即使在 AsyncTask 中运行时"android.os.NetworkOnMainThreadException"

Question

即使我在asynctask thread中运行代码，我也会得到android.os.os.os.os.os.os.os.os.os.os.os.os.os.os.os.os.os.os。这基本上只是代码的一部分，应该从网站上返回我的价值。

package com.jasperhonkasalo.merenpinnankorkeusvsa;
import android.os.AsyncTask;
import android.util.Log;
import org.w3c.dom.Document;
import org.xml.sax.InputSource;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.StringReader;
import java.net.URL;
import java.util.Iterator;
import javax.xml.namespace.NamespaceContext;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathFactory;


public class NewBackgroundTask {

    public static class BackgroundTask extends AsyncTask<String, Void, String> {
        String value = "presetvalue";
        @Override
        protected String doInBackground(String... urls) {
            class NamespaceResolver implements NamespaceContext {
                private Document document;
                public NamespaceResolver(Document doc) {
                    document = doc;
                }
                public String getNamespaceURI(String prefix) {
                    if (prefix.equals("")) {
                        return document.lookupNamespaceURI(null);
                    } else {
                        return document.lookupNamespaceURI(prefix);
                    }
                }
                public String getPrefix(String namespaceURI) {
                    return document.lookupPrefix(namespaceURI);
                }
                public Iterator<String> getPrefixes(String namespaceURI) {
                    return null;
                }
            }

            try {
                String url = "http://opendata.fmi.fi/wfs/fin?service=WFS&version=2.0.0&request=GetFeature&storedquery_id=fmi::observations::mareograph::timevaluepair&fmisid=134223&";
                StringBuilder sb = new StringBuilder();
                try (BufferedReader r = new BufferedReader(new InputStreamReader(new URL(url).openStream(), "UTF-8"))) {
                    String line;
                    while ((line = r.readLine()) != null) {
                        sb.append(line + "n");
                    }
                }
                String xml = sb.toString();
                DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
                factory.setNamespaceAware(true);
                Document document = factory.newDocumentBuilder().parse(new InputSource(new StringReader(xml)));
                XPath xpath = XPathFactory.newInstance().newXPath();
                xpath.setNamespaceContext(new NamespaceResolver(document));
                String time = xpath.evaluate("//wml2:MeasurementTimeseries[@gml:id='obs-obs-1-1-WATLEV']/wml2:point[last()]//wml2:time", document);
                value = xpath.evaluate("//wml2:MeasurementTimeseries[@gml:id='obs-obs-1-1-WATLEV']/wml2:point[last()]//wml2:value", document);
                System.out.format("time = %s; value = %sn", time, value);
                return value;
            } catch (Exception e) {
                Log.d("EXCEPTIONFAIL", e.toString());
                return "FAIL: " + e.toString();
            }
        }
        @Override
        protected void onPostExecute(String feed) {
            // TODO: do something with the feed
        }
    }
}

我得到的值：

static String eeee = new NewBackgroundTask.BackgroundTask().value;

" value" - 变量的值保持相同(预先值(，也给我一个android.os.networkonmainthreadexception错误。我真的是任何Java编程的新手，都希望得到任何帮助。

Answer 1

您需要执行异步，您不能简单地访问它的静态字段，并期望它给您任何结果。您需要做这样的事情：

new BackgroundTask().execute();

由于这是一个异步调用，因此无法直接返回结果(这就是异步的含义(。相反，您可以在onPostExecute()方法中获得结果，该方法在主线程中执行。

如果您真的是Java编程的新手，则可能不应从异步方法调用开始。那是某种东西，甚至有经验的开发人员有时都有问题。