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Pax Logging API Library is a collection of logging APIs from different libraries/facades. It supports SLF4J, Commons Logging, JULI Logging, Log4J1 API, Log4J2 API, JBoss Logging and Avalon APIs. Additionally, Pax Logging specific library is available as backend implementation with its specific configuration mechanisms, but it's not required.
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/*
 * 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.ops4j.pax.logging;

import java.util.HashMap;
import java.util.Map;

/**
 * The MDC class that provides mapped diagnostic contexts.
 *
 * A Mapped Diagnostic Context, 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.
 *
 * The MDC is managed on a per thread basis. A child thread automatically
 * inherits a copy of the mapped diagnostic context of its parent (with normal
 * {@link InheritableThreadLocal} child inherits the same references). That behavior can be switched 
 * off by setting the system property org.ops4j.pax.logging.threadContextMapInheritable to
 * false (default is true).
 *
 * 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.
 *
 * @author Ceki Gülcü
 * @since 1.2
 */
public class PaxContext {

    static final int HT_SIZE = 7;

    final ThreadLocal> tlm;

    public PaxContext() {
        if (Boolean.parseBoolean(System.getProperty(PaxLoggingConstants.LOGGING_CFG_INHERIT_THREAD_CONTEXT_MAP, "true"))) {
            tlm = new ThreadLocalMap();
        } else {
            tlm = new ThreadLocal<>();
        }
    }

    /**
     * Sets all values from passed map in this thread-bound MDC context.
     * This methods preserves existing values.
     * @param context
     */
    public void putAll(Map context) {
        Map ht = tlm.get();
        if (ht == null) {
            ht = new HashMap(HT_SIZE);
            tlm.set(ht);
        }
        ht.putAll(context);
    }

    /**
     * Sets single value in this thread-bound MDC context.
     * @param key
     * @param o
     */
    public void put(String key, Object o) {
        Map ht = tlm.get();
        if (ht == null) {
            ht = new HashMap(HT_SIZE);
            tlm.set(ht);
        }
        ht.put(key, o);
    }

    /**
     * Gets a value from this thread-bound MDC context.
     * @param key
     * @return
     */
    public Object get(String key) {
        Map ht = tlm.get();
        if (ht != null && key != null) {
            return ht.get(key);
        } else {
            return null;
        }
    }

    /**
     * Removes a value from this thread-bound MDC context.
     * @param key
     */
    public void remove(String key) {
        Map ht = tlm.get();
        if (ht != null) {
            ht.remove(key);
        }
    }

    /**
     * Gets full map of values related to this thread-bound MDC context.
     * @return
     */
    public Map getContext() {
        return tlm.get();
    }

    /**
     * Gets a copy of full map of calues related to this thread-bound MDC context.
     * @return
     */
    public Map getCopyOfContextMap() {
        Map ht = tlm.get();
        if (ht != null) {
            return new HashMap(ht);
        } else {
            return null;
        }
    }

    /**
     * Clears entire thread-bound MDC context.
     */
    public void clear() {
        Map ht = tlm.get();
        if (ht != null) {
            ht.clear();
        }
    }

    /**
     * Sets all values from passed map in this thread-bound MDC context.
     * This methods always clears existing values first.
     * @param contextMap
     */
    public void setContextMap(Map contextMap) {
        Map ht = tlm.get();
        if (ht == null) {
            ht = new HashMap(HT_SIZE);
            tlm.set(ht);
        } else {
            ht.clear();
        }
        ht.putAll(contextMap);
    }

    /**
     * Version of {@link InheritableThreadLocal} that creates a copy of parent thread's map before
     * starting child thread. That's why we may use not-synchronized version of {@link Map}.
     */
    static class ThreadLocalMap extends InheritableThreadLocal> {

        @Override
        protected Map childValue(Map parentValue) {
            if (parentValue != null) {
                return new HashMap(parentValue);
            } else {
                return null;
            }
        }
    }

}