Android中WAP PUSH的实现分析

1 WAP PUSH 流程简介
WAP Push分为两种：SI(Service Initiate) 和SL(Service Load)。都是服务器端向客户端推送消息的一种方式。
先看SI:
‘图发不上来’
图1.1
服务器通过网关采用OTA协议把信息发送到手机，手机存储解析并存储信息，然后提示给用户。

而SL流程如下：
‘图发不上来’
图1.2
在接收到SL消息后，同样也会存储并提示用户（视情况具体对待），区别在于客户端会主动调用浏览器打开SL中附带的连接。

2 WAP PUSH消息体剖析
下面我们以SL为例，来看看WAP PUSH的消息。
一个SL的WAP PUSH消息主体大致如下：
<!--
Service Loading (SL) Document Type Definition.
SL is an XML language.  Typical usage:
   <?xml version="1.0"?>
   <!DOCTYPE sl PUBLIC "-//WAPFORUM//DTD SL 1.0//EN"
          "http://www.wapforum.org/DTD/sl.dtd">
   <sl>
   href %URI; #REQUIRED
  action (execute-low|execute-high|cache) "execute-low"
   </sl>
大家都看到了，其实内容很少，包括一个连接和一个action控制执行的优先级。
由于考虑到节约网络数据的传输，往往都是将这些文本转化为WBXML格式再进行传输。WBXML是一种压缩的二进制表示方式，有严格的定义。对于标签和属性，定义如下表：
Tag Name Token
sl 5

Attribute Name Attribute Value Prefix Token
action execute-low 5
action execute-high 6
action cache 7
href 8
href http:// 9
href http://www. A
href https:// B
href https://www. C

Attribute Value Token
.com/ 85
.edu/ 86
.net/ 87
.org/ 88

文本中的对应Tag和Attribute value会被Token代替，以减少文本大小。我们看下面一段内容：
<?xml version="1.0"?>
     <!DOCTYPE sl PUBLIC "-//WAPFORUM//DTD SL 1.0//EN"
                  "http://www.wapforum.org/DTD/sl.dtd">
<sl href="http://www.xyz.com/ppaid/123/abc.wml"></sl>
总共有159个字节。
我们使用WBXML格式来表示，如下表对应关系：
Token Stream Description
02 Version number - WBXML version 1.2
06 SL 1.0 Public Identifier
6A Charset=UTF-8 (MIBEnum 106)
00 String table length
05 sl, with attributes
0A Token for "href="http://www."
03 Inline string follows
‘x’, ‘y’, ‘z’, 00 String
85 Token for ".com/"
03 Inline string follows
‘p’, ‘p’, ‘a’, ‘i’, ‘d’, ‘/’, ‘1’, ‘2’, ‘3’, ‘/’, ‘a’, ‘b’, ‘c’, ‘.’, ‘w’, ‘m’, ‘l’, 00 String
01 END (of sl attribute list)
这样一来，就是：
02  06  6A  00  05  0A  03  'x' 'y' 'z' 00  85  03  'p' 'p' 'a'
'i' 'd' '/' '1' '2' '3' '/' 'a' 'b' 'c' '.' 'w' 'm' 'l' 00  01 
总共才需要32个字节, 大大减少了数据量。
以上的只是消息主体内容，在传输过来的时候，还需要加上一些附加的信息，如
mimeType: 表示是SI还是SL, application/vnd.wap.sic或者 application/vnd.wap.slc (text/vnd.wap.sl).
transactionId: 用于分段发送大数据量的消息， 这些消息具有相同的trasactionId.
pduType: 未识别
header: 附加信息，用于标示特定的业务。
3 Android中解析WAP PUSH
按照以上所述，网络传输过来的是WBXML数据，通常我们需要按照Token表对应的进行解析，才可以得到XML格式的正文，从而获取href和action。 庆幸的是Android中在framework层的WapPushOverSms.java中已经完成了对消息的部分解析，把mimeType, trasactionId, pduType, header 和 data分离出来，放在intent的传递参数中了。看以下代码：
private void dispatchWapPdu_PushCO(byte[] pdu, int transactionId, int pduType,
                                       int headerStartIndex, int headerLength) {
        byte[] header = new byte[headerLength];
        System.arraycopy(pdu, headerStartIndex, header, 0, header.length);

        Intent intent = new Intent(Intents.WAP_PUSH_RECEIVED_ACTION);
        intent.setType(WspTypeDecoder.CONTENT_MIME_TYPE_B_PUSH_CO);
        intent.putExtra("transactionId", transactionId);
        intent.putExtra("pduType", pduType);
        intent.putExtra("header", header);
        intent.putExtra("data", pdu);

        mSmsDispatcher.dispatch(intent, "android.permission.RECEIVE_WAP_PUSH");
    }

我们在应用层只需要添加对WAP_PUSH_RECEIVED_ACTION的监听，便可获取到这个WAP PUSH。 下面以接收中国移动DCD业务的WAP PUSH为例，看看是如何实现的。
需要写一个DcdWapPushReceiver.java:
public class DcdWapPushReceiver extends BroadcastReceiver {

    private static final String LOGTAG = "DcdPushReceiver";
   
    public static final String CONTENT_MIME_TYPE_B_PUSH_SL = "application/vnd.wap.slc";
    public static final String APPLICATION_DCD_ID = "application/x-oma-DCD:DCD.ua";
   
    public void onReceive(Context context, Intent intent) {
        //only deal with SL WAP PUSH
        if (intent.getAction().equals(WAP_PUSH_RECEIVED_ACTION)
                && CONTENT_MIME_TYPE_B_PUSH_SL.equals(intent.getType())) {
                // Start a new AsyncTask to process the data.
                if (APPLICATION_DCD_ID.equals(getAppIdFromIntent(intent))) {
                    new ReceivePushTask(context).execute(intent);
                }
        }
       
    }
   
    /*
     * get application id from intent header.
     * <==header sample==>
     * B0 B4 87 AF application/x-oma-DCD:DCD.ua 00 Encoding-Version 00
     */
    public static String getAppIdFromIntent(Intent intent) {
        byte[] header = intent.getByteArrayExtra("header");
        if (header == null) {
            return null;
        }
       
        String str = new String(header);
        int start = str.indexOf("application/");
        if (start > 0) {
            //application id end with 00.
            int end = str.indexOf(0x00);
            if (end > 0) {
                str = str.substring(start, end);
                return str;
            }
        }
        return null;
    }
private class ReceivePushTask extends AsyncTask<Intent, Void, Void> {
      
        private Context mContext;
       
        public ReceivePushTask(Context context) {
            mContext = context;
        }
       
        protected Void doInBackground(Intent... intents)
        {
            Intent intent = intents[0];
            // step1. obtain wbxml data from intent.
            byte[] pushData = intent.getByteArrayExtra("data");
           
            String wbxmlData = "";
            for (byte by: pushData) {
                wbxmlData = wbxmlData + by + " ";
            }
            Debug.print("wap push data = "+wbxmlData);
           
                       
            // step2. pass the data to WapPushParser and get the parsing result.
            DcdWapPushParser parser = new DcdWapPushParser(pushData);
            DcdWapPushMsg pushMsg = null;

            if (CONTENT_MIME_TYPE_B_PUSH_SL.equals(intent.getType())) {
                pushMsg = parser.parse(DcdWapPushMsg.WAP_PUSH_TYPE_SL);
            }
                       
            if (null == pushMsg) {
                Debug.error("Invalid WAP PUSH data");
                return null;
            }
           
            //get href
            String href = pushMsg.getAttributeValueString(DcdWapPushMsg.WAP_PUSH_PROJECTION_HREF);
            Debug.print("href = " + href);
            Intent i = new Intent(mContext, DcdCmService.class);
           
            //step3. sync invoked by wap push, so set sync type to SVR
            //i.putExtra("syncType", DcdCmService.SYNC_TYPE_SVR);
            //i.putExtra("href", href);
            Bundle bundle = new Bundle();
            bundle.putBoolean(DcdCmService.NOTIFICATION_WAPPUSH, true);
            bundle.putString("syncType", DcdRequest.REQUEST_TYPE_SVR);
            bundle.putString("href", href);
            i.putExtras(bundle);           
            //start sync service
            mContext.startService(i);
           
            return null;
        }
    }
}
在AndroidManifest.xml中注册这个接收器：
<receiver android:name=".contentmanager.DcdWapPushReceiver">
            <intent-filter>
                <action android:name="android.provider.Telephony.WAP_PUSH_RECEIVED" />
                <data android:mimeType="application/vnd.wap.slc" />
            </intent-filter>
        </receiver>