MDC(Mapped Diagnostic Context，映射调试上下文)是 log4j 、logback及log4j2 提供的一种方便在多线程条件下记录日志的功能。MDC 可以看成是一个与当前线程绑定的哈希表，可以往其中添加键值对。MDC 中包含的内容可以被同一线程中执行的代码所访问。当前线程的子线程会继承其父线程中的 MDC 的内容。当需要记录日志时，只需要从 MDC 中获取所需的信息即可。
@Component public class LogInterceptor implements HandlerInterceptor { @Override public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { //如果有上层调用就用上层的ID String traceId = request.getHeader(TraceIdUtil.TRACE_ID); if (StringUtil.isEmpty(traceId)) { TraceIdUtil.setTraceId(TraceIdUtil.generateTraceId()); } else { TraceIdUtil.setTraceId(traceId); } return true; } @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { } @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { //调用结束后删除 TraceIdUtil.remove(); } }public class TraceIdUtil { public static final String TRACE_ID = "requestId"; public static String getTraceId() { String traceId =(String) MDC.get(TRACE_ID); return traceId == null ? "" : traceId; } public static void setTraceId(String traceId) { MDC.put(TRACE_ID,traceId); } public static void remove() { MDC.remove(TRACE_ID); } public static void clear() { MDC.clear(); } public static String generateTraceId() { return UUID.randomUUID().toString().replace("-", ""); }<property name="LOG_PATTERN" value="%date{yyyy-MM-dd HH:mm:ss.SSS} [%thread] %-5level [%X{requestId}] %logger{36} - %msg%n" />重点是%X{requestId},requestId和MDC中的键名称保持一致。
至此基本的功能已经实现，但是存在一下几个问题
问题重现
@LogAnnotation(model="用户管理",func="查询用户信息",desc="根据用户名称") @GetMapping("getUserByName") public Result getUserByName(@RequestParam String name) { //主线程日志 logger.info("getUserByName paramter name:"+name); for(int i=0;i<5;i++) { //子线程日志 threadPoolTaskExecutor.execute(()->{ logger.info("child thread:{}",name); userService.getUserByName(name); }); } return Result.success(); }运行结果
2022-03-13 12:45:44.156 [http-nio-8089-exec-1] INFO [ec05a600ed1a4556934a3afa4883766a] c.s.fw.controller.UserController - getUserByName paramter name:1 2022-03-13 12:45:44.173 [Pool-A1] INFO [] c.s.fw.controller.UserController - child thread:1从运行的结果来看，子线程打印日志，日志中的traceId信息已经丢失。
解决方案
子线程在打印日志的过程中traceId将丢失，解决方案为重写线程池(对于直接new Thread 创建线程的情况不考略),实际开发中也需要禁止这种情况。
public class ThreadPoolExecutorMdcWrapper extends ThreadPoolTaskExecutor { private static final long serialVersionUID = 3940722618853093830L; @Override public void execute(Runnable task) { super.execute(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap())); } @Override public <T> Future<T> submit(Callable<T> task) { return super .submit(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap())); } @Override public Future<?> submit(Runnable task) { return super .submit(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap())); } }因为Spring Boot ThreadPoolTaskExecutor 已经对ThreadPoolExecutor进行封装，只需要继承ThreadPoolTaskExecutor重写相关的执行方法即可。
public class ThreadMdcUtil { public static void setTraceIdIfAbsent() { if (MDC.get(TraceIdUtil.TRACE_ID) == null) { MDC.put(TraceIdUtil.TRACE_ID, TraceIdUtil.generateTraceId()); } } public static <T> Callable<T> wrap(final Callable<T> callable, final Map<String, String> context) { return () -> { if (context == null) { MDC.clear(); } else { MDC.setContextMap(context); } setTraceIdIfAbsent(); try { return callable.call(); } finally { MDC.clear(); } }; } public static Runnable wrap(final Runnable runnable, final Map<String, String> context) { return () -> { if (context == null) { MDC.clear(); } else { MDC.setContextMap(context); } //设置traceId setTraceIdIfAbsent(); try { runnable.run(); } finally { MDC.clear(); } }; } }代码说明：
线程池配置
@Configuration public class ThreadPoolTaskExecutorConfig { //最大可用的CPU核数 public static final int PROCESSORS = Runtime.getRuntime().availableProcessors(); @Bean public ThreadPoolExecutorMdcWrapper getExecutor() { ThreadPoolExecutorMdcWrapper executor =new ThreadPoolExecutorMdcWrapper(); executor.setCorePoolSize(PROCESSORS *2); executor.setMaxPoolSize(PROCESSORS * 4); executor.setQueueCapacity(50); executor.setKeepAliveSeconds(60); executor.setThreadNamePrefix("Task-A"); executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy()); executor.initialize(); return executor; } }重新运行结果发现子线程能够正常获取traceid信息进行跟踪。
2022-03-13 13:19:30.688 [Task-A1] INFO [482929425cbc4476a4e7168615af7890] c.s.fw.controller.UserController - child thread:1 2022-03-13 13:19:31.003 [Task-A1] INFO [482929425cbc4476a4e7168615af7890] c.s.fw.service.impl.UserServiceImpl - name:1HTTP调用第三方服务接口时traceId丢失，需要在发送请求时在Request Header中添加traceId，在被调用方添加拦截器获取header中的traceId添加到MDC中。
HTTP调用有多种方式，比较常见的有HttpClient、OKHttp、RestTemplate，以RestTemplate调用为例。
public class RestTemplateTraceIdInterceptor implements ClientHttpRequestInterceptor { @Override public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution clientHttpRequestExecution) throws IOException { String traceId=MDC.get("requestId"); if(traceId!=null) { request.getHeaders().set("requestId", traceId); } else { request.getHeaders().set("requestId", UUID.randomUUID().toString().replace("-", "")); } return clientHttpRequestExecution.execute(request, body); } }RestTemplate添加拦截器即可。
restTemplate.setInterceptors(Arrays.asList(new RestTemplateTraceIdInterceptor()));2.第三方服务需要添加拦截器。
@Component public class LogInterceptor implements HandlerInterceptor { @Override public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { //如果有上层调用就用上层的ID String traceId = request.getHeader(TraceIdUtil.TRACE_ID); if (StringUtil.isEmpty(traceId)) { TraceIdUtil.setTraceId(TraceIdUtil.generateTraceId()); } else { TraceIdUtil.setTraceId(traceId); } return true; } @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { } @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { //调用结束后删除 TraceIdUtil.remove(); } }其他HttpClient、OKHttp的实现方式与RestTemplate基本相同，这里就不一一列举。 Spring boot +logback+MDC实现全链路跟踪内容已经讲完了。在实际开发中，我也发现了大家对于日志的使用存在一些问题，下一章将讲解开发中日志使用规约。