Mapped Diagnostic Contexts (MDC)
One of the design goals of logback is to audit and debug complex distributed applications.
It lets the developer place information in a diagnostic context that can be subsequently retrieved by certain logback components.
Chapter 8: Mapped Diagnostic Context
Java 常用的日志工具，logback log4j slf4j 都支持MDC 特性。本文分析 logback MDC。
Code Insight
// MDC operations such as put() and get() affect only the MDC of the current thread, and the children of the current thread.
// Thus, there is no need for the developer to worry about thread-safety or synchronization when programming with the MDC because it handles these issues safely and transparently.
public class LogbackMDCAdapter implements MDCAdapter {
// The internal map is copied so as
// We wish to avoid unnecessarily copying of the map. To ensure
// efficient/timely copying, we have a variable keeping track of the last
// operation.
final ThreadLocal<Map<String, String>> copyOnThreadLocal = new ThreadLocal<Map<String, String>>();
}
使用方法
Simple
// 可以在任何地方put value, 由上述实现可以知道，存取和具体的日志技术无关。
MDC.put("first", "Dorothy");
Logger logger = LoggerFactory.getLogger(SimpleMDC.class);
MDC.put("last", "Parker");
logger.info("Check enclosed.");
<appender name="CONSOLE" class="ch.qos.logback.core.ConsoleAppender">
<layout>
<Pattern>%X{first} %X{last} - %m%n</Pattern>
</layout>
</appender>
打印结果：Dorothy Parker - Check enclosed.
Web Application
logback 内置的Filter, 可以实现请求信息的提取和打印，帮助统计和排查问题。
ch.qos.logback.classic.helpers.MDCInsertingServletFilter
<!-- web.xml 配置，必须配置到filters 首位，保障后续打印都能使用MDC 保存的信息 -->
<filter>
<filter-name>MDCInsertingServletFilter</filter-name>
<filter-class>
ch.qos.logback.classic.helpers.MDCInsertingServletFilter
</filter-class>
</filter>
<filter-mapping>
<filter-name>MDCInsertingServletFilter</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>
打印pattern 配置 %X{req.remoteHost} %X{req.requestURI}%n%d - %m%n
public class MDCInsertingServletFilter implements Filter {
public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
// put request information
insertIntoMDC(request);
try {
chain.doFilter(request, response);
} finally {
// 从当前线程移除请求信息，防止复用线程信息混淆
clearMDC();
}
}
}
More
- LogbackMDCAdapter 实现，why track of the last operation？
和使用 InheritableThreadLocal 有什么区别？
参考： org.slf4j.helpers.BasicMDCAdapter