Re: streaming problem and thread freeze

From:

Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 09 May 2011 20:08:08 +0200

Message-ID:

<iq9ai7$t2s$1@dont-email.me>

On 09/05/2011 16:56, Fly allegedly wrote:

Hello,
I have a client program that read an XML from a STREAM every 60 seconds.
From the main Thread I do an infinite loop where I create a new working thread
executing all the operations: read, parse, etc..

This is what I do:

<code>
  DownloaderTask tempTask = null;

         while (true) {

             if (tempTask != null) {
                 if (tempTask.isAlive()) {
                     tempTask.cancel();
                 }
             }

             tempTask = new DownloaderTask();

             tempTask.start();

             try {
                 sleep(frequency * 1000);
             } catch (InterruptedException ex) {
                 log.severe("Main sleep failed");
             }

         }

</code>

Inside the Run method of the DownloaderTask, I first read from the Stream (a class field) with a BufferedReader, in this way:

<code>
  private String readDocumentFromStream(BufferedReader reader) throws IOException {
             char[] buffer = new char[4 * 1024];
             int charsRead = -1;
             String retVal = "";
             log.finer("---START reading XML ---");

             while ((charsRead = reader.read(buffer, 0, 4 * 1024)) != -1)
             {
                 log.log(Level.FINER, "CharsRead: {0}", charsRead);
                 retVal += String.copyValueOf(buffer, 0, charsRead);
             }
             log.finer("---STOP reading XML---");

             return retVal;
         }
</code>

In the DownloaderTask class I wrote a cancel() method, called by the main thread in the case the working thread was still alive after the sleep().

It's definition:

<code>
    synchronized public void cancel() {

             this.interrupt();
             try {
                 if (rd != null) {
                     rd.close();
                 }
             } catch (IOException ex) {
                 log.severe(ex.getMessage());
             }

             http_conn.disconnect();

             log.log(Level.SEVERE, "Timeout. Thread {0} is being canceled and resource released.", this.getId());

         }
</code>

My problems.

Sometimes I see my program active, doing nothing, blocked probably in the read(), because if I check all logs, the last line written is "CharsRead: xxx" and no more.
Here I post one of my saved log (where the read buffer was 1024 bytes):

<LOG>
||-- 2011-05-03 06:44:59 FINE: Updating events data... --||
||-- 2011-05-03 06:44:59 FINE: Events updated! --||
||-- 2011-05-03 06:44:59 INFO: Release date: 2011-05-03 06:45:15 | Received: 187 Inserted: 0 Updated: 0 Deleted: 0 ||| Errors: 2 --||
||-- 2011-05-03 06:44:59 FINER: Normal closing thread 1.723 --||
||-- 2011-05-03 06:44:59 FINER: Thread 1.723 Closing --||
||-- 2011-05-03 06:45:52 FINER: Thread 1.725 Opening --||
||-- 2011-05-03 06:45:52 FINE: Opening server connection to: mysite.com:80 --||
||-- 2011-05-03 06:46:19 FINE: Reading From Stream --||
||-- 2011-05-03 06:46:19 FINER: ---START reading XML --- --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 929 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 1.024 --||
||-- 2011-05-03 06:46:19 FINER: CharsRead: 872 --||
</LOG>

I noticed another strange thing in this log, that is I can't see the Thread Id: 1724.
Can you help me to find where I am wrong? In the logic or maybe in the syntax of the program?

Thanks in advance,
Flavio

A few things:

Firstly, dunno if this will help the problem, but your main loop ought
to look something like this:

   DownloaderTask tempTask = null;
   for( Thread t = Thread.currentThread(); ! t.isInterrupted(); ){
     if (tempTask != null) {
       if (tempTask.isAlive()) {
         tempTask.cancel();
       }
     }

     tempTask = new DownloaderTask();
     tempTask.start();

     try {
       TimeUnit.SECONDS.sleep( frequency ); // ;)
     }
     catch (InterruptedException ex) {
       *t.interrupt();*
     }
   }

Always re-interrupt when you've caught an InterruptedException. The
reason is that catching that exception *clears* the interrupted status.

You possibly might as well try a join() on the worker thread, BTW.

And have a look at java.util.concurrent.Executors.

Secondly, although it is only a minor thing and only my personal
opinion, this:

   } catch (IOException ex) {
     log.severe(ex.getMessage());
   }

is a *very* bad idea. Log the full stack. The message is next to useless
as soon as the code becomes a little complex.

HTH,
--
DF.
An escaped convict once said to me:
"Alcatraz is the place to be"