CODEWITHSUNDEEP
javaexecstdoutstdin
MattS
MattS

Reputation: 717

Running external program with redirected stdin and stdout from Java

I'm trying to running an external program from a Java program and I'm having trouble. Basically what I'd like to do would be this:

 Runtime.getRuntime().exec("./extprogram <fileIn >fileOut");

However I've found that that doesn't work - Java apparentls needs to use a Process with input and output streams and other things which I'm not experienced with.

I've looked at a number of examples across the internet (many of which are from SO), and there doesn't seem to be a simple standard way of doing this, which for someone who doesn't fully understand what's going on, can be quite frustrating.

I'm also having trouble trying to build my own code off the examples of other people's code because generally it seems most other people 1. aren't interested in redirecting stdin, and 2. aren't necessarily redirecting stdout to a file, but instead to System.out.

So, would anyone be able to point me in the direction of any good simple code templates for calling external programs and redirecting stdin and stdout? Thanks.

Upvotes: 5

Views: 13839

Answers (3)

pb2q
pb2q

Reputation: 59617

If you must use Process, then something like this should work:

public static void pipeStream(InputStream input, OutputStream output)
   throws IOException
{
   byte buffer[] = new byte[1024];
   int numRead = 0;

   do
   {
      numRead = input.read(buffer);
      output.write(buffer, 0, numRead);
   } while (input.available() > 0);

   output.flush();
}

public static void main(String[] argv)
{
   FileInputStream fileIn = null;
   FileOutputStream fileOut = null;

   OutputStream procIn = null;
   InputStream procOut = null;

   try
   {
      fileIn = new FileInputStream("test.txt");
      fileOut = new FileOutputStream("testOut.txt");

      Process process = Runtime.getRuntime().exec ("/bin/cat");
      procIn = process.getOutputStream();
      procOut = process.getInputStream();

      pipeStream(fileIn, procIn);
      pipeStream(procOut, fileOut);
   }
   catch (IOException ioe)
   {
      System.out.println(ioe);
   }
}

Note:

  • Be sure to close the streams
  • Change this to use buffered streams, I think the raw Input/OutputStreams implementation may copy a byte at a time.
  • The handling of the process will probably change depending on your specific process: cat is the simplest example with piped I/O.

Upvotes: 7

Snake
Snake

Reputation: 805

have you tried System.setIn and System.setOut? has been around since JDK 1.0.

public class MyClass
{
    System.setIn( new FileInputStream( "fileIn.txt" ) );
    int oneByte = (char) System.in.read();
    ...

    System.setOut( new FileOutputStream( "fileOut.txt" ) );
    ...

Upvotes: 0

corsiKa
corsiKa

Reputation: 82579

You could try something like this:

ProcessBuilder pb = new ProcessBuilder();
pb.redirectInput(new FileInputStream(new File(infile));
pb.redirectOutput(new FileOutputStream(new File(outfile));
pb.command(cmd);
pb.start().waitFor();

Upvotes: 9

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