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Database Connection Pooling

As previously discussed, database connections should usually not be created within the servlet's service(), doGet() , or doPost() method. Creating a new database connection to service each new request greatly reduces servlet performance. Of course, this performance penalty can be avoided by defining the database Connection object as a servlet instance variable and establishing the connection from within the servlet's init() method. This approach was first demonstrated in Listing 16.2. Though it may work well for low-traffic Web sites, there is a major disadvantage to this approach. The drawback is that all clients must share a single database connection. For heavy-traffic sites, any number of requests may arrive concurrently. If the first request grabs the database connection, all other requests are queued and forced to wait until the connection is available. In addition, this approach may present thread safety concerns. Fortunately, connection pooling provides a solution to these problems.

NOTE

Support for connection pooling was added to JDBC 2.0 as part of the JDBC 2.0 Optional Package. This package is based on the JDBC 2.0 Standard Extension API specification. The Standard Extension API allows JDBC 2.0 driver vendors to implement connection pooling in a standard way such that code written to take advantage of connection pools is portable across JDBC drivers and databases. The JDBC 2.0 Standard Extension API is contained within the javax.sql package as opposed to the basic java.sql package. Connection pooling and other advanced functionality was added as a standard extension in order to keep the core JDBC API as small and simple as possible. Unlike JDBC 2.0, the javax.sql package is included in the basic JDBC3.0 distribution. Though available with many new drivers, connection pooling is demonstrated here for educational purposes (and is useful for older drivers).

Database connection pooling is the process of establishing a set, or pool, of database connections before they are needed. This pool of connections is often stored in a connection broker object that controls access to the connections. When a client requires a database connection, it simply asks the connection broker for an available (or free) connection. The broker selects a free connection from the pool, flags the connection as unavailable (or busy), and returns it to the client. Since it was created previously, the broker can return a connection almost instantly without the overhead required to establish a new connection. When the connection is no longer needed, the client returns it to the broker, who now flags it as available. In addition to handing out connections, the broker is responsible for all details regarding the maintenance of the connection pool. This may include occasionally refreshing database connections by dropping and recreating them or establishing new connections when the pool of available connections is exhausted by numerous concurrent requests. Listing 16.11 presents a connection broker object called ConnectionPool.

Listing 16.11 ConnectionPool manages a pool of database connections.

import java.sql.*; 
import java.util.*; 

/** 
 * ConnectionPool creates a pool of connections of the specified 
 * size to the specified database. The connection pool object 
 * allows the client to specify the JDBC driver, database, 
 * username, and password. In addition, the client can also 
 * specify the number of connections to create when this class is 
 * instantiated, the number of additional connections to create 
 * if all connections are exhausted, and the absolute maximum 
 * number of connections. 
 * 
 * @author Dustin R. Callaway 
 */ 
public class ConnectionPool 
{ 
private String jdbcDriver = ""; 
private String dbUrl = ""; 
private String dbUsername = ""; 
private String dbPassword = ""; 
private String testTable = ""; 
private int initialConnections = 10; 
private int incrementalConnections =5; 
private int maxConnections = 50; 
private Vector connections = null; 
/** 
 * Constructor stores the parameters passed by the calling 
 * object. 
 * 
 * @param jdbcDriver String containing the fully qualified name 
 * of the jdbc driver (class name and full package info) 
 * @param dbUrl String containing the database URL 
 * @param dbUsername String containing the username to use when 
 * logging into the database 
 * @param dbPassword String containing the password to use when 
 * logging into the database 
 */ 
public ConnectionPool(String jdbcDriver, String dbUrl, 
String dbUsername, String dbPassword) 
{ 
this.jdbcDriver = jdbcDriver; 
this.dbUrl = dbUrl; 
this.dbUsername = dbUsername; 
this.dbPassword = dbPassword;
} 
/** 
 * Returns the initial number of connections to create. 
 * 
 	 * @return Initial number of connections to create. 
  	 */ 
public int getInitialConnections() 
{ 
return initialConnections; 
} 

/** 
 	 * Sets the initial number of connections to create. 
 * 
 * @param initialConnections Initial number of connections to 
 * create 
 */ 
public void setInitialConnections(int initialConnections) 
{ 
this.initialConnections = initialConnections; 
} 

/**
 * Returns the number of incremental connections to create if 
 * the initial connections are all in use. 
 * 
 * @return Number of incremental connections to create. 
 */ 
public int getIncrementalConnections() 
{ 
return incrementalConnections; 
} 

/** 
 * Sets the number of incremental connections to create if 
 * the initial connections are all in use. 
 * 
 *@param incrementalConnections Number of incremental 
 * connections to create. 
 */ 
public void setIncrementalConnections( 
int incrementalConnections) 
{ 
this.incrementalConnections = incrementalConnections; 
} 

/** 
 * Returns the absolute maximum number of connections to 
 	 * create. If all connections are in use, the getConnection() 
 * method will block until one becomes free. 
 * 
 * @return Maximum number of connections to create. 
 */
public int getMaxConnections() 
{ 
return maxConnections; 
} 

/** 
 * Sets the absolute maximum number of connections to create. 
 * If all connections are in use, the getConnection() method 
 * will block until one becomes free. 
 * 
 * @param maxConnections Maximum number of connections to 
 * create. 
 */ 
public void setMaxConnections(int maxConnections) 
{ 
this.maxConnections = maxConnections; 
} 

/** 
 * Returns the name of the table that should be tested to 
 * insure that the database connection is still open. 
 * 
 * @return Name of the database table used to test the 
 * connection. 
 */ 
public String getTestTable() 
{ 
return testTable; 
} 

/**
 * Sets the name of the table that should be tested to insure 
 * that the database connection is still open. 
 * 
 * @param testTable Name of the database table used to test the 
 * connection. 
 */ 
public void setTestTable(String testTable) 
{ 
this.testTable = testTable;
} 

/** 
 * Creates a pool of connections. Number of connections is 
 * determined by the value of the initialConnections property. 
 */ 
public synchronized void createPool() throws Exception
{ 
//make sure that createPool hasn't already been called 
if (connections "` null) 
{ 
return; //the pool has already been created, return 
} 

//instantiate JDBC driver object from init param jdbcDriver 
Driver driver = (Driver) 
(Class.forName(jdbcDriver).newInstance()); 

DriverManager.registerDriver(driver); //register JDBC driver 

connections = new Vector(); 
  
//creates the proper number of initial connections 
createConnections(initialConnections); 
} 

/** 
 * Creates the specified number of connections, places them in
 * a PooledConnection object, and adds the PooledConnection to 
 * the connections vector. 
 * 
 * @param numConnections Number of connections to create. 
 */ 
private void createConnections(int numConnections) throws 
SQLException 
{ 
//create the specified number of connections 
for (int x=0; x < numConnections; x++) 
{ 
//have the maximum number of connections been created? 
//a maxConnections value of zero indicates no limit 
if (maxConnections > 0 && 
connections.size() >= maxConnections) 
{ 
break; //break out of loop because we're at the maximum 
} 

//add a new PooledConnection object to connections vector 
connections.addElement(new PooledConnection(
newConnection())); 

System.out.println("Database connection created..."); 
} 
} 

/** 
 * Creates a new database connection and returns it. 
 * 
 * @return New database connection. 
 */ 
private Connection newConnection() throws SQLException 
{ 
//create a new database connection 
Connection conn = DriverManager.getConnection (dbUrl, 
dbUsername, dbPassword); 

//if this is the first connection, check the maximum number 
//of connections supported by this database/driver 
if (connections.size()== 0) 
{ 
DatabaseMetaData metaData = conn.getMetaData(); 
int driverMaxConnections = metaData.getMaxConnections(); 

//driverMaxConnections value of zero indicates no maximum 
//or unknown maximum 
if (driverMaxConnections > 0 && 
maxConnections > driverMaxConnections) 
{ 
maxConnections = driverMaxConnections; 
} 
} 
return conn; //return the new connection 
} 
	
/** 
 * Attempts to retrieve a connection from the connections 
 * vector by calling getFreeConnection(). If no connection is
 * currently free, and more can not be created, getConnection() 
 * waits for a short amount of time and tries again. 
 * 
 * @return Connection object 
 */ 
public synchronized Connection getConnection()throws 
SQLException 
{ 
//make sure that createPool has been called 
if (connections == null) 
{ 
return null; //the pool has not been created 
} 

Connection conn = getFreeConnection();//get free connection 

while (conn == null) //no connection was currently free 
{ 

//sleep for a quarter of a second and then check to see if 
//a connection is free 
wait(250); 

 			conn = getFreeConnection(); //try again to get connection 
} 

return conn; 
} 

 	/** 
 * Returns a free connection from the connections vector. If no 
 * connection is available, a new batch of connections is 
 * created according to the value of the incrementalConnections 
 * variable. If all connections are still busy after creating 
 * incremental connections, the method will return null. 
 * 
 * @return Database connection object 
 */ 
private Connection getFreeConnection() throws SQLException 
{ 
//look for a free connection in the pool 
Connection conn = findFreeConnection(); 

if (conn == null) 
{ 
//no connection is free, create additional connections 
createConnections(incrementalConnections); 

//try again to find a free connection 
conn = findFreeConnection();

if (conn == null) 
{ 
//there are still no free connections, return null 
return null; 
} 
} 

 		return conn; 
} 

/** 
 * Searches through all of the pooled connections looking for 
 * a free connection. If a free connection is found, its 
 * integrity is verified and it is returned. If no free 
 * connection is found, null is returned. 
 * 
 * @return Database connection object. 
 */ 
private Connection findFreeConnection()throws SQLException 
{ 
Connection conn = null; 
PooledConnection pConn = null; 

Enumeration enum = connections.elements(); 

//iterate through the pooled connections looking for free one 
while (enum.hasMoreElements()) 
{ 
pConn =(PooledConnection)enum.nextElement(); 

if (!pConn.isBusy()) 
{
//this connection is not busy, get a handle to it 
conn = pConn.getConnection(); 

pConn.setBusy(true); //set connection to busy 

//test the connection to make sure it is still valid 
if	(!testConnection(conn)) 
{ 
//connection is no longer valid, create a new one 
conn = newConnection(); 

//replace invalid connection with new connection 
pConn.setConnection(conn); 
} 

break; //we found a free connection, stop looping 
} 
} 
  
return conn; 
} 
  
/** 

 * Test the connection to make sure it is still valid. If not, 
 * close it and return FALSE. 
 *
 * @param conn Database connection object to test. 
 * @return True indicates connection object is valid. 
 */ 
private boolean testConnection(Connection conn) 
{ 
try 
{ 
//determine if a test table has been designated 
if (testTable.equals("")) 
{ 
//There is no table to test the database connection so 
//try setting the auto commit property. This verifies 
//a valid connection on some databases. However, the 
//test table method is much more reliable. 
conn.setAutoCommit(true); 
} 
else 
{ 
//check if this connection is valid 
Statement stmt = conn.createStatement(); 
stmt.execute("select count(*) from " + testTable); 
} 
} 
catch(SQLException e) 
{ 
//connection is no longer valid, attempt to close it 
closeConnection(conn); 

return false; 
} 

return true; 
} 

 	/** 
 * Turns off the busy flag for the current pooled connection. 
 * All ConnectionPool clients should call returnConnection() as 
 * soon as possible following any database activity (within a 
 * finally block). 
 * 
 * @param conn Connection object 
 */ 
public void returnConnection(Connection conn)
{ 
//make sure that createPool has been called 
if (connections== null) 
{ 
return; //the pool has not been created 
} 

PooledConnection pConn = null; 

Enumeration enum = connections.elements(); 

//iterate through the pooled connections looking for the 
//returned connection 
while (enum.hasMoreElements()) 
{ 
pConn =(PooledConnection)enum.nextElement(); 
 
//determine if this pooled connection contains the returned 
//connection 
if (conn == pConn.getConnection()) 
{ 
//the connection has been returned, turn off busy flag 
pConn.setBusy(false); 

break; 
} 
}
} 

/** 
 * Refreshes all of the connections in the connection pool. 
 */ 
public synchronized void refreshConnections()throws 
SQLException 
{ 
//make sure that createPool has been called 
if (connections == null) 
{ 
return; //the pool has not been created 
} 

PooledConnection pConn = null; 

Enumeration enum = connections.elements(); 

while (enum.hasMoreElements()) 
{ 
pConn =(PooledConnection)enum.nextElement(); 

if (!pConn.isBusy()) 
{
wait(10000); //wait 5 seconds 
} 

closeConnection(pConn.getConnection()); 

 		pConn.setConnection(newConnection()); 

 		pConn.setBusy(false); 
} 
} 
/** 
 * Closes all of the connections and empties the connection 
 * pool. Once this method has been called, the createPool() 
 * method can again be called. 
 */ 
public synchronized void closeConnections() throws SQLException 
{ 
//make sure that createPool has been called 
if (connections == null) 
{ 
return; //the pool has not been created 
} 

PooledConnection pConn = null; 

Enumeration enum = connections.elements(); 

while (enum.hasMoreElements()) 
{ 
pConn = (PooledConnection)enum.nextElement(); 

if (!pConn.isBusy()) 
{ 
wait(5000); //wait 5 seconds 
} 
closeConnection(pConn.getConnection()); 

connections.removeElement(pConn); 
} 

connections = null; 
} 
  
/** 
 * Closes a database connection. 
 * 
 * @param conn Database connection to close. 
 */ 
private void closeConnection(Connection conn) 
{ 
try 
{ 
conn.close();
} 
catch (SQLException e) 
{ 
} 
} 
  
/** 
 * Sleeps for a specified number of milliseconds. 
 * 
 *@param mSeconds Number of seconds to sleep. 
 */ 
private void wait(int mSeconds) 
{ 
try 
{ 
Thread.sleep(mSeconds); 
} 
catch (InterruptedException e) 
{ 
} 
} 

/** 
 * Inner class encapsulating the properties of a pooled 
 * connection object. These properties include a JDBC database 
 * connection object and a flag indicating whether or not the 
 * database object is currently in use (busy). 
 */ 
class PooledConnection 
{ 
Connection connection = null; 
boolean busy = false; 

public PooledConnection(Connection connection) 
{ 
this.connection = connection; 
} 

public Connection getConnection() 
{ 
return connection; 
} 

  		public void setConnection(Connection connection) 
{ 
this.connection = connection; 
} 
  
public boolean isBusy() 
{ 
return busy; 
} 
  
public void setBusy(boolean busy) 
{ 
this.busy = busy; 
} 
} 
}

The ConnectionPool class is a typical connection broker that provides methods to retrieve available connections, return connections to the pool, refresh connections, and close connections. Listing 16.12 presents a version of Listing 16.10 that has been adapted to use the ConnectionPool class to manage a database connection pool. The output created by this servlet is identical to that shown in Figure 16.8.

Listing 16.12 Servlet that employs database connection pooling.

import javax.servlet.*; 
import javax.servlet.http.*; 
import java.io.*; 
import java.sql.*; 

/** 
 * EmployeeInfo returns employee data presented in an HTML table 
 * that is dynamically created with help from the 
 * ResultSetMetaData object. This servlet utilizes the
 * ConnectionPool class to manage a pool of database connections. 
 */
public class EmployeeInfo extends HttpServlet 
{ 
ConnectionPool connectionPool = null; 

/** 
 * Creates a connection pool. 
 */ 
public void init() throws ServletException 
{ 
String jdbcDriver = "org.gjt.mm.mysql.Driver"; 
String dbURL ="jdbc:mysql://localhost/phonebook"; 

try 
{ 
//instantiate the connection pool object by passing the 
//jdbc driver, database URL, username, and password 
connectionPool = new ConnectionPool(jdbcDriver, dbURL, 
"", ""); 
  
//specify the initial number of connections to establish 
connectionPool.setInitialConnections(5); 

//specify number of incremental connections to create if 
//pool is exhausted of available connections 
connectionPool.setIncrementalConnections(5); 

//specify absolute maximum number of connections to create 
connectionPool.setMaxConnections(20); 

//specify a database table that can be used to validate the 
//database connections (this is optional) 
connectionPool.setTestTable("EMPLOYEE"); 
connectionPool.createPool(); //create the connection pool 
} 
catch(Exception e) 
{ 
System.out.println("Error: " + e); 
} 
}

/** 
 * Dynamically constructs an HTML table from a ResultSet object 
 * containing employee information. 
 */ 
public void doGet(HttpServletRequest request, 
HttpServletResponse response) throws ServletException, 
IOException 
{ 
Connection dbConn = null; 
 
response.setContentType("text/html"); 
  
PrintWriter out = response.getWriter(); 

try 
{ 
//get free connection from pool 
dbConn = connectionPool.getConnection(); 

Statement stmt = dbConn.createStatement(); 

//create ResultSet containing employee information 
String sql = "select LAST_NAME, FIRST_NAME, PHONE, " + 
"EMAIL from EMPLOYEE order by LAST_NAME, FIRST_NAME"; 
ResultSet rs = stmt.executeQuery(sql); 

//get ResultSetMetaData object from ResultSet 
ResultSetMetaData rsMeta = rs.getMetaData(); 

//get number of columns in ResultSet 
int cols = rsMeta.getColumnCount(); 

out.println("<TABLE BORDER=\"1\">"); //begin dynamic table 

//create header row containing column titles 
out.println("<TR>"); 
for (int i = 1; i <= cols; i++) 
{ 
out.println("<TH>" + rsMeta.getColumnLabel(i) +"</TH>"); 
} 
out.println("</TR>"); 

//create a row for each row in result set 
while (rs.next()) 
{ 
out.println("<TR>"); 
for (int i = 1; i <= cols; i++) 
{ 
out.println("<TD>" + rs.getString(i) + "</TD>"); 
} 
out.println("</TR>"); 
} 

out.println("</TABLE>"); //end dynamic table 

out.close(); 
} 
catch (Exception e) 
{ 
System.out.println(e.getMessage()); 
} 
finally 
{ 
//return connection to pool (always within a finally block) 
connectionPool.returnConnection(dbConn); 
 	} 
 	} 
}

NOTE

It is imperative that you always return connections to the pool. Any connections that are not returned properly (using the ConnectionPool object's returnConnection() method) will become permanently unavailable. If connections are not returned, the entire pool can be quickly exhausted. In addition, connections should be returned as soon as possible to ensure that the maximum number of connections are available to service other requests. Finally, always return connections to the pool from within a finally block. This practice guarantees that all connections will be returned, even if an unexpected error occurs.

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