/dev/blog

Ever seen duplicate lines in your log4j output? Maybe something like this:

2010-06-28 21:56:11,743 [main] INFO  org.quartz.impl.StdSchedulerFactory - Quartz scheduler version: 1.8.1
2010-06-28 21:56:11,743 [main] INFO  org.quartz.impl.StdSchedulerFactory - Quartz scheduler version: 1.8.1
2010-06-28 21:56:11,750 [main] INFO  org.quartz.core.QuartzScheduler - Scheduler MyScheduler_$_1 started.
2010-06-28 21:56:11,750 [main] INFO  org.quartz.core.QuartzScheduler - Scheduler MyScheduler_$_1 started.

Well, the obvious cause is probably duplicate loggers your log4j configuration:

log4j.rootLogger=INFO, DefaultConsoleAppender
log4j.logger.org.quartz=DEBUG, DefaultConsoleAppender

As the properties are inherited from the root logger, this is telling log4j that all quartz-classes should send their log to the DefaultConsoleAppender (which the root logger is doing anyway). So we simply have to remove the appender from the second logger definition:

log4j.rootLogger=INFO, DefaultConsoleAppender
log4j.logger.org.quartz=DEBUG

So watch your inherited log levels and appenders in your log4j configuration!

If anyone knows, why these duplicate lines appear from time to time in heavily multi-threaded environments (although log4j claims to be thread-safe) despite correct property files, please drop me a mail or leave a comment.

Since Sun’s API search is a real pain in the arse and even DocWeb doesn’t allow you to quickly search the Java API, this firefox plugin is a substantial improvement: https://addons.mozilla.org/en-US/firefox/addon/60675/

It basically redirects the search query to Google’s “I’m Feeling Lucky” feature to search Java’s API for the class information.

Together with Second Search querying the Java API is now lightning fast: https://addons.mozilla.org/en-US/firefox/addon/4096/

At first sight, implementing equals() doesn’t seem fairly hard. Unfortunately, it turns out that writing a correct equality method is surprisingly difficult:

http://java.sun.com/javase/6/docs/api/java/lang/Object.html

The equals method implements an equivalence relation on non-null object references:

• It is reflexive: for any non-null reference value x, x.equals(x) should return true.
• It is symmetric: for any non-null reference values x and y, x.equals(y) should return true if and only if y.equals(x) returns true.
• It is transitive: for any non-null reference values x, y, and z, if x.equals(y) returns true and y.equals(z) returns true, then x.equals(z) should return true.
• It is consistent: for any non-null reference values x and y, multiple invocations of x.equals(y) consistently return true or consistently return false, provided no information used in equals comparisons on the objects is modified.
• For any non-null reference value x, x.equals(null) should return false.

In fact, after studying a large body of Java code, the authors of a 2007 paper concluded that almost all implementations of equals methods are faulty.¹

There are quite a lot of HowTos available on the web. One of the best might be the article “How to Write an Equality Method in Java” by Martin Odersky, Lex Spoon, and Bill Venners.

So instead of repeating all the pitfalls you have to avoid, I’ll give you a sample implementation² of the equals()-method that follows the general contract using the example of an imaginary class Point:

public boolean equals (final Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (this.getClass() != obj.getClass()) {
      return false;
    }
    final Point other = (Point) obj;
    if (this.x != other.x) {
      return false;
    }
    if (this.y != other.y) {
      return false;
    }
    return true;
  }

Be careful: Whenever you overwrite equals() it is generally necessary to override the hashCode()-method, too!

If you want to learn more about this topic, I suggest, you have a look at Joshua Bloch’s Effective Java Second Edition. Addison-Wesley, 2008.

[1] Vaziri, Mandana, Frank Tip, Stephen Fink, and Julian Dolby. “Declarative Object Identity Using Relation Types.” In Proc. ECOOP 2007, pages 54–78. 2007.
[2] http://www.artima.com/forums/flat.jsp?forum=226&thread=259279&start=30&msRange=15

Thread safety is a very hot topic for Java programmers right now. But I’ve seen quite a few folks using the rather complex collections from java.util.concurrent when they actually needed just a thread-safe implementation of a Set.

Of course, the HashSet implementation is non-thread-safe:

http://java.sun.com/javase/6/docs/api/java/util/HashSet.html

Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be “wrapped” using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:

So getting a thread-safe representation of the HashSet class is pretty easy:

   Set s = Collections.synchronizedSet(new HashSet(...));

This returns a synchronized set backed by the specified set. But be careful: In order to guarantee serial access, it is critical that all access to the backing set is accomplished through the returned set.

A further pitfall is the use of the class’s iterator

Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

So it is imperative that you manually synchronize on the returned set when iterating over it:

  Set s = Collections.synchronizedSet(new HashSet());
      ...
  synchronized(s) {
      Iterator i = s.iterator(); // Must be in the synchronized block
      while (i.hasNext())
          foo(i.next());
  }
 

Failure to follow this advice may result in non-deterministic behavior.

For those of you, who want to understand what goes on inside google adsense’s show_ads.js, here is the complete source code:

google_ad_url = '';
google_random = (new Date()).getTime();
google_org_error_handler = window.onerror;

function quoted(str) {
  return (str != null) ? '"' + str + '"' : '""';
}

function google_encodeURIComponent(str) {
  if (typeof(encodeURIComponent) == 'function') {
    return encodeURIComponent(str);
  } else {
    return escape(str);
  }
}

function google_write_tracker(tracker_event) {
  var img_url = window.google_ad_url.replace(/pagead\/ads/, 'pagead/imp.gif');
  var img_src = img_url + '&event=' + tracker_event;
  var img_tag = '<i' + 'mg height="1" width="1" border="0" ' +
                'src=' + quoted(img_src) +
                ' />';
  document.write(img_tag);
}

function google_append_url(param, value) {
  if (value) {
    window.google_ad_url += '&' + param + '=' + value;
  }
}

function google_append_url_esc(param, value) {
  if (value) {
    google_append_url(param, google_encodeURIComponent(value));
  }
}

function google_append_color(param, value) {
  if (value && typeof(value) == 'object') {
    value = value[window.google_random % value.length];
  }
  google_append_url('color_' + param, value);
}

function google_show_ad() {
  var w = window;
  w.onerror = w.google_org_error_handler;

  if (w.google_num_ad_slots) {
    w.google_num_ad_slots = w.google_num_ad_slots + 1;
  } else {
    w.google_num_ad_slots = 1;
  }

  if (w.google_num_ad_slots > 3) {
    return;
  }

  w.google_ad_url = 'http://pagead2.googlesyndication.com/pagead/ads?';
  w.google_ad_client = w.google_ad_client.toLowerCase();
  if (w.google_ad_client.substring(0,3) != 'ca-') {
     w.google_ad_client = 'ca-' + w.google_ad_client;
  }
  w.google_ad_url += 'client=' + escape(w.google_ad_client) +
                     '&random=' + w.google_random;

  google_append_url('hl', w.google_language);
  if (w.google_country) {
    google_append_url('gl', w.google_country);
  } else {
    google_append_url('gl', w.google_gl);
  }
  google_append_url('gr', w.google_region);
  google_append_url_esc('gcs', w.google_city);
  google_append_url_esc('hints', w.google_hints);
  google_append_url('adsafe', w.google_safe);
  google_append_url('oe', w.google_encoding);
  google_append_url('lmt', w.google_last_modified_time);
  google_append_url_esc('alternate_ad_url', w.google_alternate_ad_url);
  google_append_url('alt_color', w.google_alternate_color);

  if (w.google_skip) {
    google_append_url("skip", w.google_skip);
  } else if (w.google_prev_ad_formats) {
    google_append_url_esc('prev_fmts', w.google_prev_ad_formats.toLowerCase());
  }

  if (w.google_ad_format) {
    google_append_url_esc('format', w.google_ad_format.toLowerCase());
    if (w.google_prev_ad_formats) {
      w.google_prev_ad_formats = w.google_prev_ad_formats + ',' + w.google_ad_format;
    } else {
      w.google_prev_ad_formats = w.google_ad_format;
    }
  }

  google_append_url('num_ads', w.google_max_num_ads);
  google_append_url('output', w.google_ad_output);
  google_append_url('adtest', w.google_adtest);
  if (w.google_ad_channel) {
    google_append_url_esc('channel', w.google_ad_channel.toLowerCase());
  }
  google_append_url_esc('url', w.google_page_url);
  google_append_color('bg', w.google_color_bg);
  google_append_color('text', w.google_color_text);
  google_append_color('link', w.google_color_link);
  google_append_color('url', w.google_color_url);
  google_append_color('border', w.google_color_border);
  google_append_color('line', w.google_color_line);
  google_append_url('kw_type', w.google_kw_type);
  google_append_url_esc('kw', w.google_kw);
  google_append_url_esc('contents', w.google_contents);
  google_append_url('num_radlinks', w.google_num_radlinks);
  google_append_url('max_radlink_len', w.google_max_radlink_len);
  google_append_url('rl_filtering', w.google_rl_filtering);
  google_append_url('ad_type', w.google_ad_type);
  google_append_url('image_size', w.google_image_size);
  google_append_url('feedback_link', w.google_feedback);

  w.google_ad_url = w.google_ad_url.substring(0, 1000);
  w.google_ad_url = w.google_ad_url.replace(/%\w?$/, '');

  if (google_ad_output == 'js' && w.google_ad_request_done) {
    document.write('<scr' + 'ipt language="JavaScript1.1"' +
                   ' src=' + quoted(google_ad_url) +
                   '></scr' + 'ipt>');
  } else if (google_ad_output == 'html') {
    if (w.name == 'google_ads_frame') {
      google_write_tracker('reboundredirect');
    } else {
      document.write('<ifr' + 'ame' +
                     ' name="google_ads_frame"' +
                     ' width=' + quoted(w.google_ad_width) +
                     ' height=' + quoted(w.google_ad_height) +
                     ' frameborder=' + quoted(w.google_ad_frameborder) +
                     ' src=' + quoted(w.google_ad_url) +
                     ' marginwidth="0"' +
                     ' marginheight="0"' +
                     ' vspace="0"' +
                     ' hspace="0"' +
                     ' allowtransparency="true"' +
                     ' scrolling="no">');
      google_write_tracker('noiframe');
      document.write('</ifr' + 'ame>');
    }
  }

  w.google_ad_frameborder = null;
  w.google_ad_format = null;
  w.google_page_url = null;
  w.google_language = null;
  w.google_gl = null;
  w.google_country = null;
  w.google_region = null;
  w.google_city = null;
  w.google_hints = null;
  w.google_safe = null;
  w.google_encoding = null;
  w.google_ad_output = null;
  w.google_max_num_ads = null;
  w.google_ad_channel = null;
  w.google_contents = null;
  w.google_alternate_ad_url = null;
  w.google_alternate_color = null;
  w.google_color_bg = null;
  w.google_color_text = null;
  w.google_color_link = null;
  w.google_color_url = null;
  w.google_color_border = null;
  w.google_color_line = null;
  w.google_adtest = null;
  w.google_kw_type = null;
  w.google_kw = null;
  w.google_num_radlinks = null;
  w.google_max_radlink_len = null;
  w.google_rl_filtering = null;
  w.google_ad_type = null;
  w.google_image_size = null;
  w.google_feedback = null;
  w.google_skip = null;
}

function google_error_handler(message, url, line) {
  google_show_ad();
  return true;
}

window.onerror = google_error_handler;

if (window.google_ad_frameborder == null) {
  google_ad_frameborder = 0;
}

if (window.google_ad_output == null) {
  google_ad_output = 'html';
}

if (window.google_ad_format == null && window.google_ad_output == 'html') {
  google_ad_format = google_ad_width + 'x' + google_ad_height;
}

if (window.google_page_url == null) {
  google_page_url = document.referrer;
  if (window.top.location == document.location) {
    google_page_url = document.location;
    google_last_modified_time = Date.parse(document.lastModified) / 1000;
  }
}
google_show_ad();

Found on http://www.koders.com.