Siam's Blog

Timer in Android ( The Better Way )

2009-03-19T03:50:00.004+06:00

The usual Timer class is available in Android, but unless you are doing non-trivial stuff, its better to use Handler for scheduling task. Why? Well, Timer class introduces a new thread and while developing mobile application, you should really think twice (or more than that!) before using it.

So, how jobs can be scheduled using Handler class? There are two different approaches. First, You can ask it to send messages at some later time by using sendMessageAtTime(Message, long) or sendMessageDelayed(Message, long) , which then can be processed as shown in my previous post, effectively acting as a timer. Secondly, you can directly ask it to run some scheduled task by using postAtTime(Runnable, long) and postDelayed(Runnable, long).

The methods described here lets you schedule a single shot task. To repeat a scheduled task, you have to register it again to run for the next time. So, assuming that you want to repeat a task after one second delay, we can do something like this,


private Handler handler = new Handler();

 
private Runnable runnable = new Runnable() {

 public void run() {

  doStuff();

  /*
   * Now register it for running next time
   */

  handler.postDelayed(this, 1000);
 }

   
};

To stop a repeated task you can use removeCallbacks(Runnable r), which removes all pending Runnable r in the message queue.

For First Time GSoC Applicants

2009-02-15T13:54:00.012+06:00

Google Summer of Code for 2009 has been announced. In GSoC 2008, I applied and worked for Jato. It was really fun. Moreover it helped me to be a better developer than I had been. While doing my GSoC, I have learned git, under-the-hood details about Java and become a more matured C programmer. It also helped to shake off the initial intimidation factors in joining open source community as a developer for the first time. Before GSoC, I didn't even know the name of Jato, but now I am an active developer continuing my GSoC works.

Thats why I want to share what I have learned. I know there are plenty of resources available for GSoC wanna be applicants, but for the first-timers the initial process may be very confusing. Thats the audience on which I will focus.

So here it goes.

1. Start Early

Yes, start early, start Now ! Do not wait for the last moment. Go through the faqs, tutorials. Dig into the previous 2008, 2007, 2006, 2005 GSoC pages. Get an idea about the mentors that participated and projects that have been proposed. But it is important to keep in mind that no previous mentor organization is guaranteed to have a slot in GSoC.

2. Come up with a short list

As there are many organizations participating in GSoC now, it is necessary to make a list of organizations that you think suits you most. And I think, its better to make that list even before the announcement of accepted mentor list. Because without any sort of prior list and plan, you may run out of time if you need to go through all the accepted mentors and their projects one by one. Of course, there is a certain not-so-amazing possibility that your selected mentor failed to get a slot this year but, hey, you can certainly make changes in that case.

You can start sorting by using language. If you abhor loosely typed language there is no point in applying php, JavaScript focused mentors. Here is a list of mentors in 2008, categorized by language.

Another important factor in choosing the mentors apart from language that they use, might be the type of the work they do. Are you interested in working on compilers ? How about diving into GPS/Geospatial projects? Here is another nice categorized list of 2008 mentors that might help you to guess.

While traversing previous years' mentors list, you can find that even though officially no mentoring organization is guaranteed to have a slot, most well known mentors, for example apache, mozilla or pidgin, are almost sure to have slots. Thats the source of another dilemma. Because, well known mentors mean more fierce competition, while lesser known mentors are less likely to have slots. I had to face this problem. I chose to work on Plan 9 from Bell Labs. After reading lots of documents, doing lots of trying and testing with Inferno, it failed to get a grant. So, my advice is to come up with a balanced list.

3. Get in touch now

After choosing the list, its necessary to get in touch with the organization. Join the mailing list, hang in the IRC channel.

Read the mails carefully, watch the conversations in IRC. Most probably the 'hot' topics for upcoming gsoc will be discussed. Find out the steps and protocols for committing code.

And most importantly, try to get the culture in the organization. Some organization may be very strict. You might get burnt for Top-Posting or get a sharp 'RTFM' reply for asking something which has been answered else where. But thicken your skin for that. We all certainly make mistake, do say things which we later regret. The important thing is to learn from the mistakes and not to let the intimidation factor trade on you.

And particularly if this is your first approach to an open source community, you may lay low for few days both in IRC and mailing lists. Try to understand the environment, the pattern of conversations, which is allowed to ask and which is not. But don't hesitate to participate if the topic is in your level. It'll certainly help you to bond with your new developer colleagues.

4. Land on an Idea

Try hard to come up with an idea. Certainly, you can choose ideas from the proposed idea-list. But coming up with an idea can and will boost your chance to get accepted. While proposing idea, try hard to come up with original and ambitious idea which the organization actually 'needs'.

To come up with such ideas, you may need to start reading the development docs and go through few source code files. It is necessary to do so to find out how and where your idea fits and whether its feasible to complete within the GSoC time line (3 months approximately).

After coming up with an idea, the most important thing is to communicate with the organization. Propose the idea in details, get a feedback. That will make a strong case for your application.

I think it concludes the initial preparation. The next important step will be writing application. Here is some link for that

WeakReference? What is that !

2009-02-03T17:06:00.008+06:00

Even though weak references are massively useful features available for fairly long time, I think they are pretty overlooked. ( Yeah, I am pretty bemused by getting multiple "WeakReference? huh! whats that?!" responses on a single day.) So, lets refresh on what weak references are and what they do.

In short, weak references can save you from a lot of problems in case of unintentional object retention. If you don't understand "Unintentional Object Retention" mumbo-jumbo, here is a scenario for you.

Lets say, you need to use gigantic images in your application. But, as you know loading images each time from disk is pretty expensive, you decided to cache it. And as a sane person, you don't want the gigantic images using up a large chunk of memory when there is no reference to it. Now, how can you do that? If you use normal reference to the image ( to place it in vector or hash map or in your custom cache class ), when GC will run, it will find that it has at least a reference to that image and would refrain itself from finalizing and reclaiming the memory. Now what you can do is, keeping some kind of in house count of the references that point to that image and when there is no other reference pointing, you can remove the holding reference from your cache store making it eligible for garbage collection. Is it fun to mimic the garbage collector? Hell, No !

Here comes the mighty weak reference into play. To put simply weak references are not strong enough to force the referent object to remain in memory. Another mumbo-jumbo? Well, lets go into details.

Each ordinary reference in Java is a strong reference. What make them strong is the way they interact with GC. More particularly, if an object is reachable through a chain of strong reference it will not be claimed by GC. So for two ordinary references x and y, the following statement


x = y;

will make sure that the object to which y points would be kept in memory as long as x is reachable.

But, for a weak reference, that is not the case. If all the references to a particular object are weak references ( we call this situation "weakly reachable"), then garbage collector is eligible to claim the object. So if you use WeakReference, like the following


WeakReference<SomeOtherRef> weakReference = new WeakReference(someOtherRef);

you are making sure that garbage collector's reachability has been increased and later you can get the reference by using


weakReference.get();

The point to remember is that it is not enough strong to prevent from garbage collection, so don't surprise if get() all on a sudden, starts returning null.

So, by now you have got what you need for that caching mechanism. In your caching store house, you will use WeakReference so that when there is no other strong references, the gigantic image is gracefully removed by the GC.

Java provides more control over garbage collection by providing PhantomReference, SoftReference, ReferenceQueue and WeakHashMap. But I think I have enough for today, so all these have to wait.

A first look into Android Thread

2009-02-01T20:49:00.010+06:00

I have started looking into Android. Late to join the show? well, may be, but surely its pretty exciting.

I have tried to build a utility class that can do Network I/O. And grasping things was little hard at first. Network I/O or other heavy-duty stuff in Android should be done on other worker thread. Because doing it in main thread (or the UI thread) can ( and will ) make your application unresponsive and may be killed as the system is persuaded to think that it has hung. For every Android developer, this is a must-read.

So you have to do the long running operations in separate thread. And to interact between threads you have to resort to Handler. A Handler is used to send message or runnable to a particular thread. The thing to remember is that a Handler is associated with the MessageQueue of the single thread which has created it. After creating a Handler, it can be used to post message or runnable to that particular thread.

Here is an example


public class MyActivity extends Activity {  

   void startHeavyDutyStuff() {

      // Here is the heavy-duty thread
      Thread t = new Thread() {

          public void run() {
              while (true) {

                  mResults = doSomethingExpensive();

                  //Send update to the main thread
                  messageHandler.sendMessage(Message.obtain(messageHandler, mResults)); 
              }

          }
      };
      t.start();
  }

  // Instantiating the Handler associated with the main thread.
  private Handler messageHandler = new Handler() {

      @Override
      public void handleMessage(Message msg) {  
          switch(msg.what) {
              //handle update
              //.....
          }
      }

  };
}

So what is done here? Very simple. A Handler has been created which is bound to the message queue of the main thread. And by using the handler, from the heavy-duty, worker thread we send message to the main thread to be processed.

Feeling like drinking from hose-pipe? Dont worry, I'll post more realistic example soon.

Providing synchronization support in Jato

2008-08-12T14:34:00.003+06:00

When I started to work on monitorenter and monitorexit, I thought it would be quite easy. Get the reference, call the lock/unlock function, done. But, soon after beginning, I was chanting "nothing-is-what-it-seems" mantra.

For monitorenter and monitorexit, it is necessary to provide support for goto and athrow bytecode instructions. Here is why.

Providing support for athrow instruction was easy. As Jato still does not support exception handling, actually nothing but function for bytecode consuming and dummy instruction selection rule was necessary.

But, for goto, I ran into a bug in control flow analyzer. So, after few hours of debugging, did fix that bug. And every thing was easy there after.

Then sent flood of patches! YAY !

( I made some nasty formatting horrors, which were fixed by Pekka. Thanks Pekka. )

The 64 Bit and premature optimaization anomaly !

2008-07-28T17:24:00.003+06:00

I am currently working to improve my undergrad research on Neural Network (which has been accepted in Fifth International Symposium On Neural Networks ).We have done a lot of brain storming in last few months. And then I was selected to translate the whole idea to code ! Having the sole responsibility for developing the whole idea does involve twist and turns, but it is fun too ! After finishing developing , I have a feeling that it is gonna be a good work.

And last night, my research partner informed me that , result has improved for every case but for benchmark heart attack problem. For this data set the program falls into an infinite loop.

In constructing neural network, instead of maintaining a explicit relationship of which node connected to which one, I have packed the connection information in unsigned integer. So storing and retrieving the connection information is easy, just a few left or right shift and masking .

As a matter of fact, this optimization is not very necessary here, because

1) Storing and retrieving connection information do not occur that much.

2) As it is a research on how neural network can predict result more accurately , result is of more concern not the speed.

But now, as heart attack benchmark problem has 35 input nodes, way more than other benchmark problems, I am left shifting an unsigned integer by 35 bits !. Phew ! Guess what Donald. E. Knuth has told us

"We should forget about small efficiencies, say about 97% of the time: premature optimization is the root of all evil." (Knuth, Donald. Structured Programming with go to Statements, ACM Journal Computing Surveys, Vol 6, No. 4, Dec. 1974. p.268.)

But as changing it would cause an avalanche of more changes, I did the obvious, changed the data type from unsigned integer to unsigned long long. And the result is same, trapped in the infinite loop.

So after more debugging for 2 hours, when I am cursing under my breath, the following line just hit me

connection_matrix_value+=1<<(edge->get_forward_node()->get_node_id()+

    shifting_value_for_forward_node);

Here, 1 is of type int having length 32 bits, so shifting it by 32 or more bits, nothing retains, the net result is a big fat 0 !. So what i need to do is making 1 as unsigned long long type ( 1ULL ).

And, YAY ! everything works !

Update On Jato

2008-07-25T14:36:00.002+06:00

Well , everything seems little scattered.

<*>store and <*>load is almost done , but cant check whether it actually works or not because of its dependency on register allocation.

instanceof is done but a little code re-structuring is needed.

For monitorenter and monitorexit , parsing and code generation is done, but need to handle athrow as well.

Haven't looked into checkcast yet.

And the deadline is August 18. Gotta run !.

multianewarray byte instruction in Jato

2008-06-25T14:18:00.006+06:00

I think, multianewarray instruction was the most tough one that I have confronted so far while working on Jato. But with the help of Pekka Enberg, finally I was able to creep out of the abyss.

Initially , I assumed (very naively) that the counts for multianewarray instruction would be pushed in expression stack as EXPR_VALUE expressions. But then, Pekka pointed out that it does not work for every situation. For example, it will break in the following code :

   public int[][] newArray(int x, int y) {
   return new int[x][y];
   }

The better way would be, as Pekka suggested, to 'abuse' the EXPR_ARG. That is , converting each count expression to argument expression, so that instruction selector can automatically push appropriate values on the stack.

So in multianewarrray byte code handler, what I do is, after popping dimension values from the expression stack, wrap each expression into EXPR_ARGS. Then, in instruction selection and code emission phase, as all the count values are already pushed on stack, all I need to do is, make a call to array allocation function in jamvm.

Easy as pie , when you are in right track !

SynchMaster 920nw in My Fedora

2008-06-16T17:23:00.003+06:00

Just got a SyncMaster 920nw 19" wide screen . A very nice monitor to have , bright clear distinct view all the times. But after plugging it to my computer , it shows a black area on my left ,as if it starts rendering from some (x,0) pixel.

So , I started to look into X configuration , and after a lot of head scratching , when I set the refresh rate to 75
Hz , it started to work perfectly . Weird !

Debugging Jato

2008-06-05T10:45:00.011+06:00

In last few weeks , while working on arraylength bytecode instruction , I had to debug Jato . But instruction provided in readme file is rather obsolete . So I had to go through the run-suite.sh and ''reinvent the wheel" .

To debug , you have to point gnu.classpath.boot.library.path to <gnu_classpath_root>/lib/classpath and bootclasspath should contain <gnu_classpath_root>/share/classpath/glibj.zip and <jato_root>/lib/classes.zip.

The easiest way to do so is , setting the following values



GNU_CLASSPATH_ROOT=`<JATO_ROOT>/tools/classpath-config`

GLIBJ=$GNU_CLASSPATH_ROOT/share/classpath/glibj.zip

BOOTCLASSPATH=<JATO_ROOT>/lib/classes.zip:$GLIBJ

then applying usual gdb command ,i.e.,



gdb --args <JATO_ROOT>/java \
-Dgnu.classpath.boot.library.path=:$GNU_CLASSPATH_ROOT/lib/classpath \
-Xbootclasspath:$BOOTCLASSPATH -cp <JATO_ROOT>/regression class_file_name_to_debug

will launch it in debug mode. You can additionally pass any other command line option ( -Xint , -Xtrace:jit ... ) also.

The Future Of Search ?

2008-05-09T11:47:00.001+06:00

Hmm , well , I'll be waiting for this http://futureofsearch.federatedmedia.net/archives/5

How to run a company ( Or How not to )

2008-04-27T12:28:00.001+06:00

From a developer's perspective ;)
http://torjo.blogspot.com/2008/03/paying-your-people.html

Jato , The First Look

2008-04-08T16:36:00.006+06:00

Usually looking into any OpenSource project is rather intimidating. But ,Jato, at the first look, did not seem that scary to me. The code base is not that large and I was somehow familiar with the field (at least I thought that !). And particularly the project owner was very nice to work with.

So , armed with git , I downloaded the source code , and started to hack on it (Who wants to go to the beach in a sunny day , when you can code ;) ) So far I have submitted patch for anewarray .

Initially ,it took times to grasp the idea . But thanks to Pekka Enberg , with his help , the ride so far is fun.

Setting Java plugin for Firefox

2008-02-14T17:57:00.004+06:00

If you have JDK or JRE installed, enabling java in Firefox is easy.
Just make a symbolic link of libjavaplugin_oji.so resided in your JRE to your Firefox-Plugin directory.

In short ,
ln -s $JAVA_HOME/jre/plugin/i386/ns7-gcc29/libjavaplugin_oji.so /usr/lib/firefox-2.0.0.3/plugins/

will enable java in your Firefox.

Setting Java Path

2008-02-13T13:42:00.002+06:00

To set Java path for all user, it is needed to edit /etc/profile

Assuming that you have installed Java at /usr/java/jdk1.6.0_02
append the following lines at /etc/profile

export JAVA_HOME=/usr/java/jdk1.6.0_02
export PATH=$JAVA_HOME/bin:$PATH
export CLASSPATH=$JAVA_HOME/lib

You can also edit .bashrc to provide user specific Java path setup.

J2ME

2008-01-23T23:49:00.001+06:00

We've released new LivTv version last week. Encountering J2ME technology for the first time was fun ( I mean it ). But device dependency was the real pain in the ass.Here are some learned-in-hard-way facts for the newbies in the brave new world of J2ME.

Never ever believe your emulator . Build incrementally and run the program in the widest number of devices possible.

If any program runs smoothly in other devices but seems to hang in Nokia N70, check if there is any UnsupportedOperationException(default action in newly created functions in NetBeans) anywhere . NokiaN70 stalls at the very sight of UnsupportedOperationException even if it has not been thrown during runtime.

Do not calculate the height or width of the canvas in the constructor of any class derived from abstract Canvas class. Some phones returns wrong result when Canvas.getHeight() or Canvas.getWidth() is called in the constructor .

Never ever draw anything using absolute pixel distance . It is destined to make a mess for phones having different resolutions. Instead make it as general as possible . For example if you are drawing a string surrounded by a rectangle , then use stringWidth() or getHeight() methods declared in Font class to calculate the height and width of surrounding rectangle. Remember the first law, Do not believe the emulator.

Trying to access camera in a Samsung device by using J2ME API may turn out as a waste of time.

Some Samsung devices place icons and other informations at the top of the canvas. So dont just start to draw from (0,0), make it dynamical too.

J2ME threads are somehow different than Java threads. In J2ME , one thread can not stop, pause or resume another thread, keeping that in mind during design may save from lot of head scratching later.

Adding multiple commands in an alert may cause weird behavior in NokiaN90.

If URL contains "|" ( pipe symbol ) , NokiaN95 fails to open that Http Connection . One way around it is using Base64 encoder and decoder .

Hope, this helps.

কম bandwidth এর লাইনে yum ব্যবহার

2007-10-06T03:32:00.000+06:00

fedora থেকে software ইনস্টল অথবা আপডেট করার সবচেয়ে সহজ এবং নির্ভরযোগ্য উপায় yum ব্যবহার করা। কিন্তু bandwidth যথেষ্ট বেশি না হলে default কনফিগারেশনে yum ব্যবহার করা প্রায় অসম্ভব, ক্রমাগত 'timed out' মেসেজ এ জীবন জেরবার হয়ে যাবে আপনার।

তবে কনফিগারেশনে কিছু পরিবর্তন করলেই yum কম bandwidth এর লাইন থেকেও ব্যবহার করতে পারবেন।
প্রথমেই root user হয়ে আপনার প্রিয় text editor এ /etc/yum.conf ওপেন করুন।

retries বাড়িয়ে দিন। যেমন,retries=10

timeout বাড়িয়ে দিন।যেমন,timeout=300

সেভ করুন।

লিনাক্সে মোবাইল থেকে ইন্টারনেট

2007-10-04T22:45:00.000+06:00

[This Post contains Bangla text, If you see garbled characters, please, set up Bangla Unicode support]

pppd command ব্যবহার করে লিনাক্স থেকে মোবাইলের মাধ্যমে ইন্টারনেটে সংযুক্ত হওয়া যায়।
কিন্তু, pppd configure করা একটু ঝামেলার এবং কোনো গ্রাফিকাল ইন্টারফেস না থাকায়
সাধারণ ব্যবহারকারীরা মনে হয় KPPP তেই সাচ্ছন্দ্যবোধ করবেন।

এর জন্য আপনাকে জানতে হবে আপনার মোবাইলটি কোন সিরিয়াল ডিভাইসের সাথে সংযুক্ত হয়েছে ,
যা নির্ভর করবে আপনার মোবাইল ও pc এর সাথে connection এর উপর।

সাধারণত

ইনফ্রারেড (IrDA) ডিভাইসের জন্য /dev/ircomm0

ব্লুটুথ ডিভাইসের জন্য /dev/ttyUB0

USB ডিভাইসের জন্য /dev/ttyUSB0

USB সিরিয়াল ডিভাইসের জন্য /dev/ttyACM0

USB cable দিয়ে সংযুক্ত হলে সাধারণত /dev/ttyACM0 ব্যবহৃত হয়।
তবে নিশ্চিত হওয়ার জন্য মোবাইলটিকে কম্পিউটারের সাথে লাগিয়ে ,root user হয়ে টার্মিনালে
dmesg|tail|grep tty কমান্ডটি লিখে দেখুন।

এবার টার্মিনালে গিয়ে
kppp &
লিখে অথবা প্যানেল এর মেনুতে গিয়ে kppp চালু করুন।

প্রথমে kppp কনফিগার করে নিতে হবে। এর জন্য configure এ ক্লিক করুন,
তারপর account ট্যাব এ গিয়ে new তে ক্লিক করুন,
Wizard নাকি Manual Setup এর Option এ Manual Setup সিলেক্ট করুন।

Dial ট্যাবে গিয়ে Connection Name দিন (যা ইচছা :D )

Phone Number এ add করুন *99***1#( আপনার নেটওয়ার্ক অপারেটর এর ডায়াল নাম্বার )

Authentication এ গিয়ে Script-based সিলেক্ট করুন।

ok করুন

এবার Modem ট্যাব এ যান, new তে ক্লিক করুন এবং Device ট্যাব এ যান,

Modem Name দিন (যা খুশি :D)

Modem device এ গিয়ে মোবাইলটি যে সিরিয়াল ডিভাইস এর সাথে সংযুক্ত হয়েছে সেটি সিলেক্ট করুন।

Connection Speed সিলেক্ট করুন

এবার Modem tab এ গিয়ে Query Modem এ ক্লিক করুন
এখানে যদি কোনে সমস্যা হয়, তাহলে Modem device এ ঠিক ডিভাইস সিলেক্ট করা আছে কিনা দেখুন।

এবার Modem Commands এ যান,

Dial String এ ATDT থাকলে ATD করুন।

GP connection যারা ব্যবহার করছেন তারা Initialization String 2: তে AT+CGDCONT=1,"IP","gpinternet","",0,0 লিখুন। অন্যান্য অপারেটরদের ক্ষেত্রে কিছু করতে হবে না।

ok করুন।

এবার Misc ট্যাবে গিয়ে Dock into Panel On Connect সিলেক্ট করুন ।
(না করলেও চলে ,কিন্তু আমার প্রিয় Option এটা ;) )
ok করুন।

Connect এ ক্লিক করুন। ব্যস ,আপনি কানেক্টেড :D