为什么我的结果集类型仅是正向时可以使用first()方法?

问题描述

当我执行JDBC调用时，我正在使用默认的ResultSet.令我惊讶的是，在使用next()迭代ResultSet之后，我可以调用first()返回第一行.这并不意味着只使用正向ResultSet?

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?

我的代码很简单:

  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();

我正在使用8.0.11版的mysql连接器

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>

推荐答案

MySQL Connector/J的默认行为是，一旦调用.executeQuery，就会将ResultSet的全部内容加载到内存中.因此，即使我们的ResultSet是TYPE_FORWARD_ONLY，MySQL JDBC开发人员显然还是决定很好"，并允许我们在这种情况下使用.first，.absolute等(因为整个ResultSet在内存，并且随时可用)，即使JDBC规范说了

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

但是，请注意，如果不能保证整个ResultSet都在内存中，例如，如果我们在滚动浏览ResultSet时使用st.setFetchSize(Integer.MIN_VALUE)来流式传输" ResultSet，则MySQL Connector/J会获胜除了.next之外，我们什么都不能使用

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

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