Why Does \'Operation not allowed after ResultSet closed\' Occur in JDBC and How Can It Be Fixed?

Operation not allowed after MySQL Result Set has closed

When interacting with a database using JDBC, it's crucial to handle ResultSet objects with care to avoid exceptions like "Operation not allowed after ResultSet closed." This error arises when you attempt to perform operations on a ResultSet that has already been closed.

Understanding the Exception

The exception is thrown when you try to iterate over a ResultSet using its next() method after it has been closed. This happens because closing a ResultSet releases its resources, including its internal cursor, making it invalid for further operations.

Causes of Closed Result Sets

• Explicitly calling the close() method on the ResultSet
• Re-executing queries on the same Statement object
• Using the Statement object to create multiple ResultSets

Solution in the Provided Code

The error in the provided code occurs within the MySQLDonation class while cycling through the donations table. The ResultSet obtained from the SELECT query is closed when the Statement is re-executed for subsequent DELETE queries.

To resolve this issue, separate the query execution and result processing steps:

// In MySQLDatabase class
private void handleDonation(Client client, int creditamount) {
    try (Statement statement = getConnection().createStatement()) {
        // Execute DELETE query with new Statement
        statement.executeUpdate(String.format("DELETE ..."));
    }
}

By creating a new Statement for each query, you ensure that the ResultSet remains open until the query is complete.

Additional Notes

• Consider using try-with-resources blocks to automatically close statements and result sets.
• If you need to iterate over multiple ResultSets, execute them on different Statement objects.
• Consult the JDBC documentation for best practices on handling ResultSets.

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