Servlets and sessions - good practices
Hello,
This is a newbie question.
The scenario:
Let's say I made a bank application that allows a user to log in and view 2 pages; the simple home page with just some static content and then a dynamic page that lists the last 10 transactions fetched from a database.
When the user goes to view the transactions, a servlet is invoked in which I create a connection object and query the db for the last 10 transactions, store the Collection in the session and then forward to displayTransactions.jsp.
The question:
Every time a user goes to view the last 10 transactions page, should I query the database each time or should I check to see if the list (Collection) was already loaded and present in the session, and if so, avoid the potentially expensive i/o operation of reading from a database and display the list from the session?
If the answer is to use the session, my follow up question is this: what if a transaction has been made since I queried the database and stored the results in the session. I wouldn't see the new transaction until I kill the session and query the database again right?
Best regards,
-b
If you want the last 10 transactions, you cannot (and should not) use the session. If you use the session the data can go stale, as you have figured out yourself. You need to fetch them on each request. Unless you have hundreds of visitors going to the page at the same time, it is far from expensive though, so why bother trying to optimize it? If your query is optimal (indexed, no queries that need full table scans, etc.), you shouldn't have any problems.
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Hey,
I'm running a poker script using applet/servlets and it works great. But for some reason about about 8 hours that database layer stops working. At first I thought it was the connections to mySQL that were timing out (because im using connection pooling) but after turning pooling off (I now create the connection each time) I'm still seeing that same error (I can create a connection but when I do an action ex. like a select statment I get an error). What i'm wondering could it be that the driver I load with Class.forName() some how unloads it's self after x amount of time not being used? Not sure if that is it but if anyone could give me some insight that would be great. The Error i recieve is below:
INFO: Database Event:DatabaseController: Error executing database query.
ERROR: Communications link failure due to underlying exception:
** BEGIN NESTED EXCEPTION **
java.net.SocketException
MESSAGE: Software caused connection abort: recv failed
STACKTRACE:
java.net.SocketException: Software caused connection abort: recv failed
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(Unknown Source)
at com.mysql.jdbc.util.ReadAheadInputStream.fill(ReadAheadInputStream.java:104)
at com.mysql.jdbc.util.ReadAheadInputStream.readFromUnderlyingStreamIfNecessary(ReadAheadInputStream.java:144)
at com.mysql.jdbc.util.ReadAheadInputStream.read(ReadAheadInputStream.java:172)
at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1839)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2288)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2784)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1531)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1622)
at com.mysql.jdbc.Connection.execSQL(Connection.java:2370)
at com.mysql.jdbc.Connection.execSQL(Connection.java:2297)
at com.mysql.jdbc.Statement.executeQuery(Statement.java:1183)
at com.softnet.database.DatabaseController.executeDatabaseQuery(DatabaseController.java:190)
at com.softnet.games.GameServer.validateUser(GameServer.java:438)
at com.softnet.games.GameServer.handleData(GameServer.java:113)
at com.softnet.network.HttpConnectionThread.run(HttpServletListener.java:191)
** END NESTED EXCEPTION **
I know the query is good because it works all other times just not after about 8 hours.
--Z3r0CooLHey,
Thanks for the responces. For the connection pooling I would open 5 connections and keep them open. So i though maybe after 8 hours after not being used they would timeout. Thats why i turned off conection pooling and create a new connection each time. Anyways i'll post the code below incase i made a mistake somewhere.
package com.softnet.database;
/************************ DatabaseControler **************************/
import java.sql.*;
import java.util.*;
import com.softnet.database.DatabaseConnectionPool;
import com.softnet.database.DatabaseSettings;
public class DatabaseController
implements DatabaseListener
//Used to make sure the database driver is loaded
private boolean databaseDriverState = false;
//Used to store a database connection
private Connection databaseConnection = null;
//If to user connection pooling or not
private boolean useConnectionPooling = false;
//Used to hold the connection pool varible
private DatabaseConnectionPool connectionPool = null;
//Used to store database settings
private DatabaseSettings databaseSettings;
//Used to hold the DatabaseController listeners
private List databaseControllerListeners = new ArrayList();
//min number of connections for connection pool
private int minNumberOfConnections = 1;
//max number of connections for connection pool -1 is unlimited
private int maxNumberOfConnections = -1;
//DatabaseController Constructors
public DatabaseController(DatabaseSettings databaseSettings)
this.databaseSettings = databaseSettings;
databaseDriverState = loadDatabaseDriver(databaseSettings.getDatabaseDriver());
public DatabaseController(DatabaseSettings databaseSettings, boolean useConnectionPooling, int minNumberOfConnections, int maxNumberOfConnections)
this.databaseSettings = databaseSettings;
this.useConnectionPooling = useConnectionPooling;
this.minNumberOfConnections = minNumberOfConnections;
this.maxNumberOfConnections = maxNumberOfConnections;
if(useConnectionPooling == true)
connectionPool = new DatabaseConnectionPool(databaseSettings, minNumberOfConnections, maxNumberOfConnections);
connectionPool.addDatabaseListener(this);
else
databaseDriverState = loadDatabaseDriver(databaseSettings.getDatabaseDriver());
public DatabaseController() {}
//Database Settings Get/Set
public DatabaseSettings getDatabaseSettings()
return databaseSettings;
public void setDatabaseSettings(DatabaseSettings databaseSettings)
this.databaseSettings = databaseSettings;
//Connection Pooling Get/Set
public boolean getConnectionPooling()
return useConnectionPooling;
public void setConnectionPooling(boolean useConnectionPooling, int minNumberOfConnections, int maxNumberOfConnections)
this.useConnectionPooling = useConnectionPooling;
this.minNumberOfConnections = minNumberOfConnections;
this.maxNumberOfConnections = maxNumberOfConnections;
if(useConnectionPooling == true)
if(connectionPool == null)
connectionPool = new DatabaseConnectionPool(databaseSettings, minNumberOfConnections, maxNumberOfConnections);
connectionPool.addDatabaseListener(this);
else
if(connectionPool != null)
connectionPool.destroyConnections();
connectionPool.removeDatabaseListener(this);
connectionPool = null;
//Return if there connected
public boolean isConnected()
boolean isConnected;
if(databaseConnection != null)
isConnected = true;
else
isConnected = false;
return isConnected;
//Used to connect to database or get a connection for the connection pool
public void connect()
if(databaseDriverState == false)
databaseDriverState = loadDatabaseDriver(databaseSettings.getDatabaseDriver());
//If we dont have a current connection, make one
if(databaseConnection == null && databaseDriverState == true)
if(useConnectionPooling == false)
try
databaseConnection = DriverManager.getConnection(databaseSettings.getDatabaseURL(), databaseSettings.getUserName(), databaseSettings.getUserPassword());
catch (SQLException sqle)
//Raise event
raiseDatabaseEvent("DatabaseController: Error connecting to database. \nERROR: " + sqle.getMessage());
databaseConnection = null;
else
databaseConnection = connectionPool.getConnection();
//Used to disconnect from the database or give back the connection to the connection pool
public void disconnect()
if(databaseConnection != null)
if(useConnectionPooling == false)
try
//Close DB Connection
databaseConnection.close();
catch(SQLException ignore) {}
finally
databaseConnection = null;
else
connectionPool.returnConnection(databaseConnection);
databaseConnection = null;
public ResultSet executeDatabaseQuery(String sSQL)
ResultSet databaseResult = null;
if(databaseConnection != null)
try
Statement databaseStatement = databaseConnection.createStatement();
databaseResult = databaseStatement.executeQuery(sSQL);
catch(SQLException sqle)
//Raise event
raiseDatabaseEvent("DatabaseController: Error executing database query.\nSQL: " + sSQL + "\nERROR: " + sqle.getMessage());
return databaseResult;
public int executeDatabaseUpdate(String sSQL)
int rowsAffected = -1;
if(databaseConnection != null)
try
Statement databaseStatement = databaseConnection.createStatement();
rowsAffected = databaseStatement.executeUpdate(sSQL);
catch(SQLException sqle)
//Raise event
raiseDatabaseEvent("DatabaseController: Error executing database update.\nSQL: " + sSQL + "\nERROR: " + sqle.getMessage());
return rowsAffected;
//Used to load the Database Driver
private boolean loadDatabaseDriver(String databaseDriver)
boolean driverLoaded;
if(databaseDriver.equals("") == false)
try
//Load Database Driver
Class.forName(databaseDriver).newInstance();
driverLoaded = true;
catch (Exception e)
//Raise event
raiseDatabaseEvent("DatabaseController: Error loading database driver. \nERROR: " + e.getMessage());
driverLoaded = false;
else
driverLoaded = false;
return driverLoaded;
//Wrap the DatabaseConnectionPool Error to the DatabaseController
public void databaseEventOccurred(DatabaseEvent de)
raiseDatabaseEvent(de.getErrorMessage());
//Event Handling Code
//Used to add database listeners (Its sync'd so you can change the listeners when firing an event)
public synchronized void addDatabaseListener(DatabaseListener databaseControllerListener)
databaseControllerListeners.add(databaseControllerListener);
//Used to remove a listener from the list (Its sync'd so you can change the listeners when firing an event)
public synchronized void removeDatabaseListener(DatabaseListener databaseControllerListener)
databaseControllerListeners.remove(databaseControllerListener);
//Used to send the raise event to the listeners
private synchronized void raiseDatabaseEvent(String databaseError)
DatabaseEvent databaseEvent = new DatabaseEvent(this, databaseError);
Iterator listeners = databaseControllerListeners.iterator();
while(listeners.hasNext())
DatabaseListener listener = (DatabaseListener) listeners.next();
listener.databaseEventOccurred(databaseEvent);
/********************* DatabaseConnectionPool **************/
package com.softnet.database;
import java.io.*;
import java.sql.*;
import java.util.*;
import com.softnet.database.*;
import com.softnet.database.DatabaseSettings;
public class DatabaseConnectionPool
//min number of connections
private int minNumberOfConnections = 1;
//max number of connections -1 is unlimited
private int maxNumberOfConnections = -1;
//Store the connections
protected Hashtable databaseConnections = null;
//Database Info
protected DatabaseSettings databaseSettings;
//to hold Driver state
private boolean databaseDriverState = false;
//To hold connection checker
private DatabaseConnectionCheck connectionChecker = null;
//Used to hold the DatabaseConnectionPool listeners
private List databaseConnectionPoolListeners = new ArrayList();
public DatabaseConnectionPool(DatabaseSettings databaseSettings, int minNumberOfConnections, int maxNumberOfConnections)
this.databaseSettings = databaseSettings;
this.minNumberOfConnections = minNumberOfConnections;
this.maxNumberOfConnections = maxNumberOfConnections;
//Load Driver
databaseDriverState = loadDatabaseDriver(databaseSettings.getDatabaseDriver());
//Create connection
createConnections();
public DatabaseConnectionPool(int minNumberOfConnections, int maxNumberOfConnections)
this.minNumberOfConnections = minNumberOfConnections;
this.maxNumberOfConnections = maxNumberOfConnections;
//Database Settings Get/Set
public DatabaseSettings getDatabaseSettings()
return databaseSettings;
public void setDatabaseSettings(DatabaseSettings databaseSettings)
this.databaseSettings = databaseSettings;
//Driver State Get
public boolean getDatabaseDriverState()
return databaseDriverState;
public void createConnections()
if(databaseDriverState == false)
databaseDriverState = loadDatabaseDriver(databaseSettings.getDatabaseDriver());
//Create all connections and load the minimum in the Hashtable
if(databaseConnections == null)
if(databaseDriverState == true && minNumberOfConnections != 0)
databaseConnections = new Hashtable();
for(int i = 0; i < minNumberOfConnections; i++)
try
databaseConnections.put(DriverManager.getConnection(databaseSettings.getDatabaseURL(), databaseSettings.getUserName(), databaseSettings.getUserPassword()), Boolean.FALSE);
catch(SQLException sqle)
//Problem break loop and destroy any connections
destroyConnections();
//Raise event
raiseDatabaseEvent("DatabaseConnectionPool: Error creating database connections. \nERROR: " + sqle.getMessage());
break;
//If no connection check exists create one
if(connectionChecker == null)
connectionChecker = new DatabaseConnectionCheck(this);
connectionChecker.start();
public Connection getConnection()
Connection connection = null;
boolean errorWithConnection = false;
Enumeration connections = databaseConnections.keys();
synchronized (databaseConnections)
while(connections.hasMoreElements())
errorWithConnection = false;
connection = (Connection) connections.nextElement();
Boolean state = (Boolean) databaseConnections.get(connection);
//If connection is not used, use it.
if(state == Boolean.FALSE)
try
connection.setAutoCommit(true);
catch(SQLException e)
//Problem with connection remove connection and replace it
databaseConnections.remove(connection);
try
connection = DriverManager.getConnection(databaseSettings.getDatabaseURL(), databaseSettings.getUserName(), databaseSettings.getUserPassword());
catch(SQLException sqle)
errorWithConnection = true;
if(errorWithConnection == false)
// Update the Hashtable to show this one's taken
databaseConnections.put(connection, Boolean.TRUE);
// Return the connection
return connection;
//All connections being used check to max to see if we can make a new one
if(maxNumberOfConnections == -1 || maxNumberOfConnections > databaseConnections.size())
try
connection = DriverManager.getConnection(databaseSettings.getDatabaseURL(), databaseSettings.getUserName(), databaseSettings.getUserPassword());
catch(SQLException sqle)
errorWithConnection = true;
if(errorWithConnection == false)
databaseConnections.put(connection, Boolean.TRUE);
return connection;
//If not connections free and max connections reached wait for a free connection
return getConnection();
public void returnConnection(Connection connection)
boolean errorWithConnection = false;
//Make sure connection still works
try
connection.setAutoCommit(true);
catch(SQLException e)
//Problem with connection remove connection and replace it
databaseConnections.remove(connection);
try
connection = DriverManager.getConnection(databaseSettings.getDatabaseURL(), databaseSettings.getUserName(), databaseSettings.getUserPassword());
catch(SQLException sqle)
errorWithConnection = true;
if(errorWithConnection == false)
databaseConnections.put(connection, Boolean.FALSE);
public void destroyConnections()
Connection connection = null;
if(databaseConnections != null)
//Close all connections
Enumeration connections = databaseConnections.keys();
while (connections.hasMoreElements())
connection = (Connection) connections.nextElement();
try
connection.close();
catch(SQLException ignore) {}
//Free up hashtable
databaseConnections = null;
private boolean loadDatabaseDriver(String databaseDriver)
boolean driverLoaded;
if(databaseDriver.equals("") == false)
try
//Load Database Driver
Class.forName(databaseDriver);
driverLoaded = true;
catch (ClassNotFoundException cnfe)
//Raise event
raiseDatabaseEvent("DatabaseController: Error loading database driver. \nERROR: " + cnfe.getMessage());
driverLoaded = false;
else
driverLoaded = false;
return driverLoaded;
//Event Handling Code
//Used to add database listeners (Its sync'd so you can change the listeners when firing an event)
public synchronized void addDatabaseListener(DatabaseListener databaseConnectionPoolListener)
databaseConnectionPoolListeners.add(databaseConnectionPoolListener);
//Used to remove a listener from the list (Its sync'd so you can change the listeners when firing an event)
public synchronized void removeDatabaseListener(DatabaseListener databaseConnectionPoolListener)
databaseConnectionPoolListeners.remove(databaseConnectionPoolListener);
//Used to send the raise event to the listeners
private synchronized void raiseDatabaseEvent(String databaseError)
DatabaseEvent databaseEvent = new DatabaseEvent(this, databaseError);
Iterator listeners = databaseConnectionPoolListeners.iterator();
while(listeners.hasNext())
DatabaseListener listener = (DatabaseListener) listeners.next();
listener.databaseEventOccurred(databaseEvent);
class DatabaseConnectionCheck extends Thread
private DatabaseConnectionPool connectionPool;
DatabaseConnectionCheck(DatabaseConnectionPool connectionPool)
this.connectionPool = connectionPool;
public void run()
try
while(true)
//check threads every 30 seconds
this.sleep(300000);
if(connectionPool.databaseConnections != null)
Connection connection = null;
Enumeration connections = connectionPool.databaseConnections.keys();
synchronized (connectionPool.databaseConnections)
while(connections.hasMoreElements())
connection = (Connection) connections.nextElement();
Boolean state = (Boolean) connectionPool.databaseConnections.get(connection);
//If connection is not used, use it.
if(state == Boolean.FALSE)
try
connection.setAutoCommit(true);
catch(SQLException e)
//Problem with connection remove connection and replace it
connectionPool.databaseConnections.remove(connection);
try
connection = DriverManager.getConnection(connectionPool.databaseSettings.getDatabaseURL(), connectionPool.databaseSettings.getUserName(), connectionPool.databaseSettings.getUserPassword());
catch(SQLException sqle)
connection = null;
// Update the Hashtable with new connection if its not null
if(connection != null)
connectionPool.databaseConnections.put(connection, Boolean.FALSE);
catch(InterruptedException ignored) {}
Basicly the why it works is the connection pool hold the database connections. When the user needs a connection they use the database controller to request a connection (By create a instance and called the connect() method) and the connection is either created or grabed from the connection pool. After the user is done with the connection they call the disconnect() method which closes the connection or returns it to the connection pool.
--Z3r0CooL
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