MDC日志跟踪

MDC是log4j，logback和java.util.logging中提供的一种方便在多线程下记录日志的功能，全称是Mapped Diagnostic Context，可以看做是一个与当前线程绑定的哈希表，依赖于ThreadLocalMap

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("traceId");
        if (traceId == null) {
            traceId = TraceIdUtil.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 {
        //调用结束后删除
        MDC.remove("traceId");
    }
}

在配置日志格式时，使用%X{}来获取MDC中的值

1	<PatternLayout charset="UTF-8" pattern="[%-5p %d{yyyy-MM-dd HH:mm:ss.SSS} [%X{traceId}]] %l [%m]%n"/>

如果想要子线程获取到父线程的MDC内容，在父线程新建子线程之前调用MDC.getCopyOfContextMap()方法将MDC内容取出来传给子线程，子线程在执行操作前先调用MDC.setContextMap()方法将父线程的MDC内容设置到子线程