--- /dev/null
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.log4j;
+
+import org.apache.log4j.helpers.Loader;
+import org.apache.log4j.helpers.ThreadLocalMap;
+
+import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.util.Hashtable;
+
+/**
+ * The MDC class is similar to the {@link NDC} class except that it is
+ * based on a map instead of a stack. It provides <em>mapped
+ * diagnostic contexts</em>. A <em>Mapped Diagnostic Context</em>, or
+ * MDC in short, is an instrument for distinguishing interleaved log
+ * output from different sources. Log output is typically interleaved
+ * when a server handles multiple clients near-simultaneously.
+ * <p/>
+ * <p><b><em>The MDC is managed on a per thread basis</em></b>. A
+ * child thread automatically inherits a <em>copy</em> of the mapped
+ * diagnostic context of its parent.
+ * <p/>
+ * <p>The MDC class requires JDK 1.2 or above. Under JDK 1.1 the MDC
+ * will always return empty values but otherwise will not affect or
+ * harm your application.</p>
+ *
+ * <p>Attention: the application is required to clean up. In web applications
+ * this can happen with creating a Servlet Filter and overriding the
+ * onFilter method like:</p>
+ *
+ * <pre>
+ * try {
+ * MDC.put(myKey);
+ * chain.doFilter(request, response);
+ * } finally {
+ * MDC.remove(myKey);
+ * }
+ * </pre>
+ *
+ * <p>Please also see: {@link http://logging.apache.org/log4j/1.2/faq.html#mdcmemoryleak}</p>
+ *
+ * @author Ceki Gülcü
+ * @since 1.2
+ */
+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
+ }
+ }
+
+ /**
+ * Put a context value (the <code>o</code> parameter) as identified
+ * with the <code>key</code> parameter into the current thread's
+ * context map.
+ * <p/>
+ * <p>If the current thread does not have a context map it is
+ * created as a side effect.
+ */
+ public static void put(String key, Object o) {
+ if (mdc != null) {
+ mdc.put0(key, o);
+ }
+ }
+
+ /**
+ * Get the context identified by the <code>key</code> parameter.
+ * <p/>
+ * <p>This method has no side effects.
+ */
+ public static Object get(String key) {
+ if (mdc != null) {
+ return mdc.get0(key);
+ }
+ return null;
+ }
+
+ /**
+ * Remove the the context identified by the <code>key</code>
+ * parameter.
+ */
+ public static void remove(String key) {
+ if (mdc != null) {
+ mdc.remove0(key);
+ }
+ }
+
+
+ /**
+ * Get the current thread's MDC as a hashtable. This method is
+ * intended to be used internally.
+ */
+ public static Hashtable getContext() {
+ if (mdc != null) {
+ return mdc.getContext0();
+ } else {
+ return null;
+ }
+ }
+
+ /**
+ * Remove all values from the MDC.
+ *
+ * @since 1.2.16
+ */
+ 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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