Databases
StackOverflow2013
Postsdbo
POExec'ing multiple processes from Java: outputs mixed up?
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POExec'ing multiple processes from Java: outputs mixed up?
    primarykey
    data
    text
    <p>I have a servlet which creates a new Action object inside doGet(), and this object uses exec() to run external processes. There may be several requests to the servlet at the same time, so I might have several Action objects where each one is running an external process at the same time. Occasionally when this happens I find that the output from one process gets mixed up with the output from one of the others.</p> <p>Each Action creates a unique temporary directory and runs the process with this as its current directory. Separate threads then read the output streams from the process into a string buffer. The code that the Action object executes looks basically like this:</p> <pre><code>Process proc = null; ReaderThread stdout = null; ReaderThread stderr = null; StringBuffer buff = new StringBuffer(); int exit = -1; try { // // Run the process // proc = Runtime.getRuntime().exec(command,null,directory); // // Read the output from the process // stdout = new ReaderThread(proc.getInputStream(),buff); stderr = new ReaderThread(proc.getErrorStream(),buff); stdout.start(); stderr.start(); // // Get the exit code // exit = proc.waitFor(); // // Wait for all the output to be read // stdout.join(); stderr.join(); } catch (InterruptedException e) { if (proc != null) { proc.destroy(); } } catch (Exception e) { buff.append(e.getClass() + ": " + e.getMessage()); if (proc != null) { proc.destroy(); } } </code></pre> <p>So each request uses a separate Action object to run a process, and this has its own StringBuffer "buff" that the output of the process is accumulated into by the two ReaderThreads. But what I find is that, when two requests are running two processes at the same time, the output of one will sometimes end up in the StringBuffer of the thread that is running the other one, and one of the two servlet requests will see output intended for the other one. It basically behaves as if Runtime.exec() provides a single global pipe to which the output streams of all the processes are connected.</p> <p>The ReaderThread looks like this:</p> <pre><code>public class ReaderThread extends Thread { private BufferedReader reader; private StringBuffer buffer; public ReaderThread (InputStream stream, StringBuffer buffer) { this.reader = new BufferedReader(new InputStreamReader(stream)); this.buffer = buffer; } @Override public void run () { try { String line; while ((line = reader.readLine()) != null) { synchronized (buffer) { buffer.append(line + "\n"); } } } catch (IOException e) { synchronized (buffer) { buffer.append(e.getMessage() + "\n"); } } } } </code></pre> <p>Can anyone suggest what I can do to fix this?</p>
    singulars
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USuser1636349
    plurals
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. COplease show full Servlet and Action code. i think the mistake lies there, because these snippets are correct.
      singulars
      1. POExec'ing multiple processes from Java: outputs mixed up?
      1. USMichele Mariotti
    2. COIf you use ProcessBuilder to execute your command, you can make use of the [`ProcessBuilder.redirectErrorStream`](http://docs.oracle.com/javase/7/docs/api/java/lang/ProcessBuilder.html#redirectErrorStream%28boolean%29) method, which will allow you to simply your own code by having only one ReaderThread per process, plus you can remove all the synchronization from your ReaderThread class. Simplifying the code should make the problem easier to track down.
      singulars
      1. POExec'ing multiple processes from Java: outputs mixed up?
      1. USVGR
    3. COKind of hard, unless you want me to post about 35,000 lines of code! Trust me, the servlet creates a brand-new Action in doGet(), the Action reads a pile of XML from a database, and one of the XML nodes creates the process as above. If these snippets are correct, and I can't see anything wrong with them either, the only other explanation I can think of is a bug in Runtime.exec().
      singulars
      1. POExec'ing multiple processes from Java: outputs mixed up?
      1. USuser1636349
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload