源码跟踪-springmvc(一)：DispatcherSevlet

背景

日常coding中，要解决项目当中遇到的问题，难免会需要写一个Converter，aspect，或者扩展一个MediaType等等。这时候需要写一个侵入性小的扩展，就需要了解源码。我找了很多博客文章，甚至看了《看透Spring MVC：源代码分析与实践》，写的很好，但是视角都是从整体框架出发，大而全，而我仅仅只是想解决当前的问题，所以我以代码跟踪的视角记录下这篇文章，免得下次忘了还要重新跟踪源码，直接过来看就好了。

目的

从源码中提取可能用到的工具，特别是标注好注意事项，下次扩展可以查阅。

准备

springboot 2.1.1.RELEASE的demo
pom依赖


            org.springframework.boot
            spring-boot-starter-actuator
        
        
            org.springframework.boot
            spring-boot-starter-web

示例类Employee，省略getter/setter

public class Employee {

    private Long id;
    private String name;
    private Integer age;
    private Date birthday;
    private Date createTime;
}

Controller示例类

@RestController
@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @PostMapping("/employee")
    public ResponseEntity insert(Employee employee){
        employee.setId(1L);
        employee.setCreateTime(new Date());
        return ResponseEntity.ok(employee);
    }

}

断点

如图上断点

源码跟踪-springmvc(一)：DispatcherSevlet_第1张图片

请求参数如图

源码跟踪-springmvc(一)：DispatcherSevlet_第2张图片

ALT+左键点击employee检查参数

源码跟踪-springmvc(一)：DispatcherSevlet_第3张图片

FrameworkServlet

我们在Debugger视图的Frames里从底部往上，找到第一个属于spring-webmvc包的类，FrameworkServlet
通过查看源码，我们GET到第一个工具类，HttpMethod，这个枚举类包含了所有的http方法。

protected void service(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        HttpMethod httpMethod = HttpMethod.resolve(request.getMethod());
        if (httpMethod == HttpMethod.PATCH || httpMethod == null) {
            processRequest(request, response);
        }
        else {
            super.service(request, response);
        }
    }

在Frames视图继续向上推，可以发现从 HttpServlet 转了一遭到了 FrameworkServlet.doPost ，然后来到了 FrameworkServlet.processRequest （删减版）

protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        // (1)
        LocaleContext previousLocaleContext = LocaleContextHolder.getLocaleContext();
        LocaleContext localeContext = buildLocaleContext(request);

        RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
        ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);

        initContextHolders(request, localeContext, requestAttributes);

        try {
            // (2)
            doService(request, response);
        }
        catch (ServletException | IOException ex) {
        }
        catch (Throwable ex) {
        }

        finally {
            resetContextHolders(request, previousLocaleContext, previousAttributes);
            if (requestAttributes != null) {
                requestAttributes.requestCompleted();
            }
            // (3)
            publishRequestHandledEvent(request, response, startTime, failureCause);
        }
    }

看得出来，做了三件事

ContextHolder的设置和重置
具体见：源码跟踪-springmvc(二)：LocaleContextHolder和RequestContextHolder
执行doService方法，也是真正执行handler的方法。
执行了publishRequestHandledEvent，代码如下

private void publishRequestHandledEvent(HttpServletRequest request, HttpServletResponse response,
            long startTime, @Nullable Throwable failureCause) {

        if (this.publishEvents && this.webApplicationContext != null) {
            // Whether or not we succeeded, publish an event.
            long processingTime = System.currentTimeMillis() - startTime;
            this.webApplicationContext.publishEvent(
                    new ServletRequestHandledEvent(this,
                            request.getRequestURI(), request.getRemoteAddr(),
                            request.getMethod(), getServletConfig().getServletName(),
                            WebUtils.getSessionId(request), getUsernameForRequest(request),
                            processingTime, failureCause, response.getStatus()));
        }
    }

我们知道webApplicationContext继承了ApplicationEventPublisher，拥有了发布事件的能力，我把发布的事件打印出来看一下

@EventListener
    public void printServletRequestHandledEvent(ServletRequestHandledEvent event){
        System.out.println(event);
    }

打印结果如下

ServletRequestHandledEvent: url=[/employee]; client=[127.0.0.1]; method=[POST]; servlet=[dispatcherServlet]; session=[null]; user=[null]; time=[52ms]; status=[OK]

如果发现打印的内容满足需要，我们就不需要再写个aop用来记录日志啦。

DispatcherSevlet

现在进入了DispatcherSevlet.doService方法（删减版）

protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {
        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));
                }
            }
        }

        request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());

        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);
                }
            }
        }
    }

这里有两件事

如果满足一定的条件（WebUtils.isIncludeRequest(request)），会把request的attributes做一份快照备份（attributesSnapshot），执行完handler后还原备份（restoreAttributesAfterInclude(request, attributesSnapshot)）。但是这里如果没有成立，也就没有执行，就先不管。但是很明显，这个手法和上面的ContextHolder如出一辙。这时候其实能够象出来，这样做的目的是为了安全。
在request中设置了一堆的attributes，有一个特别显眼，request.setAttribute(WEB_APPLICATION_CONTEXT_ATTRIBUTE, getWebApplicationContext());，这个时候我们又get到一个获取webApplicationContext的办法。

WebApplicationContext wac = (WebApplicationContext) request.getAttribute(DispatcherServlet.WEB_APPLICATION_CONTEXT_ATTRIBUTE);

进入了DispatcherSevlet.doDispatch方法（删减版）

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
// ....
        try {
//...
            try {
                // (1)
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // (2)
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // (3)
                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;
                    }
                }

                // (4)
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                // (5)
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                // (6)
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            // (7)
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            // (7)
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            // (7)
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
    }

这里非常重要了，对应代码中的注释，解释如下

获取到mappedHandler，其实也就HandlerExecutionChain
具体见源码跟踪-springmvc(三)：RequestMappingHandlerMapping
获取处理器适配器，就是RequestMappingHandlerAdapter
http协议中的缓存实现，可以看Spring mvc HTTP协议之缓存机制
分别调用mappedHandler中的三个拦截器的preHandle方法
执行真正的handler
具体见：源码跟踪-springmvc(四)：RequestMappingHandlerAdapter
执行拦截器的postHandle方法
执行拦截器的afterCompletion方法

源码跟踪-springmvc(一)：DispatcherSevlet

背景

目的

准备

断点

FrameworkServlet

DispatcherSevlet

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