lookup.html

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RMI-IIOP, including <CODE>rmi</CODE> in the URL will save confusion later on.
Once you have a reference to <CODE>SellerHome</CODE>, you can call its methods.
<P>
In contrast to the JNDI lookup performed by
<CODE>AuctionServlet.java</CODE>,
which requires a two-stage lookup to create a context and then the
actual lookup, RMI initializes the connection to the RMI name server,
<CODE>rmiregistry</CODE>, and also gets the remote reference with one call.

<P>
This remote reference is leased to the client from the
<CODE>rmiregistry</CODE>.
The lease means that unless the client informs the server it still
needs a reference to the object, the lease expires and the memory is
reclaimed.
This leasing operation is transparent to the user, but can be tuned by
setting the server property <CODE>java.rmi.dgc.leaseValue</CODE> value
in milliseconds when starting the server as follows:

<PRE>
  java -Djava.rmi.dgc.leaseValue=120000 myAppServer
</PRE>

<A NAME="iiop"></A>
<H3>RMI Over Internet Inter-ORB Protocol (IIOP)</H3>

The advent of RMI over Internet Inter-ORB Protocol (IIOP), means
existing RMI code can reference and look up an object with the CORBA
<CODE>CosNaming</CODE> service. This gives you greater interoperability
between architectures with little change to your existing RMI code.

<BLOCKQUOTE>
<HR>
<STRONG>Note:</STRONG>
The <CODE>rmic</CODE> compiler provides the <CODE>-iiop</CODE> option to
generates the stub and tie classes necessary for RMI-IIOP.
<HR>
</BLOCKQUOTE>

<H4>IIOP Server</H4>

The RMI-IIOP protocol is implemented as a JNDI plug-in, so as before,
you need to create an <CODE>InitialContext</CODE>:

<PRE>
  Hashtable env = new Hashtable();
  env.put("java.naming.factory.initial",
        "com.sun.jndi.cosnaming.CNCtxFactory");
  env.put("java.naming.provider.url",
        "iiop://localhost:1091");
  Context ic = new InitialContext(env);
</PRE>

The naming factory should look familiar as it is the same CORBA naming
service used in the CORBA section. The main difference is the addition of a
URL value specifing the naming service to which to connect. The naming
service used here is the <CODE>tnameserv</CODE> program
started on port 1091.

<PRE>
  tnameserv -ORBInitialPort 1091
</PRE>

<P>
The other main change to the server side is to replace calls
to <CODE>Naming.rebind</CODE> to use the JNDI <CODE>rebind</CODE>
method in the <CODE>InitialContext</CODE> instance. For example:

<P>
Old RMI code:
<PRE>
  SellerHome shome=(SellerHome)Naming.lookup(
  "rmi://appserver:1090/seller");
</PRE>

New RMI code:
<PRE>
  Hashtable env = new Hashtable();
  env.put("java.naming.factory.initial",
	"com.sun.jndi.cosnaming.CNCtxFactory");
  env.put("java.naming.provider.url",
	"iiop://localhost:1091");
  Context ic = new InitialContext(env);

  SellerHome shome= 
	(SellerHome)PortableRemoteObject.narrow(
	ic.lookup("seller"), SellerHome)
</PRE>

<H4>IIOP Client</H4>

On the client side, the RMI lookup is changed to use an instance of
the <CODE>InitialContext</CODE> in the place of RMI
<CODE>Naming.lookup</CODE>.
The return object is mapped to the requested object by using the
<CODE>narrow</CODE> method of the
<CODE>javax.rmi.PortableRemoteObject</CODE> class.
<CODE>PortableRemoteObject</CODE> replaces
<CODE>UnicastRemoteObject</CODE> that was previously available in the RMI
server code.

<P>
Old RMI lookup code:
<PRE>
  SellerHome shome= new SellerHome("seller");
  Naming.rebind("seller", shome);
</PRE>

<P>
New RMI code:
<PRE>
   Hashtable env = new Hashtable();
   env.put("java.naming.factory.initial",
	"com.sun.jndi.cosnaming.CNCtxFactory");
   env.put("java.naming.provider.url", 
	"iiop://localhost:1091");
   Context ic = new InitialContext(env);
   
   SellerHome shome= new SellerHome("seller");
   ic.rebind("seller", shome);
</PRE>

The <CODE>PortableRemoteObject</CODE> replaces
<CODE>UnicastRemoteObject</CODE>
previously available in the RMI server code. The RMI code would either
extend
<CODE>UnicastRemoteObject</CODE> or call the <CODE>exportObject</CODE>
method
from the <CODE>UnicastRemoteObject</CODE> class. The
<CODE>PortableRemoteObject</CODE>
also contains an equivalent <CODE>exportObject</CODE> method. In the
current
implementation, is is best to explicitly remove unused objects by
calling <CODE>PortableRemoteObject.unexportObject()</CODE>.

<A NAME="jini"></A>
<H3>JINI lookup services</H3>

(To be done later)


<A NAME="perf"></A>
<H3>Improving Lookup Performance</H3>

When you run your application, if you find it would be faster
to walk the object to the other computer on a floppy, you
have a network configuration problem. The source of the problem
is how host names and IP addresses are resolved, and there is
a workaround.  

<P>
RMI and other naming services use the <CODE>InetAddress</CODE> class to
obtain resolved host name and IP addresses. <CODE>InetAddress</CODE> 
caches lookup results to improve subsequent calls, but when it is 
passed a new IP address or host name, it performs a cross-reference
between the IP address and the host name to prevent address spoofing. 
If you supply the host name as an IP address, <CODE>InetAddress</CODE> 
still tries to verify the name of the host.

<P>
To workaround this problem, include the host name and IP address
in a hosts file on the client.

<P>
<STRONG>Unix Systems:</STRONG>
On Unix, the hosts file is usually <CODE>/etc/hosts</CODE>.

<P>
<STRONG>Windows:</STRONG>
On Winddows 95 or 98, the hosts file is 
<CODE>c:\windows\hosts</CODE>, (the <CODE>hosts.sam</CODE>
file is a sample
file). On Windows NT, the hosts file is
<CODE>c:\winnt\system32\drivers\etc\hosts</CODE>

<P>
All you need to do is put these lines in your hosts file. 
The <CODE>myserver1</CODE> and <CODE>myserver2</CODE> entries
are the hosts running the remote server and <CODE>rmiregistry</CODE>

<PRE>
127.0.0.1  localhost
129.1.1.1  myserver1
129.1.1.2  myserver2
</PRE>

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