Re: executeQuery() throws "Statement has been closed"

Re: executeQuery() throws "Statement has been closed"

Hannes Erven  wrote:

> sometimes my application (jdbc-1101, hibernate, desktop application)
> reports the following "Statement has been closed" error:

> ... I'm wondering why there is even checkClosed() called before
> returning the value of a field?
> Could that check be safely removed?

Not according to the API documentation:

http://docs.oracle.com/javase/7/docs/api/java/sql/Statement.html#getMaxRows%28%29

| Throws:
|     SQLException - if a database access error occurs or this method is called on a closed Statement

Besides, if a Statement has been closed, why would you expect it to 
still be usable?

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Re: executeQuery() throws "Statement has been closed"

Hannes Erven  wrote:

> As Kevin pointed out (thanks!), getMaxRows() is required by the
> spec to check whether the connection is still open.

It's more strict than that -- it is required to throw an exception
if the *Statement* has been closed.  Of course, closing a
Connection will automatically close all Statement objects
associated with that connection, but the Statement object can, and
often is, closed before the Connection.

So, something is closing the Statement (directly or indirectly) and
then there is an attempt to use it.  You need to figure out how
that happens.

--
Kevin Grittner
EDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

executeQuery() throws "Statement has been closed"

Hi everyone,


sometimes my application (jdbc-1101, hibernate, desktop application) 
reports the following "Statement has been closed" error:


Caused by: org.postgresql.util.PSQLException: Die Anweisung wurde 
geschlossen.
	at 
org.postgresql.jdbc2.AbstractJdbc2Statement.checkClosed(AbstractJdbc2Statement.java:2631)
	at 
org.postgresql.jdbc2.AbstractJdbc2Statement.getMaxRows(AbstractJdbc2Statement.java:659)
	at 
org.postgresql.jdbc4.Jdbc4Statement.createResultSet(Jdbc4Statement.java:34)
	at 
org.postgresql.jdbc2.AbstractJdbc2Statement$StatementResultHandler.handleResultRows(AbstractJdbc2Statement.java:219)
	at 
org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1839)
	at 
org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:255)
	at 
org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:559)
	at 
org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:417)
	at 
org.postgresql.jdbc2.AbstractJdbc2Statement.executeQuery(AbstractJdbc2Statement.java:302)
	at 
org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.extract(ResultSetReturnImpl.java:80)
	... 19 more



When looking at the code of getMaxRows() 
(https://github.com/pgjdbc/pgjdbc/blob/master/org/postgresql/jdbc2/AbstractJdbc2Statement.java 
line 665):

     public int getMaxRows() throws SQLException
     {
         checkClosed();
         return maxrows;
     }


... I'm wondering why there is even checkClosed() called before 
returning the value of a field?
Could that check be safely removed?

It may be the case that there are concurrent calls from multiple threads 
to the same Connection object, but shouldn't that be thread-safe?

(Un)fortunately the exception is logged very seldomly, so there has not 
yet a pattern emerged when they happen.


Thanks for your comments,
best regards

	-hannes

Re: executeQuery() throws "Statement has been closed"

Hi Dave,


 > Well if another thread has closed the connection which I suspect is
> happening here, then the results of checkClosed is correct.

The JDBC connection is only closed when the application exits. Since the 
message is "statement closed" (as opposed to "connection closed"), I 
quite sure thats not happening here.

It may well be a concurrency issue, since we are sharing one JDBC 
connections among multiple Hibernate-Sessions.


thanks again,

	-hannes

Re: executeQuery() throws "Statement has been closed"

Dave,


 > Sharing one connection across multiple hibernate sessions seems
 > fraught with danger ?

sorry, I should have explained better. This is a read-only ("SELECT") 
workload, so there are no commits etc. interfering between threads. This 
is a session-per-view pattern so the indiviual sessions have minimal 
size and may be closed and garbage collected independently.
Access to the backend is again pooled through a pgbouncer statement pool 
(which might be another place to look at).


According to http://jdbc.postgresql.org/documentation/81/thread.html , 
JDBC connections are threadsafe and can be used by multiple threads 
concurrently.

That page also reads,
"If a thread attempts to use the connection while another one is using 
it, it will wait until the other thread has finished its current 
operation. If the operation is a regular SQL statement, then the 
operation consists of sending the statement and retrieving any ResultSet 
(in full)."


As I understand it, even if one thread closes the connection, a call to 
executeQuery() should either fail early or complete. It appears to me 
that is is not the case in these rare instances where the exception is 
thrown only when the results have already been processed successfully.


As Kevin pointed out (thanks!), getMaxRows() is required by the spec to 
check whether the connection is still open.
So let me rephrase the question: why is 
AbstractJdbc4Statement.createResultSet() calling getMaxResult() and 
friends [which in turn check for openness] instead of directly using the 
already prepared field values?


In any case, I'll see if I can come up with a test case to demonstrate.


Thanks again,
best regards

	-hannes

Re: executeQuery() throws "Statement has been closed"

Hi,


 > http://jdbc.postgresql.org/documentation/81/thread.html
 >
> "If a thread attempts to use the connection while another one is using
> it, it will wait until the other thread has finished its current
> operation. If the operation is a regular SQL statement, then the
> operation consists of sending the statement and retrieving any ResultSet
> (in full)."


Consider this test case: one thread calls executeQuery("SELECT ..."), 
and while that query is still executing, a second thread calls close() 
on the statement.

Given the documentation above, I would expect the regular SQL statement 
to complete, the statement then to be closed, and no exception thrown.


I'll attach a sample code for this scenario. With the current git 
driver, this fails at getMaxRows().

When you change Jdbc4Statement.createResultset() so it uses this.maxrows 
instead of getMaxRows() and hence skip checkClosed, then that test will 
pass. Patch also attached, although I have no idea whether this really 
is the right thing to do.


Still, I will also check my code. I think this happens when a user 
closes a view while the data inside is (re)loading, so I'll probably 
have the error handling discard that exception in this case instead of 
ringing the alarm bell...


Thanks for your comments, David and Kevin!

Best regards,

	-hannes

Re: executeQuery() throws "Statement has been closed"

Re: executeQuery() throws "Statement has been closed"

Re: executeQuery() throws "Statement has been closed"

Re: executeQuery() throws "Statement has been closed"