java - XmlPullParserException "expected: START_DOCUMENT {null}articles (position:START_TAG"

Question

当我试图实现一个从 Android 的“Parsing XML Data”培训页面派生的简单 xml 解析器时，我遇到了这个错误。由于我是一名新的 Android 开发人员——并且是一名非常业余的编码人员——当我尝试实现我自己的修改版本时抛出的异常无助于指导我解决问题。

这是抛出的确切 XmlPullParserException:

04-06 19:10:42.772  19279-19279/com.example.reader.app E/Main-Activity﹕ expected: START_DOCUMENT {null}articles (position:START_TAG <articles>@2:11 in java.io.InputStreamReader@41ec5dd8)

这是我的测试 xml 文件:

<?xml version="1.0" encoding="utf-8"?>
<articles>
    <article>
        <title>Test Article Number 1</title>
        <description>This is a short description of the article.</description>
        <text>This is some text.</text>
    </article>
    <article>
        <title>Test Article Number 2</title>
        <description>This is a short description of the article.</description>
        <text>This is some text.</text>
    </article>
</articles>

这是我的 MainActivity.java 文件的 onCreate 方法，它打开 inputStream 并实例化我的 ArticleListParser 类。

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    mNavigationDrawerFragment = (NavigationDrawerFragment)
            getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
    mTitle = getTitle();

    // Set up the drawer.
    mNavigationDrawerFragment.setUp(
            R.id.navigation_drawer,
            (DrawerLayout) findViewById(R.id.drawer_layout));

    // try to access the data in the xml file
    Log.d(TAG, "Starting to execute your code...");
    try {
        InputStream minputStream = getResources().openRawResource(R.raw.test);
        ArticleListParser marticleParser = new ArticleListParser();
        List mlistOfArticles = marticleParser.parse(minputStream);
    } catch (Exception e) {
        Log.e(TAG, e.getMessage());
    }
}

这是我的 ArticleListParser.java 类。由 MainActivity 实例化的此类的实例不会使其通过 parse() 方法(即，不会执行将由 return readArticles(parser); parse 中的行触发的任何其他方法())...

package com.example.reader.app;
import android.util.Log;
import android.util.Xml;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.List;

/**
 * Created by jasondblack on 4/5/14.
 */

public class ArticleListParser {
    // We don't use namespaces
    private static final String ns = null;

    // log tag
    private static final String TAG = "ArticleListParser";

    public List parse(InputStream in) throws XmlPullParserException, IOException {
        try {
            // get a new xml pull parser
            XmlPullParser parser = Xml.newPullParser();

            // indicate that this xml does not have namespaces
            parser.setFeature(XmlPullParser.FEATURE_PROCESS_NAMESPACES, false);

            // set the input stream of the pull parser
            parser.setInput(in, null);

            // go to the first element
            parser.nextTag();

            // start reading the xml
            return readArticles(parser);
        } finally {
            // no matter what, close the input stream
            in.close();
        }
    }

    private List readArticles(XmlPullParser parser) throws XmlPullParserException, IOException {
        // create List that will be returned
        List articles = new ArrayList();

        parser.require(XmlPullParser.START_DOCUMENT, ns, "articles");
        while(parser.next() != XmlPullParser.END_TAG) {
            if(parser.getEventType() != XmlPullParser.START_TAG) {
                continue;
            }
            String thisName = parser.getName();

            if(thisName.equals("article")) {
                articles.add(readArticle(parser));
            }
            else {
                skip(parser);
            }
        }
        return articles;
    }

    public static class Article {
        public final String title;
        public final String description;
        public final String text;

        private Article (String thisTitle, String thisDescription, String thisText) {
            this.title = thisTitle;
            this.description = thisDescription;
            this.text = thisText;
        }
    }

    private Article readArticle(XmlPullParser parser) throws XmlPullParserException, IOException {
        // make sure the xml parser is at the beginning of a tag
        parser.require(XmlPullParser.START_TAG, ns, "article");
        String title = null;
        String description = null;
        String text = null;

        // step through xml file until you reach the end tag
        while(parser.getEventType() != XmlPullParser.END_TAG) {
            // if it is the start tag, continue onto next element in parser
            // ??? weird because we have already required a start tag in the first line of this method
            if(parser.getEventType() != XmlPullParser.START_TAG) {
                continue;
            }

            // get the name of the element in the angled brackets
            String name = parser.getName();

            // check to see if it is one of the titles we want ... if so extract the contents
            if(name.equals("title")) {
                title = readTitle(parser);
            } else if (name.equals("description")) {
                description = readDescription(parser);
            } else if (name.equals("text")) {
                text = readText(parser);
            } else {
                skip(parser);
            }
        }
        // return an Article object
        return new Article (title, description, text);
    }

    // TODO: stopped here don't be confused by duplicate methods implement your own readTitle(), readDescription(), readText(), and extractText() [their "readTExt()" method]

    // Processes title tags in the feed
    private String readTitle(XmlPullParser parser) throws IOException, XmlPullParserException {
        parser.require(XmlPullParser.START_TAG, ns, "title");
        String thisTitle = readText(parser);
        parser.require(XmlPullParser.END_TAG, ns, "title");
        return thisTitle;
    }

    // Processes description tags in the feed.
    private String readDescription(XmlPullParser parser) throws IOException, XmlPullParserException {
        parser.require(XmlPullParser.START_TAG, ns, "description");
        String thisDescription = readText(parser);
        parser.require(XmlPullParser.END_TAG, ns, "description");
        return thisDescription;
    }

    // For the tags title and summary, extracts their text values.
    private String readText(XmlPullParser parser) throws IOException, XmlPullParserException {
        String result = "";
        // TODO: possible that this will snag on my xml because I don't have type="text" in my xml elements
        if (parser.next() == XmlPullParser.TEXT) {
            result = parser.getText();
            parser.nextTag();
        }
        return result;
    }

    private void skip(XmlPullParser parser) throws XmlPullParserException, IOException {
        if (parser.getEventType() != XmlPullParser.START_TAG) {
            throw new IllegalStateException();
        }
        int depth = 1;
        while (depth != 0) {
            switch (parser.next()) {
                case XmlPullParser.END_TAG:
                    depth--;
                    break;
                case XmlPullParser.START_TAG:
                    depth++;
                    break;
            }
        }
    }
}

Answer 1

如果您在 readArticles() 中调用 parser.require() 行之前添加以下内容，您可以看到您实际上并不在 START_DOCUMENT 事件中那个点。

int eventType = parser.getEventType();
Log.i("TAG", "The event type is: " + eventType);

while (eventType != XmlPullParser.START_DOCUMENT) {
    eventType = parser.next();            
    Log.i("TAG", "The event type is: " + eventType);
}

在 parse() 中注释掉 parser.nextTag(); 行后，此日志记录显示您现在位于 START_DOCUMENT。但是随后 require() 调用失败，出现您问题中提到的异常。似乎对命名空间参数使用 null ns 是无效的。将命名空间和名称参数都设置为 null 确实允许 require() 成功，但可能不是您想要的。