Oracle – Connection pool opens more connections then maximum pool size

The exception indicates a typical case of application code which leaks database connections. You need to ensure that you acquire and close all of them (Connection, Statement and ResultSet) in a try-with-resources block in the very same method block according the normal JDBC idiom.

public void create(Entity entity) throws SQLException {
    try (
        Connection connection = dataSource.getConnection();
        PreparedStatement statement = connection.prepareStatement(SQL_CREATE);
    ) { 
        statement.setSomeObject(1, entity.getSomeProperty());
        // ...
        statement.executeUpdate();
    }
}

Or when you're not on Java 7, in a try-finally block. Closing them in finally will guarantee that they are also closed in case of exceptions.

public void create(Entity entity) throws SQLException {
    Connection connection = null;
    PreparedStatement statement = null;

    try { 
        connection = dataSource.getConnection();
        statement = connection.prepareStatement(SQL_CREATE);
        statement.setSomeObject(1, entity.getSomeProperty());
        // ...
        statement.executeUpdate();
    } finally {
        if (statement != null) try { statement.close(); } catch (SQLException logOrIgnore) {}
        if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
    }
}

Yes, you still need to close connections yourself, even when using connection pooling. It's a common mistake among starters that they think that it will then automatically handle the close. This is not true. The connection pool namely returns a wrapped connection which does something like the following in the close():

public void close() throws SQLException {
    if (this.connection is still eligible for reuse) {
        do not close this.connection, but just return it to pool for reuse;
    } else {
        actually invoke this.connection.close();
    }
}

Not closing them would cause the connection not being released back to the pool for reuse and thus it will acquire a new one again and again until the DB runs out of connections which will cause your application to crash.

See also:

• How often should Connection, Statement and ResultSet be closed in JDBC?
• Is it safe to use a static java.sql.Connection instance in a multithreaded system?
• Closing JDBC Connections in Pool

Seems that you have a connection leak somewhere in your application.

When you borrow a connection from the connection pool and forget to properly close it, it remains marked as unavailable, causing the exception you are experiencing.

To locate the leak, you should monitor the connection usage through the JMX connector surely exposed by your application server.