Am I Using JDBC Connection Pooling?

As a follow up to BalusC's solution, below is an implementation that I can be used within an application that requires more than one connection, or in a common library that would not know the connection properties in advance...

import org.apache.commons.dbcp.BasicDataSource;

import java.sql.Connection;
import java.sql.SQLException;
import java.util.concurrent.ConcurrentHashMap;

public final class Database {

    private static final ConcurrentHashMap<String, BasicDataSource> dataSources = new ConcurrentHashMap();

    private Database() {
        //
    }

    public static Connection getConnection(String connectionString, String username, String password) throws SQLException {

        BasicDataSource dataSource;

        if (dataSources.containsKey(connectionString)) {
            dataSource = dataSources.get(connectionString);
        } else {
            dataSource = new BasicDataSource();
            dataSource.setDriverClassName("com.mysql.jdbc.Driver");
            dataSource.setUrl(connectionString);
            dataSource.setUsername(username);
            dataSource.setPassword(password);
            dataSources.put(connectionString, dataSource);
        }

        return dataSource.getConnection();

    }

}

Looks like a DBCP usage. If so, then yes. It's already pooled. And here is the default pool property value of the DBCP.

/**
* The default cap on the number of "sleeping" instances in the pool.
* @see #getMaxIdle
* @see #setMaxIdle
*/
public static final int DEFAULT_MAX_IDLE  = 8;
/**
* The default minimum number of "sleeping" instances in the pool
* before before the evictor thread (if active) spawns new objects.
* @see #getMinIdle
* @see #setMinIdle
*/
public static final int DEFAULT_MIN_IDLE = 0;
/**
* The default cap on the total number of active instances from the pool.
* @see #getMaxActive
*/
public static final int DEFAULT_MAX_ACTIVE  = 8;

Doesn't seem like it's pooled. You should store the DataSource in DatabaseConnection instead of creating a new one with each getConnection() call. getConnection() should return datasource.getConnection().

Assuming that it's the BasicDataSource is from DBCP, then yes, you are using a connection pool. However, you're recreating another connection pool on every connection acquirement. You are not really pooling connections from the same pool. You need to create the connection pool only once on application's startup and get every connection from it. You should also not hold the connection as an instance variable. You should also close the connection, statement and resultset to ensure that the resources are properly closed, also in case of exceptions. Java 7's try-with-resources statement is helpful in this, it will auto-close the resources when the try block is finished.

Here's a minor rewrite:

public final class Database {

    private static final BasicDataSource dataSource = new BasicDataSource();

    static {
        dataSource.setDriverClassName("com.mysql.jdbc.Driver");
        dataSource.setUrl("jdbc:mysql://localhost:3306/data");
        dataSource.setUsername("USERNAME");
        dataSource.setPassword("PASSWORD");
    }

    private Database() {
        //
    }

    public static Connection getConnection() throws SQLException {
        return dataSource.getConnection();
    }

}

(this can if necessary be refactored as an abstract factory to improve pluggability)

and

private static final String SQL_EXIST = "SELECT * FROM users WHERE username=? AND password=?";

public boolean exist(User user) throws SQLException {
    boolean exist = false;

    try (
        Connection connection = Database.getConnection();
        PreparedStatement statement = connection.prepareStatement(SQL_EXIST);
    ) {
        statement.setString(1, user.getUsername());
        statement.setString(2, user.getPassword());

        try (ResultSet resultSet = preparedStatement.executeQuery()) {
            exist = resultSet.next();
        }
    }       

    return exist;
}

which is to be used as follows:

try {
    if (!userDAO.exist(username, password)) {
        request.setAttribute("message", "Unknown login. Try again.");
        request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response);
    } else {
        request.getSession().setAttribute("user", username);
        response.sendRedirect("userhome");
    }
} catch (SQLException e) {
    throw new ServletException("DB error", e);
}

In a real Java EE environement you should however delegate the creation of the DataSource to the container / application server and obtain it from JNDI. In case of Tomcat, see also for example this document: http://tomcat.apache.org/tomcat-6.0-doc/jndi-resources-howto.html