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The Pax Logging API Library is to allow for the Pax Logging Service to be reloaded without stopping the many dependent bundles. It also contains the OSGi Log Service API and the Knopflerfish Log API.
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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.apache.logging.log4j.spi;

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

import org.apache.logging.log4j.util.PropertiesUtil;

/**
 * The actual ThreadContext Map. A new ThreadContext Map is created each time it is updated and the Map stored is always
 * immutable. This means the Map can be passed to other threads without concern that it will be updated. Since it is
 * expected that the Map will be passed to many more log events than the number of keys it contains the performance
 * should be much better than if the Map was copied for each event.
 */
public class DefaultThreadContextMap implements ThreadContextMap {
    /**
     * Property name ({@value} ) for selecting {@code InheritableThreadLocal} (value "true") or plain
     * {@code ThreadLocal} (value is not "true") in the implementation.
     */
    public static final String INHERITABLE_MAP = "isThreadContextMapInheritable";

    private final boolean useMap;
    private final ThreadLocal> localMap;

    public DefaultThreadContextMap(final boolean useMap) {
        this.useMap = useMap;
        this.localMap = createThreadLocalMap(useMap);
    }

    // LOG4J2-479: by default, use a plain ThreadLocal, only use InheritableThreadLocal if configured.
    // (This method is package protected for JUnit tests.)
    static ThreadLocal> createThreadLocalMap(final boolean isMapEnabled) {
        final PropertiesUtil managerProps = PropertiesUtil.getProperties();
        final boolean inheritable = managerProps.getBooleanProperty(INHERITABLE_MAP);
        if (inheritable) {
            return new InheritableThreadLocal>() {
                @Override
                protected Map childValue(final Map parentValue) {
                    return parentValue != null && isMapEnabled //
                    ? Collections.unmodifiableMap(new HashMap<>(parentValue)) //
                            : null;
                }
            };
        }
        // if not inheritable, return plain ThreadLocal with null as initial value
        return new ThreadLocal<>();
    }

    @Override
    public void put(final String key, final String value) {
        if (!useMap) {
            return;
        }
        Map map = localMap.get();
        map = map == null ? new HashMap() : new HashMap<>(map);
        map.put(key, value);
        localMap.set(Collections.unmodifiableMap(map));
    }

    @Override
    public String get(final String key) {
        final Map map = localMap.get();
        return map == null ? null : map.get(key);
    }

    @Override
    public void remove(final String key) {
        final Map map = localMap.get();
        if (map != null) {
            final Map copy = new HashMap<>(map);
            copy.remove(key);
            localMap.set(Collections.unmodifiableMap(copy));
        }
    }

    @Override
    public void clear() {
        localMap.remove();
    }

    @Override
    public boolean containsKey(final String key) {
        final Map map = localMap.get();
        return map != null && map.containsKey(key);
    }

    @Override
    public Map getCopy() {
        final Map map = localMap.get();
        return map == null ? new HashMap() : new HashMap<>(map);
    }

    @Override
    public Map getImmutableMapOrNull() {
        return localMap.get();
    }

    @Override
    public boolean isEmpty() {
        final Map map = localMap.get();
        return map == null || map.size() == 0;
    }

    @Override
    public String toString() {
        final Map map = localMap.get();
        return map == null ? "{}" : map.toString();
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        final Map map = this.localMap.get();
        result = prime * result + ((map == null) ? 0 : map.hashCode());
        result = prime * result + Boolean.valueOf(this.useMap).hashCode();
        return result;
    }

    @Override
    public boolean equals(final Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (obj instanceof DefaultThreadContextMap) {
            final DefaultThreadContextMap other = (DefaultThreadContextMap) obj;
            if (this.useMap != other.useMap) {
                return false;
            }
        }
        if (!(obj instanceof ThreadContextMap)) {
            return false;
        }
        final ThreadContextMap other = (ThreadContextMap) obj;
        final Map map = this.localMap.get();
        final Map otherMap = other.getImmutableMapOrNull();
        if (map == null) {
            if (otherMap != null) {
                return false;
            }
        } else if (!map.equals(otherMap)) {
            return false;
        }
        return true;
    }
}