盡管目前的無線網絡不夠理想，手機聯網還是給我們開發人員不小的震撼的。畢竟這真的是件神奇的事情，不是嗎？本文將講述如何應用J2ME平台中的通用聯網框架開發聯網的應用程序。
首先，必須說明一點：MIDP中規定，任何移動信息設備都必須提供通過http協議的支持，而像其他的通信方式例如socket是設備相關的。有些手機會支持，有些則不支持。這裡只大概的說明一下http協議相關的內容，如果不了解這個方面的知識請參考http協議。在Javax.microedition.io裡面是大量的接口，只有一個connector類，當然在midp2.0裡面添加了對push技術的支持，這個留做以後講。connector類提供的最重要的方法是open()方法，它的返回值為Connection，你可以對他進行轉換得到你需要的類型，比如我們以http協議訪問服務器。
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 gamesn".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();
}
}
上面的代碼是我取自API doc（建議多讀一下api doc)。
下面根據自己的經驗說明一下聯網中比較重要的問題：
我們應該明白這是如何工作的，手機發送請求通過無線網絡傳輸到運營商的WAP網關，WAP網關將請求轉發到web服務器，服務器可以用CGI,ASP,servlet/JSP等構建。服務器處理後會把響應轉發到WAP網關，WAP網關再把它發送到手機上。WAP網關對我們開發人員來說是透明的我們不用管它。
如果在你的聯網程序上看不到Thread,Runnable這樣的字眼，那麼你的程序是不能運行的。因為考慮到網絡的因素，為了避免操作堵塞。你必須把聯網動作放到另外一個線程去運行，而不能在主線程運行。最好當聯網的時候提供給用戶一個等待的界面比如作一個動畫界面。我下面提供的例子中沒有用，因為我想把這個單獨出來以後談。
通常聯網的應用程序的界面是比較多的，最好我們使用MVC的模式來實現界面的導航。關於這個方面的說明我以前有文章講述
考慮好你想如何傳遞你數據，這一點是非常重要的。你可以用GET方法也可以使用POST方法，推薦後者。因為get方法只能是通過URL編碼的傳輸。而POST更加靈活，配合DataInputStream、DataOutputStream來使用更是方便。必須清楚我們如何接受數據是跟數據如何發送過來相關的，例如在clIEnt端writeUTF(message);writeInt(4);writeBoolean(true),那麼接受就應該readUTF();readInt();readBoolean();如果發送過來數據長度是可用的，那麼我們可以建立一個適當的數組來接受，如果不可用我們就要一個適當容量的數組來接受。
下面我提供一個實例來說明以上的問題，在應用程序中我們輸入任意字符，通過連接server得到響應並顯示出來。server我用servlet寫的非常簡單，只是在受到的內容後面加上“haha”,程序我安裝到自己的手機上運行沒有任何問題，就是GPRS網絡不夠快。Server是tomcat5實現的，關於如何部署servlet的問題超出了本文的討論范圍。我只提供代碼，推薦用Eclipse+Lomboz開發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);
}
}
聯網的時候一定按照如下的流程做
建立連接，設置傳輸方式推薦POST,設置方法頭
打開輸出流，傳輸數據給服務器
判斷相應的狀態碼，進入不同流程控制，注意錯誤處理。如果OK則開始接受數據
關閉連接和流
下面是客戶端的代碼，對錯誤處理沒有考慮太多。一共是5個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);
}
}
