Servlet I/O Stream problem: cannot write into file, error 500

Question

I am a new Java developer and now, I am facing a problem regarding the error 500. I try to grab records from a table with a date as a constraint. There are two servlets involed: a) saveRecord and b) getRecord. Servlet getRecord will grab data from a database while another servlet, saveRecord will get the data from servlet getRecord and save it into a file. The codes are as below:
=========================================================================================================================================
// Servlet Name : saveRecord
BufferedInputStream instr     = null;
BufferedOutputStream outstr     = null;
PostMethod http_post          = null;
http_post = new PostMethod("getRecord");
http_client.executeMethod(http_post);
instr     = new BufferedInputStream(http_post.getResponseBodyAsStream());
outstr     = new BufferedOutputStream(new FileOutputStream("c:\student.csv"), false));
int byte_at;
while (-1 != (byte_at = instr.read())) {
     outstr.write(byte_at);
=========================================================================================================================================
// Servlet Name : getRecord
ServletOutputStream out     = response.getOutputStream();
StringBuffer str_buf     = new StringBuffer();
str_buf = generateStudentAc();
out = response.getOutputStream();
FileStream(out, response, str_buf);
// Function Called
private StringBuffer generateStudentAc() {
     StringBuffer str_buf = new StringBuffer();
     String date = "2000-01-01";
     statement = connection.createStatement();
     resultset = statement.executeQuery("SELECT NAME, ADDRESS FROM STUDENT WHERE DATE>'" + date + "';
     // Write column headers               
str_buf.append("NAME,ADDRESS");
String data_row;
while (resultset.next()) {
          data_row = "\n";
          data_row += "\"" + resultset.getString("STUDENT") + "\"";
          data_row += ",\"" + resultset.getString("ADDRESS") + "\"";
          // Write row
          str_buf.append(data_row);
     return str_buf;
// Function called
private void FileStream(ServletOutputStream out, HttpServletResponse response, StringBuffer str_buf) {
     BufferedOutputStream buf_out = null;
// Create input/output streams
     buf_out     = new BufferedOutputStream(out);
     int length     = str_buf.length();
     // Read/Write
     for (int i = 0; i < length; i++) {
          buf_out.write(str_buf.charAt(i));
=========================================================================================================================================
Then, the result will be post back to a JSP page. It didn't work when the records in database growing bigger ( > 100 000 records) but works nice while the records are still in a small quantities ( < 20 000 records). It seems that no record can be inserted into the file and the JSP page becomes blank. When I opened the csv file, I found a message"Error 500:" inside it.
Hope you all experts can help to figure out this problem. TQ

Answer 1

I am amatuer in Java Programming, so I don't know how to implement serializable interface but I will look through it.
However, as I mention before, the code run perfectly if there are not much records un the table. But once there are more and more table added in (> 1000 000 records) in the table, it becomes unstable - sometime it works but most of the time it doesn't. So, I didn't find any different event I replaced the code with String date = "=2000-01-01".
Anyway, thanks for your help.