What are the resources consumed by a left-open result set / statement if a statement is created before each query?
Are result sets / statements closed by the garbage collection in this case?
There is a possibility that you may have consequences for without closing connection. maximum open cursors exceeded exception you may get.
from Java doc
By default, only one ResultSet object per Statement object can be open
at the same time. Therefore, if the reading of one ResultSet object is
interleaved with the reading of another, each must have been generated
by different Statement objects. All execution methods in the Statement
interface implicitly close a statment's current ResultSet object if an
open one exists.
So you need to close all JDBC resources explicitly.
Related
I wanted to know, does calling close method in Statement class also aborts the sql query in database, similar to how cancel method do?
By aborting a sql query, I mean sending something like HTTP DELETE request to the server to cancel a query.
Does something like this should also be done by close() method if in case application does not call cancel() method.
Close method in Statement class it is used to free up the resources used, this is because Connections to Derby are resources external to an application, and the garbage collector will not close them automatically. Note that a close method causes Derby to abort an in-flight transaction but a does NOT roll back the transaction.
You can see a detailed explanation in https://docs.oracle.com/javadb/10.8.3.0/devguide/cdevconcepts839085.html
Closing a statement closes the statement handle and should end and de-allocate any statement resources held by the database systems for that specific statement handle.
I'm not sure what you mean with 'abort', but if you attempt to close a statement while a method call of execute, executeQuery or executeUpdate is currently running, the behaviour is driver-dependent, but likely it will wait until that method call completes before it closes the statement. If you want to abort an in-progress statement execution, you should use Statement.cancel().
If you attempt to close a statement while a result set is still open (that is executeQuery finished, but you haven't read or explicitly closed the result set), the result set will be closed, and any server-resources associated with that result set (e.g. a server-side cursor) will be closed as well. However, similar to an execute, if a fetch is in-progress (e.g. as part of a ResultSet.next() call), the close may wait until that fetch is complete, and use of Statement.cancel() may be necessary to abort such a fetch.
Let's say we have a class that writes in a database a log message. This class is called from different parts of the code and executes again and again the same INSERT statement. It seems that is calling to use a PreparedStatement.
However I am wondering what is the right usage of it. Do I still get the benefit of using it, like the DBMS using the same execution path each time it is executed, even if I create a new PreparedStatement each time the method is called or should I have a PreparedStatement as a class member and never close it in order to re use it and get benefit from it?
Now, if the only way to obtain benefit using the PreparedStatement in this scenario is to keeping it opened as class member, may the same connection have different PreparedStatement's (with different queries) opened at the same time? What happens when two of these PreparedStatements are executed at the same time? Does the JDBC driver queue the execution of the PreparedStatements?
Thanks in advance,
Dani.
For all I know and experienced, statements don't run in parallel on one connection. And as you observed correctly, PreparedStatements are bound to the Connection they were created on.
As you probably don't want to synchronize your logging call (one insert at a time plus locking overhead), you'd have to keep the connections reserved for this logging statement.
But having a dedicated pool for only one statement seems very wasteful - don't want to do that as well.
So what options are left?
prepare the statement for every insert. As you'll have I/O operations to send data to the db, the overhead of preparing is relatively small.
prepare the statement inside your pool on creating a new connection and build a Map <Connection,PreparedStatement> to reference them later. Makes creating new connections a bit slower but allowes to recycle the statement.
Use some async way to queue your logs (JMS) and do the Insert as batch inside a message driven bean or similar
Probably some more options - but that's all I could think of right now.
Good luck with that.
I am curious to know that, if I create SQL statement for SQLITE database file in java as
public Statement GetStatement()
{
if(connection==null || connection.isClosed())
{
connection = DriverManager.getConnection("jdbc:sqlite:"+filePath);
}
return connection.createStatement(); //Connection is private variable of type java.sql.connection
}
This statement returned by this function is used in executing insert, select, update SQL in different scenarios and multiple reads or multiple functions will be inserting, updating or selecting from database.
Now if I do not close the statements then are there chances of memory leaks.
But I close all the result set objects got by executing select SQL.
I know it is good practice to close the statements but what are counter effects if I do not do it?
SQLite enforces sequential write-access to the database (one process at a time). This makes it vital that you close the database connection when you have completed any INSERT or UPDATE operations. If you don't, you might receive "DatabaseObjectNotClosed" exceptions when your script next attempts to write to it. Not to mention memory leaks, and possible performance decrease.
Let's say I get a PreparedStatement from a Connection object, and then later I overwrite the reference with another PreparedStatement. Then, later, I close() the reference. Will the first PreparedStatement (the one I lost the reference to) remain open? Or does some protocol or garbage collection take care of that?
For example:
PreparedStatement ps = connection.prepareStatement(MY_QUERY);
// do stuff
ps.execute();
ps = connection.prepareStatement(MY_OTHER_QUERY);
// do stuff
ps.execute();
ps.close();
Does the first PreparedStatement object, the one used to execute MY_QUERY, remain open?
Yes, the PreparedStatement will be left open until it gets garbage collected* (if at all).
That's why you always see the rule to always release (close) external resources that you acquire.
JDBC resources (such as this prepared statement, connections, results sets) and IO resources (sockets, streams, ...) are the two most common resources that you manually need to manage in Java.
* strictly speaking it could be closed in the finalize method which could be slightly before it gets GCed, but after it becomes eligible for the GC, but for this discussion this is close enough.
It'll be closed when the object is eventually garbage collected. Since you have little control over the garbage collection process, it is a good idea to close statements as soon as you're done with them.
From the documentation for Statement.close():
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.
It will stay open, and there is no guarantee that it will be closed when garbaged collected. It depends on implementation. Implementation needs to override finalize like in eg FileInputStream.finalize which
"...ensures that the close method of this file input stream is called when there are no more references to it."
But Connection.close can close PreparedStatement, see API for Connection.close:
"..releases this Connection object's database and JDBC resources immediately instead of waiting for them to be automatically released.."
but only if it is not pooled Connection.
In the tutorial "Using Prepared Statements" it states that they should always be closed. Suppose I have a function
getPrice() {
}
that I expect to be called multiple times per second. Should this method be opening and closing the PreparedStatement with every single method call? This seems like a lot of overhead.
First of all, PreparedStatement are never opened. It's just a prepared Statement that is executed. The statement is sent to the RDBMS that executes the SQL statement compiled by the PreparedStatement. The connection to the SQL statement should be opened during the duration of the SQL querying and closed when no other RDMS calls is needed.
You can send many Statement/PreparedStatement as you require provided that you finally close its ResultSet and PreparedStatement once you're completed with them and then close the RDBMS connection.
Should this method be opening and closing the PreparedStatement with every single method call?
If you are creating the PreparedStatement object within the method, then you must close it, once you are done with it. You may reuse the PreparedStatement object for multiple executions, but once you are done with it, you must close it.
This is because, although all Statement objects (including PreparedStatements) are supposed to be closed on invoking Connection.close(), it is rarely the case. In certain JDBC drivers, especially that of Oracle, the driver will be unable to close the connection if the connection has unclosed ResultSet and Statement objects. This would mean that, on these drivers:
You should never lose a reference to a PreparedStatement object. If you do, then the connection will not be closed, until garbage collection occurs. If you are reusing PreparedStatement instances for different SQL statements, it is easy to forget this.
You should close the PreparedStatement once you no longer need it. Only then can the Connection.close() actually tear down the physical connection.
As the example in the tutorial shows you should close it after all your queries have been performed.
Once the statement is closed the RDMS may release all resources associated with your statement. Thus to use it further you'd have to re-prepare the very same statement.
I think that, after every database interaction, every component like statement, resultset must be closed, except for connection, if u tend to perform more operation.
And there is no need to worry, if you are creting the prepared statement again and again, because as you will be using the same statement again and again, there wont be any performannce issue.
Yes..No issues are there if you are creating the prepared statement n number of times, because as you will be using the same statement at all the places. No need to have any observation here regarding performance
Thanks