Apache Log4j provides the interface that applications should code to and provides the adapter components required for implementers to create a logging implementation.

Bytecode (Java 1.4) for Apache Log4j is provided in a JAR file like log4j-1.2.17.jar.

Source Code files for Apache Log4j are provided in both binary packge like log4j-1.2.17.zip and source package like log4j-1.2.17-sources.zip. You can download them at Apache Log4j Website.

You can also browse Source Code files for Apache Log4j 1.2.17 below.

⏎ org/apache/log4j/pattern/CachedDateFormat.java

/*
 * 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.log4j.pattern;

import java.text.DateFormat;
import java.text.FieldPosition;
import java.text.NumberFormat;
import java.text.ParsePosition;
import java.util.Date;
import java.util.TimeZone;


/**
 * CachedDateFormat optimizes the performance of a wrapped
 * DateFormat.  The implementation is not thread-safe.
 * If the millisecond pattern is not recognized,
 * the class will only use the cache if the
 * same value is requested.
 *
 */
public final class CachedDateFormat extends DateFormat {
  /**
   *  Serialization version.
  */
  private static final long serialVersionUID = 1;
  /**
   *  Constant used to represent that there was no change
   *  observed when changing the millisecond count.
   */
  public static final int NO_MILLISECONDS = -2;

  /**
   *  Supported digit set.  If the wrapped DateFormat uses
   *  a different unit set, the millisecond pattern
   *  will not be recognized and duplicate requests
   *  will use the cache.
   */
  private static final String DIGITS = "0123456789";

  /**
   *  Constant used to represent that there was an
   *  observed change, but was an expected change.
   */
  public static final int UNRECOGNIZED_MILLISECONDS = -1;

  /**
   *  First magic number used to detect the millisecond position.
   */
  private static final int MAGIC1 = 654;

  /**
   *  Expected representation of first magic number.
   */
  private static final String MAGICSTRING1 = "654";

  /**
   *  Second magic number used to detect the millisecond position.
   */
  private static final int MAGIC2 = 987;

  /**
   *  Expected representation of second magic number.
   */
  private static final String MAGICSTRING2 = "987";

  /**
   *  Expected representation of 0 milliseconds.
   */
  private static final String ZERO_STRING = "000";

  /**
   *   Wrapped formatter.
   */
  private final DateFormat formatter;

  /**
   *  Index of initial digit of millisecond pattern or
   *   UNRECOGNIZED_MILLISECONDS or NO_MILLISECONDS.
   */
  private int millisecondStart;

  /**
   *  Integral second preceding the previous convered Date.
   */
  private long slotBegin;

  /**
   *  Cache of previous conversion.
   */
  private StringBuffer cache = new StringBuffer(50);

  /**
   *  Maximum validity period for the cache.
   *  Typically 1, use cache for duplicate requests only, or
   *  1000, use cache for requests within the same integral second.
   */
  private final int expiration;

  /**
   *  Date requested in previous conversion.
   */
  private long previousTime;

  /**
   *   Scratch date object used to minimize date object creation.
   */
  private final Date tmpDate = new Date(0);

  /**
   *  Creates a new CachedDateFormat object.
   *  @param dateFormat Date format, may not be null.
   *  @param expiration maximum cached range in milliseconds.
   *    If the dateFormat is known to be incompatible with the
   *      caching algorithm, use a value of 0 to totally disable
   *      caching or 1 to only use cache for duplicate requests.
   */
  public CachedDateFormat(final DateFormat dateFormat, final int expiration) {
    if (dateFormat == null) {
      throw new IllegalArgumentException("dateFormat cannot be null");
    }

    if (expiration < 0) {
      throw new IllegalArgumentException("expiration must be non-negative");
    }

    formatter = dateFormat;
    this.expiration = expiration;
    millisecondStart = 0;

    //
    //   set the previousTime so the cache will be invalid
    //        for the next request.
    previousTime = Long.MIN_VALUE;
    slotBegin = Long.MIN_VALUE;
  }

  /**
   * Finds start of millisecond field in formatted time.
   * @param time long time, must be integral number of seconds
   * @param formatted String corresponding formatted string
   * @param formatter DateFormat date format
   * @return int position in string of first digit of milliseconds,
   *    -1 indicates no millisecond field, -2 indicates unrecognized
   *    field (likely RelativeTimeDateFormat)
   */
  public static int findMillisecondStart(
    final long time, final String formatted, final DateFormat formatter) {
    long slotBegin = (time / 1000) * 1000;

    if (slotBegin > time) {
      slotBegin -= 1000;
    }

    int millis = (int) (time - slotBegin);

    int magic = MAGIC1;
    String magicString = MAGICSTRING1;

    if (millis == MAGIC1) {
      magic = MAGIC2;
      magicString = MAGICSTRING2;
    }

    String plusMagic = formatter.format(new Date(slotBegin + magic));

    /**
     *   If the string lengths differ then
     *      we can't use the cache except for duplicate requests.
     */
    if (plusMagic.length() != formatted.length()) {
      return UNRECOGNIZED_MILLISECONDS;
    } else {
      // find first difference between values
      for (int i = 0; i < formatted.length(); i++) {
        if (formatted.charAt(i) != plusMagic.charAt(i)) {
          //
          //   determine the expected digits for the base time
          StringBuffer formattedMillis = new StringBuffer("ABC");
          millisecondFormat(millis, formattedMillis, 0);

          String plusZero = formatter.format(new Date(slotBegin));

          //   If the next 3 characters match the magic
          //      string and the expected string
          if (
            (plusZero.length() == formatted.length())
              && magicString.regionMatches(
                0, plusMagic, i, magicString.length())
              && formattedMillis.toString().regionMatches(
                0, formatted, i, magicString.length())
              && ZERO_STRING.regionMatches(
                0, plusZero, i, ZERO_STRING.length())) {
            return i;
          } else {
            return UNRECOGNIZED_MILLISECONDS;
          }
        }
      }
    }

    return NO_MILLISECONDS;
  }

  /**
   * Formats a Date into a date/time string.
   *
   *  @param date the date to format.
   *  @param sbuf the string buffer to write to.
   *  @param fieldPosition remains untouched.
   * @return the formatted time string.
   */
  public StringBuffer format(
    Date date, StringBuffer sbuf, FieldPosition fieldPosition) {
    format(date.getTime(), sbuf);

    return sbuf;
  }

  /**
   * Formats a millisecond count into a date/time string.
   *
   *  @param now Number of milliseconds after midnight 1 Jan 1970 GMT.
   *  @param buf the string buffer to write to.
   * @return the formatted time string.
   */
  public StringBuffer format(long now, StringBuffer buf) {
    //
    // If the current requested time is identical to the previously
    //     requested time, then append the cache contents.
    //
    if (now == previousTime) {
      buf.append(cache);

      return buf;
    }

    //
    //   If millisecond pattern was not unrecognized 
    //     (that is if it was found or milliseconds did not appear)   
    //    
    if (millisecondStart != UNRECOGNIZED_MILLISECONDS &&
      //    Check if the cache is still valid.
      //    If the requested time is within the same integral second
      //       as the last request and a shorter expiration was not requested.
        (now < (slotBegin + expiration)) && (now >= slotBegin)
          && (now < (slotBegin + 1000L))) {
        // 
        //    if there was a millisecond field then update it
        //
        if (millisecondStart >= 0) {
          millisecondFormat((int) (now - slotBegin), cache, millisecondStart);
        }

        //
        //   update the previously requested time
        //      (the slot begin should be unchanged)
        previousTime = now;
        buf.append(cache);

        return buf;
    }

    //
    //  could not use previous value.  
    //    Call underlying formatter to format date.
    cache.setLength(0);
    tmpDate.setTime(now);
    cache.append(formatter.format(tmpDate));
    buf.append(cache);
    previousTime = now;
    slotBegin = (previousTime / 1000) * 1000;

    if (slotBegin > previousTime) {
      slotBegin -= 1000;
    }

    //
    //    if the milliseconds field was previous found
    //       then reevaluate in case it moved.
    //
    if (millisecondStart >= 0) {
      millisecondStart =
        findMillisecondStart(now, cache.toString(), formatter);
    }

    return buf;
  }

  /**
   *   Formats a count of milliseconds (0-999) into a numeric representation.
   *   @param millis Millisecond coun between 0 and 999.
   *   @param buf String buffer, may not be null.
   *   @param offset Starting position in buffer, the length of the
   *       buffer must be at least offset + 3.
   */
  private static void millisecondFormat(
    final int millis, final StringBuffer buf, final int offset) {
    buf.setCharAt(offset, DIGITS.charAt(millis / 100));
    buf.setCharAt(offset + 1, DIGITS.charAt((millis / 10) % 10));
    buf.setCharAt(offset + 2, DIGITS.charAt(millis % 10));
  }

  /**
   * Set timezone.
   *
   * Setting the timezone using getCalendar().setTimeZone()
   * will likely cause caching to misbehave.
   * @param timeZone TimeZone new timezone
   */
  public void setTimeZone(final TimeZone timeZone) {
    formatter.setTimeZone(timeZone);
    previousTime = Long.MIN_VALUE;
    slotBegin = Long.MIN_VALUE;
  }

  /**
   *  This method is delegated to the formatter which most
   *  likely returns null.
   * @param s string representation of date.
   * @param pos field position, unused.
   * @return parsed date, likely null.
   */
  public Date parse(String s, ParsePosition pos) {
    return formatter.parse(s, pos);
  }

  /**
   * Gets number formatter.
   *
   * @return NumberFormat number formatter
   */
  public NumberFormat getNumberFormat() {
    return formatter.getNumberFormat();
  }

  /**
   * Gets maximum cache validity for the specified SimpleDateTime
   *    conversion pattern.
   *  @param pattern conversion pattern, may not be null.
   *  @return Duration in milliseconds from an integral second
   *      that the cache will return consistent results.
   */
  public static int getMaximumCacheValidity(final String pattern) {
    //
    //   If there are more "S" in the pattern than just one "SSS" then
    //      (for example, "HH:mm:ss,SSS SSS"), then set the expiration to
    //      one millisecond which should only perform duplicate request caching.
    //
    int firstS = pattern.indexOf('S');

    if ((firstS >= 0) && (firstS != pattern.lastIndexOf("SSS"))) {
      return 1;
    }

    return 1000;
  }
}

⏎ org/apache/log4j/pattern/CachedDateFormat.java

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File name: log4j-1.2.17-sources.jar
File size: 481200 bytes
Release date: 2012-05-06
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