Implementing Connection Pooling : Java

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In one of the interviews that I faced,I was asked to implement connection pooling. So approach was this:

  1. Create a List or HashMap
  2. Create predefined number of connections
  3. Add them to the collection.
  4. Now when the ConnectionImpl getConnection() method of ConnectionPoolingImpl class is invoked return a connection reference.

Now when someone returns the connection (releaseConnection(ConnectionImpl O)) how can I ensure that when the same application again tries to reuse the connection object, my implementation throws an exception?

The same connection object might have been returned to a new application and that should be able to use it.

My point of view would be to maintain a flag variable in another array kind of structure for each Connectionimpl object and set that variable to a valid value. When user returns the connection object I would make that some invalid value. For every operation in my ConnectionImpl, I will have to verify if the user had a valid flag.

What would you say to that approach?

pooling connection java
4 Answers

ConnectionPool implemenation

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.List;



/** A Connection Pool with 5 Available Connections **/
class ConnectionPool {

    private List<Connection>availableConnections = 
                                new ArrayList<Connection>();
    private List<Connection>usedConnections = new ArrayList<Connection>();
    private final int MAX_CONNECTIONS = 5;

    private String URL;
    private String USERID;
    private String PASSWORD;


    /** Initialize all 5 Connections and put them in the Pool **/
    public ConnectionPool(String Url, String UserId, String password)
            throws SQLException {
        this.URL = Url;
        this.USERID = UserId;
        this.PASSWORD = password;

        for (int count = 0; count <MAX_CONNECTIONS; count++) {
            availableConnections.add(this.createConnection());
        }

    }






/** Private function, 
    used by the Pool to create new connection internally **/

    private Connection createConnection() throws SQLException {
        return DriverManager
                .getConnection(this.URL, this.USERID, this.PASSWORD);
    }




    /** Public function, used by us to get connection from Pool **/
    public Connection getConnection() {
        if (availableConnections.size() == 0) {
            System.out.println("All connections are Used !!");
            return null;
        } else {
            Connection con = 
            availableConnections.remove(
                availableConnections.size() - 1);
            usedConnections.add(con);
            return con;
        }
    }



    /** Public function, to return connection back to the Pool **/
    public boolean releaseConnection(Connection con) {
        if (null != con) {
            usedConnections.remove(con);
            availableConnections.add(con);
            return true;
        }
        return false;
    }





    /** Utility function to check the number of Available Connections **/
    public int getFreeConnectionCount() {
        return availableConnections.size();
    }
}
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