This deals with try/catch blocks and finallys. I always believed that "no
matter what" a finally block is always called. While this seems to remain
true, it does not seem to necessarily properly call the code within the
block. In the section below I have some code and explanation of how a
finally gets called, but does not execute the code within.
There seems to be an issue with closing a JDBC connection if a method
returns in the middle of the try/catch block. It seems to call the finally,
but does not close the connection. It does seem to call static methods like
System.out.println() perfectly fine. I don't know if it being static has
anything to do with it. I haven't researched it too far since it was really
hard to find. First off you have to have a Factory like a ConnectionPool
that spawns out inscances of an object for other classes to use. Then it
must need to have the user close it's connection at some point so the
Factory can control resources. The only example I really have on hand is a
JDBC connection pool in a J2EE app server. This is where the problem occurd
for us. If the methods "returns" are moved outside the try/catch block, the
Connection.close() happens properly and the ConnectionPool does not
complain.
I realize the getColumnClose() is the better way to implement the code
ayway, but I wanted to show everyone this problem in case they accidentally
do it in their code somewhere. Any returns in the try or catches seemed to
make the finally not be properly executed. I seem to remember some wierd
rule about this at JavaOne last year, but my head is still racing from all
that information. :-)
Any comments?
package test;
import java.sql.*;
import javax.sql.DataSource;
import javax.naming.*;
/**
* <p>Title: TryCatchTest</p>
* <p>Description: This is a test application for try/catch problem. If
using
* a Connection Pool in JDBC and you try to "return" a value within a
try/catch
* that is inside a method, the finally of that try/catch is not completely
run.
* It seems to go through the finally(I.E. Sytem.out.println() works, but
the
* Connection.close() is either never called or not executed. If the
"return"
* is moved to the outside of the catch block, the connection is properly
closed.
* <br>
* We found this because a J2EE server which had a connection pool was
telling
* us we were not closing our connections. I dn't know if there is some
rule that
* when a "return" has been called only static methods or something like
that can
* be called.</p>
* <p>Copyright: Copyright (c) 2002</p>
* <p>Company: Western Electronics </p>
* @author <a href="mail:[EMAIL PROTECTED]">Greg Davis</a>
* @version 1.0
*/
public class TryCatchTest
{
Context jndiContext;
public TryCatchTest()
{
try
{
jndiContext = new InitialContext();
}
catch(NamingException ne)
{}
}
public String getColumnNoClose()
{
Connection conn = null;
PreparedStatement ps = null;
ResultSet rs = null;
try
{
String sql ="SELECT table_name FROM sequence";
conn
=((DataSource)jndiContext.lookup("jdbc/mySourceDS")).getConnection();
ps = conn.prepareStatement(sql);
rs = ps.executeQuery();
if(rs != null && rs.next())
{
//This return causes the ?non-static? code within the finally to not
to be run.
return rs.getString("table_name");
}
//same here, the finally ?non-static? code is not called.
return null;
}
catch(NamingException ne)
{
//same here, the finally ?non-static? code is not called.
return null;
}
catch(SQLException sqle)
{
sqle.printStackTrace();
//The same problem occurs here. The finally ?non-static? code is not
called
return null;
}
finally
{
//This system out works, but the conn.close() does not. The only way
I
//know how to check this is to create a ConnectionPool yourself that
watches
//the connections it spawns out, or user a pre-built one that does the
same.
//We use Orion 1.5.4 J2EE App server (free for development). if you
want
//to test this out.
System.out.println("Finally called in noClose()");
try
{
if(rs != null)
rs.close();
if(ps != null)
ps.close();
if(conn != null)
conn.close();
}
catch(SQLException sqle)
{
sqle.printStackTrace();
}
}
}
public String getColumnClose()
{
String returnValue = null;
Connection conn = null;
PreparedStatement ps = null;
ResultSet rs = null;
try
{
String sql ="SELECT table_name FROM sequence";
conn
=((DataSource)jndiContext.lookup("jdbc/mySourceDS")).getConnection();
ps = conn.prepareStatement(sql);
rs = ps.executeQuery();
if(rs != null && rs.next())
{
//This does call the finally
returnValue = rs.getString("table_name");
}
}
catch(NamingException ne)
{}
catch(SQLException sqle)
{
//This allso calles the finally if I break the quey
sqle.printStackTrace();
}
finally
{
//This one works normally
System.out.println("Finally called in Close()");
try
{
if(rs != null)
rs.close();
if(ps != null)
ps.close();
if(conn != null)
conn.close();
}
catch(SQLException sqle)
{
sqle.printStackTrace();
}
}
return returnValue;
}
public static void main(String[] args)
{
TryCatchTest tct1 = new TryCatchTest();
System.out.println("tct1.getColumnNoClose():" +
tct1.getColumnNoClose());
System.out.println("tct1.getColumnClose():" +tct1.getColumnClose());
}
}
