尽管目前的无线网络不够理想，手机联网还是给我们开发人员不小的震撼的。毕竟这真的是件神奇的事情，不是吗？本文将讲述如何应用J2ME平台中的通用联网框架开发联网的应用程序。
首先，必须说明一点：MIDP中规定，任何移动信息设备都必须提供通过http协议的支持，而像其他的通信方式例如socket是设备相关的。有些手机会支持，有些则不支持。这里只大概的说明一下http协议相关的内容，如果不了解这个方面的知识请参考http协议。在javax.microedition.io里面是大量的接口，只有一个connector类，当然在midp2.0里面添加了对push技术的支持，这个留做以后讲。connector类提供的最重要的方法是open()方法，它的返回值为Connection，你可以对他进行转换得到你需要的类型，比如我们以http协议访问服务器。
void postViaHttpConnection(String url) throws IOException {
HttpConnection c = null;
InputStream is = null;
OutputStream os = null;
int rc;
try {
c = (HttpConnection)Connector.open(url);
// Set the request method and headers
c.setRequestMethod(HttpConnection.POST);
c.setRequestProperty("If-Modified-Since",
"29 Oct 1999 19:43:31 GMT");
c.setRequestProperty("User-Agent",
"Profile/MIDP-2.0 Configuration/CLDC-1.0");
c.setRequestProperty("Content-Language", "en-US");
// Getting the output stream may flush the headers
os = c.openOutputStream();
os.write("LIST games\n".getBytes());
os.flush(); // Optional, getResponseCode will flush
// Getting the response code will open the connection,
// send the request, and read the HTTP response headers.
// The headers are stored until requested.
rc = c.getResponseCode();
if (rc != HttpConnection.HTTP_OK) {
throw new IOException("HTTP response code: " + rc);
}
is = c.openInputStream();
// Get the ContentType
String type = c.getType();
processType(type);
// Get the length and process the data
int len = (int)c.getLength();
if (len > 0) {
int actual = 0;
int bytesread = 0 ;
byte[] data = new byte[len];
while ((bytesread != len) && (actual != -1)) {
actual = is.read(data, bytesread, len – bytesread);
bytesread += actual;
}
process(data);
} else {
int ch;
while ((ch = is.read()) != -1) {
process((byte)ch);
}
}
} catch (ClassCastException e) {
throw new IllegalArgumentException("Not an HTTP URL");
} finally {
if (is != null)
is.close();
if (os != null)
os.close();
if (c != null)
c.close();
}
}
上面的代码是我取自API doc（建议多读一下api doc)。
下面根据自己的经验说明一下联网中比较重要的问题：
我们应该明白这是如何工作的，手机发送请求通过无线网络传输到运营商的WAP网关，WAP网关将请求转发到web服务器，服务器可以用cgi,asp,servlet/jsp等构建。服务器处理后会把响应转发到WAP网关，WAP网关再把它发送到手机上。WAP网关对我们开发人员来说是透明的我们不用管它。
如果在你的联网程序上看不到Thread,Runnable这样的字眼，那么你的程序是不能运行的。因为考虑到网络的因素，为了避免操作堵塞。你必须把联网动作放到另外一个线程去运行，而不能在主线程运行。最好当联网的时候提供给用户一个等待的界面比如作一个动画界面。我下面提供的例子中没有用，因为我想把这个单独出来以后谈。
通常联网的应用程序的界面是比较多的，最好我们使用MVC的模式来实现界面的导航。关于这个方面的说明我以前有文章讲述
考虑好你想如何传递你数据，这一点是非常重要的。你可以用GET方法也可以使用POST方法，推荐后者。因为get方法只能是通过URL编码的传输。而POST更加灵活，配合DataInputStream、DataOutputStream来使用更是方便。必须清楚我们如何接受数据是跟数据如何发送过来相关的，例如在client端writeUTF(message);writeInt(4);writeBoolean(true),那么接受就应该readUTF();readInt();readBoolean();如果发送过来数据长度是可用的，那么我们可以建立一个适当的数组来接受，如果不可用我们就要一个适当容量的数组来接受。
下面我提供一个实例来说明以上的问题，在应用程序中我们输入任意字符，通过连接server得到响应并显示出来。server我用servlet写的非常简单，只是在受到的内容后面加上“haha”,程序我安装到自己的手机上运行没有任何问题，就是GPRS网络不够快。Server是tomcat5实现的，关于如何部署servlet的问题超出了本文的讨论范围。我只提供代码，推荐用Eclipse+Lomboz开发j2ee程序。
package com.north;
import java.io.DataInputStream;
import java.io.DataOutputStream;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class MyServlet extends HttpServlet {
protected void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
DataInputStream dis = new DataInputStream(request.getInputStream());
String result = dis.readUTF();
DataOutputStream dos = new DataOutputStream(response.getOutputStream());
dos.writeUTF(result+"haha");
dos.close();
dis.close();
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response) throws ServletException, IOException {
doGet(request,response);
}
}
联网的时候一定按照如下的流程做
建立连接，设置传输方式推荐POST,设置方法头
打开输出流，传输数据给服务器
判断相应的状态码，进入不同流程控制，注意错误处理。如果OK则开始接受数据
关闭连接和流
下面是客户端的代码，对错误处理没有考虑太多。一共是5个class
import javax.microedition.midlet.MIDlet;
import javax.microedition.midlet.MIDletStateChangeException;
public class HttpCommMIDlet extends MIDlet
{
private UIController uicontroller;
protected void startApp() throws MIDletStateChangeException
{
uicontroller = new UIController(this);
uicontroller.init();
}
protected void pauseApp()
{
}
protected void destroyApp(boolean arg0) throws MIDletStateChangeException
{
}
}
import java.io.IOException;
import javax.microedition.lcdui.Display;
import javax.microedition.lcdui.Displayable;
public class UIController
{
private HttpCommMIDlet midlet;
private InputCanvas inputUI;
private DisplayCanvas displayUI;
private Display display;
private HttpCommHandler httpHandler;
public UIController(HttpCommMIDlet midlet)
{
this.midlet = midlet;
}
public static class EventID
{
public static final int CONNECT_TO_SERVER = 0;
public static final int DISPLAY_BACK_TO_INPUT = 1;
}
public void init()
{
display = Display.getDisplay(midlet);
httpHandler = new HttpCommHandler(
"http://yourip:8088/http/myservlet");
inputUI = new InputCanvas(this);
displayUI = new DisplayCanvas(this);
display.setCurrent(inputUI);
}
public void setCurrent(Displayable disp)
{
display.setCurrent(disp);
}
public void handleEvent(int EventID, Object[] obj)
{
new EventHandler(EventID, obj).start();
}
private class EventHandler extends Thread
{
private int eventID;
private Object[] obj;
private Displayable backUI;
public EventHandler(int eventID, Object[] obj)
{
this.eventID = eventID;
this.obj = obj;
}
public void run()
{
synchronized (this)
{
run(eventID, obj);
}
}
private void run(int eventID, Object[] obj)
{
switch (eventID)
{
case EventID.CONNECT_TO_SERVER:
{
try
{
String result = httpHandler
.sendMessage((String) obj[0]);
displayUI.init(result);
setCurrent(displayUI);
break;
} catch (IOException e)
{
}
}
case EventID.DISPLAY_BACK_TO_INPUT:
{
setCurrent(inputUI);
break;
}
default:
break;
}
}
}
}
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.StringItem;
import javax.microedition.lcdui.TextField;
public class InputCanvas extends Form implements CommandListener
{
private UIController uicontroller;
private TextField inputField;
private StringItem result;
public static final Command okCommand = new Command("OK", Command.OK, 1);
public InputCanvas(UIController uicontroller)
{
super("Http Comunication");
this.uicontroller = uicontroller;
inputField = new TextField("Input:", null, 20, TextField.ANY);
this.append(inputField);
this.addCommand(okCommand);
this.setCommandListener(this);
}
public void commandAction(Command arg0, Displayable arg1)
{
if (arg0 == okCommand)
{
String input = inputField.getString();
uicontroller.handleEvent(UIController.EventID.CONNECT_TO_SERVER,
new Object[] { input });
}
}
}
import java.io.*;
import javax.microedition.io.Connector;
import javax.microedition.io.HttpConnection;
public class HttpCommHandler
{
private String URL;
public HttpCommHandler(String URL)
{
this.URL = URL;
}
public String sendMessage(String message) throws IOException
{
HttpConnection httpConn;
DataInputStream input;
DataOutputStream output;
String result;
try
{
httpConn = open();
output = this.openDataOutputStream(httpConn);
output.writeUTF(message);
output.close();
input = this.openDataInputStream(httpConn);
result = input.readUTF();
closeConnection(httpConn,input,output);
return result;
}
finally
{
}
}
public HttpConnection open() throws IOException
{
try
{
HttpConnection connection = (HttpConnection) Connector.open(URL);
connection.setRequestProperty("User-Agent", System
.getProperty("microedition.profiles"));
connection.setRequestProperty("Content-Type",
"application/octet-stream");
connection.setRequestMethod(HttpConnection.POST);
return connection;
} catch (IOException ioe)
{
throw ioe;
}
}
private DataInputStream openDataInputStream(HttpConnection conn)
throws IOException
{
int code = conn.getResponseCode();
if (code == HttpConnection.HTTP_OK)
{
return conn.openDataInputStream();
} else
{
throw new IOException();
}
}
private DataOutputStream openDataOutputStream(HttpConnection conn)
throws IOException
{
return conn.openDataOutputStream();
}
private void closeConnection(HttpConnection conn, DataInputStream dis,
DataOutputStream dos)
{
if(conn!= null)
{
try
{
conn.close();
}
catch(IOException e)
{}
}
if(dis!=null)
{
try
{
dis.close();
}
catch(IOException e)
{}
}
if(dos!=null)
{
try
{
dos.close();
}
catch(IOException e)
{}
}
}
}
import javax.microedition.lcdui.Command;
import javax.microedition.lcdui.CommandListener;
import javax.microedition.lcdui.Displayable;
import javax.microedition.lcdui.Form;
import javax.microedition.lcdui.StringItem;
public class DisplayCanvas extends Form implements CommandListener
{
private UIController uicontroller;
private StringItem result;
private int index = 0;
private boolean first = true;
public static Command backCommand = new Command("Back", Command.BACK, 2);
public DisplayCanvas(UIController uicontroller)
{
super("Result");
this.uicontroller = uicontroller;
result = new StringItem("you have input:", null);
this.addCommand(backCommand);
this.setCommandListener(this);
}
public void init(String message)
{
if (first)
{
result.setText(message);
index = this.append(result);
first = false;
}
else
{
this.delete(index);
result.setText(message);
this.append(result);
}
}
public void commandAction(Command arg0, Displayable arg1)
{
uicontroller.handleEvent(UIController.EventID.DISPLAY_BACK_TO_INPUT,
null);
}
}