请求在被Servlet处理之前会先被过滤器处理,之后调用Servlet的service方法来对相应的请求进行处理响应。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-mvcservlet-name>
<servlet-class>org.springframework.web.servlet.DispatcherServletservlet-class>
<init-param>
<param-name>contextConfigLocationparam-name>
<param-value>classpath:learn-spring-mvc.xmlparam-value>
init-param>
<load-on-startup>1load-on-startup>
servlet>
<servlet-mapping>
<servlet-name>spring-mvcservlet-name>
<url-pattern>/url-pattern>
servlet-mapping>
所有的请求(除静态资源)将由DispatcherServlet处理。
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。
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
if (logger.isDebugEnabled()) {
String resumed = WebAsyncUtils.getAsyncManager(request).hasConcurrentResult() ? " resumed" : "";
logger.debug("DispatcherServlet with name '" + getServletName() + "'" + resumed +
" processing " + request.getMethod() + " request for [" + getRequestUri(request) + "]");
}
// 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)) {
attributesSnapshot = new HashMap<String, Object>();
Enumeration<?> attrNames = request.getAttributeNames();
while (attrNames.hasMoreElements()) {
String attrName = (String) attrNames.nextElement();
if (this.cleanupAfterInclude || attrName.startsWith(DEFAULT_STRATEGIES_PREFIX)) {
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()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
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:
将ServletRequest和ServletResponse转换为HttpServletRequest和HttpServletResponse
因为web开发,用HTTP协议,所以需要HttpServletRequest和HttpServletResponse
接下来就是调用service(HttpServletRequest request, HttpServletResponse response),
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:
根据请求的方法类型转换对应的枚举类
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:
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);
}
}
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
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);
}
}
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。
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);
}
};
}
}
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);
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
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);
}
}
}
}
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