SpringMVC之请求处理源码分析从service到doDispatch(一)

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SpringMVC之请求处理源码分析从service到doDispatch(一)

木叶之荣 2017-07-16 11:07:43 浏览698
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说明:本文所用的SpringMVC版本为4.3.4.RELEASE,应用服务器为TomCat8.0.33。

前面几篇文章我们简单的分析了一下SpringMVC初始化的过程,接下来的这几篇文章我们从源码的角度分析一下SpringMVC对请求的处理过程。这一篇文章我们来分析请求从service到DispatcherServlet的doDispatch的过程。我们做web开发的同学大都对Servlet的生命周期比较了解,对Servlet规范也有一些了解。我们知道请求在被Servlet处理之前会先被过滤器处理,过滤器处理完之后会调用Servlet的service方法来对相应的请求进行处理响应(关于过滤器可以看这里)。所以我们这里分析的入口是Servlet的service方法。我们在用SpringMVC的时候,通常都会在web.xml中进行这样的配置:

	<servlet>
		<servlet-name>spring-mvc</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<param-name>contextConfigLocation</param-name>
			<param-value>classpath:learn-spring-mvc.xml</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>
	<servlet-mapping>
		<servlet-name>spring-mvc</servlet-name>
		<url-pattern>/</url-pattern>
	</servlet-mapping>
我们了解Servlet的同学都知道这样配置的话,所有的请求(静态资源除外)将由DispatcherServlet进行处理。即我们这里处理请求的Servlet类是DispatcherServlet。下面首先我们来看一下DispatcherServlet的UML类图:


在上面的类图中我们先关注蓝色线的这一部分,即左面的这一部分内容。从上面的类图中我们可以看到DispatcherServlet继承了FrameworkServlet,FrameworkServlet继承了HttpServletBean,HttpServletBean继承了HttpServlet,HttpServlet继承了GenericServlet,GenericServlet则实现了我们最顶级的接口Servlet和ServletConfig。请求的主要时序图如下:


从DispatcherServlet的源码中我们没有找到service(ServletRequest req, ServletResponse res)这个方法,但是我们在DispatcherServlet的父类HttpServlet中找到了这个方法,我们去HttpServlet中看看这个方法的内容:

HttpServlet#service(ServletRequest req, ServletResponse res)

    @Override
    public void service(ServletRequest req, ServletResponse res)
        throws ServletException, IOException {

        HttpServletRequest  request;
        HttpServletResponse response;

        try {
            request = (HttpServletRequest) req;
            response = (HttpServletResponse) res;
        } catch (ClassCastException e) {
            throw new ServletException("non-HTTP request or response");
        }
        service(request, response);
    }
}
service这个方法的内容很简单,就是将ServletRequest和ServletResponse转换为HttpServletRequest和HttpServletResponse。因为我们是做web开发,通常用的是HTTP协议,所以这里我们需要的时候HttpServletRequest和HttpServletResponse。接下来就是调用service(HttpServletRequest request, HttpServletResponse response),我们在HttpServlet和FrameworkServlet中都找到了这个方法,但是HttpServlet是FrameworkServlet的父类,即FrameworkServlet中重写了service这个方法,所以我们这里取FrameworkServlet中去看看这个方法的内容:

FrameworkServlet#service(HttpServletRequest request, HttpServletResponse response)

	protected void service(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
		if (HttpMethod.PATCH == httpMethod || httpMethod == null) {
			processRequest(request, response);
		}
		else {
			super.service(request, response);
		}
	}
这个方法的内容也很简单,第一步根据请求的方法类型转换对应的枚举类。我们可以看一下HttpMethod这个枚举类:
public enum HttpMethod {
	GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;
	private static final Map<String, HttpMethod> mappings = new HashMap<String, HttpMethod>(8);
	static {
		for (HttpMethod httpMethod : values()) {
			mappings.put(httpMethod.name(), httpMethod);
		}
	}
	public static HttpMethod resolve(String method) {
		return (method != null ? mappings.get(method) : null);
	}
	public boolean matches(String method) {
		return (this == resolve(method));
	}
}
HttpMethod这个定义了这样的几种枚举类型:GET, HEAD, POST, PUT, PATCH, DELETE, OPTIONS, TRACE;而这些也是RFC标准中几种请求类型。如果请求类型为PATCH或者没有找到相应的请求类型的话,则直接调用processRequest这个方法。但是这种情况我们很少很少会遇到。所以这里会执行super.service这个方法。即调用HttpServlet中的service方法。我们看一下HttpServlet中这个service方法的内容:

HttpServlet#service(HttpServletRequest req, HttpServletResponse resp)

    protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
		//获取请求类型
        String method = req.getMethod();
		//如果是get请求
        if (method.equals(METHOD_GET)) {
			//检查是不是开启了页面缓存 通过header头的 Last-Modified/If-Modified-Since
			//获取Last-Modified的值
            long lastModified = getLastModified(req);
            if (lastModified == -1) {
                // servlet doesn't support if-modified-since, no reason
                // to go through further expensive logic
				//没有开启页面缓存调用doGet方法
                doGet(req, resp);
            } else {
                long ifModifiedSince;
                try {
					//获取If-Modified-Since的值
                    ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
                } catch (IllegalArgumentException iae) {
                    ifModifiedSince = -1;
                }
                if (ifModifiedSince < (lastModified / 1000 * 1000)) {
                    // If the servlet mod time is later, call doGet()
                    // Round down to the nearest second for a proper compare
                    // A ifModifiedSince of -1 will always be less
					//更新Last-Modified
                    maybeSetLastModified(resp, lastModified);
					//调用doGet方法
                    doGet(req, resp);
                } else {
					//设置304状态码 在HttpServletResponse中定义了很多常用的状态码
                    resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
                }
            }
        } else if (method.equals(METHOD_HEAD)) {
			//调用doHead方法
            long lastModified = getLastModified(req);
            maybeSetLastModified(resp, lastModified);
            doHead(req, resp);
        } else if (method.equals(METHOD_POST)) {
			//调用doPost方法
            doPost(req, resp);
        } else if (method.equals(METHOD_PUT)) {
		   //调用doPost方法
            doPut(req, resp);
        } else if (method.equals(METHOD_DELETE)) {
		    //调用doPost方法
            doDelete(req, resp);
        } else if (method.equals(METHOD_OPTIONS)) {
			//调用doPost方法
            doOptions(req,resp);
        } else if (method.equals(METHOD_TRACE)) {
			//调用doPost方法
            doTrace(req,resp);
        } else {
			//服务器不支持的方法 直接返回错误信息
            String errMsg = lStrings.getString("http.method_not_implemented");
            Object[] errArgs = new Object[1];
            errArgs[0] = method;
            errMsg = MessageFormat.format(errMsg, errArgs);
            resp.sendError(HttpServletResponse.SC_NOT_IMPLEMENTED, errMsg);
        }
    }
这个方法的主要作用是根据请求类型调用响应的请求方法如果GET类型,调用doGet方法;POST类型,调用doPost方法。这些方法都是在HttpServlet中定义的,平时我们做web开发的时候主要是继承HttpServlet这个类,然后重写它的doPost或者doGet方法。我们的FrameworkServlet这个子类就重写了这些方法中的一部分:doGet、doPost、doPut、doDelete、doOption、doTrace。这里我们只说我们最常用的doGet和doPost这两个方法。通过翻开源码我们发现,这两个方法体的内容是一样的,都是调用了processRequest这个方法。processRequest这个方法是我们接下来要分析的:

FrameworkServlet#processRequest(HttpServletRequest request, HttpServletResponse response)

	protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
			throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
		Throwable failureCause = null;
		LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
		//国际化
		LocaleContext localeContext = buildLocaleContext(request);
		RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
		//构建ServletRequestAttributes对象
		ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
		//异步管理
		WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
		asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
        //初始化ContextHolders
		initContextHolders(request, localeContext, requestAttributes);
		//执行doService
		try {
			doService(request, response);
		}
		finally {
			//重新设置ContextHolders
			resetContextHolders(request, previousLocaleContext, previousAttributes);
			if (requestAttributes != null) {
				requestAttributes.requestCompleted();
			}
			//发布请求处理事件
			publishRequestHandledEvent(request, response, startTime, failureCause);
		}
	}
在这个方法里大概做了这样几件事:国际化的设置,创建ServletRequestAttributes对象,初始化上下文holders(即将Request对象放入到线程上下文中),调用doService方法。我们分析主要的几个:
国际化的设置主要是在DispatcherServlet#buildLocaleContext这个方法中完成的,其源码如下:
	protected LocaleContext buildLocaleContext(final HttpServletRequest request) {
		if (this.localeResolver instanceof LocaleContextResolver) {
			return ((LocaleContextResolver) this.localeResolver).resolveLocaleContext(request);
		}
		else {
			return new LocaleContext() {
				@Override
				public Locale getLocale() {
					return localeResolver.resolveLocale(request);
				}
			};
		}
	}
根据我们之前文章的分析,如果我们没有配置国际化解析器的话,那么它会使用默认的解析器:AcceptHeaderLocaleResolver,即从Header中获取国际化的信息。除了AcceptHeaderLocaleResolver之外,SpringMVC中还提供了这样的几种解析器:CookieLocaleResolver、SessionLocaleResolver、FixedLocaleResolver。分别从cookie、session中去国际化信息和JVM默认的国际化信息(Local.getDefault())。
initContextHolders这个方法主要是将Request请求、ServletRequestAttribute对象和国际化对象放入到上下文中。其源码如下:
	private void initContextHolders(
			HttpServletRequest request, LocaleContext localeContext, RequestAttributes requestAttributes) {
		if (localeContext != null) {
			LocaleContextHolder.setLocaleContext(localeContext, this.threadContextInheritable);//threadContextInheritable默认为false
		}
		if (requestAttributes != null) {//threadContextInheritable默认为false
			RequestContextHolder.setRequestAttributes(requestAttributes, this.threadContextInheritable);
		}
	}
RequestContextHolder这个类有什么用呢?有时候我们想在某些类中获取HttpServletRequest对象,比如在AOP拦截的类中,那么我们就可以这样来获取Request的对象了,
HttpServletRequest request = (HttpServletRequest) RequestContextHolder.getRequestAttributes().resolveReference(RequestAttributes.REFERENCE_REQUEST);

DispatcherServlet#doService

我们接下来分析一下DispatcherServlet中的doService这个方法
	@Override
	protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
		//include这种的
		Map<String, Object> attributesSnapshot = null;
		if (WebUtils.isIncludeRequest(request)) {
			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));
				}
			}
		}
		//Spring上下文
		request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());
		//国际化解析器
		request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
		//主题解析器
		request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
		//主题
		request.setAttribute(THEME_SOURCE_ATTRIBUTE, getThemeSource());
		//重定向的数据
		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 {
			//调用doDispatch方法-核心方法
			doDispatch(request, response);
		}
		finally {
			if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
				// Restore the original attribute snapshot, in case of an include.
				if (attributesSnapshot != null) {
					restoreAttributesAfterInclude(request, attributesSnapshot);
				}
			}
		}
	}
在这个方法中主要做了这几件事:处理include标签的请求,将上下文放到request的属性中,将国际化解析器放到request的属性中,将主题解析器放到request属性中,将主题放到request的属性中,处理重定向的请求数据最后调用doDispatch这个核心的方法对请求进行处理,我们在下一章中详细分析一下doDispatch这个方法。

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