在分析任何框架源码之前，首先要做的就是搭建一个可以本地运行的示例，通过接合示例去阅读源码，这样效果会好很多。
简单介绍以下搭建Spring Boot示例步骤：
1、开发工具这里使用的是Idea，创建project，使用maven构建。
2、如果需要创建多模块工程，project创建好之后，然后再创建Module，可以在project目录下pom里引入基础依赖：
org.springframework.boot
spring-boot-starter-parent
2.0.2.RELEASE
3、在具体的Module模块引入所需要的依赖：
org.springframework.boot
spring-boot-starter-web
4、创建启动类WebMvcBootstrap和测试的WebMvcController
目录结构如下：
image.png
/**
* spring boot webmvc启动类
* @author codingroad
* @date 2019-08-31
*/
@SpringBootApplication(scanBasePackages = "com.codingroad.webmvc")
public class WebMvcBootstrap {
public static void main(String[] args) {
SpringApplication.run(WebMvcBootstrap.class, args);
}
}
测试WebMvcController内容：
/**
* WebMvcController
* @author codingroad
* @date 2019-08-31
*/
@RestController
public class WebMvcController {
@RequestMapping("/hello")
public String hello() {
return "hello spring boot webmvc";
}
}
简单的web示例就创建好了，启动WebMvcBootstrap，启动完成之后，可以直接通过浏览器输入url路径进行访问测试：
http://localhost:8080/hello
image.png
Demo很简单，但是框架内部的运行原理非常复杂，下面是执行流程图：
image.png
1、客户端第一次发送请求，Servlet容器执行StandardWrapper的initServlet()方法，
具体方法代码如下：
/**
* servelt DispatcherServlet
*/
private synchronized void initServlet(Servlet servlet)
throws ServletException {
if (instanceInitialized && !singleThreadModel) return;
// Call the initialization method of this servlet
try {
if( Globals.IS_SECURITY_ENABLED) {//没有设置SecurityManager，所以走下面的else逻辑
boolean success = false;
try {
Object[] args = new Object[] { facade };
SecurityUtil.doAsPrivilege("init",
servlet,
classType,
args);
success = true;
} finally {
if (!success) {
// destroy() will not be called, thus clear the reference now
SecurityUtil.remove(servlet);
}
}
} else {
servlet.init(facade); //进入下一个流程GenericServlet的init方法
}
instanceInitialized = true;
} catch (UnavailableException f) { }
}
2、上面方法执行到servlet.init()，然后进入GenericServlet的init方法，该方法源码如下：
@Override
public void init(ServletConfig config) throws ServletException {
this.config = config;
this.init();
}
3、继续下一个流程HttpServletBean的init()方法，为了方便查看，删除掉部分异常及日志代码，源码如下：
@Override
public final void init() throws ServletException {
// Set bean properties from init parameters.
PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
if (!pvs.isEmpty()) {// 没有通过init-parameters设置属性，所以pvs是空的
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) {
throw ex;
}
}
// Let subclasses do whatever initialization they like.
initServletBean(); // 进入FrameworkServlet的initServletBean方法
}
4、进入FrameworkServlet的initServletBean()方法，同上面一样去掉异常及日志打印相关方法：
@Override
protected final void initServletBean() throws ServletException {
try {
this.webApplicationContext = initWebApplicationContext();
initFrameworkServlet();
}
catch (ServletException | RuntimeException ex) {
throw ex;
}
}
进入initWebApplicationContext()方法，源码如下：
protected WebApplicationContext initWebApplicationContext() {
WebApplicationContext rootContext =
WebApplicationContextUtils.getWebApplicationContext(getServletContext());
WebApplicationContext wac = null;
if (this.webApplicationContext != null) {
// A context instance was injected at construction time -> use it
wac = this.webApplicationContext;
if (wac instanceof ConfigurableWebApplicationContext) {
ConfigurableWebApplicationContext cwac = (ConfigurableWebApplicationContext) wac;
if (!cwac.isActive()) {
// The context has not yet been refreshed -> provide services such as
// setting the parent context, setting the application context id, etc
if (cwac.getParent() == null) {
// The context instance was injected without an explicit parent -> set
// the root application context (if any; may be null) as the parent
cwac.setParent(rootContext);
}
configureAndRefreshWebApplicationContext(cwac);
}
}
}
if (wac == null) {
wac = findWebApplicationContext();
}
if (wac == null) {
// No context instance is defined for this servlet -> create a local one
wac = createWebApplicationContext(rootContext);
}
if (!this.refreshEventReceived) {
// refreshed -> trigger initial onRefresh manually here.
onRefresh(wac); //进入DispatcherServlet的onRefresh()方法
}
if (this.publishContext) {
// Publish the context as a servlet context attribute.
String attrName = getServletContextAttributeName();
getServletContext().setAttribute(attrName, wac);
}
return wac;
}
5、进入DispatcherServlet的onRefresh()方法，方法源码：
@Override
protected void onRefresh(ApplicationContext context) {
initStrategies(context);
}
重点关注initStrategies(context)方法；
/**
* 初始化servlet需要使用的对象
* 子类也可以扩展该方法，进行定制化实现
*/
protected void initStrategies(ApplicationContext context) {
initMultipartResolver(context);
initLocaleResolver(context);
initThemeResolver(context);
initHandlerMappings(context);
initHandlerAdapters(context);
initHandlerExceptionResolvers(context);
initRequestToViewNameTranslator(context);
initViewResolvers(context);
initFlashMapManager(context);
}
可以看到上面按顺序初始化servlet使用的对象，这里主要看下initHandlerMappings(context)和initHandlerAdapters(context)。
在initHandlerMappings方法初始化完成后，DispatcherServlet的属性List
image.png
在initHandlerAdapters方法初始化完成后，DispatcherServlet的属性List
image.png
上面一系列servlet对象初始化完成之后，相当于servlet的init()方法执行完成，紧接着开始执行service方法。
本篇内容先介绍到这里，客户端第一次发送请求，会调用servlet的init方法进行初始化servlet所需要的对象，初始化完成之后，紧接着调用service方法，下一篇开始介绍service方法的处理逻辑。
结合上一篇内容继续，前面讲到客户端第一次发送请求时，会先调用servlet的init方法将所需对象进行初始化，初始化操作完成之后，紧接着调用servlet的service方法开始处理，处理流程包括获取Handler、HandlerAdapter，对url请求的解析，然后通过反射的方式调用Controller里的method方法，方法逻辑处理完成以后，再进行响应返回信息给客户端。下面开始介绍servlet的service方法处理逻辑。
1、进入HttpServlet的service(ServletRequest,ServletResponse)方法，源码如下：
@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); // 接着执行FrameworkServlet的service方法
}
2、进入FrameworkServlet的service方法，源码如下：
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod()); // 直接通过浏览器访问，所以httpMethod=GET,进入else流程
if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
processRequest(request, response);
}
else {
super.service(request, response); // 调用父类HttpServlet的service方法
}
}
3、进入HttpServlet的service方法，源码如下：
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod(); // 通过浏览器输入网址，method即GET
if (method.equals(METHOD_GET)) {
long lastModified = getLastModified(req);//该方法默认返回-1
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
doGet(req, resp);// 进入子类FrameworkServlet的doGet方法
} else {
long ifModifiedSince;
try {
ifModifiedSince = req.getDateHeader(HEADER_IFMODSINCE);
} catch (IllegalArgumentException iae) {
// Invalid date header - proceed as if none was set
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
maybeSetLastModified(resp, lastModified);
doGet(req, resp);
} else {
resp.setStatus(HttpServletResponse.SC_NOT_MODIFIED);
}
}
} else if (method.equals(METHOD_HEAD)) {
long lastModified = getLastModified(req);
maybeSetLastModified(resp, lastModified);
doHead(req, resp);
} else if (method.equals(METHOD_POST)) {
doPost(req, resp);
} else if (method.equals(METHOD_PUT)) {
doPut(req, resp);
} else if (method.equals(METHOD_DELETE)) {
doDelete(req, resp);
} else if (method.equals(METHOD_OPTIONS)) {
doOptions(req,resp);
} else if (method.equals(METHOD_TRACE)) {
doTrace(req,resp);
} else {
//
// Note that this means NO servlet supports whatever
// method was requested, anywhere on this server.
//
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);
}
}
4、进入FramewokrServlet的doGet()方法，源码如下：
@Override
protected final void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
processRequest(request, response);
}
protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
long startTime = System.currentTimeMillis();
Throwable failureCause = null;
LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
LocaleContext localeContext = buildLocaleContext(request);
RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.registerCallableInterceptor(FrameworkServlet.class.getName(), new RequestBindingInterceptor());
initContextHolders(request, localeContext, requestAttributes);
try {
doService(request, response);//进入子类DispatcherServlet的doService方法
}
catch (ServletException | IOException ex) {
throw ex;
}
catch (Throwable ex) {
throw new NestedServletException("Request processing failed", ex);
}
finally {
resetContextHolders(request, previousLocaleContext, previousAttributes);
if (requestAttributes != null) {
requestAttributes.requestCompleted();
}
publishRequestHandledEvent(request, response, startTime, failureCause);
}
}
5、进入DispatcherServlet的doService方法，源码如下：
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
// Keep a snapshot of the request attributes in case of an include,
// to be able to restore the original attributes after the include.
Map attributesSnapshot = null;
if (WebUtils.isIncludeRequest(request)) {
attributesSnapshot = new HashMap<>();
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());
if (this.flashMapManager != null) {
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); //开发方法的处理，处理完之后返回信息给客户端
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
HttpServletRequest processedRequest = request;
HandlerExecutionChain mappedHandler = null;
boolean multipartRequestParsed = false;
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
try {
ModelAndView mv = null;
Exception dispatchException = null;
try {
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
mappedHandler = getHandler(processedRequest);
if (mappedHandler == null) {
noHandlerFound(processedRequest, response);
return;
}
// Determine handler adapter for the current request.
HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());
// Process last-modified header, if supported by the handler.
String method = request.getMethod();
boolean isGet = "GET".equals(method);
if (isGet || "HEAD".equals(method)) {
long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
if (logger.isDebugEnabled()) {
logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
}
if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
mappedHandler.applyPostHandle(processedRequest, response, mv);
}
catch (Exception ex) {
dispatchException = ex;
}
catch (Throwable err) {
// As of 4.3, we're processing Errors thrown from handler methods as well,
// making them available for @ExceptionHandler methods and other scenarios.
dispatchException = new NestedServletException("Handler dispatch failed", err);
}
processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
}
catch (Exception ex) {
triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
}
catch (Throwable err) {
triggerAfterCompletion(processedRequest, response, mappedHandler,
new NestedServletException("Handler processing failed", err));
}
finally {
if (asyncManager.isConcurrentHandlingStarted()) {
// Instead of postHandle and afterCompletion
if (mappedHandler != null) {
mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
}
}
else {
// Clean up any resources used by a multipart request.
if (multipartRequestParsed) {
cleanupMultipart(processedRequest);
}
}
}
}
DispatcherServlet的doDispatch()方法下面详细介绍一下，首先获取handler对象，
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
for (HandlerMapping hm : this.handlerMappings) {
if (logger.isTraceEnabled()) {
logger.trace(
"Testing handler map [" + hm + "] in DispatcherServlet with name '" + getServletName() + "'");
}
HandlerExecutionChain handler = hm.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
通过上面的方法可以看到，遍历之前init方法初始化赋值的handlerMappings集合，里面包含7中HanderMapping对象，在遍历过程中匹配到RequestMappingHandlerMapping。
image.png
取到handlerMapping对象之后，调用HandlerMapping的getHandler方法，该方法源码如下：
@Override
@Nullable
public final HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
Object handler = getHandlerInternal(request); //这里取到的handler对象包含请求对应的Controller信息及method
if (handler == null) {
handler = getDefaultHandler();
}
if (handler == null) {
return null;
}
// Bean name or resolved handler?
if (handler instanceof String) {
String handlerName = (String) handler;
handler = obtainApplicationContext().getBean(handlerName);
}
HandlerExecutionChain executionChain = getHandlerExecutionChain(handler, request);
if (CorsUtils.isCorsRequest(request)) {
CorsConfiguration globalConfig = this.globalCorsConfigSource.getCorsConfiguration(request);
CorsConfiguration handlerConfig = getCorsConfiguration(handler, request);
CorsConfiguration config = (globalConfig != null ? globalConfig.combine(handlerConfig) : handlerConfig);
executionChain = getCorsHandlerExecutionChain(request, executionChain, config);
}
return executionChain;
}
handler对象获取到后，根据handler开始获取HandlerAdapter对象，源码如下：
protected HandlerAdapter getHandlerAdapter(Object handler) throws ServletException {
if (this.handlerAdapters != null) {
for (HandlerAdapter ha : this.handlerAdapters) {
if (logger.isTraceEnabled()) {
logger.trace("Testing handler adapter [" + ha + "]");
}
if (ha.supports(handler)) {
return ha;
}
}
}
throw new ServletException("No adapter for handler [" + handler +
"]: The DispatcherServlet configuration needs to include a HandlerAdapter that supports this handler");
}
通过上面的源码可以看到，也是遍历之前在init方法初始赋值的handlerAdapters集合对象，里面包含三种HandlerAdapter，如下图：
image.png
遍历过程中只要匹配到一种HandlerAdapter就直接返回，这里可以看到匹配的是RequestMappingHandlerAdapter对象，获取到HandlerAdapter之后，开始进行方法的调用：
image.png
接着调用方法handleInternal，源码如下：
@Override
protected ModelAndView handleInternal(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ModelAndView mav;
checkRequest(request);
// Execute invokeHandlerMethod in synchronized block if required.
if (this.synchronizeOnSession) {
HttpSession session = request.getSession(false);
if (session != null) {
Object mutex = WebUtils.getSessionMutex(session);
synchronized (mutex) {
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No HttpSession available -> no mutex necessary
mav = invokeHandlerMethod(request, response, handlerMethod);
}
}
else {
// No synchronization on session demanded at all...
mav = invokeHandlerMethod(request, response, handlerMethod);
}
if (!response.containsHeader(HEADER_CACHE_CONTROL)) {
if (getSessionAttributesHandler(handlerMethod).hasSessionAttributes()) {
applyCacheSeconds(response, this.cacheSecondsForSessionAttributeHandlers);
}
else {
prepareResponse(response);
}
}
return mav;
}
接着执行invokeHandlerMethod方法，源码如下：
@Nullable
protected ModelAndView invokeHandlerMethod(HttpServletRequest request,
HttpServletResponse response, HandlerMethod handlerMethod) throws Exception {
ServletWebRequest webRequest = new ServletWebRequest(request, response);
try {
WebDataBinderFactory binderFactory = getDataBinderFactory(handlerMethod);
ModelFactory modelFactory = getModelFactory(handlerMethod, binderFactory);
ServletInvocableHandlerMethod invocableMethod = createInvocableHandlerMethod(handlerMethod);
if (this.argumentResolvers != null) {
invocableMethod.setHandlerMethodArgumentResolvers(this.argumentResolvers);
}
if (this.returnValueHandlers != null) {
invocableMethod.setHandlerMethodReturnValueHandlers(this.returnValueHandlers);
}
invocableMethod.setDataBinderFactory(binderFactory);
invocableMethod.setParameterNameDiscoverer(this.parameterNameDiscoverer);
ModelAndViewContainer mavContainer = new ModelAndViewContainer();
mavContainer.addAllAttributes(RequestContextUtils.getInputFlashMap(request));
modelFactory.initModel(webRequest, mavContainer, invocableMethod);
mavContainer.setIgnoreDefaultModelOnRedirect(this.ignoreDefaultModelOnRedirect);
AsyncWebRequest asyncWebRequest = WebAsyncUtils.createAsyncWebRequest(request, response);
asyncWebRequest.setTimeout(this.asyncRequestTimeout);
WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
asyncManager.setTaskExecutor(this.taskExecutor);
asyncManager.setAsyncWebRequest(asyncWebRequest);
asyncManager.registerCallableInterceptors(this.callableInterceptors);
asyncManager.registerDeferredResultInterceptors(this.deferredResultInterceptors);
if (asyncManager.hasConcurrentResult()) {
Object result = asyncManager.getConcurrentResult();
mavContainer = (ModelAndViewContainer) asyncManager.getConcurrentResultContext()[0];
asyncManager.clearConcurrentResult();
if (logger.isDebugEnabled()) {
logger.debug("Found concurrent result value [" + result + "]");
}
invocableMethod = invocableMethod.wrapConcurrentResult(result);
}
invocableMethod.invokeAndHandle(webRequest, mavContainer);
if (asyncManager.isConcurrentHandlingStarted()) {
return null;
}
return getModelAndView(mavContainer, modelFactory, webRequest);
}
finally {
webRequest.requestCompleted();
}
}
可以看到上面方法的主要流程是构建ServletInvocableHandlerMethod对象，然后调用该对象的invokeAndHandle方法，最终会进入invokeForRequest方法，该方法主要是获取请求参数，然后通过反射的方式调用请求Controller对应的method方法，源码如下：
@Nullable
public Object invokeForRequest(NativeWebRequest request, @Nullable ModelAndViewContainer mavContainer,
Object... providedArgs) throws Exception {
Object[] args = getMethodArgumentValues(request, mavContainer, providedArgs);
if (logger.isTraceEnabled()) {
logger.trace("Invoking '" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"' with arguments " + Arrays.toString(args));
}
Object returnValue = doInvoke(args);
if (logger.isTraceEnabled()) {
logger.trace("Method [" + ClassUtils.getQualifiedMethodName(getMethod(), getBeanType()) +
"] returned [" + returnValue + "]");
}
return returnValue;
}
上面代码执行完之后，客户端的请求对应controller的method就执行完成，下一步是选取适合的HandlerMethodReturnValueHandler，最后会匹配到RequestResponseBodyMethodProcessor对象，其实这和我们刚开始创建的Controller对象是对应的，Controller当时添加的注解是@RestController，该注解是@Controller和@ResponseBody两个注解的组合，Controller里的method虽然只加了@RequestMapping注解，由于@RestController注解的作用，所以对应该Controller方法都会默认加上@ResponseBody注解，最终通过调用RequestResponseBodyMethodProcessor的父类AbstractMessageConverterMethodProcessor的writeWithMessageConverters方法将处理结果返回给客户端。