java从URL检索XML时不写入前几行

Question

我目前正在为大学编写一个基本的天气应用程序，其中包括从BBC天气RSS提要检索天气信息

我已经设置好将RSS提要输出到一个文件（output.xml）中，然后解析器类将使用该文件构建树

然而，我得到了The markup in the document following the root element must be well- formed. 运行时出错

在检查下载的XML文件时，我注意到前两个节点丢失了

以下是下载的XML：

<channel>
    <atom:link href="http://open.live.bbc.co.uk/weather/feeds/en/2656397/observations.rss" rel="self" type="application/rss+xml" />
    <title>BBC Weather - Observations for  Bangor, United Kingdom</title>
    <link>http://www.bbc.co.uk/weather/2656397</link>
    <description>Latest observations for Bangor from BBC Weather, including weather, temperature and wind information</description>
    <language>en</language>
    <copyright>Copyright: (C) British Broadcasting Corporation, see http://www.bbc.co.uk/terms/additional_rss.shtml for more details</copyright>
    <pubDate>Thu, 12 Mar 2015 05:35:08 +0000</pubDate>
    <item>
      <title>Thursday - 05:00 GMT: Thick Cloud, 10°C (50°F)</title>
      <link>http://www.bbc.co.uk/weather/2656397</link>
      <description>Temperature: 10°C (50°F), Wind Direction: South Easterly, Wind Speed: 8mph, Humidity: 90%, Pressure: 1021mb, Falling, Visibility: Very Good</description>
      <pubDate>Thu, 12 Mar 2015 05:35:08 +0000</pubDate>
      <guid isPermaLink="false">http://www.bbc.co.uk/weather/2656397-2015-03-12T05:35:08.000Z</guid>
      <georss:point>53.22647 -4.13459</georss:point>
    </item>
  </channel>
</rss>

XML在<channel>节点之前应该有以下两个节点：

<?xml version="1.0" encoding="UTF-8"?>
<rss xmlns:atom="http://www.w3.org/2005/Atom" xmlns:georss="http://www.georss.org/georss" version="2.0">

以下是我用于检索XML文件的代码：

public static void main(String[] args) throws SAXException, IOException, XPathExpressionException {
    URL url = new URL("http://open.live.bbc.co.uk/weather/feeds/en/2656397/observations.rss");
    URLConnection con = url.openConnection();
    StringBuilder builder;
    try (BufferedReader in = new BufferedReader(new InputStreamReader(con.getInputStream()))) {

        builder = new StringBuilder();
        String line;

        if (!in.readLine().isEmpty()) {
            line = in.readLine();
        }

        while ((line = in.readLine()) != null) {
            builder.append(line).append("\n");
        }

        String input = builder.toString();

        BufferedWriter out = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(new File("output.xml"))));
        out.write(input);
        out.flush();
    }
    try {
        WeatherParser parser = new WeatherParser();
        System.out.println(parser.parse("output.xml"));
    } catch (ParserConfigurationException ex) {
    }
}

下面是解析XML的代码（WeatherParser.java）：

public class WeatherParser {

    public WeatherParser() throws ParserConfigurationException {
        xpfactory = XPathFactory.newInstance();
        path = xpfactory.newXPath();
        dbfactory = DocumentBuilderFactory.newInstance();
        builder = dbfactory.newDocumentBuilder();
    }

    public String parse(String fileName) throws SAXException, IOException, XPathExpressionException {
        File f = new File(fileName);
        org.w3c.dom.Document doc = builder.parse(f);
        StringBuilder info = new StringBuilder();
        info.append(path.evaluate("/channel/item/title", doc));
        return info.toString();
    }

    private DocumentBuilderFactory dbfactory;
    private DocumentBuilder builder;
    private XPathFactory xpfactory;
    private XPath path;
}

希望提供的信息足够多

Answer 1

前两行丢失，因为您阅读了它，但没有“保存”它
去掉这个，它就会工作

    if (!in.readLine().isEmpty()) {
        line = in.readLine();
    }

在if中，您正在阅读第一行（<?xml....），但没有保留它
line = in.readLine();获取第二个变量，但当您输入while时，您将丢失line变量中的内容

Answer 2

首先，你不能操作服务器发送给你的数据流。放下StringBuilder。如果要将XML保存到磁盘，请逐字写入：

URL url = new URL("http://open.live.bbc.co.uk/weather/feeds/en/2656397/observations.rss");
URLConnection con = url.openConnection();
InputStream in = conn.getInputStream();
FileOutputStream out = new FileOutputStream("output.xml");

byte[] b = new byte[1024];
int count;
while ((count = in.read(b)) >= 0) {
    out.write(b, 0, count);
}
out.flush(); out.close(); in.close();

事实上，您根本不需要将其写入磁盘。可以直接从输入流构造XML文档

public static Document readXml(InputStream is) throws SAXException, ParserConfigurationException, IOException {
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();

    dbf.setValidating(false);
    dbf.setIgnoringComments(false);
    dbf.setIgnoringElementContentWhitespace(true);
    dbf.setNamespaceAware(true);

    DocumentBuilder db = dbf.newDocumentBuilder();
    return db.parse(is);
}

使您能够

public static void main (String[] args) throws java.lang.Exception
{
    URL observationsUrl = new URL("http://open.live.bbc.co.uk/weather/feeds/en/2656397/observations.rss");
    Document observations = readXml(observationsUrl.openConnection().getInputStream());

    XPathFactory xpf = XPathFactory.newInstance();
    XPath xpath = xpf.newXPath();

    String title = xpath.evaluate("/rss/channel/title", observations);
    System.out.println(title);

    XPathExpression rssitemsExpr = xpath.compile("/rss/channel/item");

    NodeList items = (NodeList)rssitemsExpr.evaluate(observations, XPathConstants.NODESET);
    for (int i = 0; i < items.getLength(); i++) {
        System.out.println(xpath.evaluate("./title", items.item(i)));
    }
}

我的输出：

BBC Weather - Observations for  Bangor, United Kingdom
Thursday - 06:00 GMT: Thick Cloud, 11°C (52°F)