Fix html-not-escaped bug with logback's HTMLLayout

HTMLLayout.java

import static ch.qos.logback.core.CoreConstants.LINE_SEPARATOR;

import org.apache.commons.lang.StringEscapeUtils;

import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.pattern.Converter;

/**
 * Extend the {@link ch.qos.logback.classic.html.HTMLLayout} to escape HTML content.<br/>
 * TODO remove when the bug if fixed by logback. See http://jira.qos.ch/browse/LBCLASSIC-180
 * or http://stackoverflow.com/questions/2069135/how-to-make-xml-get-escaped-in-htmllayout-of-logback
 * @author xhh
 */
public class HTMLLayout extends ch.qos.logback.classic.html.HTMLLayout {

	@SuppressWarnings("unchecked")
	public String doLayout(ILoggingEvent event) {
	    StringBuilder buf = new StringBuilder();
	    startNewTableIfLimitReached(buf);

	    boolean odd = true;
	    if (((counter++) & 1) == 0) {
	      odd = false;
	    }

	    String level = event.getLevel().toString().toLowerCase();

	    buf.append(LINE_SEPARATOR);
	    buf.append("<tr class=\"");
	    buf.append(level);
	    if (odd) {
	      buf.append(" odd\">");
	    } else {
	      buf.append(" even\">");
	    }
	    buf.append(LINE_SEPARATOR);

	    Converter<ILoggingEvent> c = head;
	    while (c != null) {
	      appendEventToBuffer(buf, c, event);
	      c = c.getNext();
	    }
	    buf.append("</tr>");
	    buf.append(LINE_SEPARATOR);

	    if (event.getThrowableProxy() != null) {
	      getThrowableRenderer().render(buf, event);
	    }
	    return buf.toString();
	  }

	  private void appendEventToBuffer(StringBuilder buf,
	      Converter<ILoggingEvent> c, ILoggingEvent event) {
	    buf.append("<td class=\"");
	    buf.append(computeConverterName(c));
	    buf.append("\">");
	    /*
	     * the following line is added by xhh to replace the original "buf.append(c.convert(event));"
	     * to escape HTML content
	     */
	    buf.append("<pre>").append(StringEscapeUtils.escapeHtml(c.convert(event))).append("</pre>");
	    buf.append("</td>");
	    buf.append(LINE_SEPARATOR);
	  }

}