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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.ops4j.pax.logging.log4jv2;

import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;

import org.apache.logging.log4j.ThreadContext.ContextStack;
import org.apache.logging.log4j.spi.MutableThreadContextStack;
import org.apache.logging.log4j.spi.ThreadContextStack;
import org.apache.logging.log4j.util.Strings;

/**
 * A copy-on-write thread-safe variant of {@code org.apache.logging.log4j.spi.ThreadContextStack} in which all mutative
 * operations (add, pop, and so on) are implemented by making a fresh copy of the underlying list.
 */
public class Log4jv2ThreadContextStack implements ThreadContextStack {

    private static final long serialVersionUID = 5050501L;

    private static final ThreadLocal STACK = new ThreadLocal();

    private final boolean useStack;

    public Log4jv2ThreadContextStack(final boolean useStack) {
        this.useStack = useStack;
    }

    private MutableThreadContextStack getNonNullStackCopy() {
        final MutableThreadContextStack values = STACK.get();
        return (MutableThreadContextStack) (values == null ? new MutableThreadContextStack() : values.copy());
    }

    @Override
    public boolean add(final String s) {
        if (!useStack) {
            return false;
        }
        final MutableThreadContextStack copy = getNonNullStackCopy();
        copy.add(s);
        copy.freeze();
        STACK.set(copy);
        return true;
    }

    @Override
    public boolean addAll(final Collection strings) {
        if (!useStack || strings.isEmpty()) {
            return false;
        }
        final MutableThreadContextStack copy = getNonNullStackCopy();
        copy.addAll(strings);
        copy.freeze();
        STACK.set(copy);
        return true;
    }

    @Override
    public List asList() {
        final MutableThreadContextStack values = STACK.get();
        if (values == null) {
            return Collections.emptyList();
        }
        return values.asList();
    }

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

    @Override
    public boolean contains(final Object o) {
        final MutableThreadContextStack values = STACK.get();
        return values != null && values.contains(o);
    }

    @Override
    public boolean containsAll(final Collection objects) {
        if (objects.isEmpty()) { // quick check before accessing the ThreadLocal
            return true; // looks counter-intuitive, but see
            // j.u.AbstractCollection
        }
        final MutableThreadContextStack values = STACK.get();
        return values != null && values.containsAll(objects);
    }

    @Override
    public ThreadContextStack copy() {
        MutableThreadContextStack values = null;
        if (!useStack || (values = STACK.get()) == null) {
            return new MutableThreadContextStack();
        }
        return values.copy();
    }

    @Override
    public boolean equals(final Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (obj instanceof Log4jv2ThreadContextStack) {
            final Log4jv2ThreadContextStack other = (Log4jv2ThreadContextStack) obj;
            if (this.useStack != other.useStack) {
                return false;
            }
        }
        if (!(obj instanceof ThreadContextStack)) {
            return false;
        }
        final ThreadContextStack other = (ThreadContextStack) obj;
        final MutableThreadContextStack values = STACK.get();
        if (values == null) {
            return false;
        }
        return values.equals(other);
    }

    @Override
    public int getDepth() {
        final MutableThreadContextStack values = STACK.get();
        return values == null ? 0 : values.getDepth();
    }

    @Override
    public int hashCode() {
        final MutableThreadContextStack values = STACK.get();
        final int prime = 31;
        int result = 1;
        // Factor in the stack itself to compare vs. other implementors.
        result = prime * result + ((values == null) ? 0 : values.hashCode());
        return result;
    }

    @Override
    public boolean isEmpty() {
        final MutableThreadContextStack values = STACK.get();
        return values == null || values.isEmpty();
    }

    @Override
    public Iterator iterator() {
        final MutableThreadContextStack values = STACK.get();
        if (values == null) {
            final List empty = Collections.emptyList();
            return empty.iterator();
        }
        return values.iterator();
    }

    @Override
    public String peek() {
        final MutableThreadContextStack values = STACK.get();
        if (values == null || values.size() == 0) {
            return Strings.EMPTY;
        }
        return values.peek();
    }

    @Override
    public String pop() {
        if (!useStack) {
            return Strings.EMPTY;
        }
        final MutableThreadContextStack values = STACK.get();
        if (values == null || values.size() == 0) {
            // Like version 1.2
            return Strings.EMPTY;
        }
        final MutableThreadContextStack copy = (MutableThreadContextStack) values.copy();
        final String result = copy.pop();
        copy.freeze();
        STACK.set(copy);
        return result;
    }

    @Override
    public void push(final String message) {
        if (!useStack) {
            return;
        }
        add(message);
    }

    @Override
    public boolean remove(final Object o) {
        if (!useStack) {
            return false;
        }
        final MutableThreadContextStack values = STACK.get();
        if (values == null || values.size() == 0) {
            return false;
        }
        final MutableThreadContextStack copy = (MutableThreadContextStack) values.copy();
        final boolean result = copy.remove(o);
        copy.freeze();
        STACK.set(copy);
        return result;
    }

    @Override
    public boolean removeAll(final Collection objects) {
        if (!useStack || objects.isEmpty()) {
            return false;
        }
        final MutableThreadContextStack values = STACK.get();
        if (values == null || values.isEmpty()) {
            return false;
        }
        final MutableThreadContextStack copy = (MutableThreadContextStack) values.copy();
        final boolean result = copy.removeAll(objects);
        copy.freeze();
        STACK.set(copy);
        return result;
    }

    @Override
    public boolean retainAll(final Collection objects) {
        if (!useStack || objects.isEmpty()) {
            return false;
        }
        final MutableThreadContextStack values = STACK.get();
        if (values == null || values.isEmpty()) {
            return false;
        }
        final MutableThreadContextStack copy = (MutableThreadContextStack) values.copy();
        final boolean result = copy.retainAll(objects);
        copy.freeze();
        STACK.set(copy);
        return result;
    }

    @Override
    public int size() {
        final MutableThreadContextStack values = STACK.get();
        return values == null ? 0 : values.size();
    }

    @Override
    public Object[] toArray() {
        final MutableThreadContextStack result = STACK.get();
        if (result == null) {
            return new String[0];
        }
        return result.toArray(new Object[result.size()]);
    }

    @Override
    public  T[] toArray(final T[] ts) {
        final MutableThreadContextStack result = STACK.get();
        if (result == null) {
            if (ts.length > 0) { // as per the contract of j.u.List#toArray(T[])
                ts[0] = null;
            }
            return ts;
        }
        return result.toArray(ts);
    }

    @Override
    public String toString() {
        final MutableThreadContextStack values = STACK.get();
        return values == null ? "[]" : values.toString();
    }

    @Override
    public void trim(final int depth) {
        if (depth < 0) {
            throw new IllegalArgumentException("Maximum stack depth cannot be negative");
        }
        final MutableThreadContextStack values = STACK.get();
        if (values == null) {
            return;
        }
        final MutableThreadContextStack copy = (MutableThreadContextStack) values.copy();
        copy.trim(depth);
        copy.freeze();
        STACK.set(copy);
    }

    /*
     * (non-Javadoc)
     *
     * @see org.apache.logging.log4j.ThreadContext.ContextStack#getImmutableStackOrNull()
     */
    @Override
    public ContextStack getImmutableStackOrNull() {
        return STACK.get();
    }
}