源码分析：Tomcat - Filter - RequestContextHolder 上下文请求

1. Filter - RequestContextHolder 上下文请求
2. Filter - SpringBoot桥接原理

前言

问1：为什么我在 Controller 方法内部可以获取 RequestContextHolder 上下文请求？

问2：这个上下文会内存泄漏吗？

阶段1：SpringBoot引导 - Tomcat

SpringApplication#run
AbstractApplicationContext#refresh
        public void refresh(){
            // ...
            try {
                // ...
                finishBeanFactoryInitialization(beanFactory);
                finishRefresh();// ***
            }
            // ...
        }
AbstractApplicationContext#finishRefresh
// getLifecycleProcessor().onRefresh();
// DefaultLifecycleProcessor#doStart
// WebServerStartStopLifecycle#start // 实现 Lifecycle#start
// AbstractProtocol#start
// NioEndpoint#startInternal
        // 1. 创建线程池 - 用于处理请求
        // createExecutor();
            // AbstractEndpoint#createExecutor
        
        // 2. Selector 接收HTTP请求，提交任务 -> 线程池
        // poller = new Poller();// Selector.open(); 
        // pollerThread.start();// NioEndpoint.Poller#run
                // selector.selectedKeys()
                // processKey(sk, socketWrapper);
                    // sc = createSocketProcessor(socketWrapper, event);// NioEndpoint#createSocketProcessor
                    // executor.execute(sc);// 参考下面

阶段2：Tomcat处理请求 - Filter

// NioEndpoint.SocketProcessor#doRun
// ↓ state = getHandler().process(socketWrapper, event);
// ↓↓ state = processor.process(wrapper, status);
// ↓↓↓ state = service(socketWrapper);// OPEN_READ
// ↓↓↓↓ getAdapter().service(request, response);// Http11Processor#service
// ↓↓↓↓↓ connector.getService().getContainer().getPipeline().getFirst().invoke(request, response);// CoyoteAdapter#service
// ↓↓↓↓↓↓ host.getPipeline().getFirst().invoke(request, response);
// ...
// StandardWrapperValve#invoke
// ApplicationFilterChain filterChain = ApplicationFilterFactory.createFilterChain(request, wrapper, servlet);
// addFilter
        for (FilterMap filterMap : filterMaps) {
            if (!matchFiltersURL(filterMap, requestPath)) {
                continue;
            }
            ApplicationFilterConfig filterConfig = (ApplicationFilterConfig) context.findFilterConfig(filterMap.getFilterName());
            filterChain.addFilter(filterConfig);
        }
// filterChain.doFilter(request.getRequest(), response.getResponse());
// internalDoFilter(request, response);
        private void internalDoFilter(ServletRequest request, ServletResponse response){
    
            // 1、调用下一个 filter
            if (pos < n) {
                ApplicationFilterConfig filterConfig = filters[pos++];
                //...
                Filter filter = filterConfig.getFilter();
                filter.doFilter(request, response, this);
                //...
                return;
            }
    
            // 2、责任链结束，调用 servlet
            servlet.service(request, response);
        }

// RequestContextFilter#doFilterInternal // 上下文拦截器 // 获取上下文信息
        @Override
        protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain){
    
            ServletRequestAttributes attributes = new ServletRequestAttributes(request, response);
            initContextHolders(request, attributes);// set
    
            try {
                filterChain.doFilter(request, response);
            }
            finally {
                resetContextHolders();// remove
                //...
            }
        }

阶段3：MVC引导 - controller

// HttpServlet#service
// ...
// FrameworkServlet#processRequest // ServletRequestAttributes - 再置
        protected final void processRequest(HttpServletRequest request, HttpServletResponse response){
    
            // ...
            
            // 旧的
            RequestAttributes previousAttributes = RequestContextHolder.getRequestAttributes();
            // 新的
            ServletRequestAttributes requestAttributes = buildRequestAttributes(request, response, previousAttributes);
            // ...
            // 放入新的
            initContextHolders(request, localeContext, requestAttributes);
    
            try {
                // controller
                doService(request, response);
            }
            // ...
            finally {
                // 恢复旧的
                resetContextHolders(request, previousLocaleContext, previousAttributes);
                // ...
            }
        }
// ...
// DispatcherServlet#doDispatch
// # 调用controller方法
// mv = ha.handle(processedRequest, response, mappedHandler.getHandler());