How can I keep Oracle SQL Developer from closing the DB connection?

Here's another Keep Connection Active extension that might be of use. (The extension mentioned above contains a number of issues that are resolved in this extn.)

http://sites.google.com/site/keepconnext/

Also sounds like a firewall problem to me.

You may have some luck with setting EXPIRE_TIME parameter in the server's SQLNET.ORA file. From the documentation:

Use parameter SQLNET.EXPIRE_TIME to specify a the time interval, in minutes, to send a probe to verify that client/server connections are active. Setting a value greater than 0 ensures that connections are not left open indefinitely, due to an abnormal client termination. If the probe finds a terminated connection, or a connection that is no longer in use, it returns an error, causing the server process to exit. This parameter is primarily intended for the database server, which typically handles multiple connections at any one time.

10g Documentation on EXPIRE_TIME

Answer

It's most likely a firewall between SQL Developer and the database that breaks things. You can solve that from SQL Developer using the SQL Developer Keepalive plugin.

You can also fix this from the Database Server by using the answers by Thomas and David Mann.

Oracle Net can be configured with Dead Connection Detection (SQLNET.EXPIRE_TIME) to workaround this problem. Set EXPIRE_TIME on the database server to a value less than the firewall connection timeout so that DCD keeps the connection to the database alive. See Note 151972.1 "Dead Connection Detection (DCD) Explained"

Original answer

I don't have the answer for this, but I'm experiencing the same problem.

The firewall between my SQL Developer and the database automaticly closes "inactive" sessions. A long running query is according to the firewall an inactive session, so he closes it. I've not, yet, found how to make SQL Developer send packets over a connection with a long running query, so that the firewall doesn't close the connection. And I don't know if this is possible at all.

So I don't think it is a SQL Developer problem, but a firewall issue.

-- UPDATE

There is an extension for SQL Developer that allows you to keep the connections active: http://sites.google.com/site/keepaliveext/

It's not totally finished yet (for example the notification you get states the same timeout no matter what timeout you have specified) but it does the trick. I've not, yet, tested it against the latest SQL Developer pre-release, but it worked with SQL Developer 2.2.x

-- UPDATE

For SQL Developer 4+ you can use: http://scristalli.github.io/SQL-Developer-4-keepalive/