ENTERPRISE JAVAHow to Use Jini Distributed Leasing

• By Kathy Kozel
• May 13, 2000

Listing 4. Client with manual lease renewal.

import java.rmi.*;

// version 2 client—manual lease renewal

public class DogClient2 implements Runnable {
      private Confirmation confirm;
      private Trainable myDog = new Dog();
      private Kennel kennel;
      private Thread renewalThread;

     // how long Dog should stay in kennel (regardless of lease durations)
      private long lengthOfStay = 1000 * 60 * 5; 
     
    // how much time to allow for a renewal attempt, or taking dog
    // back out, prior to the lease expiring.

      private long netTravelBuffer = (1000 * 30);

     public static void main (String args[]) {
           DogClient2 client = new DogClient2();
           client.startUp();
      }
    
     public void startUp() {
       try {
            kennel = (Kennel)
             Naming.lookup("rmi://127.0.0.1:1099/Kennel");
            System.out.println("got the kennel from registry");
            confirm = kennel.bookRoom(myDog, lengthOfStay);

            System.out.println("We got a lease for " +
             confirm.getLease().getExpiration());
            System.out.println("we want the dog to stay in for " +
             lengthOfStay);

            // we got a lease, so start a thread to handle renewals
            renewalThread = new Thread(this);
            renewalThread.start();

           } catch(Exception e) {
               e.printStackTrace();
             }
       
      } // close startUp

     public void run() {
       long timeToStop = lengthOfStay + System.currentTimeMillis();
        while (true) {       
          try {
            Thread.sleep(calcSleepTime());
           }catch(InterruptedException e) {}

          if (System.currentTimeMillis() >= timeToStop) break;
          try { 
            renewRoomLease(timeToStop);
           }catch(Exception e) {
             System.out.println("unable to renew");
             e.printStackTrace();
             break; //better go get Dog now!
           }
        } //close loop

      retrieveDog();

    } // close run

    protected void renewRoomLease(long timeToStop) throws
     Exception {
         confirm.getLease().renew(
		  (timeToStop - System.currentTimeMillis()) + netTravelBuffer);
         System.out.println("successfully renewed");
      }

    protected void retrieveDog() {
        try {
          myDog = (Dog) kennel.takeTrainableHome(confirm.getID(),
           confirm.getRoomNum());
          System.out.println("I got my dog back");

         }
         catch(Exception e) {
           System.out.println("couldn't get my dog!");
           e.printStackTrace();
         }
      
     }

    // calculate how long to sleep safely before trying to renew the
    // lease

    public long calcSleepTime() {

      // determines how long until current lease expires,
      // and then allows time to wake up and renew

       long nextExpiration = confirm.getLease().getExpiration();
       long remainingLeaseDuration = 
	     (nextExpiration - System.currentTimeMillis());
       long timeToSleep = remainingLeaseDuration - netTravelBuffer;

       if (timeToSleep >= 0)
         {
          // try to renew now!!
          return 0;
         }
        else
         {
         return timeToSleep;
         }
   
   } // close calcSleepTime
  } // close client