/*
 * 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 G&uuml;lc&uuml;
 * @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
                }
            }
        }
    }
}