Read live output of process in java

问题

I start a python script in my java application with

Process p = Runtime.getRuntime().exec("python script.py");

This script runs in a loop and is only canceled by an event (or user interaction). The script writes to the output every loop cycle, some text like "12:22:35 -- Heartbeat"

while True:
  print("Heartbeat")
  time.sleep(1)

In my Java application I want to read this output as it appears. My problem is, if I use the BufferReader, it will wait until the process is completed and after that it reads the output. Here is how I read it:

BufferedReader is = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line;

while ((line = is.readLine()) != null)
   System.out.println(line);

How can I read the output "live"?

For better understanding: the python-script listen on a hardware button, and when this button is pressed, some output is written out. And in my java application I want to show up this message and sent it to some clients.

回答1:

Using https://github.com/zeroturnaround/zt-exec

new ProcessExecutor().command("python", "script.py")
    .redirectOutput(new LogOutputStream() {
        @Override
        protected void processLine(String line) {
            ...
        }
    })
    .execute();

回答2:

I've often written something like:

class StreamHandler extends Thread {

    InputStream is;
    Writer writer;

    StreamHandler(InputStream is, Writer writer) {
        super("StreamHandler");

        this.is = is;
        this.writer = writer;
    }

    @Override
    public void run() {
        try (InputStreamReader isr = new InputStreamReader(is)) {

            char buffer[] = new char[256];
            int n;
            while ((n = isr.read(buffer)) > 0) {
                writer.write(buffer, 0, n);
            }
        } catch (IOException e) {
            System.err.println("StreamHandler: " + e);
        }
    }
}

While I've used this for capturing the output stream to my own buffer, you could simply echo to stdout in real time like:

Process process = Runtime.getRuntime().exec(...);
Writer writer = new OutputStreamWriter(System.out);
StreamHandler stdout_handler = new StreamHandler(process.getInputStream(), writer);
stdout_handler.start();

回答3:

try changing your python code to:

while True:
    print("Heartbeat", flush=True)
    time.sleep(1)

I've been trying to do the same as you and found this solution on this question: PumpStreamHandler can capture the process output in realtime

来源：https://stackoverflow.com/questions/41991147/read-live-output-of-process-in-java

标签

java

python

stdout