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)