Java StAX

Java XML教程 - Java StAX

StAX是为了解决SAX和DOM API中的限制而创建的。

StAX API允许我们请求下一个事件(拉动事件)，并允许状态以过程方式存储。

XML解析有两种编程模型:流和文档对象模型(DOM)。

DOM模型涉及创建表示整个文档树的内存对象。DOM树可以自由导航。 成本是一个大的内存占用。这对于小文档来说是可以的，但是当文档变得更大时，内存消耗可能会迅速上升。

流是指其中XML数据被串行解析的编程模型。在文档中，我们只能在一个位置查看XML数据。这意味着我们需要在读取XML文档之前知道XML结构。用于XML处理的流模型在存在内存限制时很有用。

拉式解析vs推式解析

当我们想获取（拉取）XML数据时，我们做流式拉解析。

当解析器发送数据时，无论客户端是否准备好使用它，我们都进行流式推送解析。

StAX拉解析器可以过滤XML文档并忽略元素不必要。

StAX是一个双向API，通过它我们可以读取和写入XML文档。 SAX是只读的。

SAX是一个推送API，而StAX是拉式。

例子

此程序演示如何使用StAX解析器。它打印XHTML网页的所有超链接链接。

import java.io.InputStream;
import java.net.URL;

import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.XMLStreamReader;

public class Main {
  public static void main(String[] args) throws Exception {
    URL url = new URL("http://www.w3c.org");
    InputStream in = url.openStream();
    XMLInputFactory factory = XMLInputFactory.newInstance();
    XMLStreamReader parser = factory.createXMLStreamReader(in);
    while (parser.hasNext()) {
      int event = parser.next();
      if (event == XMLStreamConstants.START_ELEMENT) {
        if (parser.getLocalName().equals("a")) {
          String href = parser.getAttributeValue(null, "href");
          if (href != null)
            System.out.println(href);
        }
      }
    }
  }
}

例2

下面的代码显示了如何使用XML流读取器加载XML文档。

import java.io.File;
import java.io.FileInputStream;

import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.XMLStreamReader;

public class Main {
  public static void main(String[] args) throws Exception {
    File file = new File("yourXML.xml");
    FileInputStream inputStream = new FileInputStream(file);
    XMLInputFactory inputFactory = XMLInputFactory.newInstance();
    XMLStreamReader reader = inputFactory.createXMLStreamReader(inputStream);

    System.out.println(reader.getVersion());
    System.out.println(reader.isStandalone());
    System.out.println(reader.standaloneSet());
    System.out.println(reader.getEncoding());
    System.out.println(reader.getCharacterEncodingScheme());

    parseRestOfDocument(reader);
  }

  private static void parseRestOfDocument(XMLStreamReader reader)
      throws XMLStreamException {

    while (reader.hasNext()) {
      int type = reader.next();
      switch (type) {
      case XMLStreamConstants.START_ELEMENT:
        System.out.println(reader.getLocalName());
        if (reader.getNamespaceURI() != null) {
          String prefix = reader.getPrefix();
          if (prefix == null) {
            prefix = "[None]";
          }
          System.out.println("prefix = "" + prefix + "", URI = ""
              + reader.getNamespaceURI() + """);
        }

        if (reader.getAttributeCount() > 0) {
          for (int i = 0; i < reader.getAttributeCount(); i++) {
            System.out.println("Attribute (name = ""
                + reader.getAttributeLocalName(i) + "", value = ""
                + reader.getAttributeValue(i) + "")");
            String attURI = reader.getAttributeNamespace(i);
            if (attURI != null) {
              String attPrefix = reader.getAttributePrefix(i);
              if (attPrefix == null || attPrefix.equals("")) {
                attPrefix = "[None]";
              }
              System.out.println("prefix=" + attPrefix + ",URI=" + attURI);
            }
          }
        }

        break;
      case XMLStreamConstants.END_ELEMENT:
        System.out.println("XMLStreamConstants.END_ELEMENT");
        break;
      case XMLStreamConstants.CHARACTERS:
        if (!reader.isWhiteSpace()) {
          System.out.println("CD:" + reader.getText());
        }
        break;
      case XMLStreamConstants.DTD:
        System.out.println("DTD:" + reader.getText());
        break;
      case XMLStreamConstants.SPACE:
        System.out.println(" ");
        break;
      case XMLStreamConstants.COMMENT:
        System.out.println(reader.getText());
        break;
      default:
        System.out.println(type);
      }
    }
  }
}

XMLStreamWriter

以下代码显示了如何使用 XMLStreamWriter 输出xml。

import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamWriter;

public class Main {
  public static void main(String[] args) throws Exception {
    XMLOutputFactory factory = XMLOutputFactory.newInstance();
    XMLStreamWriter writer = factory.createXMLStreamWriter(System.out);

    writer.writeStartDocument("1.0");

    writer.writeStartElement("catalog");

    writer.writeStartElement("book");

    writer.writeAttribute("id", "1");

    writer.writeStartElement("code");
    writer.writeCharacters("I01");
    writer.writeEndElement();

    writer.writeStartElement("title");
    writer.writeCharacters("This is the title");
    writer.writeEndElement();

    writer.writeStartElement("price");
    writer.writeCharacters("$2.95");
    writer.writeEndElement();

    writer.writeEndDocument();

    writer.flush();
    writer.close();
  }
}

上面的代码生成以下结果。

XMLEventReader

import java.io.FileReader;
import java.io.Reader;
import java.util.Iterator;

import javax.xml.namespace.QName;
import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.events.Attribute;
import javax.xml.stream.events.Characters;
import javax.xml.stream.events.EndElement;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.XMLEvent;

public class Main {
  public static void main(String[] args) throws Exception {
    XMLInputFactory factory = XMLInputFactory.newInstance();
    Reader fileReader = new FileReader("Source.xml");
    XMLEventReader reader = factory.createXMLEventReader(fileReader);

    while (reader.hasNext()) {
      XMLEvent event = reader.nextEvent();
      if (event.isStartElement()) {
        StartElement element = (StartElement) event;
        System.out.println("Start Element: " + element.getName());

        Iterator iterator = element.getAttributes();
        while (iterator.hasNext()) {
          Attribute attribute = (Attribute) iterator.next();
          QName name = attribute.getName();
          String value = attribute.getValue();
          System.out.println("Attribute name/value: " + name + "/" + value);
        }
      }
      if (event.isEndElement()) {
        EndElement element = (EndElement) event;
        System.out.println("End element:" + element.getName());
      }
      if (event.isCharacters()) {
        Characters characters = (Characters) event;
        System.out.println("Text: " + characters.getData());
      }
    }
  }
}

XMLEventWriter

import java.util.Arrays;
import java.util.List;

import javax.xml.stream.XMLEventFactory;
import javax.xml.stream.XMLEventWriter;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.events.Attribute;
import javax.xml.stream.events.Characters;
import javax.xml.stream.events.EndDocument;
import javax.xml.stream.events.EndElement;
import javax.xml.stream.events.StartDocument;
import javax.xml.stream.events.StartElement;

public class Main {
  public static void main(String[] args) throws Exception {
    XMLOutputFactory outputFactory = XMLOutputFactory.newInstance();

    XMLEventWriter writer = outputFactory.createXMLEventWriter(System.out);

    XMLEventFactory xmlEventFactory = XMLEventFactory.newInstance();

    StartDocument startDocument = xmlEventFactory.createStartDocument("UTF-8", "1.0");
    writer.add(startDocument);

    StartElement startElement = xmlEventFactory.createStartElement("", "", "My-list");
    writer.add(startElement);

    Attribute attribute = xmlEventFactory.createAttribute("version", "1");
    List attributeList = Arrays.asList(attribute);
    List nsList = Arrays.asList();
    StartElement startElement2 = xmlEventFactory.createStartElement("", "", "Item",
        attributeList.iterator(), nsList.iterator());
    writer.add(startElement2);

    StartElement codeSE = xmlEventFactory.createStartElement("", "", "code");
    writer.add(codeSE);
    Characters codeChars = xmlEventFactory.createCharacters("I001");
    writer.add(codeChars);
    EndElement codeEE = xmlEventFactory.createEndElement("", "", "code");
    writer.add(codeEE);

    StartElement nameSE = xmlEventFactory.createStartElement(" ", " ", "name");
    writer.add(nameSE);
    Characters nameChars = xmlEventFactory.createCharacters("a name");
    writer.add(nameChars);
    EndElement nameEE = xmlEventFactory.createEndElement("", "", "name");
    writer.add(nameEE);

    StartElement contactSE = xmlEventFactory.createStartElement("", "", "contact");
    writer.add(contactSE);
    Characters contactChars = xmlEventFactory.createCharacters("another name");
    writer.add(contactChars);
    EndElement contactEE = xmlEventFactory.createEndElement("", "", "contact");
    writer.add(contactEE);

    EndDocument ed = xmlEventFactory.createEndDocument();
    writer.add(ed);

    writer.flush();
    writer.close();
  }
}

上面的代码生成以下结果。

StreamFilter

import java.io.FileInputStream;

import javax.xml.namespace.QName;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamReader;
import javax.xml.stream.events.XMLEvent;

public class Main implements javax.xml.stream.StreamFilter {
  public static void main(String[] args) throws Exception {
    String filename = "yourXML.xml";

    XMLInputFactory xmlif = null;

    xmlif = XMLInputFactory.newInstance();
    xmlif.setProperty(XMLInputFactory.IS_REPLACING_ENTITY_REFERENCES,
        Boolean.TRUE);
    xmlif.setProperty(XMLInputFactory.IS_SUPPORTING_EXTERNAL_ENTITIES,
        Boolean.FALSE);
    xmlif.setProperty(XMLInputFactory.IS_NAMESPACE_AWARE, Boolean.TRUE);
    xmlif.setProperty(XMLInputFactory.IS_COALESCING, Boolean.TRUE);

    System.out.println("FACTORY: " + xmlif);
    System.out.println("filename = " + filename);

    FileInputStream fis = new FileInputStream(filename);

    XMLStreamReader xmlr = xmlif.createFilteredReader(
        xmlif.createXMLStreamReader(fis), new Main());

    int eventType = xmlr.getEventType();
    printEventType(eventType);

    while (xmlr.hasNext()) {
      eventType = xmlr.next();
      printEventType(eventType);
      printName(xmlr, eventType);
      printText(xmlr);

      if (xmlr.isStartElement()) {
        printAttributes(xmlr);
      }
      printPIData(xmlr);
    }
  }

  public static final String getEventTypeString(int eventType) {
    switch (eventType) {
    case XMLEvent.START_ELEMENT:
      return "START_ELEMENT";

    case XMLEvent.END_ELEMENT:
      return "END_ELEMENT";

    case XMLEvent.PROCESSING_INSTRUCTION:
      return "PROCESSING_INSTRUCTION";

    case XMLEvent.CHARACTERS:
      return "CHARACTERS";

    case XMLEvent.COMMENT:
      return "COMMENT";

    case XMLEvent.START_DOCUMENT:
      return "START_DOCUMENT";

    case XMLEvent.END_DOCUMENT:
      return "END_DOCUMENT";

    case XMLEvent.ENTITY_REFERENCE:
      return "ENTITY_REFERENCE";

    case XMLEvent.ATTRIBUTE:
      return "ATTRIBUTE";

    case XMLEvent.DTD:
      return "DTD";

    case XMLEvent.CDATA:
      return "CDATA";
    }

    return "UNKNOWN_EVENT_TYPE";
  }

  private static void printEventType(int eventType) {
    System.out.print("EVENT TYPE(" + eventType + "):");
    System.out.println(getEventTypeString(eventType));
  }

  private static void printName(XMLStreamReader xmlr, int eventType) {
    if (xmlr.hasName()) {
      System.out.println("HAS NAME: " + xmlr.getLocalName());
    } else {
      System.out.println("HAS NO NAME");
    }
  }

  private static void printText(XMLStreamReader xmlr) {
    if (xmlr.hasText()) {
      System.out.println("HAS TEXT: " + xmlr.getText());
    } else {
      System.out.println("HAS NO TEXT");
    }
  }

  private static void printPIData(XMLStreamReader xmlr) {
    if (xmlr.getEventType() == XMLEvent.PROCESSING_INSTRUCTION) {
      System.out.println(" PI target = " + xmlr.getPITarget());
      System.out.println(" PI Data = " + xmlr.getPIData());
    }
  }

  private static void printAttributes(XMLStreamReader xmlr) {
    if (xmlr.getAttributeCount() > 0) {
      System.out.println("\nHAS ATTRIBUTES: ");

      int count = xmlr.getAttributeCount();

      for (int i = 0; i < count; i++) {
        QName name = xmlr.getAttributeName(i);
        String namespace = xmlr.getAttributeNamespace(i);
        String type = xmlr.getAttributeType(i);
        String prefix = xmlr.getAttributePrefix(i);
        String value = xmlr.getAttributeValue(i);

        System.out.println("ATTRIBUTE-PREFIX: " + prefix);
        System.out.println("ATTRIBUTE-NAMESP: " + namespace);
        System.out.println("ATTRIBUTE-NAME:   " + name.toString());
        System.out.println("ATTRIBUTE-VALUE:  " + value);
        System.out.println("ATTRIBUTE-TYPE:  " + type);
      }
    } else {
      System.out.println("HAS NO ATTRIBUTES");
    }
  }

  public boolean accept(XMLStreamReader reader) {
    if (!reader.isStartElement() && !reader.isEndElement()) {
      return false;
    } else {
      return true;
    }
  }
}