迷你版Spring MVC 实现

本文参考自 写出我的第一个框架：迷你版Spring MVC ,写这篇文章用于个人学习的记录。

项目建立

首先，建立一个空Maven项目，在java目录下，建立以下包结构。

.....|--com.it|-annotation|-dao|-service|-webDispatchServlet

接下来，引入servlet依赖

<dependency><groupId>javax.servlet</groupId><artifactId>javax.servlet-api</artifactId><version>3.1.0</version></dependency>

在annotation增加如下注解：

@Autowired.java

@Documented
@Target(ElementType.FIELD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Autowired {String value();}

@Controller.java

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Controller {String value() default "";}

@Repository.java

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Repository {String value() default "";}

@RequestMapping.java

@Documented
@Target({ElementType.TYPE,ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
public @interface RequestMapping {String value();}

@Service.java

@Documented
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface Service {String value() default "";}

编写核心Servlet

在Spring MVC中，DispatcherServlet是核心，因此我们也建立一个自己的DispatcherServlet来接收自己的请求。

@WebServlet是什么？

在Servlet3.0后出现了基于注解的Servlet,配置Servlet支持注解方式。

为了方便，我这里就通过初始化参数直接将需要扫描的基包路径传入。

DispatcherServlet.java

@WebServlet(urlPatterns = "/*", loadOnStartup = 0,initParams = {@WebInitParam(name = "base-package", value = "com.it")})
public class DispatchServlet extends HttpServlet {private static final String EMPTY = "";/** 扫描的基础包*/private String basePackage = EMPTY;/** 扫描的基础包下面的类的全类名*/private List<String> packagesName = new ArrayList<>();/** key：注解里的值（默认为类名第一个字母小写）  value ：类的实例化对象*/private Map<String, Object> instanceMap = new HashMap<>();/** key：类的全类名  value：注解里的值（默认为类名第一个字母小写）*/private Map<String, String> nameMap = new HashMap<>();/** key：请求路径  value： 所调用的方法*/private Map<String, Method> urlMethodMap = new HashMap<>();/** key：controller里的方法示例  value： 当前类的全类名*/private Map<Method, String> methodPackageMap = new HashMap<>();}

接下来，需要在serlvet初始化的时候，去扫描我们注解标识的类的信息，总体步骤如下：

1）扫描制定包下所有的类，进行初始化，记录类的全类名。

2）扫描所有标注@Controller/@Service/@Repository的注解类，并且记录全类名与实例的映射关系。

3）对于有@Autowired注解的字段，我们需要为其注入相应类的实例，为了方便，在这里统一用类的名称去获取，当然在spring 中可以通过类型等种种手段去注入。这里所有的类名默认转换成了 类名首字母小写的形式。

4）对于所有@RequestMapping注解的形式，需要将请求路径和相应的方法做关联记录，这样我们可以通过请求路径映射到我们的controller方法上。

Servlet初始化

@Override
public void init(ServletConfig config) throws ServletException {basePackage = config.getInitParameter("base-package");try {scanBasePackage(basePackage);instance(packagesName);ioc();handlerUrlMethod();} catch (Exception e) {e.printStackTrace();System.out.println("框架加载失败");}
}

第一步 ，包名扫描

 private void scanBasePackage(String basePackage) {Optional<URL> resource = Optional.ofNullable(this.getClass().getClassLoader().getResource(basePackage.replaceAll("\\.", "/")));String path = resource.map(URL::getPath).orElse("");File basePackageFile = new File(path);Optional<File[]> files = Optional.ofNullable(basePackageFile.listFiles());files.ifPresent(file -> {for (File temp : file) {if (temp.isDirectory()) {scanBasePackage(basePackage + "." + temp.getName());}if (temp.isFile()) {packagesName.add(basePackage + "." + temp.getName().split("\\.")[0]);}}});}

第二步，实例化类

private void instance(List<String> packagesNames) {if (packagesNames.isEmpty()) {return;}packagesNames.forEach(packageName -> {try {Class clazz = Class.forName(packageName);String subName = clazz.getName().substring(clazz.getName().lastIndexOf(".")).replaceAll("\\.", EMPTY);String clazzName = subName.substring(0, 1).toLowerCase() + subName.substring(1);if (clazz.isAnnotationPresent(Controller.class)) {Controller controller = (Controller) clazz.getAnnotation(Controller.class);String controllerName = controller.value();if (EMPTY.equals(controllerName)) {controllerName = clazzName;}instanceMap.put(controllerName, clazz.newInstance());nameMap.put(packageName, controllerName);}if (clazz.isAnnotationPresent(Service.class)) {Service service = (Service) clazz.getAnnotation(Service.class);String serviceName = service.value();if (EMPTY.equals(serviceName)) {serviceName = clazzName;}instanceMap.put(serviceName, clazz.newInstance());nameMap.put(packageName, serviceName);}if (clazz.isAnnotationPresent(Repository.class)) {Repository repository = (Repository) clazz.getAnnotation(Repository.class);String respositoryName = repository.value();if (EMPTY.equals(respositoryName)) {respositoryName = clazzName;}instanceMap.put(respositoryName, clazz.newInstance());nameMap.put(packageName, respositoryName);}} catch (Exception e) {e.printStackTrace();System.out.println("类加载失败");}});
}

第三步，注入类的实例

private void ioc() throws IllegalAccessException {for (Map.Entry<String, Object> instance : instanceMap.entrySet()) {Field[] declaredFields = instance.getValue().getClass().getDeclaredFields();for (Field declaredField : declaredFields) {if (declaredField.isAnnotationPresent(Autowired.class)) {String value = declaredField.getAnnotation(Autowired.class).value();declaredField.setAccessible(true);declaredField.set(instance.getValue(), instanceMap.get(value));}}}
}

第四步，处理请求路径与方法的映射

private void handlerUrlMethod() {if (packagesName.isEmpty()) {return;}packagesName.forEach(packageName -> {try {Class clazz = Class.forName(packageName);if (clazz.isAnnotationPresent(Controller.class)) {Method[] methods = clazz.getMethods();StringBuffer baseUrl = new StringBuffer();if (clazz.isAnnotationPresent(RequestMapping.class)) {RequestMapping controllerClazzMapping = (RequestMapping) clazz.getAnnotation(RequestMapping.class);baseUrl.append(controllerClazzMapping.value());}for (Method method : methods) {if (method.isAnnotationPresent(RequestMapping.class)) {RequestMapping methodMapping = method.getAnnotation(RequestMapping.class);baseUrl.append(methodMapping.value());urlMethodMap.put(baseUrl.toString(), method);methodPackageMap.put(method, packageName);}}}} catch (Exception e) {e.printStackTrace();System.out.println("获取映射失败");}});
}

编写GET/POST方法

编写GET/POST方法，使请求映射到我们的Controller上

@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {doPost(req, resp);
}@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {String uri = req.getRequestURI();String contextPath = req.getContextPath();String path = uri.replace(contextPath, EMPTY);Optional<Method> method = Optional.ofNullable(urlMethodMap.get(path));method.ifPresent(m -> {String packageName = methodPackageMap.get(m);String controllerName = nameMap.get(packageName);Object controllerObj = instanceMap.get(controllerName);m.setAccessible(true);try {m.invoke(controllerObj);} catch (Exception e) {e.printStackTrace();}});
}

编写测试

UserDao.java

@Repository
public class UserDao {public void insert(){System.out.println("insert user:");}}

UserService.java

@Service
public class UserService {@Autowired("userDao")private UserDao userDao;public void insert() {userDao.insert();}
}

UserController.java

@Controller
@RequestMapping("/user")
public class UserController {@Autowired("userService")private UserService userService;@RequestMapping("/insert")public void insert() {userService.insert();}}

编写完成后，在tomcat启动，访问 IP:PORT + /user/insert 后，可以在控制台看到 insert user:的输出，说明我们的迷你版Spring MVC可以正常工作啦。

代码参考：mymvc