log4j MDC NDC应用场景

NDC（Nested Diagnostic Context）和MDC（Mapped Diagnostic Context）是log4j种非常有用的两个类，它们用于存储应用程序的上下文信息（context infomation），从而便于在log中使用这些上下文信息。

 

NDC的实现是用hashtable来存储每个线程的stack信息，这个stack是每个线程可以设置当前线程的request的相关信息，然后当前线程在处理过程中只要在log4j配置打印出%x的信息，那么当前线程的整个stack信息就会在log4j打印日志的时候也会都打印出来，这样可以很好的跟踪当前request的用户行为功能。

MDC的实现是使用threadlocal来保存每个线程的Hashtable的类似map的信息，其他功能类似。

 

NDC的实现代码：

public class NDC {


  static Hashtable ht = new Hashtable();

  
  private static Stack getCurrentStack() {
      if (ht != null) {
          return (Stack) ht.get(Thread.currentThread());
      }
      return null;
  }

  public
  static
  String pop() {
    Stack stack = getCurrentStack();
    if(stack != null && !stack.isEmpty()) 
      return ((DiagnosticContext) stack.pop()).message;
    else
      return "";
  }
  public
  static
  String peek() {
    Stack stack = getCurrentStack();
    if(stack != null && !stack.isEmpty())
      return ((DiagnosticContext) stack.peek()).message;
    else
      return "";
  }
  public
  static
  void push(String message) {
    Stack stack = getCurrentStack();
      
    if(stack == null) {
      DiagnosticContext dc = new DiagnosticContext(message, null);      
      stack = new Stack();
      Thread key = Thread.currentThread();
      ht.put(key, stack);
      stack.push(dc);
    } else if (stack.isEmpty()) {
      DiagnosticContext dc = new DiagnosticContext(message, null);            
      stack.push(dc);
    } else {
      DiagnosticContext parent = (DiagnosticContext) stack.peek();
      stack.push(new DiagnosticContext(message, parent));
    }    
  }

MDC的实现：

 

public class MDC {
  
  final static MDC mdc = new MDC();
  
  static final int HT_SIZE = 7;

  boolean java1;
  
  Object tlm;

  private Method removeMethod;

  private
  MDC() {
    java1 = Loader.isJava1();
    if(!java1) {
      tlm = new ThreadLocalMap();
    }

    try {
      removeMethod = ThreadLocal.class.getMethod("remove", null);
    } catch (NoSuchMethodException e) {
      // don't do anything - java prior 1.5
    }
  }

 
   */
  static
  public
  void put(String key, Object o) {
     if (mdc != null) {
         mdc.put0(key, o);
     }
  }

  static 
  public
  Object get(String key) {
    if (mdc != null) {
        return mdc.get0(key);
    }
    return null;
  }

  static 
  public
  void remove(String key) {
    if (mdc != null) {
        mdc.remove0(key);
    }
  }


  public static Hashtable getContext() {
    if (mdc != null) {
        return mdc.getContext0();
    } else {
        return null;
    }
  }


  public static void clear() {
    if (mdc != null) {
        mdc.clear0();
    }
  }


  private
  void put0(String key, Object o) {
    if(java1 || tlm == null) {
      return;
    } else {
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht == null) {
        ht = new Hashtable(HT_SIZE);
        ((ThreadLocalMap)tlm).set(ht);
      }    
      ht.put(key, o);
    }
  }
  
  private
  Object get0(String key) {
    if(java1 || tlm == null) {
      return null;
    } else {       
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht != null && key != null) {
        return ht.get(key);
      } else {
        return null;
      }
    }
  }

  private
  void remove0(String key) {
    if(!java1 && tlm != null) {
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht != null) {
        ht.remove(key);
        // clean up if this was the last key
        if (ht.isEmpty()) {
          clear0();
        }
      } 
    }
  }


  private
  Hashtable getContext0() {
     if(java1 || tlm == null) {
      return null;
    } else {       
      return (Hashtable) ((ThreadLocalMap)tlm).get();
    }
  }

  private
  void clear0() {
    if(!java1 && tlm != null) {
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht != null) {
        ht.clear();
      }
      if(removeMethod != null) {
          // java 1.3/1.4 does not have remove - will suffer from a memory leak
          try {
            removeMethod.invoke(tlm, null);
          } catch (IllegalAccessException e) {
            // should not happen
          } catch (InvocationTargetException e) {
            // should not happen
          }
      }
    }
  }

}

在webx框架中对于log4j的MDC的处理：

先配置一个filter，这个filter是放置web.xml的最前面

    protected void populateMDC(Map<String, String> mdc) {
        // GET or POST
        putMDC(mdc, MDC_METHOD, request.getMethod());

        
        StringBuffer requestURL = request.getRequestURL();
        String queryString = trimToNull(request.getQueryString());

        putMDC(mdc, MDC_REQUEST_URL, getRequestURL(requestURL, null));
        putMDC(mdc, MDC_REQUEST_URL_WITH_QUERY_STRING, getRequestURL(requestURL, queryString));

        
        String requestURI = request.getRequestURI();
        String requestURIWithQueryString = queryString == null ? requestURI : requestURI + "?" + queryString;

        putMDC(mdc, MDC_REQUEST_URI, requestURI);
        putMDC(mdc, MDC_REQUEST_URI_WITH_QUERY_STRING, requestURIWithQueryString);
        putMDC(mdc, MDC_QUERY_STRING, queryString);

        // client info
        putMDC(mdc, MDC_REMOTE_HOST, request.getRemoteHost());
        putMDC(mdc, MDC_REMOTE_ADDR, request.getRemoteAddr());

        // user agent
        putMDC(mdc, MDC_USER_AGENT, request.getHeader("User-Agent"));

        // referrer
        putMDC(mdc, MDC_REFERRER, request.getHeader("Referer"));

        // cookies
        Cookie[] cookies = request.getCookies();
        List<String> names = emptyList();

        if (cookies != null) {
            names = createArrayList(cookies.length);

            for (Cookie cookie : cookies) {
                names.add(cookie.getName());
                putMDC(mdc, MDC_COOKIE_PREFIX + cookie.getName(), cookie.getValue());
            }

            sort(names);
        }

        putMDC(mdc, MDC_COOKIES, names.toString());
    }

在finally中记住cleanMDC，否则可能会造成OOM。

  try {   helper.setLoggingContext();

            chain.doFilter(request, response);
        } finally {
            helper.clearLoggingContext();
        }

 

在 NDC 简介部分，我们曾经说过，%x 表示会在每个日志行上打印当前 NDC 上下文。

MDC  %X{remoteAddr}  {remoteAddr} 表示对应map中的remoteAddr的值

配置log4j：

 <param name="ConversionPattern" value="%d{yyyy-MM-dd HH\:mm\:ss} %X{remoteAddr} %X{requestURI} %X{referrer} %X{userAgent} %c %c - %m%n "/>

打印日志如下：2012-05-22 22:16:30 127.0.0.1 /cta/index.htm Mozilla/5.0 (Windows NT 6.1) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.168 Safari/535.19 com.alibaba.citrus.webx.impl.WebxRootControllerImpl com.alibaba.citrus.webx.impl.WebxRootControllerImpl - Error occurred while process request /cta/index.htm

java.lang.NullPointerException
	at com.alibaba.citrus.webx.impl.WebxControllerImpl.service(WebxControllerImpl.java:42)
	at com.alibaba.citrus.webx.impl.WebxRootControllerImpl.handleRequest(WebxRootControllerImpl.java:53)
	at com.alibaba.citrus.webx.support.AbstractWebxRootController.service(AbstractWebxRootController.java:156)
	at com.alibaba.citrus.webx.servlet.WebxFrameworkFilter.doFilter(WebxFrameworkFilter.java:141)
	at com.alibaba.citrus.webx.servlet.FilterBean.doFilter(FilterBean.java:164)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)
	at com.alibaba.citrus.webx.servlet.SetLoggingContextFilter.doFilter(SetLoggingContextFilter.java:62)
	at com.alibaba.citrus.webx.servlet.FilterBean.doFilter(FilterBean.java:164)
	at org.eclipse.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1322)