Android入门：通过XML数据与服务器进行通信

一、与服务器通信

类似于浏览器与服务器通信，Android应用也可以与Web服务器通信，比如Android应用可以访问Servlet、JSP、图片等等；

Web服务器通过指定Content-Type可以指定返回的数据的类型，比如text/html表示网页，text/xml表示XML数据，image/jpeg为图片；

二、通过XML让客户端和服务器交流数据

比如Android客户端打开相亲网，则返回的是多个人的序列，包含人的姓名、年龄等信息，而怎么组织这些数据返回给客户端呢？用XML很方便；

三、代码实现

服务器使用了dom4j组织XML数据，并返回给客户端；

代码结构如下：

package org.xiazdong.servlet;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.dom4j.Document;
import org.dom4j.DocumentHelper;
import org.dom4j.Element;
import org.xiazdong.domain.Person;


@WebServlet("/ListServlet")
public class ListServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		doPost(request,response);
	}

	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		List<Person> list = new ArrayList<Person>();
		for(int i=0;i<4;i++){
			Person person = new Person(i,"xiazdong-"+i,20+i);
			list.add(person);
		}
		Document document = DocumentHelper.createDocument();
		Element persons = DocumentHelper.createElement("persons");
		document.add(persons);
		for(int i=0;i<4;i++){
			persons.addElement("person").addAttribute("id", list.get(i).getId()+"");
			persons.addElement("name").addText(list.get(i).getName());
			persons.addElement("age").addText(list.get(i).getAge()+"");
		}
		String xml = document.asXML();	//用dom4j组织一个XML字符串
		response.setContentType("text/xml; charset=UTF-8");	//设置返回值的类型
		response.getOutputStream().write(xml.getBytes("UTF-8"));	//设置返回值
	}

}

如果让浏览器访问，效果如下：

（注：Chrome打开总是显示不出XML声明。请用IE打开）

接下来，我们可以开发客户端，目标是用列表显示这些数据；

在AndroidManifest.xml中加入：

<uses-permission android:name="android.permission.INTERNET" />

每个item的布局如下：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal" >

    <TextView
        android:layout_width="50dp"
        android:layout_height="wrap_content" 
        android:id="@+id/id"
        />
    <TextView
        android:layout_width="100dp"
        android:layout_height="wrap_content" 
        android:id="@+id/name"
        /><TextView
        android:layout_width="50dp"
        android:layout_height="wrap_content" 
        android:id="@+id/age"
        />

</LinearLayout>

MainActivity.java

package org.xiazdong.personlist;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import org.xiazdong.domain.Person;
import org.xmlpull.v1.XmlPullParser;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
import android.util.Xml;
import android.widget.ListView;
import android.widget.SimpleAdapter;

public class MainActivity extends Activity {
	private ListView listView;

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		listView = (ListView) this.findViewById(R.id.listview);
		List<Person> persons = null;
		try {
			persons = getXMLData();
		} catch (Exception e) {
			e.printStackTrace();
		}
		List<Map<String, Object>> maps = new ArrayList<Map<String, Object>>();
		for (Person person : persons) {
			HashMap<String, Object> map = new HashMap<String, Object>();
			map.put("id", person.getId());
			map.put("name", person.getName());
			map.put("age", person.getAge());
			maps.add(map);
		}
		SimpleAdapter adapter = new SimpleAdapter(this, maps, R.layout.item,
				new String[] { "id", "name", "age" }, new int[] { R.id.id,
						R.id.name, R.id.age });
		listView.setAdapter(adapter);
	}

	//读取XML数据并转为List<Person>
	private List<Person> getXMLData() throws Exception {
		List<Person> persons = new ArrayList<Person>();
		Person person = null;
		URL url = new URL("http://192.168.0.103:8080/Server/ListServlet");
		HttpURLConnection conn = (HttpURLConnection) url.openConnection();
		conn.setRequestMethod("GET");
		if (conn.getResponseCode() == 200) {
			InputStream in = conn.getInputStream();
			XmlPullParser parser = Xml.newPullParser();
			parser.setInput(in, "UTF-8");
			int event = parser.getEventType();
			while (event != XmlPullParser.END_DOCUMENT) {
				Log.i("start_document", "start_document");
				switch (event) {
				case XmlPullParser.START_TAG:
					if ("person".equals(parser.getName())) {
						person = new Person();
						person.setId(Integer.parseInt(parser.getAttributeValue(0)));
					}
					if (("name").equals(parser.getName())) {
						person.setName(parser.nextText());
					}
					if ("age".equals(parser.getName())) {
						person.setAge(Integer.parseInt(parser.nextText()));
					}
					break;
				case XmlPullParser.END_TAG:
					if ("person".equals(parser.getName())) {
						persons.add(person);
						Log.i("person", person.toString());
					}
					break;
				}
				event = parser.next();
			}
		}
		return persons;

	}
}

效果如下：