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

import org.apache.logging.log4j.ThreadContext.ContextStack;

/**
 *
 */
public class MutableThreadContextStack implements ThreadContextStack {

    private static final long serialVersionUID = 50505011L;

    /**
     * The underlying list (never null).
     */
    private final List list;
    private boolean frozen;

    /**
     * Constructs an empty MutableThreadContextStack.
     */
    public MutableThreadContextStack() {
        this(new ArrayList());
    }

    public MutableThreadContextStack(final List list) {
        this.list = new ArrayList<>(list);
    }

    private MutableThreadContextStack(final MutableThreadContextStack stack) {
        this.list = new ArrayList<>(stack.list);
    }

    private void checkInvariants() {
        if (frozen) {
            throw new UnsupportedOperationException("context stack has been frozen");
        }
    }

    @Override
    public String pop() {
        checkInvariants();
        if (list.isEmpty()) {
            return null;
        }
        final int last = list.size() - 1;
        final String result = list.remove(last);
        return result;
    }

    @Override
    public String peek() {
        if (list.isEmpty()) {
            return null;
        }
        final int last = list.size() - 1;
        return list.get(last);
    }

    @Override
    public void push(final String message) {
        checkInvariants();
        list.add(message);
    }

    @Override
    public int getDepth() {
        return list.size();
    }

    @Override
    public List asList() {
        return list;
    }

    @Override
    public void trim(final int depth) {
        checkInvariants();
        if (depth < 0) {
            throw new IllegalArgumentException("Maximum stack depth cannot be negative");
        }
        if (list == null) {
            return;
        }
        final List copy = new ArrayList<>(list.size());
        final int count = Math.min(depth, list.size());
        for (int i = 0; i < count; i++) {
            copy.add(list.get(i));
        }
        list.clear();
        list.addAll(copy);
    }

    @Override
    public ThreadContextStack copy() {
        return new MutableThreadContextStack(this);
    }

    @Override
    public void clear() {
        checkInvariants();
        list.clear();
    }

    @Override
    public int size() {
        return list.size();
    }

    @Override
    public boolean isEmpty() {
        return list.isEmpty();
    }

    @Override
    public boolean contains(final Object o) {
        return list.contains(o);
    }

    @Override
    public Iterator iterator() {
        return list.iterator();
    }

    @Override
    public Object[] toArray() {
        return list.toArray();
    }

    @Override
    public  T[] toArray(final T[] ts) {
        return list.toArray(ts);
    }

    @Override
    public boolean add(final String s) {
        checkInvariants();
        return list.add(s);
    }

    @Override
    public boolean remove(final Object o) {
        checkInvariants();
        return list.remove(o);
    }

    @Override
    public boolean containsAll(final Collection objects) {
        return list.containsAll(objects);
    }

    @Override
    public boolean addAll(final Collection strings) {
        checkInvariants();
        return list.addAll(strings);
    }

    @Override
    public boolean removeAll(final Collection objects) {
        checkInvariants();
        return list.removeAll(objects);
    }

    @Override
    public boolean retainAll(final Collection objects) {
        checkInvariants();
        return list.retainAll(objects);
    }

    @Override
    public String toString() {
        return String.valueOf(list);
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((this.list == null) ? 0 : this.list.hashCode());
        return result;
    }

    @Override
    public boolean equals(final Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (!(obj instanceof ThreadContextStack)) {
            return false;
        }
        final ThreadContextStack other = (ThreadContextStack) obj;
        final List otherAsList = other.asList();
        if (this.list == null) {
            if (otherAsList != null) {
                return false;
            }
        } else if (!this.list.equals(otherAsList)) {
            return false;
        }
        return true;
    }

    @Override
    public ContextStack getImmutableStackOrNull() {
        return copy();
    }

    /**
     * "Freezes" this context stack so it becomes immutable: all mutator methods will throw an exception from now on.
     */
    public void freeze() {
        frozen = true;
    }
    
    public boolean isFrozen() {
        return frozen;
    }
}