Re: Java DAO pattern: singleton and threadsafe?

Lew <>
Tue, 15 Apr 2008 08:32:08 -0400
<> wrote:

package nl.nedcar.apollo.server.dao;

import java.sql.ResultSet;

public class FirstDAO extends AbstractDAO {

    private static FirstDAO theInstance;

Here's your trouble. You will also note than none of the methods or other
accesses to shared state are synchronized. This code was designed to fail.

     public static synchronized FirstDAO getInstance() {
        if(theInstance == null) {
            theInstance = new FirstDAO();
        return theInstance;

    public String getSomethingFromDatabase() throws Exception {
        try {
            ResultSet s = getStatement().executeQuery("select something from

Note that

[a] ResultSet object is automatically closed when the Statement object that
generated it is closed, re-executed, or used to retrieve the next result from
a sequence of multiple results.

if( {
                return s.getString("username");
            return null;
        finally {


Yep. Designed to fail.


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