利用IBM Java Toolbox for i实现针对数据库文件的记录级访问
从文件系统的角度，IBM i 提供了对 POSIX 类文件系统 IFS（或集成文件系统）的强大支持。从数据库的角度，IBM i 内置有关系数据库 DB2。作为 IBM i 平台应用普遍的数据库文件，则是以文件的视图访问数据库。
对于从事 IBM i 平台的应用开发而言，如何使用 IBM Java Toolbox for i 访问文件是非常重要的技能。本文的目标是系统性介绍如何利用 IBM Java Toolbox for i 实现针对数据库文件的记录级访问（RLA 或 Record Level Access），并指出 JDBC 与 RLA 两种技术的区别。
从文件系统的角度，IBM i 提供了对 POSIX 类文件系统 IFS（或集成文件系统）的强大支持。从数据库的角度，IBM i 内置有关系数据库 DB2。作为 IBM i 平台应用普遍的数据库文件，则是以文件的视图访问数据库。对于从事 IBM i 平台的数据库应用开发而言，如何使用 IBM Java Toolbox for i 访问数据库文件是非常重要的技能。本文的目标是系统性介绍如何利用 IBM Java Toolbox for i 实现针对数据库文件的记录级访问（RLA 或 Record Level Access），并指出 JDBC 与 RLA 两种技术的区别。
从文章结构上，本文首先介绍 IBM i 数据库文件的基础知识。接下来，以客户信息数据库为例，指导读者如何使用 IBM Java Toolbox for i 创建数据库文件，然后添加、检索、更新与删除客户记录。另外，关于 IBM Java Toolbox for i 对数据库锁，游标，事务的相关支持，本文也所涉及。
IBM i 数据库文件
我们从文件系统与数据库这两个维度分别介绍 IBM i 数据库文件的基本概念。
IFS 文件 vs 数据库文件
首先，从文件的角度，IBM i 提供两种文件资源形式：IFS 文件与数据库文件。所谓 IFS（或 Integrated File System）指的是一种 POSIX 类文件系统，文件存在形式是一个字节序列，没有任何数据格式。IFS 的访问方式分为顺序与随机两种。而数据库文件具有指定的数据格式。我们称数据库文件的访问方式为记录级访问。有关 IFS 文件与数据库文件的区别，见图 1。
图 1. IFS 文件 vs 数据库文件
DB2 vs 数据库文件
接下来，我们从数据库的角度对比数据库文件与数据库表。表 1 描述了 IBM i 系统对象和 DB2 对象的对应关系。结合清单 1，我们可以简单地认为，IBM i 数据库文件有若干数据记录组成，这些数据记录以文件成员（Member）的形式呈现，每一个成员提供访问数据记录的路径。而每一条记录由各个字段组成，其对应关系为：行（Row） – 记录（Record）, 列（Column） – 字段（Field）。从这个角度而言，数据库表的记录格式与物理文件的记录格式完全相同。
表 1 IBM i 系统对象和 DB2 对象的对应关系
| DB2 对象 | IBM i 对象 |
|---|---|
| 模式（Schema） | 库（Library） |
| 表（Table） | 数据库文件（Database Files） |
JDBC 访问 vs 基于 RLA 访问
对于数据库的操作，无论从关系型数据库表的角度，还是 IBM i 数据库文件的角度，IBM Java Toolbox for i 均提供了 OO 编程级别的支持。
JDBC 是一种以 SQL 语句为核心的标准编程接口。IBM Java Toolbox for i 提供了标准的 JDBC 数据库编程实现。RLA（Record Level Access）则是一种以 SQL API 为核心的数据库文件访问技术。换句话说，不同于 IBM Java Toolbox for i 与数据库主机基于 Socket 的请求 / 相应式交互模式，RLA 直接调用 IBM i 底层的 Call Level Interface（调用级接口），属于直接调用 IBM i 本地 API 的方式，因此数据库的访问性能上要优于前者。
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