ItemReader

尽管概念简单，ItemReader 却是从多种不同类型输入提供数据的一种方法。最常见的示例包括：

Although a simple concept, an ItemReader is the means for providing data from many different types of input. The most general examples include:

  • Flat File: Flat-file item readers read lines of data from a flat file that typically describes records with fields of data defined by fixed positions in the file or delimited by some special character (such as a comma).

  • XML: XML ItemReaders process XML independently of technologies used for parsing, mapping and validating objects. Input data allows for the validation of an XML file against an XSD schema.

  • Database: A database resource is accessed to return resultsets which can be mapped to objects for processing. The default SQL ItemReader implementations invoke a RowMapper to return objects, keep track of the current row if restart is required, store basic statistics, and provide some transaction enhancements that are explained later.

还有更多可能性，但我们专注于本章中的基础知识。可以在Appendix A中找到所有可用 `ItemReader`实现的完整列表。

There are many more possibilities, but we focus on the basic ones for this chapter. A complete list of all available ItemReader implementations can be found in Appendix A.

ItemReader 是用于普通输入操作的基本接口，如下面的接口定义所示：

ItemReader is a basic interface for generic input operations, as shown in the following interface definition:

public interface ItemReader<T> {

    T read() throws Exception, UnexpectedInputException, ParseException, NonTransientResourceException;

}

read 方法定义了 ItemReader 的最基本约定。调用它可返回一个项或（如果没有更多项）返回 null。一项可以表示文件一行、数据库一行或 XML 文件中的一个元素。通常，这些项会映射到一个可用的域对象（例如 Trade、Foo 或其他对象），但约定中没有此要求。

The read method defines the most essential contract of the ItemReader. Calling it returns one item or null if no more items are left. An item might represent a line in a file, a row in a database, or an element in an XML file. It is generally expected that these are mapped to a usable domain object (such as Trade, Foo, or others), but there is no requirement in the contract to do so.

ItemReader 接口的实现有望仅限于前向操作。但是，如果底层资源是事务性的（例如 JMS 队列），则在回滚场景中调用 read 可能会在后续调用中返回相同逻辑项。值得注意的是，ItemReader 没有可处理的项并不会导致引发异常。例如，使用返回 0 个结果的查询配置的数据库 ItemReader 会在第一次调用 read 时返回 null。

It is expected that implementations of the ItemReader interface are forward only. However, if the underlying resource is transactional (such as a JMS queue) then calling read may return the same logical item on subsequent calls in a rollback scenario. It is also worth noting that a lack of items to process by an ItemReader does not cause an exception to be thrown. For example, a database ItemReader that is configured with a query that returns 0 results returns null on the first invocation of read.