java - xmlreader 不运行 startelement 方法

Question

我对 xmlreader 有疑问。我可以运行它并且 url 是正确的但它没有运行 startelement 方法。返回的所有值为空。我想知道为什么会发生这种情况以及解决方案。谢谢!

package com.headfirstlabs.nasadailyimage;

import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.UnknownHostException;
import java.util.jar.Attributes;

import javax.xml.parsers.SAXParser;
import javax.xml.parsers.SAXParserFactory;

import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.XMLReader;
import org.xml.sax.helpers.DefaultHandler;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.StrictMode;

public class IotdHandler extends DefaultHandler {
    private String url = "http://www.nasa.gov/rss/image_of_the_day.rss";
    private boolean inUrl = false;
    private boolean inTitle = false;
    private boolean inDescription = false;
    private boolean inItem = false;
    private boolean inDate = false;
    private Bitmap image = null;
    private String title = null;
    private StringBuffer description = new StringBuffer();
    private String date = null;

    public void processFeed() {
    try {
        StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();

        StrictMode.setThreadPolicy(policy); 
    SAXParserFactory factory =
    SAXParserFactory.newInstance();
    SAXParser parser = factory.newSAXParser();
    XMLReader reader = parser.getXMLReader();
    reader.setContentHandler(this);
    InputStream inputStream = new URL(url).openStream();
    reader.parse(new InputSource(inputStream));
    }
    catch(UnknownHostException e)
    {
        title="UnknownHostException";
    }
    catch(IOException e)
    {
        title="IOException";
    }
    catch(SAXException e)
    {
        title="SAXException";
    }
    catch (Exception e) {

        title="Exception";
        System.out.println(e);
    }
    }

    private Bitmap getBitmap(String url) {
        try {
        HttpURLConnection connection =
        (HttpURLConnection)new URL(url).openConnection();
        connection.setDoInput(true);
        connection.connect();
        InputStream input = connection.getInputStream();
        Bitmap bitmap = BitmapFactory.decodeStream(input);
        input.close();
        return bitmap;
        } catch (IOException ioe) { return null; }
    }

    public void startElement(String uri, String localName, String qName,
    Attributes attributes) throws SAXException {
        if (localName.equals("url")) { inUrl = true; }
        else { inUrl = false; }
        if (localName.startsWith("item")) { inItem = true; }
        else if (inItem) {
        if (localName.equals("title")) { inTitle = true; }
        else { inTitle = false; }
        if (localName.equals("description")) { inDescription = true; }
        else { inDescription = false; }
        if (localName.equals("pubDate")) { inDate = true; }
        else { inDate = false; }
        }
    }

    public void characters(char ch[], int start, int length) {
    String chars = new String(ch).substring(start, start + length);
    if (inUrl && url == null) { image = getBitmap(chars); }
    if (inTitle && title == null) { title = chars; }
    if (inDescription) { description.append(chars); }
    if (inDate && date == null) { date = chars; }
    }


    public Bitmap getImage() { return image; }
    public String getTitle() { return title; }
    public StringBuffer getDescription() { return description; }
    public String getDate() { return date; }
}

Answer 1

问题出在这一行:

import java.util.jar.Attributes;

应该是

import org.xml.sax.Attributes;

因为 startElement 的第四个参数的类型错误，所以您的方法没有覆盖 DefaultHandler 中的 startElement，并且因此 DefaultHandler 中 startElement 的默认实现被调用而不是您的方法。

您可以使用 @Override 注释来指示方法应该覆盖父类(super class)中的方法。如果带有 @Override 注释的方法没有覆盖父类(super class)方法，您将得到一个编译器错误。事实上，如果将此注释放在 startElement 方法上，就会出现这样的错误。