logback统一扩展打印自定义属性字段的方式

记录下采用logback统一扩展打印自定义属性字段的方式, 通常打印日志，自己可以采用拼字符串的方式，或者String.format等格式化字符串方式。但其实logback还提供了一种比较好玩的方式MDC. 特使是针对那些格式比较统一的日志，在WEB中用一个拦截器就可以实现, 比如要统一打印登录的用户：

import java.io.IOException;
import java.security.Principal;

import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;

import org.slf4j.MDC;

public class UserServletFilter implements Filter {

  private final String USER_KEY = "username";
  
  public void destroy() {
  }

  public void doFilter(ServletRequest request, ServletResponse response,
    FilterChain chain) throws IOException, ServletException {

    boolean successfulRegistration = false;

    HttpServletRequest req = (HttpServletRequest) request;    
    Principal principal = req.getUserPrincipal();
    // Please note that we could have also used a cookie to 
    // retrieve the user name

    if (principal != null) {
      String username = principal.getName();
      successfulRegistration = registerUsername(username);
    } 

    try {
      chain.doFilter(request, response);
    } finally {
      if (successfulRegistration) {
        MDC.remove(USER_KEY);
      }
    }
  }

  public void init(FilterConfig arg0) throws ServletException {
  }
  

  /**
   * Register the user in the MDC under USER_KEY.
   * 
   * @param username
   * @return true id the user can be successfully registered
   */
  private boolean registerUsername(String username) {
    if (username != null && username.trim().length() > 0) {
      MDC.put(USER_KEY, username);
      return true;
    }
    return false;
  }
}

loback 的日志配置中，增加自己需要的字段,类似如下：

%-4r [%thread] %-5level C:%X{username} - %msg%n

参考资料：
https://logback.qos.ch/manual/mdc.html

From：一号门