Web应用程序的单个数据库连接

Question

I am developing a simple CRUD application, using JDBC to establish connection and perform basic CRUD operations. In that process, created a DatabaseListener to create a connection object at startup and storing it in the context attribute for reuse.

Below is the code.

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import org.apache.log4j.Logger;

public class DatabaseInitListner implements ServletContextListener {

    private static final Logger LOG = Logger.getLogger(DatabaseInitListner.class);

    private DBUtil databaseUtil = null;

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        databaseUtil.closeConnection();
    }

    @Override
    public void contextInitialized(ServletContextEvent contextinitEvent) {

        ServletContext servletContext = contextinitEvent.getServletContext();
        String database = servletContext.getInitParameter("db_name");
        String url = servletContext.getInitParameter("db_url")
                + database;
        String username = servletContext.getInitParameter("db_user");
        String password = servletContext.getInitParameter("db_password");
        String driverName = servletContext.getInitParameter("db_driver");

        databaseUtil = new DBUtil(url, username, password,
                driverName);
        servletContext.setAttribute("databaseSingleConnectionObject",
                databaseUtil.getConnection());
    }
}

public class DBUtil {
    private Connection connection = null;
    private static final Logger LOG = Logger.getLogger(DatabaseUtil.class);
    public DatabaseUtil(String url, String username, String password,
            String driver) {
        try {
            Class.forName(driver);
            this.connection = DriverManager.getConnection(url, username,
                    password);
            LOG.debug("Connection Established... ");
        } catch (ClassNotFoundException | SQLException e) {
            LOG.error("Could not create connection... ", e);
        }
    }
    public Connection getConnection() {
        return connection;
    }
    public void closeConnection() {
        if (connection != null) {
            try {
                connection.close();
            } catch (SQLException e) {
                LOG.error("Unable to close connection... ", e);
            }
        }
    }
}

I am accessing the connection in servlets like this

Connection jdbcConnection = (Connection) getServletContext().getAttribute("databaseSingleConnectionObject");

I am not sure if this is right approach. What are the effects of single database connection?

Answer 1

When you use a single database connection like this you make your application slow and brittle.

Slow: because the connection implementation is synchronized, each user has to wait until the connection is free. If one user's query takes a while to come back that directly increases the time any other concurrent users spend waiting. If there were multiple connections available from a pool then the time spent by one user would not impact other users nearly as greatly (unless a query's results take all the JVM's memory or a big query bogs down the database server).

Brittle: The connection is a network connection, they tend to go down. Without a provision to create new connections any kind of timeout, network hiccup, or period of database non-availability (such as taking the database offline for maintenance) is going to require an application restart. Using a connection pool will mean your application will be able to survive these episodes and recover without outside intervention.

Answer 2

This will not be threadsafe, and if it were, performance would be really poor. Look into using a Connection Pool, like DBCP or C3PO

Answer 3

You should let your application server manage database connection. Add a JNDI datasource in its configuration file and make a lookup from your application to get a connection when needed (for instance when you instantiate a class that must access your database). You may configure the datasource to manage a connection pool so that each user session will get its own. Depending on the AS you use run a search with keywords 'JNDI' and 'datasource' and you will get further details about the AS configuration and how to implement it in your application.