Magercise: Writing Callbacks

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by MageLang Institute

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Unlike a client/server system, the objects in CORBA application are peers: none is intrinsically more important than any other. You may decide to design your program using various kinds of Client/Server metaphors, but that decision is yours to make. The ORB just sees objects communicating with each other over a network.

In this Magercise there is a pair of peer objects, PingObject and PongObject, for which Java implementations have already been written: PingObjectImpl and PongObjectImpl. Their behavior is such that when the PingObject is pinged, it pongs the given PongObject. When PongObject is ponged, it returns the favor by pinging the given PingObject. To keep things from getting out of hand, the PingObject keeps track of the number of times it has been pinged, and stops after a maximum number of pings.

Your task is to create two Java classes, PingServer and PongServer whose main methods will instantiate either a PingObject or a PongObject respectively, and call the appropriate methods to start the pings and pongs going.

Prerequisites

• Stock Example

Skeleton Code

• pingpong.idl
• PingObjectImpl.java
• PongObjectImpl.java
• PingServer.java
• PongServer.java

Tasks

1. First take a look at the IDL file, pingpong.idl, and generate it.

2. Examine the two implementation classes, PingObjectImpl and PongObjectImpl to see how they work and how to use them.

3. Using the skeleton code file as a starting point, modify the class PingServer to add all the code necessary to start up the ORB and BOA, create a PingObject instance, set the maximum number of pings, etc. There are comments to guide your way. Be sure to pick a unique name for your PingObject instance.

4. Now fill out the PongServer class. The code needed here will be very similar to that for PingServer, but you will also initiate the ping-pong exchange.

5. Compile your program and run it to see what happens.

Where help exists, the task numbers above are linked to the step-by-step help page.

The timing information printed out when the solution is run gives you some idea of the latency involved when calling methods on a distributed object. For extra credit, you can modify the implementation classes to print out the time difference between pings and pongs.

Solution Source

• solution/pingpong.idl
• solution/PingObjectImpl.java
• solution/PongObjectImpl.java
• solution/PingServer.java
• solution/PongServer.java

Demonstration

The PongServer part of your program should output something like:

PongObjectImpl[Server,oid=TransientId[repId=IDL:
PPModule/PongObject:1.0,serverId=1,creationTime=
-1557647756]] is ready.
pong:917565354109
pong:917565354219
pong:917565354429
pong:917565354619
pong:917565354820

The PingServer part of your program should output a similar set of messages.

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