技术分享 | SpringBoot 流式输出时，正常输出后为何突然报错？

项目背景

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。
该项目需要调用大语言模型进行流式输出。
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 {@Overridepublic 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);} 
}

Interceptor

@Slf4j
public class MyInterceptor implements HandlerInterceptor {@Overridepublic 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;}@Overridepublic void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {log.info("postHandle method is running..., thread: {}", Thread.currentThread());}      @Overridepublic void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {log.info("afterCompletion method is running..., thread: {}", Thread.currentThread());} 
}

WebMvcConfigurer

@Configuration
public class WebAppConfigurer implements WebMvcConfigurer {@Beanpublic MyInterceptor myInterceptor() {return new MyInterceptor();}@Overridepublic void addInterceptors(InterceptorRegistry registry) {registry.addInterceptor(myInterceptor()).addPathPatterns("/**");}@Overridepublic 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);}
}

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();}});}
}

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 ms
2024-05-06 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: REQUEST
2024-05-06 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: REQUEST
2024-05-06 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: ASYNC
2024-05-06 07:35:28.167  INFO 209108 --- [io-14000-exec-2] c.p.java.study.web.config.MyInterceptor  : preHandle dispatcherType=ASYNC
2024-05-06 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]

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

（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

（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：

将过滤器中的部分业务逻辑迁移到拦截器中。
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。

@Slf4j
public class MyInterceptor implements HandlerInterceptor {@Overridepublic 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;}@Overridepublic void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {log.info("postHandle method is running..., thread: {}", Thread.currentThread());     }@Overridepublic void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {log.info("afterCompletion method is running..., thread: {}", Thread.currentThread());} 
}