Re: Java DAO pattern: singleton and threadsafe?

"" <>
Mon, 14 Apr 2008 07:15:58 -0700 (PDT)
On Apr 14, 1:35 pm, Lew <> wrote: wrote:

Hello all,

Here's my problem:
I'm working on a legacy J2EE application that uses the DAO pattern
and implements all DAO's [sic] as singletons. There's one abstract DAO

That the DAOs are singletons and that there is one abstract superclass are

But this class holds the connection to the database as an instance
Which, in my opinion, is not thread safe.

It's a lot easier to make an instance variable thread safe than a static
variable. The problem isn't that there's an instance variable, the problem is
that you're using a singleton. Stop using the class as a singleton.

If multiple clients would make a call simultaneously, the very same
would be used for the different calls, resulting in an exception on
the app server.

I think a different connection should be gotten from the datasource
pool on everycall? And that the datasource itself should be an
instance variable.

I suggest that you provide an SSCCE - a simple, self-contained compilable
example that illustrates the points you're making. All this vague hand-waving
doesn't admit of any useful comment. The example should elucidate the
"singleton-ness" of the approach.


Hey Lew,

Thanks for your comment. To provide you with some (simplified) code:

The superclass looks like this:
(this can be abstract, although subclasses are singletons ;) )


package nl.nedcar.apollo.server.dao;

import java.sql.Connection;
import java.sql.SQLException;
import java.sql.Statement;

import javax.naming.InitialContext;
import javax.naming.NamingException;
import javax.sql.DataSource;

public abstract class AbstractDAO {

    private DataSource dataSource;
    private Connection connection;
    private final static String JNDI_NAME = "yourJndiName";

    protected AbstractDAO() {
        InitialContext initial;
        try {
            initial = new InitialContext();
            DataSource dataSource = (DataSource) initial.lookup(JNDI_NAME);
            Logger.debug(this, "Obtained ref to DataSource " + JNDI_NAME);
            this.dataSource = dataSource;
        catch (NamingException e) {

    private Connection getConnection() throws SQLException {
        return dataSource.getConnection();

    protected void releaseConnection() throws SQLException {
        if(connection != null) {
        connection = null;

    protected Statement getStatement() throws SQLException {
        if(connection != null) {
            RuntimeException rt = new
RuntimeException(this.getClass().getName() + ": Creating new statement
while previous query was not " + "properly closed. \nClosing previous
            throw rt;
        connection = getConnection();
        return connection.createStatement();


The singletons subclasses look like:


package nl.nedcar.apollo.server.dao;

import java.sql.ResultSet;

public class FirstDAO extends AbstractDAO {

    private static FirstDAO theInstance;

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



calls are made like:

String somethingFromDb =

So, the problem occurs when multiple clients are performing a call at
the moment:
client1 holds the connection, while client2 is attempting to use the
very same connection.

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