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

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    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(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 {
    //调用结束后删除
    MDC.remove(Constants.TRACE_ID);
    }
    }
    复制代码
  • 修改日志格式

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

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    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()));
    }
    }
    复制代码

    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

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    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();
    }
    };
    }
    }
    复制代码

    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

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    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();
    }
    }
    };
    }
    复制代码

    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
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public class HttpClientTraceIdInterceptor implements HttpRequestInterceptor {
@Override
public 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);
}
}
}
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​ 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

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

    通过addInterceptorFirst方法为HttpClient添加拦截器

OKHttp:

  • 实现OKHttp拦截器

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    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) {
    //添加请求体
    request = chain.request().newBuilder().addHeader(Constants.TRACE_ID, traceId).build();
    }
    Response originResponse = chain.proceed(request);

    return originResponse;
    }
    }
    复制代码

    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

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

    调用addNetworkInterceptor方法添加拦截器

RestTemplate:

  • 实现RestTemplate拦截器

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    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);
    }
    }
    复制代码

    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

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    restTemplate.setInterceptors(Arrays.asList(new RestTemplateTraceIdInterceptor()));
    复制代码

    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

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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(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);
}
}
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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:

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