java - 如何以流式传输方式迭代巨大 XML 中的节点？

Question

我有一个巨大的 XML 文件，如下所示:

<?xml version="1.0"?>
<catalog>
   <book id="bk101">
      <author>Gambardella, Matthew</author>
      <title>XML Developer's Guide</title>
   </book>
   <book id="bk102">
      <author>Ralls, Kim</author>
      <title>Midnight Rain</title>
   </book>
   [... one gazillion more entries ...]
</catalog>

我想以流的方式迭代这个文件，这样我就不必将整个文件加载到内存中，例如:

InputStream stream = new FileInputStream("gigantic-book-list.xml");
String nodeName = "book";
Iterator it = new StreamingXmlIterator(stream, nodeName);
Document bk101 = it.next();
Document bk102 = it.next();

此外，我希望它能够处理不同的 XML 输入文件，而无需创建特定对象(例如 Book.java)。

@McDowell 有一个很有前途的方法，使用 XMLStreamReader 和 StreamFilter，地址为 https://stackoverflow.com/a/16799693/13365 ，但这仅提取单个节点。

此外，Camel's .tokenizeXML正是我想要的，所以我想我应该查看源代码。

Answer 1

@XmlRootElement
public class Book {
  // TODO: getters/setters
  public String author;
  public String title;
}

假设您希望将数据作为强类型对象进行处理，您可以使用实用程序类型组合 StAX 和 JAXB:

  class ContentFinder implements StreamFilter {
    private boolean capture = false;

    @Override
    public boolean accept(XMLStreamReader xml) {
      if (xml.isStartElement() && "book".equals(xml.getLocalName())) {
        capture = true;
      } else if (xml.isEndElement() && "book".equals(xml.getLocalName())) {
        capture = false;
        return true;
      }
      return capture;
    }
  }

  class Limiter extends StreamReaderDelegate {
    Limiter(XMLStreamReader xml) {
      super(xml);
    }

    @Override
    public boolean hasNext() throws XMLStreamException {
      return !(getParent().isEndElement()
               && "book".equals(getParent().getLocalName()));
    }
  }

用法:

XMLInputFactory inFactory = XMLInputFactory.newFactory();
XMLStreamReader reader = inFactory.createXMLStreamReader(inputStream);
reader = inFactory.createFilteredReader(reader, new ContentFinder());
Unmarshaller unmar = JAXBContext.newInstance(Book.class)
    .createUnmarshaller();
Transformer tformer = TransformerFactory.newInstance().newTransformer();
while (reader.hasNext()) {
  XMLStreamReader limiter = new Limiter(reader);
  Source src = new StAXSource(limiter);
  DOMResult res = new DOMResult();
  tformer.transform(src, res);
  Book book = (Book) unmar.unmarshal(res.getNode());
  System.out.println(book.title);
}