<tr>     <td align=center><b>X</b></td>     <td>           <p>Used to output the MDC (mapped diagnostic context) associated     with the thread that generated the logging event. The <b>X</b>     conversion character <em>must</em> be followed by the key for the     map placed between braces, as in <b>%X{clientNumber}</b> where     <code>clientNumber</code> is the key. The value in the MDC     corresponding to the key will be output.</p>          <p>See {@link MDC} class for more details.     </p>          </td>   </tr>   <tr>     <td align=center><b>%</b></td>     <td>The sequence %% outputs a single percent sign.     </td>   </tr>   </table>   <p>By default the relevant information is output as is. However,   with the aid of format modifiers it is possible to change the   minimum field width, the maximum field width and justification.   <p>The optional format modifier is placed between the percent sign   and the conversion character.   <p>The first optional format modifier is the <em>left justification   flag</em> which is just the minus (-) character. Then comes the   optional <em>minimum field width</em> modifier. This is a decimal   constant that represents the minimum number of characters to   output. If the data item requires fewer characters, it is padded on   either the left or the right until the minimum width is   reached. The default is to pad on the left (right justify) but you   can specify right padding with the left justification flag. The   padding character is space. If the data item is larger than the   minimum field width, the field is expanded to accommodate the   data. The value is never truncated.   <p>This behavior can be changed using the <em>maximum field   width</em> modifier which is designated by a period followed by a   decimal constant. If the data item is longer than the maximum   field, then the extra characters are removed from the   <em>beginning</em> of the data item and not from the end. For   example, it the maximum field width is eight and the data item is   ten characters long, then the first two characters of the data item   are dropped. This behavior deviates from the printf function in C   where truncation is done from the end.   <p>Below are various format modifier examples for the category   conversion specifier.   <p>   <TABLE BORDER=1 CELLPADDING=8>   <th>Format modifier   <th>left justify   <th>minimum width   <th>maximum width   <th>comment   <tr>   <td align=center>%20c</td>   <td align=center>false</td>   <td align=center>20</td>   <td align=center>none</td>   <td>Left pad with spaces if the category name is less than 20   characters long.   <tr> <td align=center>%-20c</td> <td align=center>true</td> <td   align=center>20</td> <td align=center>none</td> <td>Right pad with   spaces if the category name is less than 20 characters long.   <tr>   <td align=center>%.30c</td>   <td align=center>NA</td>   <td align=center>none</td>   <td align=center>30</td>   <td>Truncate from the beginning if the category name is longer than 30   characters.   <tr>   <td align=center>%20.30c</td>   <td align=center>false</td>   <td align=center>20</td>   <td align=center>30</td>   <td>Left pad with spaces if the category name is shorter than 20   characters. However, if category name is longer than 30 characters,   then truncate from the beginning.   <tr>   <td align=center>%-20.30c</td>   <td align=center>true</td>   <td align=center>20</td>   <td align=center>30</td>   <td>Right pad with spaces if the category name is shorter than 20   characters. However, if category name is longer than 30 characters,   then truncate from the beginning.   </table>   <p>Below are some examples of conversion patterns.   <dl>   <p><dt><b>%r [%t] %-5p %c %x - %m%n</b>   <p><dd>This is essentially the TTCC layout.   <p><dt><b>%-6r [%15.15t] %-5p %30.30c %x - %m%n</b>   <p><dd>Similar to the TTCC layout except that the relative time is   right padded if less than 6 digits, thread name is right padded if   less than 15 characters and truncated if longer and the category   name is left padded if shorter than 30 characters and truncated if   longer.  </dl>   <p>The above text is largely inspired from Peter A. Darnell and   Philip E. Margolis' highly recommended book "C -- a Software   Engineering Approach", ISBN 0-387-97389-3.   @author <a href="mailto:cakalijp@Maritz.com">James P. Cakalic</a>   @author Ceki G&uuml;lc&uuml;   @since 0.8.2 */public class PatternLayout extends Layout {  /** Default pattern string for log output. Currently set to the      string <b>"%m%n"</b> which just prints the application supplied      message. */  public final static String DEFAULT_CONVERSION_PATTERN ="%m%n";  /** A conversion pattern equivalent to the TTCCCLayout.      Current value is <b>%r [%t] %p %c %x - %m%n</b>. */  public final static String TTCC_CONVERSION_PATTERN                                             = "%r [%t] %p %c %x - %m%n";  protected final int BUF_SIZE = 256;  protected final int MAX_CAPACITY = 1024;  // output buffer appended to when format() is invoked  private StringBuffer sbuf = new StringBuffer(BUF_SIZE);  private String pattern;  private PatternConverter head;  /**     Constructs a PatternLayout using the DEFAULT_LAYOUT_PATTERN.     The default pattern just produces the application supplied message.  */  public PatternLayout() {    this(DEFAULT_CONVERSION_PATTERN);  }  /**     Constructs a PatternLayout using the supplied conversion pattern.  */  public PatternLayout(String pattern) {    this.pattern = pattern;    head = createPatternParser((pattern == null) ? DEFAULT_CONVERSION_PATTERN :			     pattern).parse();  }   /**     Set the <b>ConversionPattern</b> option. This is the string which     controls formatting and consists of a mix of literal content and     conversion specifiers.   */  public  void setConversionPattern(String conversionPattern) {    pattern = conversionPattern;    head = createPatternParser(conversionPattern).parse();  }  /**     Returns the value of the <b>ConversionPattern</b> option.   */  public  String getConversionPattern() {    return pattern;  }  /**     Does not do anything as options become effective  */  public  void activateOptions() {    // nothing to do.  } /**     The PatternLayout does not handle the throwable contained within     {@link LoggingEvent LoggingEvents}. Thus, it returns     <code>true</code>.     @since 0.8.4 */  public  boolean ignoresThrowable() {    return true;  }  /**    Returns PatternParser used to parse the conversion string. Subclasses    may override this to return a subclass of PatternParser which recognize    custom conversion characters.    @since 0.9.0  */  protected PatternParser createPatternParser(String pattern) {    return new PatternParser(pattern);  }  /**     Produces a formatted string as specified by the conversion pattern.  */  public String format(LoggingEvent event) {    // Reset working stringbuffer    if(sbuf.capacity() > MAX_CAPACITY) {      sbuf = new StringBuffer(BUF_SIZE);    } else {      sbuf.setLength(0);    }    PatternConverter c = head;    while(c != null) {      c.format(sbuf, event);      c = c.next;    }    return sbuf.toString();  }}