Optimisation of Client/Server communication

[Greg Pata]

Hi,
Here is the code of a kind of client/server communication. It works.
However I am sure there is a better way to notify the client as soon as the message arrives.
Thank you for your help.

public class Postman{
			private boolean msgArrived;
			private long msg;
			
		...
		
		public void checkout(){	
			// A-thread begins
			msgArrived = false;
			Test.controller.requestData( new IDataHandler() {
				@Override public void retrieveData(long data) {
					// B-thread retrieves data
					msg = data;
					// B-thread already exists and still exists 
					// after the end of A-thread
					msgArrived = true;
				}
			});
			while(!msgArrived){
				try {
					Thread.sleep(250);
				} catch (InterruptedException ie) {
					return;
				}
			}
			System.out.format("The message is %d %n", msg);
		}

[Chris Hurst]

1) The code as is has a bug .. msgArrived should be AtomicBoolean or volatile or you may get odd results.

2) wait / notify is probably the simplest solution, your inner class (IDataHandler()) should synchronize on a common object and notify when setting the boolean. Then replace the sleep with synchronizing on the same object and a wait (careful with using 'this' for synchronization as new IDataHandler() is an inner class).

[Greg Pata]

Thank you, Chris, for your help!
1) Ok you are right.
2) I have tried and failed. But now, thanks to you it works . The code follows:
(slightly different from what you described)

public class Postman{
	private boolean msgArrived;
	private long msg;
	
...
public synchronized void notifyMsgArrived(){
	msgArrived = true;
	notifyAll();
}

public synchronized void checkout(){	
	// A-thread begins
	msgArrived = false;
	Test.controller.requestData( new IDataHandler() {
		@Override public void retrieveData(long data) {
			// B-thread retrieves data
			msg = data;
			notifyMsgArrived();
		}
	});
	while(!msgArrived){
		try {
			wait();
		} catch (InterruptedException ie) {
			return;
		}
	}
	System.out.format("The message is %d %n", msg);
}

[Sresh Rangi]

This can be done with the newer CompletableFuture class. Libraries like guava have similar classes if Java 8 is not available. Create a CompletableFuture inside the checkout method:

CompletableFuture<Long> message = new CompletableFuture<>();

Call

message.complete(data)

inside the callback, and

message.get()

outside it to get its value.

[Greg Pata]

Sresh Rangi wrote:This can be done with the newer CompletableFuture class.

Good to know. Thank you.