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The Apache Log4j SLF4J 2.0 API binding to Log4j 2 Core
There is a newer version: 3.0.0-beta2
/*
 * 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.slf4j;

import java.util.ArrayDeque;
import java.util.Deque;
import java.util.HashMap;
import java.util.Map;
import java.util.Objects;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.ThreadContext;
import org.apache.logging.log4j.ThreadContext.ContextStack;
import org.apache.logging.log4j.status.StatusLogger;
import org.slf4j.spi.MDCAdapter;

/**
 *
 */
public class Log4jMDCAdapter implements MDCAdapter {

    private static final Logger LOGGER = StatusLogger.getLogger();

    private final ThreadLocalMapOfStacks mapOfStacks = new ThreadLocalMapOfStacks();

    @Override
    public void put(final String key, final String val) {
        ThreadContext.put(key, val);
    }

    @Override
    public String get(final String key) {
        return ThreadContext.get(key);
    }

    @Override
    public void remove(final String key) {
        ThreadContext.remove(key);
    }

    @Override
    public void clear() {
        ThreadContext.clearMap();
    }

    @Override
    public Map getCopyOfContextMap() {
        return ThreadContext.getContext();
    }

    @Override
    public void setContextMap(final Map map) {
        ThreadContext.clearMap();
        ThreadContext.putAll(map);
    }

    @Override
    public void pushByKey(final String key, final String value) {
        if (key == null) {
            ThreadContext.push(value);
        } else {
            final String oldValue = mapOfStacks.peekByKey(key);
            if (!Objects.equals(ThreadContext.get(key), oldValue)) {
                LOGGER.warn("The key {} was used in both the string and stack-valued MDC.", key);
            }
            mapOfStacks.pushByKey(key, value);
            ThreadContext.put(key, value);
        }
    }

    @Override
    public String popByKey(final String key) {
        if (key == null) {
            return ThreadContext.getDepth() > 0 ? ThreadContext.pop() : null;
        }
        final String value = mapOfStacks.popByKey(key);
        if (!Objects.equals(ThreadContext.get(key), value)) {
            LOGGER.warn("The key {} was used in both the string and stack-valued MDC.", key);
        }
        ThreadContext.put(key, mapOfStacks.peekByKey(key));
        return value;
    }

    @Override
    public Deque getCopyOfDequeByKey(final String key) {
        if (key == null) {
            final ContextStack stack = ThreadContext.getImmutableStack();
            final Deque copy = new ArrayDeque<>(stack.size());
            stack.forEach(copy::push);
            return copy;
        }
        return mapOfStacks.getCopyOfDequeByKey(key);
    }

    @Override
    public void clearDequeByKey(final String key) {
        if (key == null) {
            ThreadContext.clearStack();
        } else {
            mapOfStacks.clearByKey(key);
            ThreadContext.put(key, null);
        }
    }

    private static class ThreadLocalMapOfStacks {

        private final ThreadLocal>> tlMapOfStacks = ThreadLocal.withInitial(HashMap::new);

        public void pushByKey(final String key, final String value) {
            tlMapOfStacks
                    .get()
                    .computeIfAbsent(key, ignored -> new ArrayDeque<>())
                    .push(value);
        }

        public String popByKey(final String key) {
            final Deque deque = tlMapOfStacks.get().get(key);
            return deque != null ? deque.poll() : null;
        }

        public Deque getCopyOfDequeByKey(final String key) {
            final Deque deque = tlMapOfStacks.get().get(key);
            return deque != null ? new ArrayDeque<>(deque) : null;
        }

        public void clearByKey(final String key) {
            final Deque deque = tlMapOfStacks.get().get(key);
            if (deque != null) {
                deque.clear();
            }
        }

        public String peekByKey(final String key) {
            final Deque deque = tlMapOfStacks.get().get(key);
            return deque != null ? deque.peek() : null;
        }
    }
}