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开发环境：windows10、idea、jdk1.8、apache-tomcat-9.0.0.M3
SpringMVC框架是基于Servlet设计的，所以如果你知道SpringMVC，但是没听过道Servlet，那你就应该先去学习下Servlet的知识点了。以下我所描述的SpringMVC框架都是简易版本SpringMVC框架。该框架主要是以请求为驱动，其核心是DispatcherServlet类，它实际上就是一个Servlet，底层实现的也是Servlet。在SpringMVC框架中DispatcherServlet主要做两件事，
第一 ：处理请求路径到各个Handler之间的映射，这里的Handler就类似我们平时使用的Controller，也就是struts里边的action，这时候需要使用一个HandlerMapping的东西，也就是一个map来保存映射关系，并且需要在初始化的时候就保存，这就使用到了Servlet生命周期中初始化阶段所调用的init()方法;
第二 ：将用户请求分发到各个具体的Handler，并且将请求交给Handler中的具体方法去处理，然后就接收返回回来的ModelAndView，查询一个或多个ViewResoler视图解析器，找到ModelAndView指定的视图，视图负责将结果显示到客户端。
首先我们先新建一个Java Enterprise项目，也就是一个java web项目
项目建立好之后，在web.xml中我们先做简单的配置，将客户端所有的请求交给DispatcherServlet这个类去处理。
<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd"version="3.1"><servlet><servlet-name>dispatcherServlet</servlet-name><servlet-class>com.zhainan.springmvc.servlet.DispatcherServlet</servlet-class></servlet><servlet-mapping><servlet-name>dispatcherServlet</servlet-name><url-pattern>/*</url-pattern></servlet-mapping></web-app>接下来就是最为重要的DispatcherServlet实现：
public class DispatcherServlet extends HttpServlet {private Map<String, HandlerBean> handlerBeanMap;@Overridepublic void init() throws ServletException {handlerBeanMap = new HashMap<String, HandlerBean>();for (String name : ClassUtils.getAllClassesFromPackage("com.zhainan.springmvc.handler")) {try {Class clz = Class.forName(name);for (Method method : clz.getDeclaredMethods()) {RequestMapping requestMapping = method.getAnnotation(RequestMapping.class);if (requestMapping != null) {String path = requestMapping.value();handlerBeanMap.put(path, new HandlerBean(name, method.getName()));}}} catch (ClassNotFoundException e) {e.printStackTrace();}}}@Overrideprotected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doDispatch(req, resp);}private void doDispatch(HttpServletRequest req, HttpServletResponse resp) throws IOException{String path = req.getPathInfo();HandlerBean handlerBean = handlerBeanMap.get(path);if (handlerBean == null)handlerBean = new HandlerBean("com.zhainan.springmvc.handler.DefaultHandler", "doService");try {Class clz = Class.forName(handlerBean.getBeanName());for (Method method : clz.getDeclaredMethods()) {if (handlerBean.getMethodName().equals(method.getName())) {Parameter[] parameters = method.getParameters();Object[] args = new Object[parameters.length];for (int i = 0; i < parameters.length; i++) {RequestParam requestParam = parameters[i].getAnnotation(RequestParam.class);if (requestParam != null) {String parameterName = requestParam.value();args[i] = req.getParameter(parameterName);}if (parameters[i].getType() == HttpServletResponse.class) {args[i] = resp;}}ModelAndView view = (ModelAndView) method.invoke(clz.newInstance(), args);new ViewResolver().render(view, req, resp);}}} catch (ClassNotFoundException e) {e.printStackTrace();} catch (InvocationTargetException e) {e.printStackTrace();} catch (InstantiationException e) {e.printStackTrace();} catch (IllegalAccessException e) {e.printStackTrace();}}
}可以看到简易SpringMVC框架中的DispatcherServlet直接继承自HttpServlet，重写了它的init()方法和doGet()方法，init()方法先保存请求路径到各个Handler之间的映射，当然这里边就涉及了类ClassUtils中的一个重要方法和一个类HandlerBean，以及一些关键注解
ClassUtils类：根据包名定位到文件夹,然后根据读取该包下所有class文件,最后根据包名和文件名,构造返回的完整类名.
public class ClassUtils {public static List<String> getAllClassesFromPackage(String packageName){List<String> classNames = new ArrayList<String>();String pkg = "com.zhainan.springmvc.handler";String relPath = pkg.replace('.', '/');URL resource = Thread.currentThread().getContextClassLoader().getResource(relPath);if (resource == null) {throw new RuntimeException("Unexpected problem: No resource for "+ relPath);}File f = new File(resource.getPath());String[] files = f.list();for (int i = 0; i < files.length; i++) {String fileName = files[i];String className = null;String fileNm = null;if (fileName.endsWith(".class")) {fileNm = fileName.substring(0, fileName.length() - 6);className = pkg + '.' + fileNm;}if (className != null) {classNames.add(className);}}return classNames;}
}HandlerBean类：具体Handler和其中处理方法的对应关系
public class HandlerBean {private String beanName;private String methodName;public HandlerBean(String beanName, String methodName) {this.beanName = beanName;this.methodName = methodName;}public String getBeanName() {return beanName;}public void setBeanName(String beanName) {this.beanName = beanName;}public String getMethodName() {return methodName;}public void setMethodName(String methodName) {this.methodName = methodName;}
}注解RequestMapping：和Controller里边的RequestMapping是一样的道理，将请求路径映射到某个具体方法
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RequestMapping {String value() default "";}注解RequestParam：和Controller里边的RequestParam一样，请求路径中的请求参数接收
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RequestParam {String value() default "";}注解PathVariable：和Controller里边的PathVariable一样，请求路径中的路径变量接收
@Target(ElementType.PARAMETER)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface PathVariable {String value() default "";}doGet()方法直接调用类里边的doDispatch()方法用于将用户请求分发到各个具体的Handler去处理。接收返回回来的ModelAndView，查询一个或多个ViewResoler视图解析器，找到ModelAndView指定的视图，视图负责将结果显示到客户端。
首先来看看我们的Handler实现，在Demo中我定义了两个Handler
DefaultHandler：用于处理默认请求
public class DefaultHandler {public void doService(HttpServletRequest request,HttpServletResponse response) throws IOException {response.getWriter().write("this is index page !");}}DemoHandler：用于处理指定路径的请求，可以在这编写相关的API
public class DemoHandler {@RequestMapping(value = "/test")public void test(HttpServletRequest request,HttpServletResponse response) throws IOException {response.getWriter().write("<h1>test</h1>");}@RequestMapping(value = "/demo")public ModelAndView demo(@RequestParam(value = "name") String name,@RequestParam(value = "age") String age) {Map<String, String> model = new HashMap<String, String>();model.put("name", name);model.put("age", age);return new ModelAndView(model, "my.view");}}除此之外，还可以去编写其他的Handler，如同Controller一样去处理各类请求。
接下来就是关于视图的处理了，先来看下ModelAndView
ModelAndView：ModelAndView中包含了模型（Model）和视图（View），Handler处理完具体的业务逻辑后可以返回对应的模型和需要返回的视图
public class ModelAndView {private Map<String, String> model;private String viewName;public ModelAndView(Map<String, String> model, String viewName) {this.model = model;this.viewName = viewName;}public Map<String, String> getModel() {return model;}public void setModel(Map<String, String> model) {this.model = model;}public String getViewName() {return viewName;}public void setViewName(String viewName) {this.viewName = viewName;}
}然后再编写一个小型的试图解析功能
先定义一个接口View
public interface View {public void render(ModelAndView view,HttpServletRequest request,HttpServletResponse response) throws IOException;
}
然后定义一个泪ViewResolver去实现View的render()方法
public class ViewResolver implements View {@Overridepublic void render(ModelAndView view, HttpServletRequest request, HttpServletResponse response) throws IOException {PrintWriter writer = response.getWriter();String viewName = view.getViewName();String content = loadTemplate(request.getServletContext().getResourceAsStream("/WEB-INF/" + viewName));content = parseTemplate(view.getModel(), content);writer.write(content);}private String loadTemplate(InputStream is) throws IOException {BufferedReader reader = new BufferedReader(new InputStreamReader(is));StringBuilder content = new StringBuilder();String line = null;while ((line = reader.readLine()) != null) {content.append(line);}return content.toString();}private String parseTemplate(Map<String, String> model, String template) {for (Map.Entry<String, String> entry : model.entrySet()) {String key = entry.getKey();String matchedKey = String.format("\\$\\{%s\\}", key);template = template.replaceAll(matchedKey, entry.getValue());}return template;}}最后就是视图文件了，demo中定义了一个简单的视图文件my.view
<html><head><title>test</title></head><body><h3>hello,my name is ${name},${age} years old.</h3></body>
</html>最终还是要展示下效果
附上项目基本结构图和别人家的SpringMVC实现原理图帮助理解
项目基本结构图：
别人家的SpringMVC实现原理图：
项目Git地址：https://gitee.com/lvchang/springmvc_demo.git
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