什么是 RowMapper、ResultSetExtractor、绑定变量和查询类型？
我知道如何使用 JDBC Template 和 DAO，但我仍然有疑问：
RowMapper和ResultSetExtractor有什么用？- 什么是绑定变量？
- 查询是列表类型吗？
I know how to use JDBC Template and DAO, but I still have questions regarding it:
- What's the use of
RowMapperandResultSetExtractor? - What is a bind variable?
- Are queries a type of List?
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Q1：这些接口与
JdbcTemplate在查询数据库时经常使用的RowCallbackHandler一起使用。您实现哪个接口、如何实现它以及在 JdbcTemplate 中使用哪种方法取决于您的数据库以及您想要执行的查询类型。来自 Spring API 文档 和一些其他注释：RowMapper：
，即当数据库中的行与结果对象之间存在一对一关系时，通常使用 RowMapper 来映射对象。结果对象。
ResultSetExtractor：
ResultSetExtractor的实现通常会从多行中创建一个对象，然后返回该对象。它是无状态的，因为实现类在方法调用之间不保留任何状态。RowCallbackHandler：
RowCallbackHandler用于更新或删除行等查询。此外，当您需要跟踪ResultSet中的状态（例如 RowCountCallbackHandler。Q1: Theses interfaces are together with
RowCallbackHandlerfrequently used by theJdbcTemplatewhen queering a database. Which interface you implement, how you implement it and which method you use in theJdbcTemplatedepends on your database and what kind of query you would like to execute. From the Spring API doc and some additional comments:RowMapper:
i.e. the
RowMapperis commonly used to map objects when there is a one-to-one relationship between a row in the database and the resulting object.ResultSetExtractor:
Implementations of the
ResultSetExtractortypically creates one object out of several rows, that is subsequently returned. It is stateless because the implementing class does not preserve any state between method calls.RowCallbackHandler:
The
RowCallbackHandleris used for queries such as updating or deleting rows. Additionally, it is used when you need to track a state across theResultSet, such as number of rows in the RowCountCallbackHandler.