使用Processbuilder从Java Web应用程序调用python脚本

Question

I am working on a simple meteo station - I want to use raspberry pi 3b+ as a host, dht22 sensor and write a web application in Java (with spring boot, then deploy it to tomcat 8) and Python for retrieving sensor's data. What I've done so far:

Python application for retrieving and displaying data. Works as expected, it just prints something like "22.5;37.4":

import Adafruit_DHT

DHT_SENSOR = Adafruit_DHT.DHT22
DHT_PIN = 4

humidity, temperature = Adafruit_DHT.read_retry(DHT_SENSOR, DHT_PIN)

if humidity is not None and temperature is not None:
    print("{0:0.1f};{1:0.1f}".format(temperature, humidity))
else:
    print("FAIL")

Then I've wrote a java application, put it into .jar and checked if I am able to get sensor's data. Not a rocket science, also works as expected when I use java -jar InputTest.jar on my raspberry pi:

public static void main(String[] args) {
    try {
        ProcessBuilder pb = new ProcessBuilder("python", "/home/pi/Desktop/input/dht_once.py");
        Process process = pb.start();
        BufferedReader reader = new BufferedReader(new InputStreamReader(process.getInputStream()));
        String line;
        while ((line = reader.readLine()) != null) {
            System.out.println("measured: " + line);
        }
        process.waitFor(); 
    } catch (IOException ) {
        System.out.println(" exception " + e.getLocalizedMessage());
    }
}

Then I've created a spring boot application, put my java code inside (logic same as above), packed as a war, deployed to tomcat 8 and run it. It turned out nothing is being printed (of course I've changed code to log output to logfile, it works fine, I can see other logs inside). No issues in logs, it looks like reader never returns a line.
I believe application does not wait for a process to produce output, but I have no idea why. Important thing: it takes up to few seconds to produce sensor's output. I've also changed python script just for test purposes to return value immediately:

print("22.4;33.0")

and it results in successful read by java web application. But when it has to wait few seconds for the output it kills process (process.isAlive() is false right after while loop). I've also tried to play with sleep() on current thread to force it to wait for python process but no success.

Do you guys have any idea what can be the reason for this behavior? Is there anything more I should check?

TLDR;
Java application which creates python process works fine until I run it as a web application - then it looks like it does not wait for a process' output

Answer 1

I haven't found a solution yet, however I've implemented workaround/cleaner solution. I've decided to separate totally java and python code and created microservice for data retrieving. I use flask for rest webservice (followed this tutorial https://docs.dataplicity.com/docs/control-gpios-using-rest-api ) and call it directly from java.
Since this does not resolve my initial question I do not mark it as an answer, however it might help someone.