第二章 定位Lookup Service

客户通过查询某个Lookup Service来定位服务。为了实现这一点，首先必须找到lookup service。另一方面，一个服务要在lookup service上注册，首先也要找到lookup service。所以客户和服务的第一步都是要查找lookup service。查找lookup service有两种方法： unicast（唯一）和broadcast（广播）.

1、一对一查找（Unicast discovery）
当你已经知道了lookup service所在的机器，就可以用一对一的查找。一般用在你知道lookup service的准确地址。编程使用包net.jini.core.discovery中的类LookupLocator ，它有两个构造函数如下：

package net.jini.core.discovery;
public Class LookupLocator {
    LookupLocator(java.lang.String url)
                  throws java.net.MalformedURLException;
    LookupLocator(java.lang.String host,int port);
}

import net.jini.core.discovery.LookupLocator;
/** 
* InvalidLookupLocator.java * *
    * Created: Tue Mar  9 14:14:06 1999
* @author Jan Newmarch 
* @version
    */

public class InvalidLookupLocator  { 
   
   static public void main(String argv[]) {
	        new InvalidLookupLocator();    
    }   

   public InvalidLookupLocator() {
	       LookupLocator lookup;	// this is valid	
       try {
	        lookup = new LookupLocator("jini://localhost");
	        System.out.println("First lookup creation succeeded");
	       } catch(java.net.MalformedURLException e) {
	        System.err.println("First lookup failed: " + e.toString());	}

	// this is probably an invalid URL, 	
// but the URL is syntactically okay	
  try {
	    lookup = new LookupLocator("jini://ABCDEFG.org");
	    System.out.println("Second lookup creation succeeded");
	} catch(java.net.MalformedURLException e) {
	    System.err.println("Second lookup failed: " + e.toString());	}
	
// this IS a malformed URL	
try {
	    lookup = new LookupLocator("A:B:C://ABCDEFG.org");
	    System.out.println("Third lookup creation succeeded");
	} catch(java.net.MalformedURLException e) {
	    System.err.println("Third lookup failed: " + e.toString());	}
	
// this is valid	
lookup = new LookupLocator("localhost", 80);
	System.out.println("Fourth lookup creation succeeded"); 
   }    
} // InvalidLookupLocator

  public ServiceRegistrar getRegistrar()
                              throws java.io.IOException,
                                     java.lang.ClassNotFoundException
   程序实例如下：

import net.jini.core.discovery.LookupLocator;
import net.jini.core.lookup.ServiceRegistrar;
/**
*UnicastRegistrar.java 
* *
*Created: Fri Mar 12 22:34:53 1999 
* 
*@author Jan Newmarch 
*@version 
*/

public class UnicastRegistrar  {
     static public void main(String argv[]) 
      {
        new UnicastRegistrar();    }  
     
     public UnicastRegistrar() {
    
  	   LookupLocator lookup = null;
       ServiceRegistrar registrar = null;        
      try {
            lookup = new LookupLocator("jini://localhost");
        } catch(java.net.MalformedURLException e) {
            System.err.println("Lookup failed: " + e.toString());
	    System.exit(1);        
      }	
     
     try {	    registrar = lookup.getRegistrar();
	    } catch (java.io.IOException e) {
            System.err.println("Registrar search failed: " + e.toString());
	        System.exit(1);	
       } catch (java.lang.ClassNotFoundException e) {
            System.err.println("Registrar search failed: " + e.toString());
	       System.exit(1);	}
	// the code takes separate routes from here for client or service    }   
} // UnicastRegistrar

2、广播式查找(Broadcast discovery)
如果lookup service的位置不知道，那就要用广播式查找了。我们要使用在包net.jini.discovery 中的类LookupDiscovery 。它只有一个构造函数如下：

     
     LookupDiscovery(java.lang.String[] groups)

2．1 DiscoveryListener
广播式查找是在网上全面搜索，网上能收到信息的Lookup Service都应该响应请求。这种查找是很费时间的，因为一般不知道能响应的lookup service的数目。为了处理这种不确定性，对象 LookupDiscovery可以注册一个监听器来监听响应信息。

   public void addDiscoveryListener(DiscoveryListener l)

该监听器必须实现接口DiscoveryListener
     package net.jini.discovery;public 
     abstract interface DiscoveryListener {
        public void discovered(DiscoveryEvent e);
        public void discarded(DiscoveryEvent e);
     }
	 

2．2 DiscoveryEvent
方法discover（）的参数是DiscoveryEvent对象。

package net.jini.discovery;

public Class DiscoveryEvent {
    public net.jini.core.lookup.ServiceRegistrar[] getRegistrars();
}

import net.jini.discovery.LookupDiscovery;
import net.jini.discovery.DiscoveryListener;
import net.jini.discovery.DiscoveryEvent;
import net.jini.core.lookup.ServiceRegistrar;
/** * MulticastRegistrar.java * *
*Created: Fri Mar 12 22:49:33 1999
* @author Jan Newmarch 
* @version 
*/
public class MulticastRegistrar implements DiscoveryListener { 
static public void main(String argv[]) {
        new MulticastRegistrar();
}

public MulticastRegistrar() {
        LookupDiscovery discover = null;
        try {
            discover = new LookupDiscovery(LookupDiscovery.ALL_GROUPS);
        } catch(Exception e) {
            System.err.println(e.toString());
	    System.exit(1);
        } 
       discover.addDiscoveryListener(this);
	// stay around long enough to receive replies
	try {
	    Thread.currentThread().sleep(1000000L);
	} catch(java.lang.InterruptedException e) {
	    // do nothing	
      }  
   }    
    
public void discovered(DiscoveryEvent evt) {
        ServiceRegistrar[] registrars = evt.getRegistrars();
        for (int n = 0; n < registrars.length; n++) {
	    ServiceRegistrar registrar = registrars[n];
	    // the code takes separate routes from here for client or service  	} 
   }

public void discarded(DiscoveryEvent evt) {    }
}// MulticastRegistrar

3、ServiceRegistrar
ServiceRegistrar是一个被每一个lookup service实现的抽象类。该类实际实现的细节和这没有关系。类ServiceRegistrar的作用是担当lookup service的代理。该代理运行在客户或服务中。在Jini中，这是第一个从一个Java进程移到另一个进程中的对象。使用RMI，该对象被从lookup service移到找到该lookup service的应用中。从那时起，它就做为应用地址空间中的一个对象运行，同时该应用对它进行正常的方法调用。当需要的时候，它可以和它的lookup service通信，但是这种通信不需要应用显式的调用RMI方法。该对象有两个主要的方法，其中一个是服务用来注册的：

public ServiceRegistration register(ServiceItem item,
                                    long leaseDuration)
                             throws java.rmi.RemoteException

public java.lang.Object lookup(ServiceTemplate tmpl)
                        throws java.rmi.RemoteException;
public ServiceMatches lookup(ServiceTemplate tmpl,
                             int maxMatches)
                      throws java.rmi.RemoteException;