Can't access ejb with more than 1 person at the same time?

Question

I am using the ejb to access the database using DBConnection Pool that I find in book. The code is as follow:
The problem is when more than one user access almost at the same time, the one who enter frst can access, but the other one can't and return context lookup error in creating EJB.
What's the problem of it, is it the ejb concurrent access problem or database concurrent access problem?
package login.database;
import java.io.*;
import java.sql.*;
import java.util.*;
import java.util.Date;
public class DBConnectionManager {
static private DBConnectionManager instance;
static private int clients;
private Vector drivers = new Vector();
private PrintWriter log;
private Hashtable pools = new Hashtable();
static synchronized public DBConnectionManager getInstance() {
if (instance == null) {
instance = new DBConnectionManager();
clients++;
return instance;
private DBConnectionManager() {
init();
public void freeConnection(String name, Connection con) {
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null){
pool.freeConnection(con);
public Connection getConnection(String name) {
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null) {
return pool.getConnection();
return null;
public Connection getConnection(String name, long time) {
DBConnectionPool pool = (DBConnectionPool) pools.get(name);
if (pool != null) {
return pool.getConnection(time);
return null;
public synchronized void release() {
if (--clients != 0) {
return;
Enumeration allPools = pools.elements();
while (allPools.hasMoreElements()) {
DBConnectionPool pool = (DBConnectionPool) allPools.nextElement();
pool.release();
Enumeration allDrivers = drivers.elements();
while (allDrivers.hasMoreElements()) {
Driver driver = (Driver) allDrivers.nextElement();
try {
DriverManager.deregisterDriver(driver);
log("Deregistered JDBC driver " + driver.getClass().getName());
} catch (SQLException e) {
log(e, "Can't deregister JDBC driver: " + driver.getClass().getName());
private void createPools(Properties props) {
Enumeration propNames = props.propertyNames();
while (propNames.hasMoreElements()) {
String name = (String) propNames.nextElement();
if (name.endsWith(".url")) {
String poolName = name.substring(0, name.lastIndexOf("."));
String url = props.getProperty(poolName + ".url");
if (url == null) {
log("No URL specified for " + poolName);
continue;
String user = props.getProperty(poolName + ".user");
String password = props.getProperty(poolName + ".password");
String maxconn = props.getProperty(poolName + ".maxconn", "0");
int max;
try {
max = Integer.valueOf(maxconn).intValue();
} catch (NumberFormatException e) {
log("Invalid maxconn value " + maxconn + "for " + poolName);
max = 0;
DBConnectionPool pool = new DBConnectionPool(poolName, url, user, password, max);
pools.put(poolName, pool);
log("Initialized pool " + poolName);
private void init(){
InputStream is = getClass().getResourceAsStream("db.properties");
Properties dbProps = new Properties();
try {
dbProps.load(is);
} catch (Exception e) {
System.err.println("Can't read the properties file. " +
"Make sure db.properties is in the CLASSPATH");
return;
String logFile = dbProps.getProperty("logfile","DBConnectionManager.log");
try {
log = new PrintWriter(new FileWriter(logFile, true), true);
} catch (IOException e) {
System.err.println("Can't open the log file: " + logFile);
log = new PrintWriter(System.err);
loadDrivers(dbProps);
createPools(dbProps);
private void loadDrivers(Properties props) {
String driverClasses = props.getProperty("drivers");
StringTokenizer st = new StringTokenizer(driverClasses);
while (st.hasMoreElements()) {
String driverClassName = st.nextToken().trim();
try {
Driver driver = (Driver) Class.forName(driverClassName).newInstance();
DriverManager.registerDriver(driver);
drivers.addElement(driver);
log("Registered JDBC driver " + driverClassName);
} catch (Exception e) {
log("Can't register JDBC driver: " + driverClassName + ", Exception: " + e);
private void log(String msg) {
log.println(new Date() + ": " + msg);
private void log(Throwable e, String msg) {
log.println(new Date() + ": " + msg);
e.printStackTrace(log);
class DBConnectionPool {
private int checkedOut;
private Vector freeConnections = new Vector();
private int maxConn;
private String name;
private String password;
private String URL;
private String user;
public DBConnectionPool(String name, String URL, String user, String password, int maxConn) {
this.name = name;
this.URL = URL;
this.user = user;
this.password = password;
this.maxConn = maxConn;
public synchronized void freeConnection(Connection con) {
freeConnections.addElement(con);
checkedOut--;
notifyAll();
public synchronized Connection getConnection() {
Connection con = null;
if (freeConnections.size() > 0) {
con = (Connection) freeConnections.firstElement();
freeConnections.removeElementAt(0);
try {
if (con.isClosed()) {
log("Removed bad connection from " + name);
con = getConnection();
} catch (SQLException e) {
log("Removed bad connection from " + name);
con = getConnection();
else if (maxConn == 0 || checkedOut < maxConn) {
con = newConnection();
if (con != null) {
checkedOut++;
return con;
public synchronized Connection getConnection(long timeout) {
long startTime = new Date().getTime();
Connection con;
while ((con = getConnection()) == null) {
try {
wait(timeout);
} catch (InterruptedException e) {}
if ((new Date().getTime() - startTime) >= timeout) {
return null;
return con;
public synchronized void release() {
Enumeration allConnections = freeConnections.elements();
while (allConnections.hasMoreElements()) {
Connection con = (Connection) allConnections.nextElement();
try {
con.close();
log("Closed connection for pool " + name);
} catch (SQLException e) {
log(e, "Can't close connection for pool " + name);
freeConnections.removeAllElements();
private Connection newConnection() {
Connection con = null;
try {
if (user == null) {
con = DriverManager.getConnection(URL);
else {
con = DriverManager.getConnection(URL, user, password);
log("Created a new connection in pool " + name);
} catch (SQLException e) {
log(e, "Can't create a new connection for " + URL);
return null;
return con;

Answer 1

Hi,
static synchronized public DBConnectionManager getInstance() {
The word "synchronized" will allow only one user at a time to access the getInstance() method which returns DBConnectionManager. Suggest u read some stuff on the same
As I am not sure which App server ur using, u cld do the following
1. Create a Conection Pool in ur App Server itself (Weblogic/Websphere/Jboss way)
2. Create a DataSource which maps to this Connection Pool.
3. Use the DataSource object which handles poolong of Connections to access the database
4. As mentioned above, Read notes/documents on "Synchronized" and "Vector" before you use this word in any code of EJB. This will solve ur problem. Read topic called "Collection Framework".
Seetesh