嵌入式系统/ARM技术:开发J2ME联网应用程序

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)
            {}
        }

    }

}

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);

}