MDC概述

Mapped Diagnostic Context（MDC）主要是在打印日志的时候，在当前代码的上文中不存在要打印的变量，而又需要打印该变量的数据。在这种需求场景下就可以通过 MDC 来解决。

MDC的注意点

MDC是基于ThreadLocal 实现的。因此在多线程环境下，在 MDC 用完后，需要 clear。

Let’s see how the combination of ThreadLocal-based MDCs and thread pools can be dangerous:

1. We get a thread from the thread pool.
2. Then we store some contextual information in MDC using MDC.put() or ThreadContext.put() .
3. We use this information in some logs, and somehow we forgot to clear the MDC context.
4. The borrowed thread comes back to the thread pool.
5. After a while, the application gets the same thread from the pool.
6. Since we didn’t clean up the MDC last time, this thread still owns some data from the previous execution.

参考

• slf4j MDC是个好东西

• Improved Java Logging with Mapped Diagnostic Context (MDC)