This article is just to showcase the SignalHandler in Java with an example of showing a Java program that prints some content infinite times and how to use it along with head, which just takes the given lines and expect to stop the execution.

Assume that you have a file of around 100M rows of data, if you would like to cat the content of the file, it might take a couple of minutes for the operation to get completed. But assume that if you have to execute the below line,

cat my_big_file.txt|head -10

It would get completed instantaneously. What happens here is that, cat start printing out the content of the file, head takes as much as it needs and says “I’m done”. cat is able to hear that and stops reading the file and the whole execution completes.

Instead of cat, assume that you have some java program that returns an infinite stream of data. And how to make it work the same way as the cat above? How would we make the Java program hear when the head says “I’m done”?

The solution might not be apparent until you have a bit of understanding about SIGPIPE. Signals are communications to the process from OS. In this context head saying “I’m done” is via SIGPIPE. I.e., head takes the first 10 lines from the pipe and gets completed. And since there is no process to read from the pipe, the process that is writing to the pipe (cat) receives a SIGPIPE signal, on which it has a choice to act on.

Given enough context about SIGPIPE, we have to explicitly write the code to handle the SIGPIPE within a Java program. This example would showcase usage. The program prints a string infinite number of times. Without the implmentation of signal handling the program would never stop.

Usage of the above program,

To Compile:

javac RespectSigPipe.java

Respecting SIGPIPE:

This prints just the first 20 lines and stops.

java RespectSigPipe | head -20