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。
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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。
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NDC的实现代码：
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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)); } }?
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MDC的实现：
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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 } } } }}?
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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
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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。
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??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：
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<param name="ConversionPattern" value="%d{yyyy-MM-dd HH\:mm\:ss} %X{remoteAddr} %X{requestURI} %X{referrer} %X{userAgent} %c %c - %m%n "/>?
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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.NullPointerExceptionat 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)?
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