在 Android Studio 中使用 AsyncTask 解析在线 XML 文件时出现 NetworkOnMainThreadException

Question

我正在构建一个货币转换器，它使用以下 XML api 来获取汇率：http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml

但是我在函数中得到 NetworkOnMainThreadException 错误。

这是我的 XMLPath 函数，它应该扫描 API 并将那里的所有费率保存到 ArrayList 中：

    private static final String CURRENCY = "currency";
private static final String RATE = "rate";
private static final String CUBE_NODE = "//Cube/Cube/Cube";

public class XMLPath extends AsyncTask<URL, Void, List<Currency_Rate>> {

    @Override
    protected List<Currency_Rate> doInBackground(URL... urls){

        List<Currency_Rate> currRateList = new ArrayList<>();

        DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();
        DocumentBuilder builder = null;

        try {
            builder = builderFactory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
        Document document = null;
        String target_url = "http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml";

        try{
            URL xmlurl = new URL(target_url);
            InputStream xml = xmlurl.openStream();
            document = builder.parse(xml);

            XPathFactory xPathfactory = XPathFactory.newInstance();
            XPath xpath = xPathfactory.newXPath();
            String xPathString = CUBE_NODE;

            XPathExpression expression = xpath.compile(xPathString);

            NodeList nodel = (NodeList) expression.evaluate(document, XPathConstants.NODESET);

            for (int i = 0; i < nodel.getLength(); i++){

                Node node = nodel.item(i);
                NamedNodeMap attr = node.getAttributes();

                if(attr.getLength() > 0) {
                    Node currencyAttr = attr.getNamedItem(CURRENCY);

                    if(currencyAttr != null){
                        String currency_text = currencyAttr.getNodeValue();
                        String rate_text = attr.getNamedItem(RATE).getNodeValue();
                        currRateList.add(new Currency_Rate(rate_text));

                        Log.i("Rate", rate_text.toString());
                    }
                }
            }
        } catch (SAXException | XPathExpressionException | IOException e) {
            e.printStackTrace();
        }

        for (Currency_Rate currency_rate : currRateList){

            System.out.println(currency_rate);
        }


        return currRateList;
    }

    @Override
    protected void onPostExecute(List<Currency_Rate> currRateList)
    {
        super.onPostExecute(currRateList);
    }

}

我收到以下行的错误：

document = builder.parse(xml);

我在清单文件中启用了互联网访问：

<uses-permission android:name="android.permission.INTERNET" />

编辑

在应用程序的 MainActivity 的 OnCreate() 中，我调用了该类：

    XMLPath xmlpath = new XMLPath();
    xmlpath.doInBackground();

对此的解决方案非常迷茫，因此我们将不胜感激！ 谢谢

Answer 1

XMLPath xmlpath = new XMLPath();
xmlpath.doInBackground();

您不应直接致电doInBackground()。而是调用 execute() 方法之一，使 asynctask 在后台线程上运行。