1. 一个 SpringBoot 项目同时使用了 Tomcat 的过滤器和 Spring 的拦截器，一些线程变量在过滤器中初始化并在拦截器中使用。

2. 该项目需要调用大语言模型进行流式输出。

3. 项目中，笔者使用 SpringBoot 的 ResponseEntity<StreamingResponseBody> 将流式输出返回前端。

问题出现

问题出现在上述第 3 点：正常输出一段内容后，后台突然报错，而报错内容由拦截器产生。

笔者仔细查看了报错日志，发现只是拦截器的问题：执行时由于某些线程变量不存在而报错。但是，这些线程变量已经在过滤器中初始化了。

那么问题来了：为什么这个接口明明可以正常通过过滤器和拦截器，并开始正常输出，却又突然在拦截器中报错呢？

场景重现

Filter

@Slf4j@Component@Order(1)public class MyFilter implements Filter {
    @Override    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {        // 要继续处理请求，必须添加 filterChain.doFilter()        log.info("doFilter method is running..., thread: {}, dispatcherType: {}", Thread.currentThread(), servletRequest.getDispatcherType());         filterChain.doFilter(servletRequest,servletResponse);    } }

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Interceptor

@Slf4jpublic class MyInterceptor implements HandlerInterceptor {
    @Override    public boolean preHandle(@NotNull HttpServletRequest request, @NotNull HttpServletResponse response, @NotNull Object handler) throws Exception {        log.info("preHandle method is running..., thread: {}, dispatcherType: {}", Thread.currentThread(), request.getDispatcherType());        if (DispatcherType.ASYNC == request.getDispatcherType()) {            log.info("preHandle dispatcherType={}", request.getDispatcherType());        }        return true;    }        @Override    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {        log.info("postHandle method is running..., thread: {}", Thread.currentThread());    }              @Override    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {        log.info("afterCompletion method is running..., thread: {}", Thread.currentThread());    } }

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WebMvcConfigurer

@Configurationpublic class WebAppConfigurer implements WebMvcConfigurer {        @Bean    public MyInterceptor myInterceptor() {        return new MyInterceptor();    }        @Override    public void addInterceptors(InterceptorRegistry registry) {        registry.addInterceptor(myInterceptor()).addPathPatterns("/**");    }        @Override    public void configureAsyncSupport(AsyncSupportConfigurer configurer) {        configurer.setDefaultTimeout(120_000L);        configurer.registerCallableInterceptors();        configurer.registerDeferredResultInterceptors();            ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();        executor.setCorePoolSize(5);        executor.setMaxPoolSize(10);        executor.setQueueCapacity(100);        executor.setThreadNamePrefix("web-async-");        executor.initialize();        configurer.setTaskExecutor(executor);    }}

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Controller

@Slf4j@RestController@RequestMapping("/test-stream")public class TestStreamController {
    @ApiOperation("流式输出示例")    @PostMapping(value = "/example", produces = MediaType.TEXT_EVENT_STREAM_VALUE)    public ResponseEntity<StreamingResponseBody> example() {        log.info("Stream method is running, thread: {}", Thread.currentThread());        return  ResponseEntity.status(HttpStatus.OK)            .contentType(new MediaType(MediaType.TEXT_EVENT_STREAM, StandardCharsets.UTF_8))            .body(outputStream -> {                log.info("Internal stream method is running, thread: {}", Thread.currentThread());                try (outputStream) {                    String msg = "To be or not to be!";                    outputStream.write(msg.getBytes(StandardCharsets.UTF_8));                    outputStream.flush();                }            });    }}

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根据以下运行日志，我们可以看到拦截器的 preHandle 确实执行了两次，并且此次调用过程共有 3 个线程（io-14000-exec-1，web-async-1，io-14000-exec-2）参与了工作。

2024-05-06 07:35:27.362  INFO 209108 --- [io-14000-exec-1] o.a.c.c.C.[.[localhost].[/java-study]    : Initializing Spring DispatcherServlet 'dispatcherServlet'2024-05-06 07:35:27.362  INFO 209108 --- [io-14000-exec-1] o.s.web.servlet.DispatcherServlet        : Initializing Servlet 'dispatcherServlet'2024-05-06 07:35:27.365  INFO 209108 --- [io-14000-exec-1] o.s.web.servlet.DispatcherServlet        : Completed initialization in 3 ms2024-05-06 07:35:27.402  INFO 209108 --- [io-14000-exec-1] com.peng.java.study.web.config.MyFilter  : doFilter method is running..., thread: Thread[http-nio-14000-exec-1,5,main], dispatcherType: REQUEST2024-05-06 07:35:28.107  INFO 209108 --- [io-14000-exec-1] c.p.java.study.web.config.MyInterceptor  : preHandle method is running..., thread: Thread[http-nio-14000-exec-1,5,main], dispatcherType: REQUEST2024-05-06 07:35:28.121  INFO 209108 --- [io-14000-exec-1] c.p.j.s.w.r.test.TestStreamController    : Stream method is running, thread: Thread[http-nio-14000-exec-1,5,main]2024-05-06 07:35:28.152  INFO 209108 --- [    web-async-1] c.p.j.s.w.r.test.TestStreamController    : Internal stream method is running, thread: Thread[web-async-1,5,main]2024-05-06 07:35:28.167  INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor  : preHandle method is running..., thread: Thread[http-nio-14000-exec-2,5,main], dispatcherType: ASYNC2024-05-06 07:35:28.167  INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor  : preHandle dispatcherType=ASYNC2024-05-06 07:35:28.174  INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor  : postHandle method is running..., thread: Thread[http-nio-14000-exec-2,5,main]2024-05-06 07:35:28.183  INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor  : afterCompletion method is running..., thread: Thread[http-nio-14000-exec-2,5,main]

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问题分析

1. 方法调用流程的差异

众所周知，SpringBoot 的普通输出接口调用流程图如图 1 所示。

图1-SpringBoot 普通输出调用流程图

结合日志，我们可以简单画出流式输出接口对应的流程图（图 2）。

图2-SpringBoot 流式输出调用流程图

2. 线程的差异

普通接口的执行时序图如图 3 所示。

图3-普通接口的时序图

而流式接口的时序图如图 4 所示。

图4-流式接口的调用时序图

解决问题

通过分析，对流式输出的情况提出两种解决方案：

1. 将过滤器中的部分业务逻辑迁移到拦截器中。

2. 根据条件，跳过第二次的拦截器 preHandle 方法。

笔者选择了第二个方案，实现代码如下。

@Slf4jpublic class MyInterceptor implements HandlerInterceptor {        @Override    public boolean preHandle(@NotNull HttpServletRequest request, @NotNull HttpServletResponse response, @NotNull Object handler) throws Exception {        log.info("preHandle method is running..., thread: {}, dispatcherType: {}", Thread.currentThread(), request.getDispatcherType());        // 如果是异步请求，则跳过        if (DispatcherType.ASYNC == request.getDispatcherType()) {            log.info("preHandle dispatcherType={}", request.getDispatcherType());            return true;        }        return true;    }        @Override    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {        log.info("postHandle method is running..., thread: {}", Thread.currentThread());         }        @Override    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {        log.info("afterCompletion method is running..., thread: {}", Thread.currentThread());    } }

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需要注意，请求线程和回调线程都需考虑清理线程变量，不然会导致内存泄漏。