(七)Logback中的MDC(Mapped Diagnostic Context)

一、爲什麼會有MDC(MappedDiagnosticContext)?

審計和調試分佈式應用是logback的設計目標之一，在多線程環境中，不同的線程會處理不同的客戶端，爲了區分不同客戶端日誌輸出，一種輕量級但不可取的做法是爲每個實例化一個新的、完全分離日誌記錄器，這種方法會產生很多的日誌記錄器而且難以管理。

更輕量級的做法是唯一標記每個來自客戶端的日誌請求，爲了唯一標記每個請求，用戶把上下文信息放置到MDC中，MDC類的核心方法如下，完整方法參考MDC javadocs。

package org.slf4j;

public class MDC {
  //Put a context value as identified by key
  //into the current thread's context map.
  public static void put(String key, String val);

  //Get the context identified by the key parameter.
  public static String get(String key);

  //Remove the context identified by the key parameter.
  public static void remove(String key);

  //Clear all entries in the MDC.
  public static void clear();
}

1、基本MDC的使用

package chapters.mdc;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;

import ch.qos.logback.classic.PatternLayout;
import ch.qos.logback.core.ConsoleAppender;

public class SimpleMDC {
  static public void main(String[] args) throws Exception {

    // You can put values in the MDC at any time. Before anything else
    // we put the first name
    MDC.put("first", "Dorothy");

    
    Logger logger = LoggerFactory.getLogger(SimpleMDC.class);
    // We now put the last name
    MDC.put("last", "Parker");

    // The most beautiful two words in the English language according
    // to Dorothy Parker:
    logger.info("Check enclosed.");
    logger.debug("The most beautiful two words in English.");

    MDC.put("first", "Richard");
    MDC.put("last", "Nixon");
    logger.info("I am not a crook.");
    logger.info("Attributed to the former US president. 17 Nov 1973.");
  }

}

logback配置文件如下：

<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender"> 
  <layout>
    <Pattern>%X{first} %X{last} - %m%n</Pattern>
  </layout> 
</appender>

注意：通過%X轉換詞可以獲取MDC設置的變量值。

日誌輸出如下：

Dorothy Parker - Check enclosed.
Dorothy Parker - The most beautiful two words in English.
Richard Nixon - I am not a crook.
Richard Nixon - Attributed to the former US president. 17 Nov 1973.