<Context>
    <Resource
            name="jdbc/list"
            type="javax.sql.DataSource"
            maxTotal="12"
            maxIdle="8"
            maxWaitMillis="10000"
            username="abc"
            password="abc"
            driverClassName="org.gjt.mm.mysql.Driver"
            url="jdbc:mysql://localhost/list" />
</Context>

<resource-ref>
        <description>MySQL DB Connection Pool</description>
        <res-ref-name>jdbc/list</res-ref-name>
        <res-type>javax.sql.DataSource</res-type>
        <res-auth>Container</res-auth>
</resource-ref>

public class ConnectionPool {

    private static final String DATASOURCE_NAME = "jdbc/list";
    private static DataSource dataSource;
    static {
        try {
            Context initContext = new InitialContext();
            Context envContext = (Context) initContext.lookup("java:/comp/env");
            dataSource = (DataSource) envContext.lookup(DATASOURCE_NAME);
        } catch (NamingException e) {
            e.printStackTrace();
        }
    }
    private ConnectionPool() {}
    public static Connection getConnection() throws SQLException {
        Connection connection = dataSource.getConnection();
        return connection;
    }
}

@WebServlet("/main")
public class Tam extends HttpServlet {
    private static final String INSERT = "INSERT INTO users (name, password) VALUES (?, ?)";
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        try {
            Connection connection = ConnectionPool.getConnection();
            PreparedStatement preparedStatement = connection.prepareStatement(INSERT);
            preparedStatement.setString(1,"abc");
            preparedStatement.setInt(2,123);
            preparedStatement.executeUpdate();
            resp.sendRedirect("/main.jsp");
        } catch (SQLException e){
            System.out.println(e);
            RequestDispatcher requestDispatcher = req.getServletContext().getRequestDispatcher("/error.jsp");
            requestDispatcher.forward(req,resp);
        }
    }
}

Connection connection = dataSource.getConnection();

Type Exception Report

Message Servlet execution threw an exception

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

javax.servlet.ServletException: Servlet execution threw an exception
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Root Cause

java.lang.AbstractMethodError: com.mysql.jdbc.Connection.isValid(I)Z
    org.apache.tomcat.dbcp.dbcp2.DelegatingConnection.isValid(DelegatingConnection.java:874)
    org.apache.tomcat.dbcp.dbcp2.PoolableConnection.validate(PoolableConnection.java:270)
    org.apache.tomcat.dbcp.dbcp2.PoolableConnectionFactory.validateConnection(PoolableConnectionFactory.java:389)
    org.apache.tomcat.dbcp.dbcp2.BasicDataSource.validateConnectionFactory(BasicDataSource.java:2398)
    org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createPoolableConnectionFactory(BasicDataSource.java:2381)
    org.apache.tomcat.dbcp.dbcp2.BasicDataSource.createDataSource(BasicDataSource.java:2110)
    org.apache.tomcat.dbcp.dbcp2.BasicDataSource.getConnection(BasicDataSource.java:1563)
    by.gsu.epamlab.services.ConnectionPool.getConnection(ConnectionPool.java:26)
    Tam.doPost(Tam.java:20)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:660)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53)
Note The full stack trace of the root cause is available in the server logs.