DatagramPacket setLength() tricky business

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Java Network Programming uses the DatagramPacket class as the data representation for UDP datagrams. Whether you want to send or receive it, it will be a DatagramPacket. In a recent exam, we asked our students to create a simple UDP echo server.  

The logic of such a server is that it waits endlessly until it receives a UDP datagram and then sends it back to the same address (IP and port number) from which it came. Nothing complicated, but the devil is in the details, and some students wrote a program that would create a new DatagramPacket for each datagram received. It is not wrong, as it works, but it is a bit wasteful, as it places more stress on memory allocation and garbage collection than needed. 

However, I was surprised when chatGPT commented on the code I got from DeepSeek-R1 as a sample solution, where a call to setLength() was made after each iteration. It did not make sense to me, but chatGPT went on for quite a while, reasoning why it was needed. Eventually, it caved in and accepted that it was not needed.

Why did it happen?

A program that wants to receive a DatagramPacket must use a DatagramSocket object. A DatagramPacket has a given buffer space, sometimes set when created, where the data of the received datagram will be stored. So once you have a DatagramPacket and a DatagramSocket object, you are set to send or receive. 

In our exercise, the server will first call to receive() and later to send() to reply to the client. Upon reception of a datagram, the program can learn about the actual size of the datagram received by calling the getLength() method, as the buffer space is usually larger than the actual data received. If you want to transmit something, a call to setLength() can be useful to adjust the number of bytes to transmit before calling the send() method, as sometimes we want to transmit fewer bytes than the underlying buffer space of the DatagramPacket. 

When a datagram is received, the length attribute is adjusted to the size of it. However, it is possible to do something special if we do not want to expose all of the buffer space to an upcoming receive() call. We do that by calling setLength() with a value smaller than the buffer space so the next call to receive() will not receive more than that number of bytes. But this "trick" only affects the first receive() call. Once completed, the length will be set to the actual size of the next datagram received. 

Apparently, chatGPT understood that the length attribute needed to be reset to the full buffer size if you wanted to reuse the same DatagramPacket, as receiving a 500-byte datagram would not allow you to receive a larger datagram later.

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