Saturday, November 18, 2006

Getting IDEAs Flowing

This is a story about starting IDEA 6 on my Gentoo GNU/Linux machine. IDEA 6 has been out a while now, but I'm not the kind of person to buy the first release of a new version of software. I like to wait until the more adventurous types take it for a spin and shake the first few waves of bugs out. So now that it's at maintenace release 2 I'm taking it out for a spin.

Getting it running was a bitch. With the default settings I could get as far as the "loading project" dialog box. It just hung there, forever, which was long enough for me to notice that they've fixed one of (a few) annoyances I have with version 5.

Aside:

Whenever IDEA is running a command that is taking a while, the "tip of the day" dialog box attaches itself to the progress meter. In 5 the dialog box is vertically too small to read the text inside it. I always have to pull the edge of the box down to see the tip. After the first few times I gave up on the "tip of the day" all together. After I realized I can actually see what's in the box I started flipping through them while I waited for IDEA to complete startup. Very insightful stuff. It turns out there are a plethora of shortcuts that I didn't know about. Which sucks. Think off all the time I could have saved w/ the new shortcuts instead of mousing around the editor.

At first I thought it was just busy importing and converting the settings and configuration from the old install. So I fired up Spider Solitaire in XP/VMWare and started listening to episode 236 of This American Life. That's how I normally pass the time when I need to wait for a computer to do something and I don't want to change my mindset to a different computer related task.

After fifteen minutes it still wasn't done and trying to close the window was getting me nowhere. So I murdered it with a 9 caliber signal. Fortunately I had started it from a shell so I could see the STDERR messages. The JVM was throwing OOM errors related to a too small perm gen size. A quick cat of {INSTALL_DIR}/bin/idea.vmoptions revealed a mandatory limit of 99MB. An easy fix. Just remove the offending line because the JVM --Sun JVM 1.5_08 Linux, to be specific-- defaults to ~130MB. Fired it up again and got to the main menu. Yippee!! Things are looking up.

Wrong!

I tried opening the last project I was working on and the project loads but the editor refuses to accept input. Then the window loses focus and one of the CPUs hit 100%. top tells me IDEA is the offending process. I figured I would give it more time. Maybe it will settle down after a while plus I've got This American Life on pause and episode 236 is quite good. So I unpause and started another round of Spider Solitaire. A few minutes go by and the JVM starts complaining the perm gen size is still too small. No problem. I run a server class machine w/ 12GB of RAM of which 4GB is in use so I've got plenty to spare. So I run echo "-XX:MaxPermSize=256m" >> {INSTALL_DIR}/bin/idea.vmoptions and rerun the start script. It gets to the main menu in a reasonable amount of time. So far so good. I reopen the project and cross my fingers. Booyah! 256MB is the magic number. Hooray!!

Friday, October 06, 2006

A Developer's Journal: Solaris/CMT #7

I got the answers I was looking for:

Solaris has major releases every 2 - 3 years and minor releases every 3 - 6 months S10 is the current major release.

...

The updates have major features integrated. To move from Update to Update you usually re-install the OS. You download the CD image or put it on your install server. You can opt for a Upgrade option as well. In between updates there are patches. These fix problems with a variety of products. There is one patch in particular called the kernel jumbo that has many of the fixes for just the OS. These can be downloaded from sun.com. There are mandatory patches to fix panics etc. These are available for free download. Others require a support contract.

A Developer's Journal: Solaris/CMT #6

I'm back in Solaris mode and I've hit a wall. Solaris 10 is Unix and that's great because visa-vi GNU/Linux I am very comfortable with Unix. Before I got my hands on my box I made the decision not for force my GNU/Linux habits onto Solaris because its probably a recipe for pointless frustration. So I'm very interested in learning the Solaris way of doing things. The issue I'm struggling with right now is patching and versioning. I don't understand how they fit together as a whole. Now I know I just said I wasn't going to force my GNU/Linux habits onto Solaris but I need to do a little compare and contrast to ensure that I'm asking the right question.

In Gentoo there is no explicit concept of a version. There are live CD releases such as 2006.1 or 2006.2 but you can't use that as a mechanism for discussing the version of Gentoo that's actually running on your box because as soon as you run emerge --sync && emerge -uD world or build your own kernel the whole version concept breaks down.

At the other extreme is Windows. You buy a copy of Windows of a specific version and it stays that way until Microsoft releases a new version and you explicitly purchase and install the new version. In the interim the most you can do is apply hot fixes and service packs. So to move from WinXP to Vista I would have to buy Vista and install clean or do an upgrade of my current WinXP install.

So which one is Solaris most like? Gentoo or Windows? Because I'm trying to get from Solaris 10 1/06 to 6/06 and I'm not sure how to get there.

Thursday, October 05, 2006

Throw more threads at it

Why Events Are A Bad Idea (for high-concurrency servers)

An interesting position paper on why parallel programming using threads are better for highly concurrent servers compared to event based approaches. Even though the paper is 3+ years old its more applicable now than it was then in large part because threads have gotten cheaper. How so?

  • High performance thread libraries are part of the Linux kernel since 2.6 (NPTL) and the Solaris 10 kernel both of whom implement a 1x1 model.
  • As memory performance and bandwidth increases the cost of context switches decreases. As a matter of fact reducing the cost of a context switch is baked into the DNA of Sun's Niagara family of processors. AMD had the bright (right) idea to include the memory controller right on the chip. They also switched to a NUMA topology which boosts performance when the data is closer to where it's needed. Intel and IBM continue to increase on chip cache sizes to reduce the frequency and thus the latency associated with having to fetch data from main memory.
  • Massively parallel super computer on a chip is around the corner. Check out:

I could go on but I'll stop here. Click on the link, read the paper, and draw your own conclusions.

Sunday, October 01, 2006

OpenBSD needs our help

Dear Reader,

I hope to appeal to your sense of decency and fairness in requesting that you support OpenBSD's ongoing campaign to get Intel to provide open access to the documentation (and in some cases binary blobs) behind their wireless chipsets [full story here]. This access will not only benefit OpenBSD but all FLOSS operating systems and strengthen the ecosystem by giving us (you and me) more choices from a hardware and operating system perspective.

Please stop buying Intel's products if possible and at a minimum send emails to majid.awad@intel.com and voice your concerns. The Internet gives you a voice and I implore you to use it.

Sincerely,


HashiDiKo

Saturday, September 30, 2006

Java Keeps Keeping On

The real motives why industry analysts love to predict Java fall... and why they will spend another 10+ years being proven wrong.

A very good entry discussing some of the reasons why Java continues to defy the doomsday predictions of so-called "industry experts".

Java not being the new kid on the block is not news it's an achievement that every professional (state of mind not state of employment) Java programmer can be proud of.

Wednesday, September 13, 2006

Concurrent Javadoc

Writing correct concurrent code is hard and what I'm finding equally challenging is documenting concurrent behavior in code in prose (javadoc to be specific).

Pre Java 1.5 all we had where java.lang.Thread, the Runnable interface, the synchronized keyword, and volatile variables. Because synchronized was the only real mechanism for concurrency control if you wanted to reason about the thread safety of a class all you had to do was look for the synchronized keyword. Some of you may be asking "but what about volatile?" volatile then was nowhere near as powerful as it is now. Back then the only thing volatile really meant was do not cache. Nowadays volatile not only means do not cache it also affects the visibility of other non volatile variables and it also imposes ordering constraints. Basically synchronized and volatile no longer act in isolation. All activities inside the JVM is now governed by a well defined memory model (JMM). It's no longer enough to look for the synchronized keyword you have to keep the memory model in mind at all times which is damn hard to document. Therefore I'm seeking feedback.

Imagine you have been asked to maintain a body of code with which you have had no previous experience and the only thing you know is it's targeted at Java 1.5 and higher (new JMM). You come across this (partial) class definition:

1    /**
2     * Ties {@link BlastMaster}, {@link BlastManager}, and {@link BlastAgent} objects together around a blast. It is the
3     * highest layer of abstraction available on a blast. It provides the hooks for the lifecycle management of a blast and
4     * is the communication channel between BlastAgent(s) and BlastManager(s) and BlastMaster.
5     * <p>
6     * The lifecycle of a context goes something like this. It is created by BlastMaster, injected into the outbound queue
7     * by BlastMaster, scheduled by BlastManager, delivered by BlastAgent(s), descheduled by BlastManager, rescheduled by
8     * BlastManager, delivered by BlastAgent(s) ....
9     *
10    * The scheduling, delivering, and descheduling steps repeat until a) all the recipients have been sent the blast or b)
11    * the duration of the blast has expired.
12    * </p>
13    *
14    * @author HashiDiKo
15    */
16   public final class BlastContext extends Constants
17   {
18       /**
19        * Keeps track of the number of domains yet to be delivered. When this number reaches zero it means the current
20        * iteration of trying to deliver the blast has ended.
21        *
22        * @see BlastAgent#decrementRemaining()
23        * @see #fire(ContextEvent.Event)
24        */
25       final AtomicInteger remaining = new AtomicInteger();
26   
27       /**
28        * Provides a mechanism for a {@link BlastManager} to be notified when the {@link BlastAgent}s executing the context
29        * stop executing it.
30        * <p> This object does a great deal in terms of establishing <em>happens-before</em> edges between a blast manager
31        * and its agents and between blast agents.</p>
32        */
33       final Set<Thread> agents = Collections.synchronizedSet( new HashSet<Thread>( MAX_BLAST_AGENTS + 1) );
34   
35       /**
36        * The message.
37        *
38        * <h4>Visibility</h4>
39        *
40        * <p>The blast manager thread is the only thread that writes this field while blast agents are the only threads
41        * to read this field. This field is <i>correctly synchronized</i> because the blast master writing this field
42        * <b>always</b> <i>happens-before</i> the blast agents reading it. This <i>happens-before</i> relationship is
43        * established in the code starting with {@link #init()} and its call to {@link #doInit()}. {@link #doInit()} is
44        * the only place in the code where this field is written. You simply need to follow the flows in
45        * {@link BlastManager} starting from where {@link #init()} is called to see synchronization the points that
46        * establishes the <i>happens-before</i> relationship between the blast manager and its blast agents in regard to
47        * this field.
48        * </p>
49        */
50       Message message;
51   
52       /**
53        * Indicates that this context is context to an old blast. In other words the system was restarted and the blast
54        * was loaded from the database.
55        *
56        * @see #init()
57        * @see #BlastContext(boolean, EmailBlast, BlastMaster)
58        */
59       private final boolean old;
60   
61       /**
62        * The home directory of the blast.
63        */
64       private final File home;
65   
66       /**
67        * The duration of the blast in milliseconds.
68        */
69       private final long durationMillis;
70   
71       /**
72        * Used to implement our backoff algorithm.
73        *
74        * <h4>Visibility</h4>
75        *
76        * <p>The field is only ever read/written by blast agents thus the visibility of this field is limited to
77        * blast agents in general and specifically the {@link #fire(ContextEvent.Event) fire} method. Even though
78        * {@link #fire(ContextEvent.Event) fire } is not <tt>synchronized</tt> and this field is non volatile it is
79        * <em>correctly synchronized</em>. There are 2 things at work that make this so. Firstly, there is never
80        * concurrent access to this field. At most 1 blast agent thread will write/read this field for a delivery cycle.
81        * This is guranteed by {@link BlastAgent#decrementRemaining()} because a) how its implemented and b)it is the only
82        * place where a blast agent get accesses this field. Finally, the fact that blast agents call
83        * {@link #register(Thread)} before they execute a blast and {@link #unregister(Thread)} when they are done
84        * executing a blast (both contained <tt>synchronized</tt> blocks using the same monitor) gurantees that the update
85        * of this field will be visible to any subsequent reads by different blast agents.
86        * </p>
87        * @see #fire(ContextEvent.Event)
88        */
89       private int magnitude;
90   
91       /**
92        * Used to implement our backoff algorithm.
93        *
94        * <h4>Visibility</h4>
95        *
96        * <p>The field is only ever read/written by blast agents thus the visibility of this field is limited to
97        * blast agents in general and specifically the {@link #fire(ContextEvent.Event) fire} method. Even though
98        * {@link #fire(ContextEvent.Event) fire } is not <tt>synchronized</tt> and this field is non volatile it is
99        * <em>correctly synchronized</em>. There are 2 things at work that make this so. Firstly, there is never
100       * concurrent access to this field. At most 1 blast agent thread will write/read this field for a delivery cycle.
101       * This is guranteed by {@link BlastAgent#decrementRemaining()} because a) how its implemented and b)it is the only
102       * place where a blast agent get accesses this field. Finally, the fact that blast agents call
103       * {@link #register(Thread)} before they execute a blast and {@link #unregister(Thread)} when they are done
104       * executing a blast (both contained <tt>synchronized</tt> blocks using the same monitor) gurantees that the update
105       * of this field will be visible to any subsequent reads by different blast agents.
106       * </p>
107       * @see #fire(ContextEvent.Event)
108       */
109      private boolean hrs;
110  
111      /**
112       * Stores the backoff timeout (the end result of the backoff algorithm computation).
113       *
114       * <h4>Visibility</h4>
115       *
116       * <p>The visibility requirements for this field is kind of funky. It's is a read/write field for
117       * {@link BlastManager}s and write only field for {@link BlastAgent}s. So the core visibilty issue is writes by
118       * blast agents being visible to blast manager. The <em>happens-before</em> edge that makes this possible is
119       * established by the monitor acquisition of {@link #agents} by the blast agent thread that writes this field and
120       * the subsequent acquisition of the same monitor by the blast manager.
121       * </p>
122       *
123       * @see #fire(ContextEvent.Event)
124       */
125      private long backoffTimeout;
126  
127      /**
128       * Indicates if {@link #doInit()} has been called.
129       * This field is only ever read/written by the blast manager thread for this context thus it is correctly
130       * synchronized.
131       */
132      private boolean didInit;
133  
134      /**
135       * Temporarily store the short domain blacklist expiration.
136       * <p>
137       * When this value is greater than zero it indicates to the blast manager thread that even though {@link #domainQ}
138       * is empty this context has undelivered recipients.
139       * </p>
140       * <p>This field is <em>correctly synchronized</em> because it is only ever read/written by the blast manager
141       * thread.</p>
142       *
143       * @see #getDomains()
144       */
145      private long shortestDomainTimeout;
146  
147      /**
148       * Gate to {@link #manager}. It establishes a <em>happens-before</em> edge with {@link #manager}.
149       */
150      private final CountDownLatch managerLatch = new CountDownLatch( 1 );
151  }
152  

So. Does the javadoc comments say enough about how non volatile class members manage to be correctly synchronized?

Saturday, September 09, 2006

A Developer's Journal; Solaris/CMT #5

I am feeling disingenuous about my "A Developer's Journal; Solaris 10 ..." title because I've made 5 entries so far and have said very little about Solaris 10. The truth of the matter is Solaris is the means and not the end. Let me explain. I have absolutely no interest in Solaris as an end to itself at this point because of Linux. And though Sun has made awesome progress with Solaris on x86 it still has a way to go before it is as seamless as Solaris on Sparc. So my key interest in Solaris is strictly with Solaris on Sparc in general and Solaris on Niagara processors specifically. Therefore all entries, starting with this one, will be titled "A Developer's Journal; Solaris/CMT ...". My apologies to any readers who have arrived here under the assumption that these are strictly Solaris 10 entries.

Friday, September 08, 2006

A Developer's Journal; Solaris 10 #4

I brought it home. It's still in the box. It's setting in my living room steering at me and refuses to blink. I know I don't stand a chance of winning the steering game but I steer anyway eyes bloodshot I steer. It's all I can do with it at the moment. I have deadlines to meet and missing deadlines is bad for business so I can't cut into my dev time to play with the it. The clock is ticking, ticking, ticking. As I look away from my monitor to rest my eyes it catches my gaze. It steers at me and I steer back neither of us blinking while the clock ticks, ticks, ticks.

Monday, September 04, 2006

A Developer's Journal; Solaris 10 #3

The T1 is here. Andrew called to let me know it's at the office. It only took 6 days. I'm impressed. I guess Sun has finally got the kinks in there supply chain hammered out.

I need to figure out where I'm going to put it. I can set it up at the office or take it down to the data center or set it up at home. I just don't know right now because my thoughts are elsewhere. I've got some integration work to do with Jetty and a heck of a lot of code to write for the web service interface for my project.

So much to do so little time.

Sunday, September 03, 2006

Latches in Java

David Holmes provides an excellent description of the difference between java.util.concurrent.CountDownLatch and java.util.concurrent.CyclicBarrier on the concurrency interest mailing list. I have posted it here for those of you who don't subscribe to the list and may not have a good grasp of these classes. And since I use CountDownLatch in code I've posted in the past and in code I'm planning on posting I'm happy to have such a clear explanation that I can link to in future entries.

... As Brian describes one notion of state "latching" is that it progresses to a terminal value from whence it no longer changes. It is permanently "latched". A synchronization object that behaves in this way is the CountDownLatch - once "open" it never closes and can't be reset.

More generally though latches can be reset - consider a digital flip-flop such as a "gated D-latch", the flip-flop latches the value of the data when the gate is pulsed/strobed; if you change the data without changing the gate then the latched value is unchanged, but change the gate and the latched value is updated.

In synchronization object terms a latch is sometimes called a "gate" - the connotation being that if the gate is open anyone/everyone can pass through; while if it is shut no one can pass through. The CountDownLatch operates this way, but a more general "gate" is the CyclicBarrier which can also be reset (and automatically does so). Of course the semantics of CountDownLatch and CyclicBarrier are somewhat different. CyclicBarrier is what can be called a "weighted gate" or "weighted bucket" - it is set up to expect N threads to arrive, when they arrive they have sufficient "weight" to open the gate, or tip the bucket - in this case the gate/bucket is spring-loaded and closes/rights-itself as soon as the threads leave, so it is ready for the next set of threads to use. CountDownLatch on the other hand is like a gate with multiple padlocks - when the last padlock is removed, the gate opens and it stays open. Aren't these analogies quaint :-) We could have defined CountDownLatch to allow reset but reset semantics are messy and usually not needed for CDL usage, in contrast barrier designs typically always use the barrier multiple times.

It seems the database folk are using the term "latch" for lightweight lock, which is an uncommon usage from a synchronization perspective and a poor choice in my view, though arguably there is an analogy between "locking a door/window" and just "latching it shut". In that sense "latching" is a weaker form of "locking". But I don't like the usage.

A Developer's Journal; Solaris 10 #2

After 2 "Try and Buy" applications Sun Microsystems has finally shipped my T1. The thought of getting my hands on it gets my geek johnson tingling. You may be wondering "why just a tingle?" Well, when I first read about CMT coming to a server near me my geek johnson pitched a tent with enough head room for Yao Ming to enter without bending his head. But the truth of the matter is I am a cynic to the core. Vendors are notorious for contorting the truth and calling it marketing.  They all claim their product is the greatest thing since Internet porn which is obviously a lie. THERE IS NO THING BETTER THAN INTERNET PORN! There. I said it. Go tell your friends. I'll wait for you in the next paragraph....

So I'll read the technical specs and the research and whitepapers on a technology/product but I don't drink the marketing Kool-Aid. So until I get my hands on the box and start playing with it you'll only get a tingle out of me.

Friday, September 01, 2006

A Developer's Journal; Solaris 10 #1

I'm testing the Solaris 10 waters. My plan for this journal is to record my Solaris journey in as much useful detail as possible. But before I start reporting my trials and tribulations I want to discuss why I'm taking this path (remember Linux is my OS of choice).  There are 5 reasons why I'm looking at Solaris 10 now (from most to least importance):

  1. Try and Buy
  2. Chip Multi-Threading (CMT)
  3. DTrace
  4. ZFS
  5. Hotspot

Try and Buy

Try and Buy made the list and snatched the number one spot because it makes number 2 accessible given my limited R&D budget.

CMT

Sun has jumped ahead of the competition (Intel/AMD/IBM) in terms of throughput computing with the release of the T1 processor, code-named Niagara. I had previously discussed the coming of throughput computing here and here and I'm surprised yet thrilled to see it come to pass so rapidly. Most of the reviews of Niagara I've read are from people who don't have a clue about multi-threading and/or what makes it great. I'm guessing they have a rudimentary understanding of what a process is and probably don't know why scaling systems with threads is more efficient and performant than doing it with processes. So I'm really excited to get a crack at throwing an application that was designed from the ground up for multi-threaded deployment at Niagara.

DTrace

I'm a performance junkie. I like to make things go fast and it is hard to make something go fast when you don't understand what the thing is doing. Before DTrace it was possible to get pieces of the puzzle and sometimes it was possible to postulate about the whole and get it right but that's usually a hit or miss and it takes years of experience to develop the facilities to get more hits than misses. With DTrace there is no need to postulate it becomes possible to know with absolute certainty what the hell is going on. DTrace takes the guessing out of performance tuning.

ZFS

The disk subsystem is probably the most mission critical part of a computer because it's where all the programs and data are stored and without programs or data a computer isn't worth squat.

Aside:

Okay I concede. You can turn it into a modern art piece or make an ugly end table but other than that it isn't worth squat!

Unfortunately the disk subsystem is also the most unreliable. So any practical technology that handles reliability w/o major performance costs is a big deal to me. For years we've been subject to either uber expensive SCSI/RAID systems or cheap unreliable (and slower) IDE systems. But that's all changing, SATA and it's ilk are catching up to SCSI in terms of reliability and performance which will eventually drive SCSI out of the picture or force the price of SCSI down to the point where there is really no difference in terms of choosing one over the other. The bottom line is ZFS makes reliable disk subsystems something everyone can afford.

Hotspot

Given that the hotspot JVM is a Sun product and so is Solaris I conjecture that hotspot works better on Solaris than anywhere else. Especially Solaris on Sparc. So even though I'm a big Linux fan I'm an even bigger Java one.

Monday, June 12, 2006

QOTD: "I’m not writing shitty code; I’m creating refactoring opportunities."

I've got nothing to add to it. It's just plain funny as hell so I thought I would share it.

Saturday, June 03, 2006

"Look Ma, No Locks!"

Brian Goetz has written an excellent introductory article on nonblocking algorithms and showcases some simple nonblocking data structures with code examples and pictures.

The core concept you should take away from the article is the importance of CAS. It sits at the root of all nonblocking data structures in the Java platform.

Saturday, April 29, 2006

Amen!

Read the title.

Click on the link.

Read the post.

"That's all folks!"

Monday, April 24, 2006

Sublime

An interesting read to say the least.

Monday, April 10, 2006

New Template

I've just switched my blogger template so if you are a regular visitor don't freak out. The new template makes better use of screen real estate and is more code friendly.

Let me know what you think.

Saturday, April 08, 2006

regex vs. XML

I needed to screen-scrape wikipedia for the list of Top Level Domains for my email app so I could build an index of suffixes that would reduce the number of martian addresses the app would have to process. So I surfed over to the TLD page to have a look-see. Low and behold those beautiful wikipedians were nice enough to produce well formed XML (actually, XHTML 1.0 Transitional) content. My heart leaped w/ joy because no screen-scraping was needed. With a little bit of XPath I would have my index in no time.

WRONG!!!!!!!!!!!

I spent the next hour+ trying to get XOM to work and spit out my list via XPath because I sure as hell wasn't going to try to manually walk the tree extracting the TLDs. Do you have any idea how much code that would take? I had a feeling the reason it doesn't work has something to do w/ how XOM handles namespaces because when I access the tree the hard way it demands that I specify the namespace URI of the document to lookup any element in the tree even though the document declares one and only one namespace. Now this isn't a knock against XOM, though I'm sure if I had used dom4j it would have worked because dom4j is more concerned with being useful than being correct. But each have there place. By this time I was so aggravated I just didn't feel like downloading dom4j and unpacking it and setting my classpath, yada, yada, yada.

Regex to the rescue

This his how I got what I needed the old fashion screen-scraping way. 5 minutes tops from concept to results:
String tld = "http://en.wikipedia.org/wiki/List_of_Internet_top-level_domains"
Matcher regex = 
    Pattern.compile( "<td><a[^>]++>\\.(\\w++)</a></td>" ).matcher( "" );
BufferedReader reader =
    new BufferedReader(
        new InputStreamReader(
            new BufferedInputStream( new URL( tld ).openStream() ), "utf-8" ) );
PrintWriter writer = 
    new PrintWriter( 
        new BufferedWriter( 
            new FileWriter( new File( "/home/HashiDiKo/temp/TLD" ) ) ) );

for( String line; null != (line = reader.readLine()); )
    if( regex.reset( line ).matches() )
        writer.println( regex.group( 1 ) );

reader.close();
writer.close();

Saturday, April 01, 2006

Monday, March 27, 2006

Gentoo is back!

I'm proud to announce my return to the Gentoo colony with my acquisition of a dual processor dual core AMD Opteron 270 box. But it is not all gravy.

Normally I stay on the KDE side of the fence but my time spent in Ubuntu land has given me a new appreciation for the Gnome desktop environment. The Ubuntu team has spent a great deal of time and effort making Gnome and all the apps in the Ubuntu distro a seamless whole. In Ubuntu most of the time I couldn't tell where Gnome ended and the application began. I'm currently running XFCE 4 and if it wasn't so damn fast I would switch to Gnome in a heartbeat. But I'm finding it difficult to give up the instant response time of XFCE. It is really fast. Click on a button any XFCE button and boom! it opens instantly, no waiting. It is intoxicating.

Even more impressive is the startup time. Booting up to a KDE/Gnome desktop is like booting up twice, once for the OS and another for the desktop. With XFCE startup is a none issue. startx and 2.5 - 3 seconds later I'm completely in my desktop. My only real gripe w/ XFCE is it doesn't come with a built-in graphical calculator and I can't find a decent one in the portage tree.

Like I said before, it's not all gravy. The box's fans are loud as hell so much so that my apartment sounds like the tarmac of an airport as planes prepare for takeoff. Which means when I 'm not working I have to turn it off or else I won't be able to hear myself think. There are positives to this though. I can't hear my neighbors.

The walls in my apartment are paper thin and I've got probably the loudest most obnoxious set of neighbors. They are constantly fighting and yelling and banging on the walls. One of them has this habit of going into the hallway to talk on his cell and it drives me absolutely insane because the hallway acts an echo chamber to his already loud and irritating voice. Like I give a shit what he has say to whoever he has to say to. If I could get away w/ it I would do not nice things to him ;)

Wow! I have no idea how I got from talking about my Gentoo environment to piss and vinegar about my neighbors. I guess they really get on my nerves. Anyway, the bottom line is Gentoo is da bomb and I'm glad to back and now w/ my tarmac apartment my neighbors are a none issue so if something happened to them it wasn't me!

Sunday, February 26, 2006

Annoyances: java.sun.com

Why are the people that run the java.sun.com site such idiots? Try to get to the JavaMail part of the J2EE stack from java.sun.com w/o using the search dialog box. Go ahead, try it! I'll wait .....

Can't be done huh? WTF you say? It's because they are idiots! I used to be able to get to any named Java technology 2 clicks from the home page but they changed it and change is good. Bullshit!!

As a company Sun is infamous for small annoyances this is one of them. And because I spend (or will be spending) so much time in Java (Sun) land the annoyances are beginning to add up. Even their own employees complain. Argh!!!!!!!!!!!

It's like there are two competing groups of people working at Sun. Some really brilliant, thoughtful, articulate people and a bunch of morons. The morons are winning! If you think I'm kidding take a look at some of the J2SE and J2EE source code. There are some parts of it where I get the feeling, these people really know what they are doing. Then there are other parts where I have to go, WTF?!!! The patients have definitely taken over the asylum.

Saturday, February 25, 2006

QOTD: The 'f' in framework

I love the English language. You can get away with saying almost anything.

Wednesday, February 15, 2006

The Long Way Around, An Exercise in Concurrent Programming

Adopting the mindset to fully utilize the classes in java.util.concurrent can be quite challenging. One of the side effects of concurrent programming is that it can require more code than it would take to achieve the same results using mutual exclusion locking.

Is concurrency worth writing more code? My current answer to the question is hell yes because I think most Java programmers need the practice it's going to take to get one's head around all the issues associated with concurrency. And if the programmer doesn't practice, how is he/she going to be able to fully exploit concurrency in the cases where it is decisive? With that in mind I would like to talk about some code that I've written, but first some background info.

Better event handling

I think Java's current event model sucks! It's biggest problem is that it tightly couples the firing of events with there processing. In other words, the thread that does the firing is usually the same thread that handles the event. This is not a very stable or scalable solution because all it takes to bring event firing/handling to its knees is one badly written event handler (think infinite loop or blocking I/O). It is for this reason that the Swing team introduce the Swing worker thread so the programmer could separate the handling of events from the firing of events without taking the main Swing thread hostage.

There is a better way based on asynchronous messages. It is not fundamentally new. It is based on how asynchronous network I/O is done on Unix operating systems. You tell the kernel your are interested in a set of events then you can either go about your business and periodically check if the things you are interested has happened (poll(2)) or you can sit around and wait until something you are interested in does happen (select(2)). The choice is yours. Plus you can be interested in as many things as you want without a) requiring a copy of yourself for each interest set or b) hogging the kernel. This is because all the kernel does is tell you that what you are interested in is ready, it is your job to do something useful with it. So how do we implement the functionality in Java? Queues!

To wait or not to wait? That is the question!

The basic idea is a class/service that fires events will provide methods for threads to receive notification of these events by registering java.util.Queue objects. You can use any Queue object but I highly recommend java.util.concurrent.BlockingQueue objects. Why BlockingQueue[s]? Simple! A BlockingQueue allows us to choose to wait indefinitely, wait temporarily, or not at all for events (take(), poll(long,TimeUnit), poll()). It's the decoupler see? It unbinds the firing of events from the processing of events. It's the service/class 's responsibility to fire the event and it's the registrant's responsibility to process them. Tada!

The code dammit! The code!

Before I present any code I need to put it in context so it makes sense to you because I'm not going to include a complete project, just snippets. The snippets are part of a class that provides availability testing services. What that means is if a thread tries to make a connection to some host on the network and the connection fails, the thread can register the host with the service and the service will continue to try connecting to the host until some pre-configured timeout has elapsed or it manages to establish a connection. The registrant will be notified of the outcome of the connection testing by the service via the notification mechanism I've been talking about.

Now that I've through all the background info lets look at some code:
    /** 

37    * Container for listeners interested all test results. 
38    */ 
39   private final Collection<Queue<Result>> allResults = new ConcurrentHashSet<Queue<Result>>(); 

40    
41   /** 
42    * Container for listeners interested in specific test results. 
43    */ 
44   private final ConcurrentMap<String, Collection<Queue<Result>>> hostResults = 

45           new ConcurrentHashMap<String, Collection<Queue<Result>>>(); 
46    
47   /** 
48    * Registers <tt>listener</tt> with <tt>this</tt> object to receive {@link Result} 

49    * notifications for all hosts. 
50    * 
51    * @param listener  The listener. 
52    * @return  <tt>true</tt> if the listener was registered, <tt>false</tt> otherwise. 

53    */ 
54   public boolean register( Queue<Result> listener ) 
55   { 
56       return allResults.add( listener ); 
57   } 

58    
59   /** 
60    * Registers <tt>listener</tt> with <tt>this</tt> object to receive {@link Result} 

61    * notifications for hosts in <tt>hosts</tt>. 
62    * 
63    * @param hosts The interest set. 
64    * @param listener  The listener. 

65    * @return  The registration results. 
66    */ 
67   public boolean[] register( String[] hosts, Queue<Result> listener ) 
68   { 

69       boolean[] registrations = new boolean[ hosts.length ]; 
70       for( int i = registrations.length; --i >= 0; ) 
71       { 

72           String host = hosts[ i ]; 
73           Collection<Queue<Result>> listeners = hostResults.get( host ); 
74           if( null == listeners ) 
75           { 

76               listeners = new ConcurrentHashSet<Queue<Result>>(); 
77               Collection<Queue<Result>> preempt = hostResults.putIfAbsent( host, listeners ); 
78               if( null != preempt ) 

79                   listeners = preempt; 
80           } 
81           registrations[ i ] = listeners.add( listener ); 
82       } 
83       return registrations; 
84   } 

85    
86   /** 
87    * Unregisters <tt>listener</tt>. 
88    * @param listener  The listener that was registered with this object via {@link #register(Queue)}. 

89    */ 
90   public void unregister( Queue<Result> listener ) 
91   { 
92       allResults.remove( listener ); 
93   } 

94    
95   /** 
96    * Unregister's <tt>listener</tt> from all <tt>hosts</tt>. 
97    * @param hosts 

98    * @param listener 
99    */ 
100  public void unregister( String[] hosts, Queue<Result> listener ) 
101  { 

102      for( String host : hosts ) 
103      { 
104          Collection<Queue<Result>> c = hostResults.get( host ); 
105          if( null != c ) 

106          { 
107              c.remove( listener ); 
108              if( c.isEmpty() ) 
109              { 
110                  Queue<Result> temp = new ConcurrentLinkedQueue<Result>(); 

111                  boolean b = register( temp ); 
112                  assert b : "IMPOSSIBLE!";// but i like to check anyway. 
113                  hostResults.remove( host ); 
114                  if( !c.isEmpty() ) 

115                  { 
116                      Collection<Queue<Result>> preempt = hostResults.putIfAbsent( host, c ); 
117                      if( null != preempt ) 
118                      { 

119                          preempt.addAll( c ); 
120                          unregister( temp ); 
121                          c = preempt; 
122                      } 
123   
124                      for( Result result : temp ) 

125                          if( result.host.equals( host ) ) 
126                              for( Queue<Result> queue : c ) 
127                                  queue.offer( result ); 
128                  } 

129                  unregister( temp ); 
130              } 
131          } 
132      } 
133  } 
134   
135  /** 

136   * Notifies registered listeners of a new event. 
137   * @param result    The event. 
138   */ 
139  private void fire( Result result ) 
140  { 

141      Collection<Queue<Result>> listeners = hostResults.get( result.host ); 
142      if( null != listeners ) 
143          for( Queue<Result> listener : listeners ) 

144              listener.offer( result ); 
145   
146      for( Queue<Result> results : allResults ) 
147          results.offer( result ); 
148  }

If you've simply skimmed pass the code you may want to go back and take a minute to digest it because I'm going to rewrite parts of it as it would have been written without the concurrency tools.

No concurrency

private final Collection<Queue<Result>> allResults = 

37       Collections.synchronizedCollection( new HashSet<Queue<Result>>() ); 
38    
39   private final Map<String, Collection<Queue<Result>>> hostResults = 

40       new Hashtable<String, Collection<Queue<Result>>>(); 
41    
42   public boolean register( Queue<Result> listener ) 
43   { 

44       return allResults.add( listener ); 
45   } 
46    
47   public boolean[] register( String[] hosts, Queue<Result> listener ) 
48   { 
49       boolean[] registrations = new boolean[ hosts.length ]; 

50       for( int i = registrations.length; --i >= 0; ) 
51       { 
52           String host = hosts[ i ]; 
53           Collection<Queue<Result>> listeners; 

54           synchronized( hostResults ) 
55           { 
56               listeners = hostResults.get( host ); 
57               if( null == listeners ) 
58               { 

59                   listeners = Collections.synchronizedCollection( new HashSet<Queue<Result>>() ); 
60                   hostResults.put( host, listeners ); 
61               } 
62           } 

63           registrations[ i ] = listeners.add( listener ); 
64       } 
65       return registrations; 
66   } 
67    
68   public void unregister( Queue<Result> listener ) 

69   { 
70       allResults.remove( listener ); 
71   } 
72    
73   public void unregister( String[] hosts, Queue<Result> listener ) 
74   { 

75       for( String host : hosts ) 
76       { 
77           synchronized( hostResults ) 
78           { 
79               Collection<Queue<Result>> c = hostResults.get( host ); 

80               if( null != c ) 
81               { 
82                   synchronized( c ) 
83                   { 
84                       c.remove( listener ); 

85                       if( c.isEmpty() ) 
86                           hostResults.remove( host ); 
87                   } 
88               } 
89           } 
90       } 

91   } 
92    
93   private void fire( Result result ) 
94   { 
95       Collection<Queue<Result>> listeners = hostResults.get( result.host ); 
96       if( null != listeners ) 

97       { 
98           synchronized( listeners ) 
99           { 
100              for( Queue<Result> listener : listeners ) 

101                  listener.offer( result ); 
102          } 
103      } 
104   
105      synchronized( allResults ) 
106      { 

107          for( Queue<Result> results : allResults ) 
108              results.offer( result ); 
109      } 
110  }

Let's compare

For the most part the non concurrent version looks pretty much like the concurrent version in terms of lines of code and complexity, except for public void unregister( String[], Queue).

It was because of this method that I started this blog entry in the first place, because I realized how much easier a synchronized version would have been to write. It just seemed to be the long way around for removing an entry from a Map. Lets walk through it.

First off, let me explain what it's supposed to be doing then (hopefully) it will make sense why it does what it does.

Basically, one host can have multiple listeners. So when all the listeners for a host have been unregistered we want to remove the mapping for the host. Otherwise, we are going to have an object retention bug on our hands. The problem is how do we remove the mapping in a thread safe way? Because we can't simply say,

if( c.isEmpty() )
    hostResults.remove( host );
and stop there because it is entirely possible that between the test and the removal another thread has registered for host. This is the kind of problem that mutual exclusion (synchronized) solves. But since we aren't using mutual exclusion we have to get creative. What we do is create a dummy Queue and register it to receive all events that get fired this way when we actual remove the mapping, if we are wrong about it being empty we can put it back and re-fire all the events that got fired between the time we removed the mapping and the time we put it back. The only draw back to all this (beside the additional lines of code) is that it is possible for a listener to receive duplicate events in the gap between,
preempt.addAll( c );
and
unregister( temp );

So is the additional complexity worth it? Maybe. If we did things the synchronized way the entire event notification mechanism can quickly become the bottle neck if a lot of threads are using it, especially if they are registering and unregistering for the same host. Now, we could replace the synchronized keyword with java.util.concurrent.locks.ReentrantLock[s]. It's a more scalable mutual exclusion mechanism than synchronized. But I've spoken to Doug Lea recently and he told me that the (Sun's) JVM team is working hard at making the synchronized keyword as performant as ReentrantLock. Ya gotta love that competition. I think everyone should email Doug and thank him and the rest of the jsr166 team for embarrassing the JVM team into trying to make contended monitor acquisition more performant.

One last thing

I want to point out there is no general purpose lock free solution/pattern to the problem that I was solving in the unregister method. I was able to get away with not using locks because of the class' support for listeners to be notified of all events regardless of host. This support wasn't put in there just for concurrency's sake but is simply a feature requirement of the class that I've leveraged to avoid locking.

Tuesday, February 14, 2006

Matching a Backslash in Java

I've done it a million times in the past but for whatever reason I keep screwing up how to properly write a backslash (\) in a regular expression. I wasted 20+ minutes yesterday trying to figure out why the character class in this
[\";:()\\[\\]<>@.\\,]
regular expression wouldn't match strings containing commas. I'm blogging the solution so I'll have a permanent record and hopefully, I'll once and for all stop screwing it up!

Solution:

[\";:()\\[\\]<>@.\\\\,]

Monday, February 13, 2006

Part 2's a come'n

I've promised a part 2 to this entry and I will deliver. Granted, nobody has complained about it so far but that is probably because you and them have better things to do with your time. Losers! Just kidding.

In the mean time I've fixed a minor bug in the code. The entry has been updated.

It was a really D'oh! (think Homer Simpson) type of bug. The code (comments) promised to return MX records sorted by preference but in reality it used lexicographical sorting only. So if a domain had two exchangers with preferences of 77 and 100 the 100 would appear before the 77. D'oh!

Saturday, February 11, 2006

Java too ubiquitous? (My Response)

I second CJAN. I think it's long overdue. I'm sick of tired of the growing size of the SDK and it's ilk (i.e. J2EE). At the rate they are going they are going to have to release a 4 DVD box set just to get the SDK to us and then we are going to have to FedEx our customers a USB external harddrive to store the SDK and the additional libraries to run the programs we write.

Back to Gentoo

I'm going back to Gentoo. As soon as my ThinkPad Z60m replacement gets here I'm putting Gentoo on it. Ubuntu/Debian is just not my bag. Even though it's GNU/Linux it feels alien to me. I think I'm just spoiled. Gentoo forces you to know your system. It's not just a black box. I feel empowered by that. I can't tell you what Gentoo has done for my sysadmin skills, though I try to avoid system adminstration at every turn. I prefer to spend my paying hours coding.

My litmus test of a good distro is how easy is it to build and install your own kernel. Nuff said! Gentoo rocks fo' shizzle!

Friday, February 10, 2006

I Am Spam No More

I'm no longer SPAM. Yeah!
Hello, Your blog has been reviewed, verified, and whitelisted so that it will no longer appear as potential spam. If you sign out of Blogger and sign back in again, you should be able to post as normal. Thanks for your patience, and we apologize for any inconvenience this has caused. Sincerely, Blogger Support

Observations, Linking Trends

Over the last year or so I've noticed that less and less of the sites I visit force new windows open or popups when I click on offsite links. The new site simply replaces the current site and a nice ordered history is maintained. This is the way it should have always been. Sometimes I get so engrossed by the content of the new site that I forget how I got there or why I'm there. So I'm really appreciative of the ability to click on the little drop down arrow to the right of my browser's back button so at a glance, I can be reminded of why I'm there.

I know the motivation for new windows and popups was to keep people on a site. Granted, it created more confusion than stickyness, but it had a purpose. So what is the motivation for this new style of site design?

Anyway, maybe I'm hallucinating and it's the same as it always was or maybe my browsing tastes of changed and the sites I frequent now are better designed. Come to think of it, I do consume significantly less Internet porn now that I'm 30.

I Am Spam

blogger.com says I am spam. Now everytime I want to post an entry I have to complete Word Verification.

Warning, edited to make it sound worse than it is!

Spam blogs can be recognized by their irrelevant, repetitive, or nonsensical text, along with a large number of links, usually all pointing to a single site.

Link to original text.

God forbid hypertext (this blog) should have hyperlinks in it. That's what I get for talking crap about big daddy (or more recently, Big Brother) Google.

3 Most Annoying Things About Firefox

  1. Press F3 to bring up the search dialog but it doesn't give the search box focus. What's the point!?! And don't tell me about [CTRL-F]
  2. Why can't I search my history, not just the title but the contents of the pages. I know it caches the data so why don't they just get it over with and make the damn thing searchable!?! I don't want a hear a peep about the dman Google toolbar. Google has enough information on me. The amount of internet porn I consume is my business and my business only! You see what Yahoo is doing to the Chinese. Google could be next. The dollar$ associated with a billion+ pontential users is damn good reason to do evil.
  3. When I scroll down a long page and click on a link on the page then go back to the previous page it starts me from the top of the page again. What the f@ck!?!

Tuesday, February 07, 2006

Making Progress

I'm happy to report that I'm making progress. It seems my breakthrough epiphany is bearing fruit. I still don't have the big picture breakdown like I want but thanx to my incessant note taking I have a bunch of little pictures that I can work on until I discover a way to tie it all together.

In the mean time I am happy to report the DNS querying code that I posted in A Breakthrough! has graduated from the Whiteboard and found it's own home. It's been fully tricked out with hostname resolution caching and all. There are a few concurrency related goodies in the code that I want to talk about to those readers still trying to figure out what the java.util.concurrency packages are good for. Since the code is 183 lines long (with comments) I'm splitting its discussion into two entries. The first entry is the one you are reading now and it contains the source code. The second entry will discuss the concurrency goodies and I'll post that sometime later today or maybe tomorrow.

Here is the code, enjoy:

(Updated 13 Feb 06, 10:14)


6    import javax.naming.CommunicationException;
7    import javax.naming.NameNotFoundException;
8    import javax.naming.NamingException;
9    import javax.naming.ServiceUnavailableException;
10   
11   import javax.naming.directory.InitialDirContext;
12   
13   import java.net.InetAddress;
14   import java.net.PortUnreachableException;
15   import java.net.UnknownHostException;
16   
17   import java.util.Enumeration;
18   import java.util.Queue;
19   import java.util.TreeSet;
20   
21   import java.util.concurrent.ConcurrentHashMap;
22   import java.util.concurrent.ConcurrentLinkedQueue;
23   import java.util.concurrent.ConcurrentMap;
24   import java.util.concurrent.CountDownLatch;
25   
26   /**
27    * Lookup service for DNS MX records.
28    * <p>
29    * <b>Thread safe:</b> yes
30    * </p>
31    *
32    * @author HashiDiKo</a>
33    */
34   public final class MXRecordQueryService
35   {
36       private static final String URL = "dns:/";
37   
38       private static final String MXATT = "MX";
39   
40       private static final String[] MXFILTER = {MXATT};
41   
42       private static final MXRecordQueryService THIS = new MXRecordQueryService();
43   
44       /**
45        * Cache queries.
46        * @see MXRecordQueryService.CacheEntry
47        */
48       private final ConcurrentMap<String, CacheEntry> cache = new ConcurrentHashMap<String, CacheEntry>();
49   
50       /**
51        * Provides a mechanism for ensuring that no 2 (or more) threads will race to query the same domain but at the same
52        * time allow concurrent queries of different domains.
53        */
54       private final ConcurrentMap<String, CountDownLatch> domainLocks = new ConcurrentHashMap<String, CountDownLatch>();
55   
56       private MXRecordQueryService()
57       {
58       }
59   
60       public static MXRecordQueryService getInstance()
61       {
62           return THIS;
63       }
64   
65       /**
66        * Returns the IP addresses of the MX records for <tt>domain</tt> sorted by preference or <tt>null</tt> if no valid
67        * hostnames exits for the MX records.
68        *
69        * @param domain The domain name.
70        *
71        * @return The  IP addresses of the MX records for <tt>domain</tt> sorted by preference or <tt>null</tt> if no valid
72        *         hostnames exits for the MX records.
73        *
74        * @throws NameNotFoundException       Indicates that the <tt>domain</tt> is invalid.
75        * @throws ServiceUnavailableException Indicates that <tt>domain</tt> is valid but no MX records exists for it.
76        * @throws CommunicationException      Indicates a problem contacting the configured DNS server(s).
77        * @throws RuntimeException            If some other <tt>javax.naming.NamingException</tt> occurs.
78        */
79       public Queue<String> get( String domain )
80               throws ServiceUnavailableException, NameNotFoundException, CommunicationException, InterruptedException
81       {
82           CacheEntry cached = cache.get( domain );
83           if( null != cached )
84           {
85               if( System.currentTimeMillis() <= cached.expiration )
86                   return cached.object;
87               cache.remove( domain, cached );
88           }
89   
90           CountDownLatch lock = domainLocks.get( domain );
91           if( null == lock )
92           {
93               lock = new CountDownLatch( 1 );
94               CountDownLatch prempted = domainLocks.putIfAbsent( domain, lock );
95               if( null == prempted )
96               {   /* Take another look because we may have preempted and it's cheaper to look than to lookup. */
97                   cached = cache.get( domain );
98                   if( null != cached && System.currentTimeMillis() <= cached.expiration )
99                       return cached.object;
100  
101                  try
102                  {
103                      TreeSet<MXRecord> sorter = new TreeSet<MXRecord>();
104                      String url = URL.concat( domain );
105                      for( Enumeration en = new InitialDirContext().getAttributes( url, MXFILTER ).get( MXATT ).getAll();
106                           en.hasMoreElements(); )
107                          sorter.add( new MXRecord( (String)en.nextElement() ) );
108  
109                      Queue<String> queue = new ConcurrentLinkedQueue<String>();
110                      for( MXRecord mx : sorter )
111                      {
112                          try
113                          {
114                              for( InetAddress addr : InetAddress.getAllByName( mx.hostname ) )
115                                  queue.offer( addr.getHostAddress() );
116                          }
117                          catch( UnknownHostException ignore )
118                          {
119                          }
120                      }
121                      cache.put( domain, new CacheEntry( queue ) );
122                      return queue;
123                  }
124                  catch( NameNotFoundException nex )
125                  {
126                      throw nex;
127                  }
128                  catch( ServiceUnavailableException sux )
129                  {
130                      throw sux;
131                  }
132                  catch( CommunicationException comx )
133                  {
134                      if( comx.getRootCause() instanceof PortUnreachableException )
135                          throw comx;
136                      throw new RuntimeException( comx );
137                  }
138                  catch( NamingException nex )
139                  {
140                      throw new RuntimeException( nex );
141                  }
142                  finally
143                  {
144                      lock.countDown();
145                      boolean invariant = domainLocks.remove( domain, lock );
146                      assert invariant : "Impossible!";
147                  }
148              }
149              else
150                  lock = prempted;
151          }
152          lock.await();
153          return get( domain );
154      }
155  
156      private static final class CacheEntry
157      {
158          final Queue<String> object;
159  
160          final long expiration;
161  
162          CacheEntry( Queue<String> object )
163          {
164              this.object = object;
165              expiration = System.currentTimeMillis() + 60000;
166          }
167      }
168  
169      /**
170       * Utility class for extracting the different parts of a MX record string.
171       */
172      private static final class MXRecord implements Comparable<MXRecord>
173      {
174          final String hostname;
175  
176          private final int pref;
177  
178          MXRecord( String mx )
179          {
180              assert mx.matches( "\\d++\\s++.+\\." ) : "Invalid MX record format: ".concat( mx );
181  
182              int offset = 0;
183              int len = mx.length() - 1;// Strip trailing dot.
184  
185              /* Extract preference integer. */
186              while( offset < len && Character.isDigit( mx.charAt( offset ) ) )
187                  offset++;
188              pref = Integer.parseInt( mx.substring( 0, offset ) );
189  
190              /* Extract domain name. */
191              while( offset < len && Character.isWhitespace( mx.charAt( offset ) ) )
192                  offset++;
193  
194              hostname = mx.substring( offset, len );
195          }
196  
197          public int compareTo( MXRecord o )
198          {
199              int diff = pref - o.pref;
200              return 0 == diff ? hostname.compareTo( o.hostname ) : diff;
201          }
202      }
203  }
204