ConnectionPool.java

package com.weebly.arjunvikramprogramming.jsp.arjmart;

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

import java.util.Iterator;
import java.util.Map.Entry;
import java.util.concurrent.ConcurrentHashMap;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;

@WebListener
public class ConnectionPool implements ServletContextListener{
    public static ServletContext servletcontext;
    public static ConcurrentHashMap<Connection,Boolean> connections;
    public static boolean init=false;
    private static final String dbURL="jdbc:mysql://localhost:3306/GroceryStore",
                                user="user",
                                password="pw",
                                driverClassName="com.mysql.jdbc.Driver";
    private static final int initialConnections=10,
                            increment=3,
                            connectionValidityTimeout=0,
                            retryCount=10;

    public static void init(){
        try{
            if(!init){
                init=true;
                Class.forName(driverClassName);
                loop:for(int i=0;i<initialConnections;i++){
                    Connection connection = DriverManager.getConnection(dbURL, user, password);
                    int j=retryCount;
                    while(!valid(connection)){
                        if(--j<=0){
                            log("Connection retry count reached during initialization");
                            continue loop;
                        }
                        log("Invalid connection during initialization - trying again");
                        connection = DriverManager.getConnection(dbURL, user, password);
                    }
                    connections.put(connection, false);
                }
            }
        }catch(ClassNotFoundException e){
            log("JDBC Driver could not be loaded:");
            e.printStackTrace(System.err);
        } catch (SQLException e) {
            log("Connection creation error during initialization:");
            e.printStackTrace(System.err);
        }
    }

    public static Connection getConnection(){
        synchronized(connections){
            Iterator<Entry<Connection,Boolean>> connectionIterator = connections.entrySet().iterator();
            while (connectionIterator.hasNext()) {
                Entry<Connection, Boolean> entry = connectionIterator.next();
                if(!entry.getValue()){
                    if(valid(entry.getKey())){
                        entry.setValue(true);
                        return entry.getKey();
                    }else{
                        log("Invalid connection - removing");
                        connectionIterator.remove();
                    }
                }
            }
            return repopulate();
        }

    }

    private static Connection repopulate() {
        try{
            loop:for(int i=0;i<increment-1;i++){
                Connection connection = DriverManager.getConnection(dbURL, user, password);
                int j=retryCount;
                while(!valid(connection)){
                    if(--j<=0){
                        log("Connection retry count reached during repopulation");
                        continue loop;
                    }
                    log("Invalid connection during repopulation - trying again");
                    connection = DriverManager.getConnection(dbURL, user, password);
                }
                connections.put(connection, false);
            }
            Connection connection = DriverManager.getConnection(dbURL, user, password);
            int j=retryCount;
            while(!valid(connection)){
                if(--j<=0){
                    log("Connection retry count reached during repopulation");
                }
                log("Invalid connection during repopulation - trying again");
                connection = DriverManager.getConnection(dbURL, user, password);
            }
            connections.put(connection, true);
            return connection;
        }catch (SQLException e) {
            log("Connection creation error while repopultating:");
            e.printStackTrace(System.err);
            return null;
        }
    }

    private static boolean valid(Connection key) {
        try {
            return key.isValid(connectionValidityTimeout);
        } catch (SQLException e) {
            return false;
        }
    }

    public static boolean returnConnection(Connection connection){
        return (connections.put(connection, false))!=null;
    }

    private static void log(String s){
        servletcontext.log(s);
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        sce.getServletContext().log("Removing all arjmart db connections - ServerletContextListener");
    }

    @Override
    public void contextInitialized(ServletContextEvent sce) {
        sce.getServletContext().log("Creating arjmart db connections - ServerletContextListener");
        servletcontext=sce.getServletContext();
        init();
        sce.getServletContext().log("Connections created successfully");
    }
}