Single db connection that does not get initialised per task

问题

public class Parser {

    ExecutorService pool = Executors.newFixedThreadPool(10);

    public void update() {

        Item item = new Item(subj.getName(), dateBuilder.toString(),
                cobBuilder.toString(), interest, count);

        pool.submit(new ItemDispatcher(item));
    }



} 



public class ItemDispatcher implements Runnable {

    private Item item;

    public ItemDispatcher(Item someItem) {

        this.item = someItem;

    }


    @Override
    public void run() {

        try {
            new Database(item).writeToDb();
        } catch (SQLException e) {
            e.printStackTrace(); 

        }

    }

}


public class Database {

    private String name;
    private String date;
    private String cob;
    private String interest;
    private String count;

    private String _url;
    private String _userId;
    private String _password;
    private String _dbLib;
    private String _dbFile;
    private Connection _conn;
    private PreparedStatement _statement;

    public Database(Item item) {

        name = item.get_item();
        date = item.get_date();
        cob = item.get_cob();
        interest = item.get_interest();
        count = item.get_count();
    }


    public void writeToDb() {

        try {

            //statment.setString(); 
        }

        } catch (SQLException e) {
            e.printStackTrace();
        } 

}

I have three different classes in package: Parser, ItemDispatcher and Database. I am using a thread pool that reuses a fixed number of threads (10). I receive updates in callback update() method in class Parser. As updates come in I create Item objects which submits as a runnable tasks for execution. From ItemDispatcher I pass the item to the Database class. I've realised there is an issue. I need Database class to connect to database, but as Threads are calling Database constructor for every item, I don't want to create connection everytime, instead use one connection which gets initialised and never again. Not sure how to do this, can someone help please with some ideas and suggestions ? I dont want parser class to know about the Database class. Where can I add the connection and preparedStatement code ?

回答1:

Create a singleton DAO (data access object) class and let this class initialize the pool. If you're unsure about either term, just ask and I'll find some code for you.

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.SQLException;

public class ItemDAO {

    private static ItemDAO dao;
    private Connection conn;
    private PreparedStatement prepStat;
    private static final String SQL_INSERT_ITEM = "INSERT INTO tbl_item SET id=?`, name = ?, count = ?;";

    private ItemDAO() {
        conn = null; /* put initialization code for the connection here */
        try {
            prepStat = conn.prepareStatement(SQL_INSERT_ITEM); // prepare a statement
        } catch (SQLException e) {
            // What do you want to do if it fails?
            e.printStackTrace();
        }
    }

    public static ItemDAO getInstance() {
        if (dao == null)
            dao = new ItemDAO();
        return dao;
    }

    public void writeItemToDB(Item item) throws SQLException {
        prepStat.setInt(1, item.getId());
        prepStat.executeUpdate();
    }
}

This code is not thread safe but it will use the same connection in each thread. If you need thread safety (you do) you can use a ConnectionPool that gives you thread safety and set the number of threads to 1. "BoneCP" is the name of a well known connection pool.

来源：https://stackoverflow.com/questions/21428512/single-db-connection-that-does-not-get-initialised-per-task