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Connections in Connection Pool don't become idle

I have a program that performs frequent requests to a database using a single-threaded workflow. The requests require obtaining connections within a second or less of each other, but one connection should always be closed before a new one is opened.
However, over time my debugging shows that the connections rack up rather quickly. It seems that connections start multiplying, and eventually reach the point where the program hangs because the connection pool's maximum (currently using BoneCP) is filled. I have the maximum set at 15, but it had similar behaviour when it was at the default number (8 I believe).
I am using try-with-resource statements, similar to the one below, to close the connections.
//Connection example
//Connection pool is configured.
DataSource source = new DataSource();
try (Connection con = source.getConnection();
PreparedStatement stmt = con.prepareStatement(query) {
//Do SQL stuff
} catch (SQLException e) {
//Handle Exception
}
If I understand try with resources right, this should automatically close the connection, and return it to the pool. Since this didn't work, I tried adding a con.close() statement in an attempt to better handle the connections. This quantitatively seemed to help, but I'm not sure it did anything in reality.
Why are so many connections being created, and how can I better manage my connections so I don't create so many?

Should a database connection stay open all the time or only be opened when needed?

I have a bukkit plugin (minecraft) that requires a connection to the database.
Should a database connection stay open all the time, or be opened and closed when needed?

The database connection must be opened only when its needed and closed after doing all the necessary job with it. Code sample:
Prior to Java 7:
Connection con = null;
try {
con = ... //retrieve the database connection
//do your work...
} catch (SQLException e) {
//handle the exception
} finally {
try {
if (con != null) {
con.close();
}
} catch (SQLException shouldNotHandleMe) {
//...
}
}
Java 7:
try (Connection con = ...) {
} catch (SQLException e) {
}
//no need to call Connection#close since now Connection interface extends Autocloseable
But since manually opening a database connection is too expensive, it is highly recommended to use a database connection pool, represented in Java with DataSource interface. This will handle the physical database connections for you and when you close it (i.e. calling Connection#close), the physical database connection will just be in SLEEP mode and still be open.
Related Q/A:
Java Connection Pooling
Some tools to handle database connection pooling:
BoneCP
c3po
Apache Commons DBCP
HikariCP

Depends on what are your needs.
Creating a connection takes some time, so if you need to access database frequently it's better to keep the connection open. Also it's better to create a pool, so that many users can access database simultaneously(if it's needed).
If you need to use this connection only few times you may not keep it open, but you will have delay when you would like to access database. So i suggest you to make a timer that will keep connection open for some time(connection timeout).

You need to close your connections after each query executions.Sometimes you need to execute multiple queries at the same time because the queries are hanging from each other.Such as "first insert task then assign it to the employees".At this time execute your queries on the same transaction and commit it, if some errors occur then rollback.By default autocommit is disabled in JDBC. Example
Use connection pooling.If you are developing a webapplication then use App Server connection pooling.App server will use the same pooling for each of your applications so you can control the connection count from the one point.Highly recommend the Apache Tomcat Connection pooling.Example
As an additional info:
Connection, Statement and ResultSet.
1.If you close connection you don't need close statement or resultset.Both of them will be closed automatically
2.If you close Statement it will close ResultSet also
3.if you use try-with-resources like this:
try (Connection con = ...) {
} catch (SQLException e) {
}
it will close the connection automatically.Because try-with-resources require autoclosable objects and Connection is autocloseable.You can see the details about try-with-resources here

Actually, it's all matter on how you write your application! It's an art, but sadly everyone takes a tutorial for a good practice like Microsoft's tutorials.
If you know what you are coding, then you keep your connection open for the lifetime of the application. It's simple, not because you have to go at work in the morning that everyday we have to build a special route just for you! You take that single route or 2 or 4 like everyone does! You judge for the traffics and you build 2, 4 or 6 routes as needed. If there is traffic with these 4 or 6 routes, you wait!
Happy coding.

The Connection should be opened only when required. If it is open before the actual need, it reduces one active connection from the connection pool..so it ultimately effects the users of the application.
So,it is always a better practice to open connection only when required and closing it after completion of process.
Always try puttting you connection close logic inside the finally block that will ensure that your connection will be closed,even if any exception occurs in the application
finally
{
connection.close()
}

java.sql.SQLException: Listener refused the connection with the following error: ORA-12519, TNS:no appropriate service handler found

I am passing Resultset object to each thread. Each thread is connecting to the database and inserting data. Untill thread 110 it is working fine. After it crosses 111 thread it throws the above exception.
I am using oracle 11g.
My sample Thread code is:
class MyThreadClass implements Runnable
{
public Connection connection;
public Statement statement2;
public ResultSet rs2;
public String cookie;
public MyThreadClass(ResultSet rs1)
{
rs2=rs1;
}
public void run()
{
try
{
cookie=rs2.getString("COOKIE");
driver = "oracle.jdbc.driver.OracleDriver";
url = "jdbc:oracle:thin:#127.0.0.1:1521:xx";
/* connection
statement2.executeUpdate("INSERT INTO visit_header VALUES ('"+cookie+"')");
}
I am not getting how to handle this exception.

Your multi-threaded application is opening too many Connections/Sessions. Hence, the listener is dropping and blocking new connections for a while.
Check your DB resource usage first:
SELECT * FROM v$resource_limit WHERE resource_name IN ('processes','sessions');
Check to see if your MAX_UTILIZATION for either your Processes or Sessions is getting too close to the LIMIT_VALUE. If yes, you should either:
Use DB Connection pooling to share Connection objects between threads. Or,
Increase the number of processes/sessions that Oracle can service simultaneously.
Actually, Connection Pooling (#1) should always be done. An application cannot scale up otherwise. Check Apache Commons DBCP for details. For #2, open a new SQL*Plus session as SYSTEM and run:
ALTER system SET processes=<n-as-per-number-of-threads> scope=spfile;
to increase backend concurrency. Then RESTART the Database. IMPORTANT!

I guess the database just don't accept more connections from your host. If I understand your question right you are making maybe 100 threads which each connects to the database in short time. Maybe you don't even close the connection correctly, or the accesses are lasting so long that a huge amount of connections are opened. The database have a limit to which it accepts connections.
You should definitely reduce the number of connections by some clever technique. Maybe reduce the number of concurrent threads and/or use a connection pool.

Try this solution at your end. It worked for me.
Close the connection in try/catch block and just after closing the connection,
write-
Thread.sleep(1000);
In this case you can write it as-
finally {
try {
if (conn != null && !conn.isClosed())
{
conn.close();
Thread.sleep(1000);
}
}
catch (SQLException e) {
e.printStackTrace();}
}

MySQL with Java: Open connection only if possible

I'm running a database-heavy Java application on a cluster, using Connector/J 5.1.14. Therefore, I have up to 150 concurrent tasks accessing the same MySQL database. I get the following error:
Exception in thread "main" com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException: Too many connections
This happens because the server can't handle so many connections. I can't change anything on the database server. So my question is: Can I check if a connection is possible BEFORE I actually connect to the database?
Something like this (pseudo code):
check database for open connection slots
if (slot is free) {
Connection cn = DriverManager.getConnection(url, username, password);
}
else {
wait ...
}
Cheers

try{
//try to connect
}catch(com.mysql.jdbc.exceptions.jdbc4.MySQLNonTransientConnectionException ex) {
//what ever you want to do
}
Also See :
Connection Pooling

Use a connection pool like as for example C3P0. A bit decent connection pool can wait a certain timeout and retry a certain amount of times until a connection is available and then return it. Also ensure that you call Connection#close() in finally block of every try where you acquire the connection. Otherwise the connection is never released to pool (or to the database when you don't use a pool) and you will run out of them anyway.

Something like should work better.
try
{
Connection cn = DriverManager.getConnection(url, username, password);
}
catch ( com.mysql.jdbc.exceptions.MySQLNonTransientConnection e )
{
// wait ...
}

Connection pooling with Java and MySQL in Tomcat web application

I recently wrote and deployed a Java web application to a server and I'm finding an unusual problem which didn't appear during development or testing.
When a user logs in after so long and goes to display data from the database, the page indicates that there are no records to see. But upon page refresh, the first x records are shown according to the pagination rules.
Checking the logs, I find:
ERROR|19 09 2009|09 28 54|http-8080-4|myDataSharer.database_access.Database_Metadata_DBA| - Error getting types of columns of tabular Dataset 12
com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:
** BEGIN NESTED EXCEPTION **
java.io.EOFException
STACKTRACE:
java.io.EOFException
at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1956)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2368)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2867)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1616)
And so on for several hundred lines.
The application is currently set for about 100 users but is not yet in full use. It uses connection pooling between the Apache Tomcat servlets / jsps and a MySQL database with the following code example forming the general arrangement of a database operation, of which there are typically several per page:
// Gets a Dataset.
public static Dataset getDataset(int DatasetNo) {
ConnectionPool_DBA pool = ConnectionPool_DBA.getInstance();
Connection connection = pool.getConnection();
PreparedStatement ps = null;
ResultSet rs = null;
String query = ("SELECT * " +
"FROM Dataset " +
"WHERE DatasetNo = ?;");
try {
ps = connection.prepareStatement(query);
ps.setInt(1, DatasetNo);
rs = ps.executeQuery();
if (rs.next()) {
Dataset d = new Dataset();
d.setDatasetNo(rs.getInt("DatasetNo"));
d.setDatasetName(rs.getString("DatasetName"));
...
}
return d;
}
else {
return null;
}
}
catch(Exception ex) {
logger.error("Error getting Dataset " + DatasetNo + "\n", ex);
return null;
}
finally {
DatabaseUtils.closeResultSet(rs);
DatabaseUtils.closePreparedStatement(ps);
pool.freeConnection(connection);
}
}
Is anyone able to advise a way of correcting this problem?
I believe it is due to MySQL leaving connection poll connections open for up to eight hours but am not certain.
Thanks
Martin O'Shea.
Just to clarify one point made about my method of connection pooling, it isn't Oracle that I'm using in my application but a class of my own as follows:
package myDataSharer.database_access;
import java.sql.*;
import javax.sql.DataSource;
import javax.naming.InitialContext;
import org.apache.log4j.Logger;
public class ConnectionPool_DBA {
static Logger logger = Logger.getLogger(ConnectionPool_DBA.class.getName());
private static ConnectionPool_DBA pool = null;
private static DataSource dataSource = null;
public synchronized static ConnectionPool_DBA getInstance() {
if (pool == null) {
pool = new ConnectionPool_DBA();
}
return pool;
}
private ConnectionPool_DBA() {
try {
InitialContext ic = new InitialContext();
dataSource = (DataSource) ic.lookup("java:/comp/env/jdbc/myDataSharer");
}
catch(Exception ex) {
logger.error("Error getting a connection pool's datasource\n", ex);
}
}
public void freeConnection(Connection c) {
try {
c.close();
}
catch (Exception ex) {
logger.error("Error terminating a connection pool connection\n", ex);
}
}
public Connection getConnection() {
try {
return dataSource.getConnection();
}
catch (Exception ex) {
logger.error("Error getting a connection pool connection\n", ex);
return null;
}
}
}
I think the mention of Oracle is due to me using a similar name.

There are a few pointers on avoiding this situation, obtained from other sources, especially from the connection pool implementations of other drivers and from other application servers. Some of the information is already available in the Tomcat documentation on JNDI Data Sources.
Establish a cleanup/reaper schedule that will close connections in the pool, if they are inactive beyond a certain period. It is not good practice to leave a connection to the database open for 8 hours (the MySQL default). On most application servers, the inactive connection timeout value is configurable and is usually less than 15 minutes (i.e. connections cannot be left in the pool for more than 15 minutes unless they are being reused time and again). In Tomcat, when using a JNDI DataSource, use the removeAbandoned and removeAbandonedTimeout settings to do the same.
When a new connection is return from the pool to the application, ensure that it is tested first. For instance, most application servers that I know, can be configured so that connection to an Oracle database are tested with an execute of "SELECT 1 FROM dual". In Tomcat, use the validationQuery property to set the appropriate query for MySQL - I believe this is "SELECT 1" (without quotes). The reason why setting the value of the validationQuery property helps, is because if the query fails to execute, the connection is dropped from the pool, and new one is created in its place.
As far are the behavior of your application is concerned, the user is probably seeing the result of the pool returning a stale connection to the application for the first time. The second time around, the pool probably returns a different connection that can service the application's queries.
Tomcat JNDI Data Sources are based on Commons DBCP, so the configuration properties applicable to DBCP will apply to Tomcat as well.

I'd wonder why you're using ConnectionPool_DBA in your code instead of letting Tomcat handle the pooling and simply looking up the connection using JNDI.
Why are you using an Oracle connection pool with MySQL? When I do JNDI lookups and connection pooling, I prefer the Apache DBCP library. I find that it works very well.
I'd also ask if your DatabaseUtils methods throw any exceptions, because if either of the calls prior to your call to pool.freeConnection() throw one you'll never free up that connection.
I don't like your code much because a class that performs SQL operations should have its Connection instance passed into it, and should not have the dual responsibility of acquiring and using the Connection. A persistence class can't know if it's being used in a larger transaction. Better to have a separate service layer that acquires the Connection, manages the transaction, marshals the persistence classes, and cleans up when it's complete.
UPDATE:
Google turned up the Oracle class with the same name as yours. Now I really don't like your code, because you wrote something of your own when a better alternative was easily available. I'd ditch yours right away and redo this using DBCP and JNDI.

This error indicates server closes connection unexpectedly. This can occur in following 2 cases,
MySQL closes idle connection after certain time (default is 8 hours). When this occurs, no thread is responsible for closing the connection so it gets stale. This is most likely the cause if this error only happens after long idle.
If you don't completely read all the responses, the connection may get returned to the pool in busy state. Next time, a command is sent to MySQL and it closes connection for wrong state. If the error occurs quite frequent, this is probably the cause.
Meanwhile, setting up an eviction thread will help to alleviate the problem. Add something like this to the Data Source,
...
removeAbandoned="true"
removeAbandonedTimeout="120"
logAbandoned="true"
testOnBorrow="false"
testOnReturn="false"
timeBetweenEvictionRunsMillis="60000"
numTestsPerEvictionRun="5"
minEvictableIdleTimeMillis="30000"
testWhileIdle="true"
validationQuery="select now()"

Is there a router between the web server and the database that transparently closes idle TCP/IP connections?
If so, you must have your connection pool either discard unused-for-more-than-XX-minutes connections from the pool, or do some kind of ping every YY minutes on the connection to keep it active.

On the off chance you haven't found your answer I've been dealing with this for the last day. I am essentially doing the same thing you are except that I'm basing my pooling off of apache.commons.pool. Same exact error you are seeing EOF. Check your mysqld error log file which is most likely in your data directory. Look for mysqld crashing. mysqld_safe will restart your mysqld quickly if it crashes so it won't be apparent that this is the case unless you look in its logfile. /var/log is not help for this scenario.
Connections that were created before the crash will EOF after the crash.