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以下课程的目的是从不同新闻网站的不同文章中获取文本。以下版本专为Android设计，但运行时会引发NetworkOnMainThread异常。当我使用专门用于在计算机上运行的此类的早期版本时，它运行良好，但我不确定网络I/O在Android上的工作方式。我已经看到关于此主题的其他一些问题的答案，但我不明白为什么在Android中程序会引发异常，但在桌面上它可以正常工作。谁能解释一下？如何在Android上使用网络I/O
package com.example.user.helloworld;
import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.select.Elements;
public class ArticleReceiver {
private ArrayList<Article> newsArticles = new ArrayList<>();
private ArrayList<String> newsLinks = new ArrayList<>();
public ArticleReceiver(int numArticles, String link) {
if (numArticles != 0) {
receiveNewsArticles(numArticles, link);
}else{
System.out.println("ERROR: numArticles request for " + link + " cannot equal 0.");
}
}
private void receiveNewsArticles(int numArticles, String urlAddress) {
URL rssUrl = null;
// if connected to Internet
if (true){//isInternetAvailable()) {
try {
// gather links
rssUrl = new URL(urlAddress);
BufferedReader in = new BufferedReader(new InputStreamReader(rssUrl.openStream()));
String line;
// fix bbc trash urls
if (urlAddress.equals(Main.BBC_URL)) {
numArticles++;
}
while ((line = in.readLine()) != null && newsLinks.size() <= numArticles) {
if (line.contains("<link>")) {
// find links through tags
int firstPos = line.indexOf("<link>");
String temp = line.substring(firstPos);
temp = temp.replace("<link>", "");
int lastPos = temp.indexOf("</link>");
temp = temp.substring(0, lastPos);
newsLinks.add(temp);
}
}
in.close();
// test if there are links and if there is remove first
// unnecessary
// link
if (!newsLinks.isEmpty()) {
if (urlAddress.equals(Main.BBC_URL)) {
newsLinks.remove(0);
newsLinks.remove(0);
}else if(urlAddress.equals(Main.CNN_URL) || urlAddress.equals(Main.FOX_URL) || urlAddress.equals(Main.ESPN_URL)){
newsLinks.remove(0);
}
} else {
System.out.println("ERROR: No Found Articles. Check If You Have Wifi.");
}
// gather articles from HTML "section" or "p" tag of article using Jsoup
for (String newsLink : newsLinks) {
// get webpage
Document doc = Jsoup.connect(newsLink).get();
// get article from different websites
String article = null;
if (urlAddress.equals(Main.FOX_URL)) {
Elements element = doc.select("p");
article = element.text();
} else if (urlAddress.equals(Main.CNN_URL)) {
Elements element = doc.select("section");
article = element.text();
} else if (urlAddress.equals(Main.BBC_URL)) {
Elements element = doc.select("p");
article = element.text();
}else if(urlAddress.equals(Main.ESPN_URL)){
Elements element = doc.select("p");
article = element.text();
}
newsArticles.add(new Article(article, Main.SUMMARY_SENTENCES));
}
} catch (IOException e) {
e.printStackTrace();
}
} else {
System.out.println("ERROR: No internet connection established.");
return;
}
}
public ArrayList<Article> getArticles() {
return newsArticles;
}
public Article getArticle(int i) {
if (newsArticles.size() <= i) {
return null;
} else {
return newsArticles.get(i);
}
}
//The method below does not recognize the "getSystemService" method, and when the method is no longer present there is a NetworkOnMainThreadException
private boolean isInternetAvailable() {
ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
return activeNetworkInfo != null && activeNetworkInfo.isConnected();
}
}
您需要在单独的线程中进行调用。 AsyncTask，Loader，Thread ..... –