使用 StAX API 解析文档
考虑以下文件:
<?xml version='1.0' encoding='UTF-8' ?>
<library>
<book id='1'>Effective Java</book>
<book id='2'>Java Concurrency In Practice</book>
<notABook id='3'>This is not a book element</notABook>
</library>
可以使用以下代码来解析它并按书 ID 构建书名的地图。
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.XMLStreamReader;
import java.io.StringReader;
import java.util.HashMap;
import java.util.Map;
public class StaxDemo {
public static void main(String[] args) throws Exception {
String xmlDocument = "<?xml version='1.0' encoding='UTF-8' ?>"
+ "<library>"
+ "<book id='1'>Effective Java</book>"
+ "<book id='2'>Java Concurrency In Practice</book>"
+ "<notABook id='3'>This is not a book element </notABook>"
+ "</library>";
XMLInputFactory xmlInputFactory = XMLInputFactory.newFactory();
// Various flavors are possible, e.g. from an InputStream, a Source, ...
XMLStreamReader xmlStreamReader = xmlInputFactory.createXMLStreamReader(new StringReader(xmlDocument));
Map<Integer, String> bookTitlesById = new HashMap<>();
// We go through each event using a loop
while (xmlStreamReader.hasNext()) {
switch (xmlStreamReader.getEventType()) {
case XMLStreamConstants.START_ELEMENT:
System.out.println("Found start of element: " + xmlStreamReader.getLocalName());
// Check if we are at the start of a <book> element
if ("book".equals(xmlStreamReader.getLocalName())) {
int bookId = Integer.parseInt(xmlStreamReader.getAttributeValue("", "id"));
String bookTitle = xmlStreamReader.getElementText();
bookTitlesById.put(bookId, bookTitle);
}
break;
// A bunch of other things are possible : comments, processing instructions, Whitespace...
default:
break;
}
xmlStreamReader.next();
}
System.out.println(bookTitlesById);
}
这输出:
Found start of element: library
Found start of element: book
Found start of element: book
Found start of element: notABook
{1=Effective Java, 2=Java Concurrency In Practice}
在这个样本中,必须要注意以下几点:
-
使用
xmlStreamReader.getAttributeValue
是有效的,因为我们首先检查了解析器是否处于START_ELEMENT
状态。在 evey 其他状态(ATTRIBUTES
除外)中,解析器被强制抛出IllegalStateException
,因为属性只能出现在元素的开头。 -
同样适用于
xmlStreamReader.getTextContent()
,它的工作原理是因为我们处于知识中,我们在本文档中知道<book>
元素没有非文本子节点。
对于更复杂的文档解析(更深层次,嵌套元素,…),将解析器委托给子方法或其他对象是一种很好的做法,例如有一个 BookParser
类或方法,并让它处理每个元素从 START_ELEMENT 到书籍 XML 标签的 END_ELEMENT。
人们还可以使用 Stack
对象来保持树上下的重要数据。