File Writing with JSP

Question

Hi all,
I'm creating a simple app that gets form data and populates it to a file. I want to make sure that each instance (thread?) is able to write to the file and not throw an exception since the file may be currently open by another instance of the jsp. Now I did basic thread programming in C++ a long time ago, so I'm aware of thread waiting conceptually, but I'm not familiar with the java implementation. Can someone show me some basic syntax on how I would go in essentially putting a lock on the file writing portion of the code.
Thanx much

Answer 1

I disagree that you should override anything in Threa, especiially the sleep method.
What you should do is handle the file writing in a producer/consumer fashion. Have a single class that extends Runnable be the consumer. It handles all the file writing issues. You will run this in its own thread.
Your JSPs (or other threads) fill in a collection, or some other holder, which informs the consumer to write to the file.
As a brief example, this is a mini logger type of program. I use a class (LogCenter) to log data put in it from other sources (client1 and client2). For the sake of using newer API, I make use of the java.util.concurrent.BlockingQueue (and java.util.concurrent.LinkedBlockingQueue) to make a thread-safe collection and reporting system for text to come in and out of the logger.
Note, this is far from production. Just something I whipped up when playing with blocking queues a while ago...
package net.thelukes.steven.thread.test;
import java.io.PrintWriter;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.LinkedBlockingQueue;
public class LoggingCenter implements Runnable {
  private PrintWriter output;
  private BlockingQueue<String> toPrint;
  public LoggingCenter() {
    toPrint = new LinkedBlockingQueue<String>(10);
  public void setOutput(PrintWriter pw) {
    if (pw == null)  {
      if (output != null)
        return; //do not replace output with null
      else //pw is null and output is null
        throw new IllegalArgumentException("Ouput PrintWriter must not be NULL");
    else {
      if (output != null) closeOutput(); //if output exists already, close it.
      output = pw;
  public void log(String text) {
    boolean added = false;
    while (!added)
      try {
        toPrint.put(text);
        added=true;
      } catch (InterruptedException ie) {
        ie.printStackTrace();
        try { Thread.sleep(300L); }
        catch (InterruptedException ie2) { ie2.printStackTrace();}
  public void run() {
    try {
      while (true) {
        printLn(toPrint.take());
    } catch (InterruptedException ie) {ie.printStackTrace();}
  private void closeOutput() {
    output.flush();
    output.close();
  private void printLn(String text) {
    if (output == null)
      throw new IllegalStateException
        ("The Output PrintWriter must be set before any output can occur.");
    output.println(text);
    output.flush();
package net.thelukes.steven.thread.test;
import java.io.File;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
public class LoggingTest {
  public static void main(String[] args) throws IOException {
    PrintWriter output = new PrintWriter(
      new FileWriter(new File("log.txt")));
    //PrintWriter output = new PrintWriter(System.out, true);
    final LoggingCenter logger = new LoggingCenter();
    logger.setOutput(output);
    Thread t = new Thread(logger);
    t.start();
    Thread client1 = new Thread(
        new Runnable() {
          public void run() {
            while (true) {
              logger.log("Client 1: "+System.currentTimeMillis());
              try {
                Thread.sleep(1250L);
              } catch (InterruptedException e) {}
    Thread client2 = new Thread(
        new Runnable() {
          public void run() {
            while (true) {
              logger.log("Client 2: "+System.currentTimeMillis());
              try {
                Thread.sleep(2500L);
              } catch (InterruptedException e) {}
    client1.start();
    client2.start();
}