
      out.write("\"/></td>\n");
      out.write("        </tr>\n");
      out.write("        <tr>\n");
      out.write("            <td>密码:</td>\n");
      out.write("            <td><input type=\"password\" name=\"password\" value=\"");
      out.write((java.lang.String) org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate("${user.password}", java.lang.String.class, (javax.servlet.jsp.PageContext)_jspx_page_context, null));
      out.write("\"/></td>\n");
      out.write("        </tr>\n");
      out.write("        <tr>\n");
      out.write("            <td>姓名:</td>\n");
      out.write("            <td><input type=\"text\" name=\"realName\" value=\"");
      out.write((java.lang.String) org.apache.jasper.runtime.PageContextImpl.proprietaryEvaluate("${user.realName}", java.lang.String.class, (javax.servlet.jsp.PageContext)_jspx_page_context, null));
      out.write("\"/></td>\n");
      out.write("        </tr>\n");
      out.write("        <tr>\n");
      out.write("            <td colspan=\"2\"><input type=\"submit\" name=\"提交\"/></td>\n");
      out.write("        </tr>\n");
      out.write("    </table>\n");
      out.write("</form>\n");
      out.write("</body>\n");
      out.write("</html>");
    } catch (java.lang.Throwable t) {
      if (!(t instanceof javax.servlet.jsp.SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          try {
            if (response.isCommitted()) {
              out.flush();
            } else {
              out.clearBuffer();
            }
          } catch (java.io.IOException e) {}
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
        else throw new ServletException(t);
      }
    } finally {
      _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }

因此,我们可以猜测,register.jsp被渲染后,通过writer.out方法将视图输出到客户端的。

作者:yangjianzhou