通过本文将了解到什么是MDC、MDC应用中存在的问题、如何解决存在的问题

MDC介绍

简介：

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。MDC 的内容则由程序在适当的时候保存进去。对于一个 Web 应用来说，通常是在请求被处理的最开始保存这些数据

API说明：

• clear() => 移除所有MDC

• get (String key) => 获取当前线程MDC中指定key的值

• getContext() => 获取当前线程MDC的MDC

• put(String key, Object o) => 往当前线程的MDC中存入指定的键值对

• remove(String key) => 删除当前线程MDC中指定的键值对

优点：

• 代码简洁，日志风格统一，不需要在log打印中手动拼写traceId，即LOGGER.info("traceId:{} ", traceId)

暂时只能想到这一点

MDC使用

• 添加拦截器

public class LogInterceptor implements HandlerInterceptor {@Overridepublic boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {//如果有上层调用就用上层的IDString traceId = request.getHeader(Constants.TRACE_ID);if (traceId == null) {traceId = TraceIdUtil.getTraceId();}MDC.put(Constants.TRACE_ID, traceId);return true;}@Overridepublic void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)throws Exception {}@Overridepublic void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)throws Exception {//调用结束后删除MDC.remove(Constants.TRACE_ID);}
}

• 修改日志格式

<property name="pattern">[TRACEID:%X{traceId}] %d{HH:mm:ss.SSS} %-5level %class{-1}.%M()/%L - %msg%xEx%n</property>

重点是%X{traceId}，traceId和MDC中的键名称一致

简单使用就这么容易，但是在有些情况下traceId将获取不到

MDC 存在的问题

• 子线程中打印日志丢失traceId

• HTTP调用丢失traceId

丢失traceId的情况，来一个再解决一个，绝不提前优化

解决MDC存在的问题

子线程日志打印丢失traceId

子线程在打印日志的过程中traceId将丢失，解决方式为重写线程池，对于直接new创建线程的情况不考虑【实际应用中应该避免这种用法】，重写线程池无非是对任务进行一次封装

• 线程池封装类：ThreadPoolExecutorMdcWrapper.java

public class ThreadPoolExecutorMdcWrapper extends ThreadPoolExecutor {public ThreadPoolExecutorMdcWrapper(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,BlockingQueue<Runnable> workQueue) {super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);}public ThreadPoolExecutorMdcWrapper(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory) {super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory);}public ThreadPoolExecutorMdcWrapper(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,BlockingQueue<Runnable> workQueue, RejectedExecutionHandler handler) {super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, handler);}public ThreadPoolExecutorMdcWrapper(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit,BlockingQueue<Runnable> workQueue, ThreadFactory threadFactory,RejectedExecutionHandler handler) {super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, threadFactory, handler);}@Overridepublic void execute(Runnable task) {super.execute(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap()));}@Overridepublic <T> Future<T> submit(Runnable task, T result) {return super.submit(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap()), result);}@Overridepublic <T> Future<T> submit(Callable<T> task) {return super.submit(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap()));}@Overridepublic Future<?> submit(Runnable task) {return super.submit(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap()));}
}

说明：

• 继承ThreadPoolExecutor类，重新执行任务的方法

• 通过ThreadMdcUtil对任务进行一次包装

• 线程traceId封装工具类：ThreadMdcUtil.java

public class ThreadMdcUtil {public static void setTraceIdIfAbsent() {if (MDC.get(Constants.TRACE_ID) == null) {MDC.put(Constants.TRACE_ID, TraceIdUtil.getTraceId());}}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);}setTraceIdIfAbsent();try {runnable.run();} finally {MDC.clear();}};}
}

说明【以封装Runnable为例】：

• 判断当前线程对应MDC的Map是否存在，存在则设置

• 设置MDC中的traceId值，不存在则新生成，针对不是子线程的情况，如果是子线程，MDC中traceId不为null

• 执行run方法

代码等同于以下写法，会更直观

public static Runnable wrap(final Runnable runnable, final Map<String, String> context) {return new Runnable() {@Overridepublic void run() {if (context == null) {MDC.clear();} else {MDC.setContextMap(context);}setTraceIdIfAbsent();try {runnable.run();} finally {MDC.clear();}}};}

重新返回的是包装后的Runnable，在该任务执行之前【runnable.run()】先将主线程的Map设置到当前线程中【 即MDC.setContextMap(context)】，这样子线程和主线程MDC对应的Map就是一样的了

HTTP调用丢失traceId

在使用HTTP调用第三方服务接口时traceId将丢失，需要对HTTP调用工具进行改造，在发送时在request header中添加traceId，在下层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，所以只给出这几种HTTP调用的解决方式

HttpClient：

• 实现HttpClient拦截器

public class HttpClientTraceIdInterceptor implements HttpRequestInterceptor {@Overridepublic void process(HttpRequest httpRequest, HttpContext httpContext) throws HttpException, IOException {String traceId = MDC.get(Constants.TRACE_ID);//当前线程调用中有traceId，则将该traceId进行透传if (traceId != null) {//添加请求体httpRequest.addHeader(Constants.TRACE_ID, traceId);}}
}

• 实现HttpRequestInterceptor接口并重写process方法

• 如果调用线程中含有traceId，则需要将获取到的traceId通过request中的header向下透传下去

• 为HttpClient添加拦截器

private static CloseableHttpClient httpClient = HttpClientBuilder.create().addInterceptorFirst(new HttpClientTraceIdInterceptor())

通过addInterceptorFirst方法为HttpClient添加拦截器

OKHttp：

• 实现OKHttp拦截器

public class OkHttpTraceIdInterceptor implements Interceptor {@Overridepublic Response intercept(Chain chain) throws IOException {String traceId = MDC.get(Constants.TRACE_ID);Request request = null;if (traceId != null) {//添加请求体request = chain.request().newBuilder().addHeader(Constants.TRACE_ID, traceId).build();}Response originResponse = chain.proceed(request);return originResponse;}
}

实现Interceptor拦截器，重写interceptor方法，实现逻辑和HttpClient差不多，如果能够获取到当前线程的traceId则向下透传

• 为OkHttp添加拦截器

private static OkHttpClient client = new OkHttpClient.Builder().addNetworkInterceptor(new OkHttpTraceIdInterceptor()).build();

调用addNetworkInterceptor方法添加拦截器

RestTemplate：

• 实现RestTemplate拦截器

public class RestTemplateTraceIdInterceptor implements ClientHttpRequestInterceptor {@Overridepublic ClientHttpResponse intercept(HttpRequest httpRequest, byte[] bytes, ClientHttpRequestExecution clientHttpRequestExecution) throws IOException {String traceId = MDC.get(Constants.TRACE_ID);if (traceId != null) {httpRequest.getHeaders().add(Constants.TRACE_ID, traceId);}return clientHttpRequestExecution.execute(httpRequest, bytes);}
}

实现ClientHttpRequestInterceptor接口，并重写intercept方法，其余逻辑都是一样的不重复说明

• 为RestTemplate添加拦截器

restTemplate.setInterceptors(Arrays.asList(new RestTemplateTraceIdInterceptor()));

调用setInterceptors方法添加拦截器

第三方服务拦截器：

HTTP调用第三方服务接口全流程traceId需要第三方服务配合，第三方服务需要添加拦截器拿到request header中的traceId并添加到MDC中

public class LogInterceptor implements HandlerInterceptor {@Overridepublic boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {//如果有上层调用就用上层的IDString traceId = request.getHeader(Constants.TRACE_ID);if (traceId == null) {traceId = TraceIdUtils.getTraceId();}MDC.put("traceId", traceId);return true;}@Overridepublic void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)throws Exception {}@Overridepublic void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)throws Exception {MDC.remove(Constants.TRACE_ID);}
}

说明：

• 先从request header中获取traceId

• 从request header中获取不到traceId则说明不是第三方调用，直接生成一个新的traceId

• 将生成的traceId存入MDC中

除了需要添加拦截器之外，还需要在日志格式中添加traceId的打印，如下：

<property name="pattern">[TRACEID:%X{traceId}] %d{HH:mm:ss.SSS} %-5level %class{-1}.%M()/%L - %msg%xEx%n</property>

需要添加%X{traceId}

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