spring mvc DispatcherServlet原理解析
spring mvc DispatcherServlet原理及源码解析
首先先讲一下servlet
Servlet 是什么？
Java Servlet 是运行在 Web 服务器或应用服务器上的程序，它是作为来自 Web 浏览器或其他 HTTP 客户端的请求和 HTTP 服务器上
的数据库或应用程序之间的中间层。
使用 Servlet，您可以收集来自网页表单的用户输入，呈现来自数据库或者其他源的记录，还可以动态创建网页。
Java Servlet 通常情况下与使用 CGI（Common Gateway Interface，公共网关接口）实现的程序可以达到异曲同工的效果。但是相比于 CGI，Servlet 有以下几点优势：
- 性能明显更好。
- Servlet 在 Web 服务器的地址空间内执行。这样它就没有必要再创建一个单独的进程来处理每个客户端请求。
- Servlet 是独立于平台的，因为它们是用 Java 编写的。
服务器上的 Java 安全管理器执行了一系列限制，以保护服务器计算机上的资源。因此，Servlet 是可信的。 - Java 类库的全部功能对 Servlet 来说都是可用的。它可以通过 sockets 和 RMI 机制与 applets、数据库或其他软件进行交互。
Servlet 架构
Servlet 任务
- 读取客户端（浏览器）发送的显式的数据。这包括网页上的 HTML 表单，或者也可以是来自 applet 或自定义的 HTTP 客户端程序的表单。
- 读取客户端（浏览器）发送的隐式的 HTTP 请求数据。这包括 cookies、媒体类型和浏览器能理解的压缩格式等等。
- 处理数据并生成结果。这个过程可能需要访问数据库，执行 RMI 或 CORBA 调用，调用 Web 服务，或者直接计算得出对应的响应。
- 发送显式的数据（即文档）到客户端（浏览器）。该文档的格式可以是多种多样的，包括文本文件（HTML 或 XML）、二进制文件（GIF 图像）、Excel 等。
- 发送隐式的 HTTP 响应到客户端（浏览器）。这包括告诉浏览器或其他客户端被返回的文档类型（例如 HTML），设置 cookies 和缓存参数，以及其他类似的任务。
Servlet 生命周期
- Servlet 通过调用 init () 方法进行初始化。
- Servlet 调用 service() 方法来处理客户端的请求。
- Servlet 通过调用 destroy() 方法终止（结束）。
- 最后，Servlet 是由 JVM 的垃圾回收器进行垃圾回收的。
其他servlet内容可以查看servlet教程
spring mvc
spring mvc原理
springmvc 是围绕 DispatcherServlet 而设计的，而DispatcherServlet是一个servlet的子类，所以springmvc依赖servlet来进行mvc的实现。
通过swagger的使用来进行DispatcherServlet类的源码解析
项目源码地址
项目主要使用knife4j对swagger2的一个增强项目来实现swagger的功能，http server 是 tomcat 9。
Dispatcher分析
类图
结合两图可以看到
顶层接口类是 Servlet定义了如图1-1的方法，根据servlet的生命周期可以知道在实际http请求时http server调用的时service方法，debug模式下看线程栈图1-2发现从tomcat服务调用了DispatcherServlet类的service的方法最终在HttpServlet类中找到了实现类
@Override
public void service(ServletRequest req, ServletResponse res)
throws ServletException, IOException {
HttpServletRequest request;
HttpServletResponse response;
try {
// 强转类型
request = (HttpServletRequest) req;
response = (HttpServletResponse) res;
} catch (ClassCastException e) {
throw new ServletException(lStrings.getString("http.non_http"));
}
调用service方法
service(request, response);
}
在ds父类FrameworkServlet找到service方法
@Override
protected void service(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
//根据http请求方法进行不同处理
if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
processRequest(request, response);
}
else {
//调用父类service方法
super.service(request, response);
}
}
调用到HttpServlet的service方法
protected void service(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
String method = req.getMethod();
if (method.equals(METHOD_GET)) {
// 默认实现为返回-1，所以如果时get请求直接就走了doGet方法，不会走else判断逻辑。
long lastModified = getLastModified(req);
if (lastModified == -1) {
// servlet doesn't support if-modified-since, no reason
// to go through further expensive logic
//调用doGet方法
doGet(req, resp);
} 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);
}
}
protected long getLastModified(HttpServletRequest req) {
return -1;
}
如果是get请求类型则调用doGet方法，因为ds类不存在doget方法则调用父类的默认实现即 FrameworkServlet，然后调用doService方法，子类实现所以调用到了ds中的doService方法。
@Override
protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
logRequest(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<>();
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);
}
finally {
if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted()) {
// Restore the original attribute snapshot, in case of an include.
if (attributesSnapshot != null) {
restoreAttributesAfterInclude(request, attributesSnapshot);
}
}
}
}
设置请求属性，然后调用doDispatch方法
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 {
// 根据content-type来进行组装流程的请求
processedRequest = checkMultipart(request);
multipartRequestParsed = (processedRequest != request);
// Determine handler for the current request.
根据流程请求，获取映射器处理器
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 (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
return;
}
}
// 如果当前请求流程存在拦截器的PreHandle方法则进行拦截，其中自定义的拦截器需要实现HandlerInterceptor重写preHandle方法，并加入到InterceptorRegistry注册器中如代码5-5
if (!mappedHandler.applyPreHandle(processedRequest, response)) {
return;
}
// Actually invoke the handler.
//处理器适配器根据流程请求和处理器映射器进行处理，根据不同处理器映射器进行不同处理，静态资源文件通过将资源数组写入response，写回前端。如果是接口资源则解析参数，反射调用接口方法后返回mv。
mv = ha.handle(processedRequest, response, mappedHandler.getHandler());
if (asyncManager.isConcurrentHandlingStarted()) {
return;
}
applyDefaultViewName(processedRequest, mv);
// 拦截器中的postHandle方法 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);
}
//处理结果，方法内进行triggerAfterCompletion对afterCompletion方法进行拦截
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);
}
}
}
}
@Nullable
protected HandlerExecutionChain getHandler(HttpServletRequest request) throws Exception {
if (this.handlerMappings != null) {
//图2-1
for (HandlerMapping mapping : this.handlerMappings) {
HandlerExecutionChain handler = mapping.getHandler(request);
if (handler != null) {
return handler;
}
}
}
return null;
}
处理器映射器返回结果如图2-2，报含处理器和拦截器列表
代码5-5
@Configuration
public class TestInterceptor implements HandlerInterceptor {
@Override
public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
System.out.println("this is preHandle");
return true;
}
@Override
public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
System.out.println("this is postHandle");
}
@Override
public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
System.out.println("this is afterCompletion");
}
}
@Configuration
public class InterceptorConfig {
@Autowired
private TestInterceptor testInterceptor;
@Bean
public WebMvcConfigurer WebMvcConfigurer() {
return new WebMvcConfigurer() {
public void addInterceptors(InterceptorRegistry registry) {
registry.addInterceptor(testInterceptor).addPathPatterns("/user/**");
}
};
}
}