如何从 URL 填充我的 .xml 文件？答案

【问题标题】：How to fill my .xml file from URL?如何从 URL 填充我的 .xml 文件？
【发布时间】：2018-03-07 13:46:59
【问题描述】：

我正在构建一个天气应用程序，我需要用来自 URL 的数据填充一个 .xml 文件。

XML 文件的 URL 是 http://vrijeme.hr/hrvatska_n.xml，我创建了一个名为 NewFile.xml 的文件。

当我手动下载此 URL 数据并将其导入 Eclipse IDE 时，它工作正常。但是就是这样的方式，我不能得到远程资源的las内容。

下面是我正在使用的代码部分：

String readXML = null;
URL url = null;
URLConnection urlconn = null;

try {
    url = new URL("http://vrijeme.hr/hrvatska_n.xml");
    urlconn = url.openConnection();
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

try {
    InputStreamReader inst = new InputStreamReader(urlconn.getInputStream());
    BufferedReader bfr = new BufferedReader(inst);
    boolean eof = false;
    while (!eof) {
        readXML = bfr.readLine();
        if (readXML == null) {
            eof = true;
        }
    }
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}

try {
    FileOutputStream fout = new FileOutputStream("NewFile.xml");
    Writer out = new OutputStreamWriter(fout, "UTF8");
    out.write(readXML);
} catch (IOException z) {
    System.out.println("Nešto se sjebalo.");
}

try {
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();
    Document doc = builder.parse("NewFile.xml");                                            //Dokument

    NodeList cityList = doc.getElementsByTagName("GradIme");
    Node cNode = cityList.item(12);

    if (cNode.getNodeType() == Node.ELEMENT_NODE) {

        Element cElement = (Element) cNode;
        String city = cElement.getTextContent();
        System.out.println("Ime grada: " + city);
    }

    NodeList tempList = doc.getElementsByTagName("Temp");                                //Element po nazivu
    Node nNode = tempList.item(12);                                                        //Item number

    if (nNode.getNodeType() == Node.ELEMENT_NODE) {                                        // provjeta tipa podataka == Element

        Element eElement = (Element) nNode;                                            // Element nNode
        String temperatura = eElement.getTextContent();                                    //uzima text iz elementa
        System.out.println("Temperatura:" + temperatura + " C");

        NodeList vlagaList = doc.getElementsByTagName("Vlaga");
        Node vNode = vlagaList.item(12);

        if (vNode.getNodeType() == Node.ELEMENT_NODE) {

            Element vElement = (Element) vNode;
            String vlaga = vElement.getTextContent();
            System.out.println("Vlaga u zraku: " + vlaga + "%");
        }
    }
} catch (ParserConfigurationException | IOException | SAXException e) {
    e.printStackTrace();
}

【问题讨论】：

你的代码有什么问题？你得到错误？还是有意想不到的结果？
VrijemeDanas.main(VrijemeDanas.java:68) 的 java.io.Writer.write（未知来源）的线程“主”java.lang.NullPointerException 中的异常
这就是我得到的。当我尝试将数据写入 NewFile.xml 时，它与 OutputStream 一致
请将异常堆栈添加到主帖中。
这是因为readXML 是null。

标签： java xml eclipse inputstream

【解决方案1】：

您应该设法清理您的 main 方法：您几乎正在分离用于文件读取和写入的代码块，这导致 readXml 输入在您尝试将其写入目标文件时变为 null。

这里是你源代码的略微修改版本：

public static void main(String[] args) {
    String readXML;
    URL url;
    URLConnection urlconn;

    try {
        url = new URL("http://vrijeme.hr/hrvatska_n.xml");
        urlconn = url.openConnection();

        BufferedReader bfr = null;
        InputStreamReader inst = null;
        FileOutputStream fout = null;
        Writer out = null;
        try { // read & write in the same block
            inst = new InputStreamReader(urlconn.getInputStream());
            bfr = new BufferedReader(inst);
            fout = new FileOutputStream("NewFile.xml");
            out = new OutputStreamWriter(fout, "UTF8");
            while ((readXML = bfr.readLine()) != null) {
                out.write(readXML);
            }
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally { // cleanup your resources
            if (out != null) {
                out.close();
            }
            if (fout != null) {
                fout.close();
            }
            if (bfr != null) {
                bfr.close();
            }
            if (inst != null) {
                inst.close();
            }
        }
    } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    try {
        DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = factory.newDocumentBuilder();
        Document doc = builder.parse("NewFile.xml");                                            //Dokument

        NodeList cityList = doc.getElementsByTagName("GradIme");
        Node cNode = cityList.item(12);

        if (cNode.getNodeType() == Node.ELEMENT_NODE) {

            Element cElement = (Element) cNode;
            String city = cElement.getTextContent();
            System.out.println("Ime grada: " + city);
        }

        NodeList tempList = doc.getElementsByTagName("Temp");                                //Element po nazivu
        Node nNode = tempList.item(12);                                                        //Item number

        if (nNode.getNodeType() == Node.ELEMENT_NODE) {                                        // provjeta tipa podataka == Element

            Element eElement = (Element) nNode;                                            // Element nNode
            String temperatura = eElement.getTextContent();                                    //uzima text iz elementa
            System.out.println("Temperatura:" + temperatura + " C");

            NodeList vlagaList = doc.getElementsByTagName("Vlaga");
            Node vNode = vlagaList.item(12);

            if (vNode.getNodeType() == Node.ELEMENT_NODE) {

                Element vElement = (Element) vNode;
                String vlaga = vElement.getTextContent();
                System.out.println("Vlaga u zraku: " + vlaga + "%");
            }
        }
    } catch (ParserConfigurationException | IOException | SAXException e) {
        e.printStackTrace();
    }
}

【讨论】：

您应该注意，这应该是一个实验性代码 sn-p，如果您需要生产就绪的代码，您应该设法清理更多。特别注意资源清理。

【解决方案2】：

你可以试试这段代码，它使用的是Java 8 nio API。

public void downloadFile(final String url, final String targetPath) throws IOException {

    final URL website = new URL(url);
    final Path target = Paths.get(targetPath);
    System.out.println(target.toAbsolutePath().toString());
    try (InputStream in = website.openStream()) {
        Files.copy(in, target, StandardCopyOption.REPLACE_EXISTING);
    }
}

//You can call the function
downloadFile("http://vrijeme.hr/hrvatska_n.xml", "NewFile.xml");

【讨论】：