Re: Java DAO pattern: singleton and threadsafe?

From:

Lew <lew@lewscanon.com>

Newsgroups:

comp.lang.java.programmer

Date:

Tue, 15 Apr 2008 08:32:08 -0400

Message-ID:

<bL2dnRe3jbPUPZnVnZ2dnUVZ_gadnZ2d@comcast.com>

koenxjans@gmail.com 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
users");

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(s.next()) {
                return s.getString("username");
            }
            return null;
        }
        finally {
            releaseConnection();
        }
    }

}

Yep. Designed to fail.

--
Lew