Android Studio ——Android 使用Pull方法解析XML文件的方法

Pull解析方法给应用程序完全的控制文档该怎么样被解析。 Android中对Pull方法提供了支持的API ，主要是
复制代码代码如下:
org.xmlpull.v1.XmlPullParser;
org.xmlpull.v1.XmlPullParserFactory;

二个类，其中主要使用的是XmlPullParser，XmlPullParserFactory是一个工厂，用于构建XmlPullParser对象。
应用程序通过调用XmlPullParser.next()等方法来产生Event，然后再处理Event。可以看到它与Push方法的不同，Push方法是由Parser自己主动产生Event，回调给应用程序。而Pull方法是主动的调用Parser的方法才能产生事件。
假如XML中的语句是这样的："<author country="United States">James Elliott</author>"，author是TAG，country是ATTRIBUTE，"James Elliott"是TEXT。
要想解析文档先要构建一个XmlPullParser对象
复制代码代码如下:
final XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
factory.setNamespaceAware(true);
final XmlPullParser parser = factory.newPullParser();

Pull解析是一个遍历文档的过程，每次调用next()，nextTag(), nextToken()和nextText()都会向前推进文档，并使Parser停留在某些事件上面，但是不能倒退。
然后把文档设置给Parser
复制代码代码如下:
parser.setInput(new StringReader("<author country=\"United States\">James Elliott</author>");

这时，文档刚被初始化，所以它应该位于文档的开始，事件应该是START_DOCUMENT，可以通过XmlPullParser.getEventType()来获取。然后调用next()会产生
START_TAG，这个事件告诉应用程序一个标签已经开始了，调用getName()会返回"author";再next()会产生
TEXT事件，调用getText()会返回"James Elliott"，再next()，会产生
END_TAG，这个告诉你一个标签已经处理完了，再next()，会产生
END_DOCUMENT，它告诉你整个文档已经处理完成了。
除了next()外，nextToken()也可以使用，只不过它会返回更加详细的事件，比如 COMMENT, CDSECT, DOCDECL, ENTITY等等非常详细的信息。如果程序得到比较底层的信息，可以用nextToken()来驱动并处理详细的事件。需要注意一点的是TEXT事件是有可能返回空白的White Spaces比如换行符或空格等。
另外有二个非常实用的方法nextTag()和nextText()
nextTag()--首先它会忽略White Spaces，如果可以确定下一个是START_TAG或END_TAG，就可以调用nextTag()直接跳过去。通常它有二个用处：当START_TAG时，如果能确定这个TAG含有子TAG，那么就可以调用nextTag()产生子标签的START_TAG事件；当END_TAG时，如果确定不是文档结尾，就可以调用nextTag()产生下一个标签的START_TAG。在这二种情况下如果用next()会有TEXT事件，但返回的是换行符或空白符。
nextText()--它只能在START_TAG时调用。当下一个元素是TEXT时，TEXT的内容会返回；当下一个元素是END_TAG时，也就是说这个标签的内容为空，那么空字串返回；这个方法返回后，Parser会停在END_TAG上。比如：
复制代码代码如下:
<author>James Elliott</author>
<author></author>
<author/>

当START_TAG时，调用 nextText() ，依次返回：
"James Elliott"
""(empty)
""(empty)
这个方法在处理没有子标签的标签时很有用。比如：
复制代码代码如下:
<title>What Is Hibernate</title>
<author>James Elliott</author>
<category>Web</category>

就可以用以下代码来处理：
复制代码代码如下:
while (eventType != XmlPullParser.END_TAG) {
switch (eventType) {
case XmlPullParser.START_TAG:
tag = parser.getName();
final String content = parser.nextText();
Log.e(TAG, tag + ": [" + content + "]");
eventType = parser.nextTag();
break;
default:
break;
}
}

这就要比用next()来处理方便多了，可读性也大大的加强了。
最后附上一个解析XML的实例Android程序
复制代码代码如下:
import java.io.IOException;
import java.io.InputStream;
import org.xmlpull.v1.XmlPullParser;
import org.xmlpull.v1.XmlPullParserException;
import org.xmlpull.v1.XmlPullParserFactory;
import android.util.Log;
public class RssPullParser extends RssParser {
private final String TAG = FeedSettings.GLOBAL_TAG;

private InputStream mInputStream;

public RssPullParser(InputStream is) {
mInputStream = is;
}

public void parse() throws ReaderBaseException, XmlPullParserException, IOException {
if (mInputStream == null) {
throw new ReaderBaseException("no input source, did you initialize this class correctly?");
}
final XmlPullParserFactory factory = XmlPullParserFactory.newInstance();
factory.setNamespaceAware(true);
final XmlPullParser parser = factory.newPullParser();

parser.setInput(mInputStream);
int eventType = parser.getEventType();
if (eventType != XmlPullParser.START_DOCUMENT) {
throw new ReaderBaseException("Not starting with 'start_document'");
}
eventType = parseRss(parser);
if (eventType != XmlPullParser.END_DOCUMENT) {
throw new ReaderBaseException("not ending with 'end_document', do you finish parsing?");
}
if (mInputStream != null) {
mInputStream.close();
} else {
Log.e(TAG, "inputstream is null, XmlPullParser closed it??");
}
}

/**
* Parsing the Xml document. Current type must be Start_Document.
* After calling this, Parser is positioned at END_DOCUMENT.
* @param parser
* @return event end_document
* @throws XmlPullParserException
* @throws ReaderBaseException
* @throws IOException
*/
private int parseRss(XmlPullParser parser) throws XmlPullParserException, ReaderBaseException, IOException {
int eventType = parser.getEventType();
if (eventType != XmlPullParser.START_DOCUMENT) {
throw new ReaderBaseException("not starting with 'start_document', is this a new document?");
}
Log.e(TAG, "starting document, are you aware of that!");
eventType = parser.next();
while (eventType != XmlPullParser.END_DOCUMENT) {
switch (eventType) {
case XmlPullParser.START_TAG: {
Log.e(TAG, "start tag: '" + parser.getName() + "'");
final String tagName = parser.getName();
if (tagName.equals(RssFeed.TAG_RSS)) {
Log.e(TAG, "starting an RSS feed <<");
final int attrSize = parser.getAttributeCount();
for (int i = 0; i < attrSize; i++) {
Log.e(TAG, "attr '" + parser.getAttributeName(i) + "=" + parser.getAttributeValue(i) + "'");
}
} else if (tagName.equals(RssFeed.TAG_CHANNEL)) {
Log.e(TAG, "\tstarting an Channel <<");
parseChannel(parser);
}
break;
}
case XmlPullParser.END_TAG: {
Log.e(TAG, "end tag: '" + parser.getName() + "'");
final String tagName = parser.getName();
if (tagName.equals(RssFeed.TAG_RSS)) {
Log.e(TAG, ">> edning an RSS feed");
} else if (tagName.equals(RssFeed.TAG_CHANNEL)) {
Log.e(TAG, "\t>> ending an Channel");
}
break;
}
default:
break;
}
eventType = parser.next();
}
Log.e(TAG, "end of document, it is over");
return parser.getEventType();
}

/**
* Parse a channel. MUST be start tag of an channel, otherwise exception thrown.
* Param XmlPullParser
* After calling this function, parser is positioned at END_TAG of Channel.
* return end tag of a channel
* @throws XmlPullParserException
* @throws ReaderBaseException
* @throws IOException
*/
private int parseChannel(XmlPullParser parser) throws XmlPullParserException, ReaderBaseException, IOException {
int eventType = parser.getEventType();
String tagName = parser.getName();
if (eventType != XmlPullParser.START_TAG || !RssFeed.TAG_CHANNEL.equals(tagName)) {
throw new ReaderBaseException("not start with 'start tag', is this a start of a channel?");
}
Log.e(TAG, "\tstarting " + tagName);
eventType = parser.nextTag();
while (eventType != XmlPullParser.END_TAG) {
switch (eventType) {
case XmlPullParser.START_TAG: {
final String tag = parser.getName();
if (tag.equals(RssFeed.TAG_IMAGE)) {
parseImage(parser);
} else if (tag.equals(RssFeed.TAG_ITEM)) {
parseItem(parser);
} else {
final String content = parser.nextText();
Log.e(TAG, tag + ": [" + content + "]");
}
// now it SHOULD be at END_TAG, ensure it
if (parser.getEventType() != XmlPullParser.END_TAG) {
throw new ReaderBaseException("not ending with 'end tag', did you finish parsing sub item?");
}
eventType = parser.nextTag();
break;
}
default:
break;
}
}
Log.e(TAG, "\tending " + parser.getName());
return parser.getEventType();
}

/**
* Parse image in a channel.
* Precondition: position must be at START_TAG and tag MUST be 'image'
* Postcondition: position is END_TAG of '/image'
* @throws IOException
* @throws XmlPullParserException
* @throws ReaderBaseException
*/
private int parseImage(XmlPullParser parser) throws XmlPullParserException, IOException, ReaderBaseException {
int eventType = parser.getEventType();
String tag = parser.getName();
if (eventType != XmlPullParser.START_TAG || !RssFeed.TAG_IMAGE.equals(tag)) {
throw new ReaderBaseException("not start with 'start tag', is this a start of an image?");
}
Log.e(TAG, "\t\tstarting image " + tag);
eventType = parser.nextTag();
while (eventType != XmlPullParser.END_TAG) {
switch (eventType) {
case XmlPullParser.START_TAG:
tag = parser.getName();
Log.e(TAG, tag + ": [" + parser.nextText() + "]");
// now it SHOULD be at END_TAG, ensure it
if (parser.getEventType() != XmlPullParser.END_TAG) {
throw new ReaderBaseException("not ending with 'end tag', did you finish parsing sub item?");
}
eventType = parser.nextTag();
break;
default:
break;
}
}
Log.e(TAG, "\t\tending image " + parser.getName());
return parser.getEventType();
}

/**
* Parse an item in a channel.
* Precondition: position must be at START_TAG and tag MUST be 'item'
* Postcondition: position is END_TAG of '/item'
* @throws IOException
* @throws XmlPullParserException
* @throws ReaderBaseException
*/
private int parseItem(XmlPullParser parser) throws XmlPullParserException, IOException, ReaderBaseException {
int eventType = parser.getEventType();
String tag = parser.getName();
if (eventType != XmlPullParser.START_TAG || !RssFeed.TAG_ITEM.equals(tag)) {
throw new ReaderBaseException("not start with 'start tag', is this a start of an item?");
}
Log.e(TAG, "\t\tstarting " + tag);
eventType = parser.nextTag();
while (eventType != XmlPullParser.END_TAG) {
switch (eventType) {
case XmlPullParser.START_TAG:
tag = parser.getName();
final String content = parser.nextText();
Log.e(TAG, tag + ": [" + content + "]");
// now it SHOULD be at END_TAG, ensure it
if (parser.getEventType() != XmlPullParser.END_TAG) {
throw new ReaderBaseException("not ending with 'end tag', did you finish parsing sub item?");
}
eventType = parser.nextTag();
break;
default:
break;
}
}
Log.e(TAG, "\t\tending " + parser.getName());
return parser.getEventType();
}
}

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