一.MDC

1.MDC介绍

MDC（Mapped Diagnostic Context，映射调试上下文）是 log4j 和 logback 提供的一种方便在多线程场景下记录日志的功能。MDC 可以看成是一个与当前线程绑定的Map，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。

简而言之，MDC就是日志框架提供的一个InheritableThreadLocal，所以它是线程安全的，在项目代码中可以将键值对放入其中，然后使用指定方式取出打印即可。
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log.info("traceId:{} ", traceId)
- 在 log4j 和 logback 的取值方式为：
```
%X{
      
      traceid}
```

2.API说明

在这里插入图片描述

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
getPropertyMap()：返回当前线程的context map的直接引用！不是拷贝副本！
getCopyOfContextMap()：返回当前线程的context map的一个副本，对这个map的修改不会影响原来copyOnInheritThreadLocal中的内容。

二.MDC使用

1.使用方式

public class Constants {
    
    
    /**
     * 日志跟踪id名。
     */
    public static final String TRACE_ID= "trace_id";

    /**
     * 请求头跟踪id名。
     */
    public static final String HTTP_HEADER_TRACE_ID = "app_trace_id";
}

public class TraceIdUtil {
    
    
    public static String getTraceId(){
    
    
        return UUID.randomUUID().toString().replace("-","");
    }
}

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 {
    
    
    @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);
    }
}

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

2.存在问题

2.1.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，当前线程创建的子线程获取不到ThreadLocal存储的键值
- 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

问题重现：

    @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

线程traceId封装工具类

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
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

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 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()));
    }
}

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:1

2.2.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

1.HttpClient

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


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);
        }
    }
}

为HttpClient添加拦截器

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

2.OKHttp

– 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

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;
    }
}

为OkHttp添加拦截器

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

3. RestTemplate

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

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);
    }
}

为RestTemplate添加拦截器

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

2.3.第三方服务需要添加拦截器

需要第三方服务配合，添加拦截器拿到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);
    }
}

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

【SpringBoot】分布式日志跟踪—通过MDC实现全链路调用日志跟踪