Create a process in java which stays alive always

[Raghu varan]

I had a scenario, where I need to start a C program in Java. This C program('command') started using ProcessBuilder, takes argument through the OutputStream and returns result through the InputStream. Here we need to start the C process once and the different operations need to be performed by changing the arguments passed to it. It is working fine for the first iteration in the below code, but the problem is the process is died after one iteration is done, i.e.,returning false for isAlive() once the single iteration is done.

ProcessBuilder pb = new ProcessBuilder(command);
Process p = pb.start();
ServerSocket server = new ServerSocket(8765);
PrintWriter pw = new PrintWriter(p.getOutputStream());
BufferedReader br = new BufferedReader( new 
InputStreamReader(p.getInputStream()) );
while(true){

    System.out.println("The p value is "+p.isAlive());

    //server waiting for client to connect
    Socket sock = server.accept();

    //reading the argument for the "command"
    DataInputStream dis = new DataInputStream(sock.getInputStream());
    String  arg=(String)dis.readUTF();
    System.out.println("data received is = "+arg);


    //code for executing the process p(command) with the arguments received from socket
    pw.println(arg);
    pw.flush();
    pw.close();

    String line = "";
    String result ="";
    //storing the result of the final_cmd with arguments in the result string                
    while ((line = br.readLine()) != null) {
        result=result+line+System.lineSeparator();
    }

    //result returning back to socket      
    DataOutputStream dout=new DataOutputStream(sock.getOutputStream());  
    dout.writeUTF(result);  
    dout.flush();  
}

[Campbell Ritchie]

Welcome to the Ranch
I shall duplicate this discussion in our threading forum.

Don't know about the process, but....
Why are you repeatedly creating data output stream objects in line 33 and not closing them? Why are you using data output streams rather than something like a print writer to write text? Do you really intend your loop to keep running forever?

[Raghu varan]

Yeah Campbell Ritchie
It is a server process which needs to be run forever.

[Tim Moores]

By "the process dies", do you mean the process no longer exists? If so, you should look into how you run this process - maybe start it outside of the Java code.

Also, it may not be a good idea to close any of the process' stream, as this is code is doing in line 23. The process may not react well to that.

The code also seems to mix running a server process with dealing with the process it started - it might be a good idea to separate those concerns.

[Raghu varan]

I tried removing the line 23 related to close(), but it is hanged at that stage itself. So I tried in that way

[Tim Moores]

You need to handle all streams of a Process, otherwise a hanging process is just one of the unintended consequences.

This article talks about what you need to watch out for, even though it's geared towards the older Runtime.exec rather than ProcessBuilder: https://www.javaworld.com/article/2071275/core-java/when-runtime-exec---won-t.html. The issues regarding Process objects are the same, though.