What is connection pooling in java ? explain with example ?

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Connection Pool Example

Code Snippet –

import java.sql.Connection;

import java.sql.DriverManager;

import java.sql.SQLException;

import java.util.concurrent.BlockingQueue;

import java.util.concurrent.LinkedBlockingQueue;

/**

 * @author Manoj

 */

public class ConnectionPool {

       private BlockingQueue<Connection> connectionPoolQueue;

       private int maximumPoolSize;

       private int initialPoolSize;

       private int currentPoolSize;

       private String dbUrl;

       private String dbUser;

       private String dbPassword;

       public ConnectionPool(int maxPoolSize, int initialPoolSize, String url, String username, String password,

                     String driverClassName) throws ClassNotFoundException, SQLException {

              if ((initialPoolSize > maxPoolSize) || initialPoolSize < 1 || maxPoolSize < 1) {

                     throw new IllegalArgumentException("Invalid pool size parameters");

              }

              this.maximumPoolSize = maxPoolSize > 0 ? maxPoolSize : 10;

              this.initialPoolSize = initialPoolSize;

              this.dbUrl = url;

              this.dbUser = username;

              this.dbPassword = password;

              this.connectionPoolQueue = new LinkedBlockingQueue<Connection>(maxPoolSize);

              initializeConnectionPool(driverClassName);

       }

       private void initializeConnectionPool(String driverClassName) throws ClassNotFoundException, SQLException {

              Class.forName(driverClassName);

              for (int i = 0; i < initialPoolSize; i++) {

                     openAndPoolConnection();

              }

       }

       private synchronized void openAndPoolConnection() throws SQLException {

              if (currentPoolSize == maximumPoolSize) {

                     return;

              }

              Connection conn = DriverManager.getConnection(dbUrl, dbUser, dbPassword);

              connectionPoolQueue.offer(new PooledConnection(conn, this));

              currentPoolSize++;

       }

       public Connection borrowConnection() throws InterruptedException, SQLException {

              if (currentPoolSize < maximumPoolSize) {

                     openAndPoolConnection();

              }

              return connectionPoolQueue.take();

       }

       public void surrenderConnection(Connection conn) {

              if (!(conn instanceof PooledConnection)) {

                     return;

              }

              connectionPoolQueue.offer(conn);

       }

}

In above code PooledConnection.java is an implementation of Connection interface

Code Snippet-

public class PooledConnection implements Connection {

    private Connection coreConnection;

    private ConnectionPool connectionPool;

    public PooledConnection(Connection coreConnection, ConnectionPool connectionPool) {

        this.connectionPool = connectionPool;

        this.coreConnection = coreConnection;

    }

    @Override

    public void close() throws SQLException {

        connectionPool.surrenderConnection(this);

    }

…….. all default method implementation of Connection interface…

Please refer below link for more details:
https://github.com/sharmaak/crashlabs/tree/master/MinimalConnectionPool/com/amitcodes/dbcp