Why does this error occur when working with Java-JDBC-Oracle?

Question

Oracle

My server written in Java receives various data from Oracle using the JDBC driver and during operation very often unexpectedly gives a Connection Wrong error and then refuses to work with Oracle, and this happens even with a small load on the database (more than 2-3 simultaneous connections do not happen) . At the same time, everything works fine through SQL Plus and SQL Developer, and the problem is solved only by restarting Oracle.
Prompt, in what there can be a reason.
The following error occurs:

oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:489)
oracle.jdbc.driver.PhysicalConnection.<init>(PhysicalConnection.java:553)
oracle.jdbc.driver.T4CConnection.<init>(T4CConnection.java:254)
oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:32)
oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:528)
java.sql.DriverManager.getConnection(Unknown Source)
java.sql.DriverManager.getConnection(Unknown Source)
com.database.JdbcConnect.Connect(JdbcConnect.java:21)
com.database.DBOperation.connect(DBOperation.java:45)
com.database.DBOperation.getNotify(DBOperation.java:293)
com.database.DBOperation.getUnreadNotifyCount(DBOperation.java:249)
com.server.ClientHandler.getUnreadNotifyCount(ClientHandler.java:223)
com.server.ClientHandler.messageReceived(ClientHandler.java:76)
com.server.ClientHandler.handle(ClientHandler.java:40)
com.sun.net.httpserver.Filter$Chain.doFilter(Unknown Source)
sun.net.httpserver.AuthFilter.doFilter(Unknown Source)
com.sun.net.httpserver.Filter$Chain.doFilter(Unknown Source)
sun.net.httpserver.ServerImpl$Exchange$LinkHandler.handle(Unknown Source)
com.sun.net.httpserver.Filter$Chain.doFilter(Unknown Source)
sun.net.httpserver.ServerImpl$Exchange.run(Unknown Source)
java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
java.lang.Thread.run(Unknown Source)

1) Connections at the dedicated server
2) limit on the maximum number of connections:

Answer 1

The problem was solved. The problem was in the overflow of allocated memory for Oracle due to the fact that open cursors were not automatically closed when disconnecting from the database.

Answer 2

Is there any statistic about how long it takes?
And another question: is this the only connection created or taken from the connection pool?