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With inspiration from other libraries
There is a newer version: 2.12.15
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2015, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */
package ch.qos.logback.classic.pattern;

import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.Context;
import ch.qos.logback.core.pattern.Converter;
import ch.qos.logback.core.pattern.ConverterUtil;
import ch.qos.logback.core.pattern.PostCompileProcessor;

public class EnsureExceptionHandling implements PostCompileProcessor {

    /**
     * This implementation checks if any of the converters in the chain handles
     * exceptions. If not, then this method adds a
     * {@link ExtendedThrowableProxyConverter} instance to the end of the chain.
     * 
     * This allows appenders using this layout to output exception information
     * event if the user forgets to add %ex to the pattern. Note that the
     * appenders defined in the Core package are not aware of exceptions nor
     * LoggingEvents.
     * 
     * If for some reason the user wishes to NOT print exceptions, then she can
     * add %nopex to the pattern.
     * 
     * 
     */
    public void process(Context context, Converter head) {
        if (head == null) {
            // this should never happen
            throw new IllegalArgumentException("cannot process empty chain");
        }
        if (!chainHandlesThrowable(head)) {
            Converter tail = ConverterUtil.findTail(head);
            Converter exConverter = null;
            LoggerContext loggerContext = (LoggerContext) context;
            if (loggerContext.isPackagingDataEnabled()) {
                exConverter = new ExtendedThrowableProxyConverter();
            } else {
                exConverter = new ThrowableProxyConverter();
            }
            tail.setNext(exConverter);
        }
    }

    /**
     * This method computes whether a chain of converters handles exceptions or
     * not.
     * 
     * @param head
     *                The first element of the chain
     * @return true if can handle throwables contained in logging events
     */
    public boolean chainHandlesThrowable(Converter head) {
        Converter c = head;
        while (c != null) {
            if (c instanceof ThrowableHandlingConverter) {
                return true;
            }
            c = c.getNext();
        }
        return false;
    }
}