Connection Pool implementation in java

Question

I am implementing a conenction pool (JDBC connections and SMPP connections).I know there are several well tested connection pools out there.But I just want to implement on my own. I am using it in multithreading environment.This is more of my personal interest.My implementation goes like this. I create a ConcurrentLinkedQueue and push the connections to the queue. Each time a thread requests a connection, connection is popped out from queue. Once job is done threads push connection back to queue. My connection poll implementation class is as shown below.

import java.util.concurrent.ConcurrentLinkedQueue;

import org.apache.log4j.Logger;
import org.jsmpp.bean.BindType;
import org.jsmpp.bean.NumberingPlanIndicator;
import org.jsmpp.bean.TypeOfNumber;
import org.jsmpp.session.BindParameter;
import org.jsmpp.session.SMPPSession;


public class SMPPConnectionPool {
    static ConcurrentLinkedQueue<SMPPSession> connectionPool = null;
    static Logger LOG = null;

    static 
    {
        LOG = LogManager.getLogger(SMPPConnectionPool.class);
        connectionPool= new ConcurrentLinkedQueue<SMPPSession>();
    }

    /* This method returns session from queue .If no sessions exist in the queue,then a new session will be created and returned.
     * This method use QueryGparams APi to read conenction related data from gparams
     * */
    public static SMPPSession getConenction()
    {
        SMPPSession session=connectionPool.poll();
        if(session!=null)
        {
            System.out.println("Thread "+Thread.currentThread().getName() +" got "+session.getSessionId());
            LOG.info("Thread "+Thread.currentThread().getName() +" got "+session.getSessionId());
            return session;
        }
        else
        {
            SMPPSession smppSession = new SMPPSession();
            try {
                String host = QueryGparams.getGparamAsString(NotificationConstants.SMSC_HOST);
                int port = QueryGparams.getGparamAsInteger(NotificationConstants.SMSC_PORT);
                String systemId = QueryGparams.getGparamAsString(NotificationConstants.SMSC_SYSTEM_ID);
                String password = QueryGparams.getGparamAsString(NotificationConstants.SMSC_PASSWORD);
                if(host == null || systemId == null || password == null || port == 0)
                {
                    String errorMessage = "Following parameters are null \n";
                    if(host == null) {
                        errorMessage = errorMessage + "host is null";
                    }
                    if(systemId == null) {
                        errorMessage = errorMessage + "systemId is null";
                    }
                    if(password == null) {
                        errorMessage = errorMessage + "password is null";
                    }
                    if(port == 0) { //TODO Need to change this if QueryGParams API will not return zero when port number is not specified 
                        errorMessage = errorMessage + "port is null";
                    }
                    throw new Exception(errorMessage);
                }
                smppSession
                .connectAndBind(host,port, new BindParameter(
                        BindType.BIND_TRX, systemId,
                        password, "",
                        TypeOfNumber.UNKNOWN,
                        NumberingPlanIndicator.UNKNOWN, null));
                LOG.info("Session has been created.Session id is "+smppSession.getSessionId());
            } catch (Exception e) {
                LOG.error(CommonUtilities.getExceptionString(e));
            }
            System.out.println("Thread "+Thread.currentThread().getName() +" got "+smppSession.getSessionId());
            LOG.info("Thread "+Thread.currentThread().getName() +" got "+smppSession.getSessionId());
            return smppSession;
        }
    }



    //This method pushes conenction back to queue ,to make it available for other threads. 
    public static void pushConenction(SMPPSession smppSession)
    {
        boolean isInserted=connectionPool.offer(smppSession);
        if(isInserted)
        {
            LOG.info("Pushed the conenction back to queue");
            System.out.println("Pushed the conenction back to queue");
        }
        else
        {
            LOG.info("Failed to push the session back to queue");
            System.out.println("Failed to push the session back to queue");
        }
    }
    public static void closeSessions()
    {
        while(connectionPool!=null && connectionPool.size()>0)
        {
            SMPPSession session=connectionPool.poll();
            session.unbindAndClose();
            LOG.info("Closed the session");
        }
    }

}

I just want to know the problems with this implementation.Please advice.I want do the same for JDBC connection pool implementation

Answer 1

1) boolean isInserted=connectionPool.offer(smppSession); and subsequent analisys does not make sense since ConcurrentLinkedQueue is unbounded and its offer() always returns true. See API

2) I would use a BlockingQueue and make threads to wait for an active thread to return a connection to pool if max active threds is reached

3) I would initialize static fields this way

static ConcurrentLinkedQueue<SMPPSession> connectionPool = new ConcurrentLinkedQueue<SMPPSession>();
static Logger LOG = LogManager.getLogger(SMPPConnectionPool.class);

4) To avoid unnecessary string concatination you could use this idiom

if (LOG.isInfoEnabled()) {
   LOG.info("Thread "+Thread.currentThread().getName() +" got "+smppSession.getSessionId());
}

5) this seems suspicious

} catch (Exception e) {
    LOG.error(CommonUtilities.getExceptionString(e));
}
System.out.println("Thread "+Thread.currentThread().getName() +" got "+smppSession.getSessionId());

you catch exception and continue as if all is OK