Running external program with redirected stdin and stdout from Java
Solution 1
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.
Solution 2
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();
MattS
Updated on June 04, 2022Comments
-
MattS almost 2 years
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 redirectingstdout
to a file, but instead toSystem.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
andstdout
? Thanks.