即使在AsyncTask中运行，也会出现“ android.os.NetworkOnMainThreadException”

Question

I'm getting the android.os.NetworkOnMainThreadException error even when I'm running the code inside AsyncTask thread. This is basically just a chunk of code that should return me a value from a website.

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 = %s\n", 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
        }
    }
}

I'm getting the value with:

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

The value of "value"- variable is staying the same (presetvalue) and also giving me a android.os.NetworkOnMainThreadException error. I'm really new to any java programming and would appreciate any help.

Answer 1

You need to execute the AsyncTask, you cannot simply access a static field of it and expect it to give you any result. You need to do something like that:

new BackgroundTask().execute();

Since this is an asynchronous call, it cannot return a result directly (that's what asynchronous means). Instead, you get the result in the onPostExecute() method, which is executed in the main thread.

If you are really completely new to Java programming, you should perhaps not start with asynchronous method calls. That's something, even experienced developers have sometimes problems with.