Why does calling Statement.close() not immediately release the ResultSet object created by a Statement object?

Tim :

From JDBC Spec

Closing a Statement object will close and invalidate any instances of ResultSet produced by that Statement object. The resources held by the ResultSet object may not be released until garbage collection runs again, so it is a good practice to explicitly close ResultSet objects when they are no longer needed.

• Does calling ResultSet.close() immediately release a ResultSet object?

• Does calling Statement.close() immediately release a Statement object?

• Calling Statement.close() releases the ResultSet objects created by a Statement object, so why does it not immediately release the ResultSet object?

Slaw :

From the Javadoc of Statement.close() (emphasis mine):

Releases this Statement object's database and JDBC resources immediately instead of waiting for this to happen when it is automatically closed. It is generally good practice to release resources as soon as you are finished with them to avoid tying up database resources.

Calling the method close on a Statement object that is already closed has no effect.

Note: When a Statement object is closed, its current ResultSet object, if one exists, is also closed.

Here, the Javadoc states that closing a Statement immediately releases its resources and also closes its current ResultSet.

And the Javadoc of ResultSet.close() (emphasis mine):

Releases this ResultSet object's database and JDBC resources immediately instead of waiting for this to happen when it is automatically closed.

The closing of a ResultSet object does not close the Blob, Clob or NClob objects created by the ResultSet. Blob, Clob or NClob objects remain valid for at least the duration of the transaction in which they are created, unless their free method is invoked.

When a ResultSet is closed, any ResultSetMetaData instances that were created by calling the getMetaData method remain accessible.

Note: A ResultSet object is automatically closed by the Statement object that generated it when that Statement object is closed, re-executed, or is used to retrieve the next result from a sequence of multiple results.

Calling the method close on a ResultSet object that is already closed is a no-op.

This Javadoc states that closing a ResultSet will release its resources immediately as well. And, as stated above, calling Statement.close() will also close the ResultSet.

The note also states that only one ResultSet can be open per Statement at any given time. The previous ResultSet will be closed automatically if you make another request (or if any of the other conditions, as stated, are met).

Note: All Javadoc quotes and links are for Java 10.