issues in threading

[mkarthik90]

i have to contact a class which uses thread to process . The java class mainly contacts the database and gets me the result set. Say i will be contacting the java class which uses a thread for twice and the main thread needs to print the result returned by the java class. The main thread is the servlet . How to do this ? If i try to make the servlet to wait Exception occurs and if i try to retrive the result set in the servlet i am can get only the empty result set.

I am using a array of resultset in the java class. Suggest me a way how to process this. I need to obtain the result set from the java class in the main Thread that is in the servlet once the 2 threads have completed its execution .

[Tolls]

We'd need to see some code otherwise we're guessing what's actually going on.

[mkarthik90]

I thought we cant stop the main thread so i passed the values to the parent thread which in turn will create two sub threads . And here is the required code.

The below is the servlet which calls the data_exe class which in turn creates a thread for executing the PreparedStatements simultaneously

package controller;

import java.io.IOException;
import java.sql.PreparedStatement;
import java.sql.SQLException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import java.sql.ResultSet;

import data_aggregator.data_exe;
import data_aggregator.data_thread;

/**
 * Servlet implementation class DataAggregators
 */
public class DataAggregators extends HttpServlet {
	private static final long serialVersionUID = 1L;
       
    /**
     * @see HttpServlet#HttpServlet()
     */
    public DataAggregators() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		
		HttpSession session = request.getSession();
		ResultSet[] rs = new ResultSet[4];
		Integer noofsub = (Integer)session.getAttribute("noofsub");
		PreparedStatement[] sub = (PreparedStatement [])session.getAttribute("subquery");
		data_exe dte = new data_exe(sub,noofsub);
		dte.start();
		rs = dte.getResult();
			for(int i=0;i<noofsub;i++)
			{
				try
				{
				while(rs[i].next())
				{
						System.out.println("Executing in ResultSet");
						System.out.println(rs[i].getString(1));
						System.out.println(rs[i].getString(2));					
			}
				}
				catch(Exception e)
				{
					e.printStackTrace();
				}
			}
	}

}

package data_aggregator;
	
import java.sql.PreparedStatement;
import java.sql.ResultSet;
public class data_exe extends Thread 
{

	PreparedStatement[] sub = new PreparedStatement[4];
	int noofsub;
	data_thread[] dt = new data_thread[4];
	ResultSet[] rs1 = new ResultSet[4];
	public data_exe(PreparedStatement[] ps,int i )
	{
		sub = ps ;
		noofsub = i;
	}
	public ResultSet[] getResult()
	{
		return rs1;
	}
	public void run()
	{
		for(int i=0;i<noofsub;i++)
		{
			dt[i] = new data_thread(sub[i],i);
			dt[i].start();
		}
		for(int i=0;i<noofsub;i++)
		{
				try
				{
					Thread.currentThread().wait();
					System.out.println(Thread.currentThread().getName());
				}
				catch(Exception e)
				{
					e.printStackTrace();
				}
			rs1[i] = data_thread.rs[i];
			try
			{
					while(rs1[i].next())
					{
						System.out.println("into data aggregator");		
						System.out.println(rs1[i].getString(0));
						System.out.println(rs1[i].getString(1));
						
					}				
			}
			catch(Exception e)
			{
				e.printStackTrace();
			}
		}
	
	}
}

package data_aggregator;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
public class data_thread extends Thread {

	private PreparedStatement[] ps = new PreparedStatement[4];
	int i=0;
	public static ResultSet[] rs = new ResultSet[4];
	
	public data_thread(PreparedStatement psi,int w) { 
	     ps[i] = psi;
	   } 

	public synchronized void run()
	{
		try
		{
		    rs[i] = ps[i].executeQuery();	
			while(rs[i].next())
				{
				System.out.println(rs[i].getString(1));
				System.out.println(rs[i].getString(2));
				}
		 Thread.currentThread ( ).notify();
		}
		catch(Exception e)
		{
			e.printStackTrace();
		}
		
	}


}

[Tolls]

You will, whatever happpens, need to wait for the data_exe (need to sort out your naming, byt the way) thread to finish before you can get the results.
Try doing this outside of a servlet, since that'll be a lot easier to test. Just a simple standalone app.

Also, if you get exceptions you need to give them here in full.

[mkarthik90]

But its a part of my project. Should i build it as a seperate app and contact it ? If so then also i need to use a servlet to contact it rite ? or is there any mechanism to call it ?

[Tolls]

No.
What I mean is write something that does this so you can get an idea of what needs doing in the servlet to get that working.
You don't need the overhead of a web server to try and do this threaded processing.
Once you have something that works then simply stick that code structure in your app.
Cut and paste...:)

[mkarthik90]

I will try .

[mkarthik90]

Atlast foound the solution. Use join method in thread to make the child threads execute one after the another and store the result in a vector. Once the child threads complete the execution retrive the resultset from the vector.
But i was able to find this one out as i first tried with a seperate app and then included in my project . Thanks Tolls for your advice.

[Tolls]

I do it that way all the time.
If i have a specific problem like this I always write a scratch program to do just the thing I'm trying to figure out.
XML parsing is a frequent one for me...:)

[mkarthik90]

oh great .. And thanks for helping me