?1.MDC是什么

先來一段原版注釋，

/*** This class hides and serves as a substitute for the underlying logging* system's MDC implementation.* * <p>* If the underlying logging system offers MDC functionality, then SLF4J's MDC,* i.e. this class, will delegate to the underlying system's MDC. Note that at* this time, only two logging systems, namely log4j and logback, offer MDC* functionality. For java.util.logging which does not support MDC,* {@link BasicMDCAdapter} will be used. For other systems, i.e slf4j-simple* and slf4j-nop, {@link NOPMDCAdapter} will be used.** <p>* Thus, as a SLF4J user, you can take advantage of MDC in the presence of log4j,* logback, or java.util.logging, but without forcing these systems as* dependencies upon your users.* * <p>* For more information on MDC please see the <a* href="http://logback.qos.ch/manual/mdc.html">chapter on MDC</a> in the* logback manual.* * <p>* Please note that all methods in this class are static.* * @author Ceki Gülcü* @since 1.4.1*/

這是MDC類上的注釋，但是沒有講這個是干什么的。具體的內容在有一個頁面：

Chapter 8: Mapped Diagnostic Context

簡單來說就是日志的增強功能，如果配置了MDC，并添加了相應的key value，就會在打日志的時候把key對應的value打印出來。

內部是用ThreadLocal來實現的，可以攜帶當前線程的context信息。

2.MDCAdapter——LogbackMDCAdapter

MDC中包裝了一個MDCAdapter，這是一個接口，不同的log包有自己對應的實現。對于不支持MDC的包，比如java.util.logging，使用默認的BasicMDCAdapter，slf4j-nop有NOPMDCAdapter。

這里說的實現是LogbackMDCAdapter。

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