Wait() to slow?

Question

I have a client/server model that use threads to handle the input and output. Basically, I used the idea behind awt's event listeners to handle when something occured that the program needed to deal with. Whenever a download of a byte array completes, the listener's downloadComplete(byte[] data) method is triggered. I have it synchronized and execute notify() in the method to tell the program that there is data to be processed. The actual thread that is created by my reciever class executes the downloadComplete(byte[]) method.
public synchronized void downloadComplete(byte[] data)
this.data = data;
notify();
}I also have a command system that just makes transfering ints quick in order to send 'commands' from client - server and vice versa.
public synchronized void commandRecieved(int command)
lastCommand = command;
notify();
}When I send a data request from the client to the server, this function executes:
public synchronized String getData(int location)
try
  client.sendCommand(201);
  wait();
  client.upload(location);
  wait();
  String temp = (String)(Convert.byteArrayToObject(data));
  //Convert is just a class with static methods from converting primitives and
  //objects into byte arrays and back.
  return temp;
catch(InterruptedException e)
}Basically, the client sends the command '201' to the server, which tells it the kind of data it wants. It then waits for the server to send back a confirmation code that it's waiting for the data's location (That trigger's the notify() in the commandRecieved(int) method). The client then uploads the location integer, then waits again. The server fetches the data, then transmits it back. When the download completes, it's stored in the global data variable and notify() is called again. The client converts the byte array data into a string and returns it.
This all works fine once. The first time it executes. The second time, I get hung up on the second wait() call. If I delay the server, it's fine. It's as if the server is getting the data and downloading it to the client before the client can go from the client.upload() call to the wait() call - even though it's a single line of code (In the client class, the final statement in the upload method is notify(), which starts the thread that handles the uploading.) I have been trying for two days to either fix this myself or find a solution online. The only thing that works is to put Thread.sleep(100) before the notify() in the downloadRecieved() method, but this slows the whole process down to unacceptable levels. Is there something about wait() I don't know?

Answer 1

I'll add the books to my list as I've been trying to learn concurrency through trial and error, but of course I had boolean controls. I originally had them in place, but as I went through tests trying to pinpoint my problem, I removed ALL the controls - and the program behaved exactly the same.
But my problem seems to be tied to my transmission threads. No download confirmation is sent back to the server when the client hangs, which means, somehow, every single thread in the client (There are minimum four at any given time) get stopped, all at the same time. I'm not going to go into my client/server code as it'll take all day to break it down. Thanks for those book suggestions.