change526

在jxta里面，所有的資源都是通過廣告來發布的，這里的服務業不例外，在這里的服務  發布里面有兩個很總要的概念，

• ModuleClassAdvertisement— defines the service class; its main purpose is to formally document the
existence of a module class. It is uniquely identified by a ModuleClassID.
• ModuleSpecAdvertisement — defines a service specification; uniquely identified by a ModuleSpecID.
Its main purpose is to provide references to the documentation needed in order to create conforming
implementations of that specification. A secondary use is to make running instances usable remotely,
by publishing any or all of the following:
• PipeAdvertisement
• ModuleSpecID of a proxy module
• ModuleSpecID of an authenticator module
• ModuleImplAdvertisement — defines an implementation of a given service specification. 

這里的 ModuleClassAdvertisement  僅僅用來告知服務的存在，對等點若是需要訪問該服務的話，還需要發現與之關聯的 ModuleSpecAdvertisement  廣告信息。

而這里的 ModuleSpecAdvertisement  則包含了 對等點節點 要訪問該服務所需要的所有相關信息，比如：管道廣告信息，通過它才能夠連接上所需要的服務。

服務端的代碼示例大抵如下：

創建發布 ModuleClassAdvertisement  ：

ModuleClassAdvertisement mcadv = (ModuleClassAdvertisement)AdvertisementFactory.newAdvertisement(ModuleClassAdvertisement.getAdvertisementType());
   mcadv.setName("JXTAMOD:JXTA-EX1");
   mcadv.setDescription("Tutorial example to use JXTA module advertisement Framework");
   ModuleClassID mcID = IDFactory.newModuleClassID();
   mcadv.setModuleClassID(mcID);//通過mcID來建立ModuleClassAdvertisement 與ModuleSpecAdvertisement 的聯系

discovery.publish(mcadv);
 discovery.remotePublish(mcadv);

創建發布 ModuleSpecAdvertisement ：

ModuleSpecAdvertisement mdadv = (ModuleSpecAdvertisement)AdvertisementFactory.newAdvertisement(ModuleSpecAdvertisement.getAdvertisementType());

mdadv.setName("JXTASPEC:JXTA-EX1");
   mdadv.setVersion("Version 1.0");
   mdadv.setCreator("sun.com");
   mdadv.setModuleSpecID(IDFactory.newModuleSpecID(mcID));
   mdadv.setSpecURI("http://www.jxta.org/Ex1");

PipeAdvertisement pipeadv = null;
   try {
    FileInputStream is = new FileInputStream("pipeserver.adv");
    pipeadv = (PipeAdvertisement)AdvertisementFactory.newAdvertisement(MimeMediaType.XMLUTF8, is);
    is.close();
   } catch (Exception e) {
    System.out.println("failed to read/parse pipe advertisement");
   }

mdadv.setPipeAdvertisement(pipeadv);

discovery.publish(mdadv);
 discovery.remotePublish(mdadv);
  myPipe = pipes.createInputPipe(pipeadv);

在客戶端，通過不斷的查找廣告（分本地查找和遠端查找）來 獲取 所需要服務的廣告信息，通過它就可以獲取 管道信息 來創建管道以達到通訊的目的。客戶端代碼示例大抵如下：

Enumeration en = null;
  while (true) {
   try {
    /* let's look first in our local cache to see if we have it! We try to discover an adverisement which as the (Name, JXTA-EX1) tag value
    en = discovery.getLocalAdvertisements(DiscoveryService.ADV,"Name","JXTASPEC:JXTA-EX1");
    //  Ok we got something in our local cache does not
    //  need to go further!
    if ((en != null) && en.hasMoreElements()) {
     break;
    }
    //  nothing in the local cache?, let's remotely query
    //  for the service advertisement.
    discovery.getRemoteAdvertisements(null,DiscoveryService.ADV,"Name","JXTASPEC:JXTA-EX1",1, null);
    //  The discovery is asynchronous as we do not know
    //  how long is going to take
    try { // sleep as much as we want. Yes we
     //  should implement asynchronous listener pipe...
     Thread.sleep(2000);
    } catch (Exception e) {}
   } catch (IOException e) {
    //  found nothing! move on
   }
   System.out.print(".");
  }
  System.out.println("we found the service advertisement");
  //  Ok get the service advertisement as a Spec Advertisement
  ModuleSpecAdvertisement mdsadv = (ModuleSpecAdvertisement)en.nextElement();
  try {
   //  let's print the advertisement as a plain text document
   StructuredTextDocument doc = (StructuredTextDocument)mdsadv.getDocument(MimeMediaType.TEXT_DEFAULTENCODING);
   StringWriter out = new StringWriter();
   doc.sendToWriter(out);
   System.out.println(out.toString());
   out.close();
   //  we can find the pipe to connect to the service
   //  in the advertisement.
   PipeAdvertisement pipeadv = mdsadv.getPipeAdvertisement();
   //  Ok we have our pipe advertiseemnt to talk to the service
   //  create the output pipe endpoint to connect  to the server
    myPipe = pipes.createOutputPipe(pipeadv, 10000);
   }

   //  send the message to the service pipe
   myPipe.send (msg);