为什么此查询不返回任何结果？

Question

I am leaning some Java for school, and for some reason the code below doesn't return any results? Is there a problem with this code? There is a single record in the mysql database, with a studentID (which is int(10) ) of 12.

public static ResultSet GetByID(int studentID) {
    // This method loads the mysql driver and establishes the database connection
    Connect();

    ResultSet results = null;

    try {
        String query = "SELECT * FROM student where studentID = ?";

        PreparedStatement statement = Connection.prepareStatement(query);

        statement.setInt(1, studentID);
        results = statement.executeQuery();
    } catch (SQLException ex) {
        LogException(ex);
    } catch (Exception ex) {
        System.out.println(ex);
    }

    // This method terminates the mysql connection.
    Disconnect();

    return results;
}

The calling code is:

@Override
public ResultSet query() {
    return DB.GetByID(getStudentID()); // this is 12
}

This does not return null, rather just an empty result set.

Answer 1

A ResultSet can only be used while the connection is still open. Read it before you "disconnect".

If fixing that doesn't give you a different result, the most likely cause is that the query didn't match any rows in the table.

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You should get into the habit of closing connections, streams, etc in finally blocks to prevent resource leakage. (Your lecturer should have explained that. Check your notes / text book.)

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Finally, since you are a beginner, it is worth pointing out that you should always conform to the accepted Java conventions for method naming. Java method names should start with a lower-case letter. GetByID should be getByID and Disconnect should be disconnect .

(And if your lecturer / tutor doesn't or didn't dock marks for that, he / she should be condemned to writing Visual Basic for the next 5 years for crimes against the Software Engineering industry.)

Answer 2

If you want to return the resultSet after closing the connection you have to use CachedRowSet. As StephenC said, your ResultSet is empty after closing the connection.