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)
