java->framework->springMVC流程Debugger

2020-10-20

SpringMVC框架流程复习

springMVC是当下web请求与响应的一个经典流程,作为一名B/S后端java开发,我们没有任何理由不去了解清楚它的详细结构与运转流程.

一、流程图

SpringMVC

二、流程描述

#1. 用户发出请求到服务器,前端控制器(dispatcher)拦截到请求,根据请求url去映射器(handlerMapping)去寻找对应的handler(Controller),映射器返回handler.

#2. 前端控制器委托处理解析器(handlerAdapter)去执行对应的handler(Controller-> service-> ...),完成后处理解析器返回ModelAndView对象.

#3. 请求视图解析器(ViewResolver)解析ModerlAndView,并返回View给前端控制器.

#4. 前端控制器将Model数据填充到View组装数据返回给用户,完成一次请求与响应流程.

三、SpringMVC源码debugger

3.1.准备工作与debugger环境:


# 	1)idea或者eclipse或其他编译器一个,jdk1.8,maven3.6.3,tomcat7.0.93.
# 	2)创建一个maven项目(war包),引入springMVC依赖,随便编写一个controller(debugger使用).
# 	3)添加并修改配置文件,web.xml,applicationContext.xml(空配置文件,但要有该配置文件),spring-mvc.xml.
#	4)为方便他人debugger,已将代码上传github,clone命令:spring mvc相关在包com.javaEE.fremwork.springMVC下.
$ git clone https://github.com/Superinterface/JavaKnowledge.git

3.2.配置文件

<!-- ########## SpringMVC maven 依赖如下 -->
	<dependency>
		<groupId>org.springframework</groupId>
		<artifactId>spring-webmvc</artifactId>
		<version>5.2.0.RELEASE</version>
	</dependency>

<!-- ########## web.xml文件如下-->

	<!-- Spring相关配置文件 -->
	<context-param>
		<param-name>contextConfigLocation</param-name>
		<param-value>classpath:applicationContext.xml</param-value>
	</context-param>
	<!-- Spring容器初始化,使用监听器 -->
	<listener>
		<listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
	</listener>
	<!-- 初始化前端控制器DispatcherServlet -->
	<servlet>
		<description></description>
		<servlet-name>dispatcherServlet</servlet-name>
		<servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
		<init-param>
			<description></description>
			<param-name>contextConfigLocation</param-name>
			<param-value>classpath:spring-mvc.xml</param-value>
		</init-param>
		<load-on-startup>1</load-on-startup>
	</servlet>
	<!-- 所有url都由servlet "dispatcherServlet" 来处理 -->
	<servlet-mapping>
		<servlet-name>dispatcherServlet</servlet-name>
		<url-pattern>/</url-pattern>
	</servlet-mapping>
</web-app>

<!-- ########## applicationContext.xml 文件如下 -->

<!-- ########## spring-mvc.xml 文件如下 -->

	<!-- 注解驱动 当配置了mvc:annotation-driven/后,Spring就知道了我们启用注解驱动。然后Spring通过context:component-scan/标签的配置,会自动为我们将扫描到的@Component,@Controller,@Service,@Repository等注解标记的组件注册到工厂中,来处理我们的请求 -->
	<mvc:annotation-driven />
	<!-- 扫描包的路径 -->
	<context:component-scan base-package="com.javaEE.fremwork.springMVC" />

3.3源码debugger解析

根据web.xml配置文件得到springMVC处理入口前端控制器(DispatcherServlet)

web.xml

进入DispatcherServlet类中找到doService方法,可以看到其中的this.doDispatch(req, res);这两个地方是dispatcher进行处理的逻辑核心.

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        this.logRequest(request);
        Map<String, Object> attributesSnapshot = null;
        if (WebUtils.isIncludeRequest(request)) {
            attributesSnapshot = new HashMap();
            Enumeration attrNames = request.getAttributeNames();
            label95:
            while(true) {
                String attrName;
                do {
                    if (!attrNames.hasMoreElements()) {
                        break label95;
                    }
                    attrName = (String)attrNames.nextElement();
                } while(!this.cleanupAfterInclude && !attrName.startsWith("org.springframework.web.servlet"));
                attributesSnapshot.put(attrName, request.getAttribute(attrName));
            }
        }
        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, this.getWebApplicationContext());
        request.setAttribute(LOCALE_RESOLVER_ATTRIBUTE, this.localeResolver);
        request.setAttribute(THEME_RESOLVER_ATTRIBUTE, this.themeResolver);
        request.setAttribute(THEME_SOURCE_ATTRIBUTE, this.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 {
            this.doDispatch(request, response);
        } finally {
            if (!WebAsyncUtils.getAsyncManager(request).isConcurrentHandlingStarted() && attributesSnapshot != null) {
                this.restoreAttributesAfterInclude(request, attributesSnapshot);
            }
        }
    }

web.xml

那么问题来了,如何知道这个doService方法就是我们要找的MVC流程呢？它是在什么时候被调用的？

// DispatcherServlet.java
public class DispatcherServlet extends FrameworkServlet {
	...
}

// FrameworkServlet.java
public abstract class FrameworkServlet extends HttpServletBean implements ApplicationContextAware {
	...
}

// HttpServletBean.java
public abstract class HttpServletBean extends HttpServlet implements EnvironmentCapable, EnvironmentAware {
	...
}

// HttpServlet.java
package javax.servlet.http;
public abstract class HttpServlet extends GenericServlet {
    ...
}

DispatcherServlet类从FrameworkServlet类继承,FrameworkServlet类从HttpServletBean类继承,HttpServletBean类从HttpServlet类继承…,顺着继承关系一直找上去不难发现,父类是HttpServlet,那么就要遵守servlet规范,我们知道servlet的生命周期是从被容器加载(例如tomcat)init(),被调用service(),最后执行销毁destroy(),现在我们想知道 DispatcherServlet类中的doService方法是如何在请求响应过程中被调用的顺序,则对其继承的各个父类的生命周期方法进行查找即可.

// DispatcherServlet 父类FrameworkServlet中有对doService()方法的调用,废话不多说,上代码.
//#########################################################################
// 类 FrameworkServlet 
protected final void processRequest(HttpServletRequest req,HttpServletResponse res) throws ServletException, IOException {

		long startTime = System.currentTimeMillis();
    
		...
            
		try {
			doService(request, response); // 此处调用doService方法
		}
    	...
}

@Override // 重写了父类的service方法
protected void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

		HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
		if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
			processRequest(request, response); // 此处调用processRequest方法
		}
		else {
			super.service(request, response); // 此处使用父类的service方法
		}
	}
@Override // 重写父类的doGet方法,doPost...一样都进行了重写.
protected final void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {

		processRequest(request, response);// 此处也调用processRequest方法
	}
//#########################################################################
// 类 HttpServlet
protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String method = req.getMethod();
        long lastModified;
        if (method.equals("GET")) {

            if (lastModified == -1L) {
                this.doGet(req, resp); 
//此处虽写this.doGet,然实际debugger时使用的是子类FrameworkServlet重写后的doGet方法,你知道这是怎么回事吗?
            } else {
                ...
            }
        } else if (method.equals("HEAD")) {
            this.doHead(req, resp);
        } else if (method.equals("POST")) {
            this.doPost(req, resp);
        } else if (method.equals("PUT")) {
            this.doPut(req, resp);
        } else if (method.equals("DELETE")) {
            this.doDelete(req, resp);
        } else if (method.equals("OPTIONS")) {
            this.doOptions(req, resp);
        } else if (method.equals("TRACE")) {
            this.doTrace(req, resp);
        } else {
        }

    }

// 梳理调用逻辑
//1.DispatcherServlet的doService方法被父类FrameworkServlet的processRequest方法调用.
//2.FrameworkServlet的processRequest方法被自身重写的service,doGet,doPost等方法调用.
//3.在FrameworkServlet的重写service方法中的else则是先调用父类HttpServlet的service,然后父类service方法中一样调用了doGet,doPost等方法,而这些方法则依然会调用processRequest方法.
//4.关于HttpServlet的service方法中的this.doGet为什么会调用子类的而不是调用自身的,是因为java的多态,创建的对象实例是子类,且子类对父类HttpServlet的doGet,doPost等方法进行了重写,所以在运行时会执行子类的重写方法.

有了调用关系,下面让我们一步一步debugger,查看具体dispatcher是如何处理请求的

1	# 浏览器发送请求到后台 http://localhost:8080/api/hello

override_service

1	# HttpServlet的service方法,调用this.doGet()

1	# this.doGet使用的是frameworkServlet的doGet

frameworkServlet_doGet

1	# framework的processRequest方法会调用doService方法

frameworkServlet_processRequest

1	# Dispatcher的doService设置一些参数之后调用doDispatch()

dispatcherServlet_doService

1
2

# 下面的getHandler(req) 方法则是前端控制器根据请求来寻找对应handler的方法.
# 可以看到源码使用for循环从handlerMapping中找到那个可以处理当前请求的handler,并返回.

Dispatcher_gethandler

1_handlerMapping

# 然后委托handlerAdapter处理器解析器去执行对应的handler(Controller),并返回ModelAndView.
# 下面是controller中的代码,当其被执行(handler被handlerAdapter执行后应该在控制台看到输出的信息)
@ResponseBody
@RequestMapping("/api/hello")
public String hello() {
	System.out.println("当前被handlerAdapter执行handler中(执行controller中)");
	return "hello";
}

2_handlerAdapter_handler

2_controller_console

1	# 执行完handler之后,将会调用视图解析器去解析视图,返回View对象.

3_viewResolver

1	# 最后则是对数据进行组合填充,进行响应,至此整个SpringMVC的流程源码debugger已经完成了,当然更细节的内容需要我们亲手去debugger.

4_response