寫(xiě)在前面

通過(guò)本文將了解到什么是MDC咨察、MDC應(yīng)用中存在的問(wèn)題、如何解決存在的問(wèn)題

MDC介紹

簡(jiǎn)介：

MDC（Mapped Diagnostic Context沛硅，映射調(diào)試上下文）是 log4j 眼刃、logback及l(fā)og4j2 提供的一種方便在多線程條件下記錄日志的功能。MDC 可以看成是一個(gè)與當(dāng)前線程綁定的哈希表摇肌，可以往其中添加鍵值對(duì)擂红。MDC 中包含的內(nèi)容可以被同一線程中執(zhí)行的代碼所訪問(wèn)。當(dāng)前線程的子線程會(huì)繼承其父線程中的 MDC 的內(nèi)容围小。當(dāng)需要記錄日志時(shí)昵骤，只需要從 MDC 中獲取所需的信息即可。MDC 的內(nèi)容則由程序在適當(dāng)?shù)臅r(shí)候保存進(jìn)去肯适。對(duì)于一個(gè) Web 應(yīng)用來(lái)說(shuō)变秦，通常是在請(qǐng)求被處理的最開(kāi)始保存這些數(shù)據(jù)

API說(shuō)明：

• clear() => 移除所有MDC
• get (String key) => 獲取當(dāng)前線程MDC中指定key的值
• getContext() => 獲取當(dāng)前線程MDC的MDC
• put(String key, Object o) => 往當(dāng)前線程的MDC中存入指定的鍵值對(duì)
• remove(String key) => 刪除當(dāng)前線程MDC中指定的鍵值對(duì)

優(yōu)點(diǎn)：

• 代碼簡(jiǎn)潔，日志風(fēng)格統(tǒng)一框舔，不需要在log打印中手動(dòng)拼寫(xiě)traceId蹦玫，即LOGGER.info("traceId:{} ", traceId)

暫時(shí)只能想到這一點(diǎn)

MDC使用

• 添加攔截器

    public class LogInterceptor implements HandlerInterceptor {
        @Override
        public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
            //如果有上層調(diào)用就用上層的ID
            String traceId = request.getHeader(Constants.TRACE_ID);
            if (traceId == null) {
                traceId = TraceIdUtil.getTraceId();
            }

            MDC.put(Constants.TRACE_ID, 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 {
            //調(diào)用結(jié)束后刪除
            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>

重點(diǎn)是%X{traceId}，traceId和MDC中的鍵名稱(chēng)一致
簡(jiǎn)單使用就這么容易刘绣，但是在有些情況下traceId將獲取不到

MDC 存在的問(wèn)題

• 子線程中打印日志丟失traceId

• HTTP調(diào)用丟失traceId

  ......丟失traceId的情況樱溉，來(lái)一個(gè)再解決一個(gè)，絕不提前優(yōu)化

解決MDC存在的問(wèn)題

子線程日志打印丟失traceId

子線程在打印日志的過(guò)程中traceId將丟失纬凤，解決方式為重寫(xiě)線程池福贞，對(duì)于直接new創(chuàng)建線程的情況不考略【實(shí)際應(yīng)用中應(yīng)該避免這種用法】，重寫(xiě)線程池?zé)o非是對(duì)任務(wù)進(jìn)行一次封裝

• 線程池封裝類(lèi)：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);
     }

     @Override
     public void execute(Runnable task) {
         super.execute(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap()));
     }

     @Override
     public <T> Future<T> submit(Runnable task, T result) {
         return super.submit(ThreadMdcUtil.wrap(task, MDC.getCopyOfContextMap()), result);
     }

     @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()));
     }
 }

說(shuō)明：

• 繼承ThreadPoolExecutor類(lèi)停士，重新執(zhí)行任務(wù)的方法
• 通過(guò)ThreadMdcUtil對(duì)任務(wù)進(jìn)行一次包裝
• 線程traceId封裝工具類(lèi)：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();
                }
            };
        }
    }

說(shuō)明【以封裝Runnable為例】：

• 判斷當(dāng)前線程對(duì)應(yīng)MDC的Map是否存在肚医，存在則設(shè)置
• 設(shè)置MDC中的traceId值绢馍，不存在則新生成，針對(duì)不是子線程的情況肠套，如果是子線程舰涌，MDC中traceId不為null
• 執(zhí)行run方法

代碼等同于以下寫(xiě)法，會(huì)更直觀

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

重新返回的是包裝后的Runnable你稚，在該任務(wù)執(zhí)行之前【runnable.run()】先將主線程的Map設(shè)置到當(dāng)前線程中【 即MDC.setContextMap(context)】瓷耙，這樣子線程和主線程MDC對(duì)應(yīng)的Map就是一樣的了

HTTP調(diào)用丟失traceId

在使用HTTP調(diào)用第三方服務(wù)接口時(shí)traceId將丟失，需要對(duì)HTTP調(diào)用工具進(jìn)行改造刁赖，在發(fā)送時(shí)在request header中添加traceId搁痛，在下層被調(diào)用方添加攔截器獲取header中的traceId添加到MDC中

HTTP調(diào)用有多種方式，比較常見(jiàn)的有HttpClient宇弛、OKHttp鸡典、RestTemplate，所以只給出這幾種HTTP調(diào)用的解決方式

HttpClient：

• 實(shí)現(xiàn)HttpClient攔截器

public class HttpClientTraceIdInterceptor implements HttpRequestInterceptor {
        @Override
        public void process(HttpRequest httpRequest, HttpContext httpContext) throws HttpException, IOException {
            String traceId = MDC.get(Constants.TRACE_ID);
            //當(dāng)前線程調(diào)用中有traceId枪芒，則將該traceId進(jìn)行透?jìng)?            if (traceId != null) {
                //添加請(qǐng)求體
                httpRequest.addHeader(Constants.TRACE_ID, traceId);
            }
        }
    }

實(shí)現(xiàn)HttpRequestInterceptor接口并重寫(xiě)process方法

如果調(diào)用線程中含有traceId彻况，則需要將獲取到的traceId通過(guò)request中的header向下透?jìng)飨氯?/p>

• 為HttpClient添加攔截器

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

通過(guò)addInterceptorFirst方法為HttpClient添加攔截器

OKHttp：

• 實(shí)現(xiàn)OKHttp攔截器

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

         return originResponse;
     }
 }

實(shí)現(xiàn)Interceptor攔截器，重寫(xiě)interceptor方法舅踪，實(shí)現(xiàn)邏輯和HttpClient差不多纽甘，如果能夠獲取到當(dāng)前線程的traceId則向下透?jìng)?

• 為OkHttp添加攔截器

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

調(diào)用addNetworkInterceptor方法添加攔截器

RestTemplate：

• 實(shí)現(xiàn)RestTemplate攔截器

    public class RestTemplateTraceIdInterceptor implements ClientHttpRequestInterceptor {
        @Override
        public 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);
        }
    }

實(shí)現(xiàn)ClientHttpRequestInterceptor接口，并重寫(xiě)intercept方法抽碌，其余邏輯都是一樣的不重復(fù)說(shuō)明

• 為RestTemplate添加攔截器

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

調(diào)用setInterceptors方法添加攔截器

第三方服務(wù)攔截器：

HTTP調(diào)用第三方服務(wù)接口全流程traceId需要第三方服務(wù)配合悍赢，第三方服務(wù)需要添加攔截器拿到request header中的traceId并添加到MDC中

public class LogInterceptor implements HandlerInterceptor {
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        //如果有上層調(diào)用就用上層的ID
        String traceId = request.getHeader(Constants.TRACE_ID);
        if (traceId == null) {
            traceId = TraceIdUtils.getTraceId();
        }

        MDC.put("traceId", 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 {
        MDC.remove(Constants.TRACE_ID);
    }
}

說(shuō)明：

• 先從request header中獲取traceId
• 從request header中獲取不到traceId則說(shuō)明不是第三方調(diào)用，直接生成一個(gè)新的traceId
• 將生成的traceId存入MDC中

除了需要添加攔截器之外货徙，還需要在日志格式中添加traceId的打印左权，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}