If a remote object is exported by a call to 
UnicastRemoteObject.exportObject(obj, port), and later unexported
by a call to UnicastRemoteObject.unexportObject(obj, true), the port is still
being used.

The following code shows the problem:

import java.rmi.registry.LocateRegistry;
import java.io.*;
import java.rmi.server.*;
import java.net.*;
import java.rmi.*;

public class ExportTest {        
    public static void main(String[] args) throws Exception {
        int port = 10001;
        ExpObj obj = (ExpObj) new ExpObjImpl();
        UnicastRemoteObject.exportObject(obj, port);        
        UnicastRemoteObject.unexportObject(obj, true);
        try {
            ServerSocket s = new ServerSocket(port);
        } catch (IOException e) {
            System.out.println("Got an exception in creating ServerSocket");
            e.printStackTrace();
        }            
    }
    
}

interface ExpObj extends java.rmi.Remote {    
}

class ExpObjImpl implements ExpObj {
    public ExpObjImpl() throws RemoteException {}
}

The output is:

Got an exception in creating ServerSocket
java.net.BindException: Address already in use
        at java.net.PlainSocketImpl.socketBind(Native Method)
        at java.net.PlainSocketImpl.bind(PlainSocketImpl.java:397)
        at java.net.ServerSocket.<init>(ServerSocket.java:170)
        at java.net.ServerSocket.<init>(ServerSocket.java:82)
        at ExportTest.main(ExportTest.java:14)