CODEWITHSUNDEEP
javamysqljdbc
Robert Li
Robert Li

Reputation: 809

why I could use first() method when my resultset type is forward only?

I'm using the default ResultSet when I do a JDBC call. I'm surprised that after using next() to iterate the ResultSet, I could call first() to back to the first row. This is not meant by using forward only ResultSet?

My code is quite simple:

  Statement st = conn.createStatement();
  rs = st.executeQuery(sql);

while (rs.next()) {
    for (int i = 1; i <= columnsNumber; i++) {
        if (i > 1) System.out.print(",  ");
        String columnValue = rs.getString(i);
        System.out.print(rsmd.getColumnName(i) + " : " + columnValue);
    }
    System.out.println("");
}
rs.first();

I am using 8.0.11 version of mysql connector

<dependency>
    <groupId>mysql</groupId>
    <artifactId>mysql-connector-java</artifactId>
    <version>8.0.11</version>
</dependency>

Upvotes: 1

Views: 2749

Answers (1)

Gord Thompson
Gord Thompson

Reputation: 123654

The default behaviour for MySQL Connector/J is to load the entire contents of the ResultSet into memory as soon as .executeQuery is called. So, even though our ResultSet is TYPE_FORWARD_ONLY the MySQL JDBC developers apparently decided to be "nice" and allow us to use .first, .absolute, etc. in that case (because the entire ResultSet is in memory and readily available), even though the JDBC spec says

For a ResultSet object that is of type TYPE_FORWARD_ONLY, the only valid cursor movement method is next. All other cursor movement methods throw an SQLException.

Note, however, that if the entire ResultSet is not guaranteed to be in memory, e.g., if we use st.setFetchSize(Integer.MIN_VALUE) to "stream" the ResultSet as we scroll through it, then MySQL Connector/J won't let us use anything but .next or we'll get

com.mysql.jdbc.OperationNotSupportedException: Operation not supported for streaming result sets

Upvotes: 3

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