read the output from java exec

Use getErrorStream().

BufferedReader in = new BufferedReader(new InputStreamReader(pr.getErrorStream()));

EDIT:

You can use ProcessBuilder (and also read the documentation)

ProcessBuilder   ps=new ProcessBuilder("java.exe","-version");//From the DOC:  Initially, this property is false, meaning that the //standard output and error output of a subprocess are sent to two //separate streamsps.redirectErrorStream(true);Process pr = ps.start();  BufferedReader in = new BufferedReader(new InputStreamReader(pr.getInputStream()));String line;while ((line = in.readLine()) != null) {    System.out.println(line);}pr.waitFor();System.out.println("ok!");in.close();System.exit(0);

Note that we're reading the process output line by line into our StringBuilder. Due to the try-with-resources statement we don't need to close the stream manually. The ProcessBuilder class let's us submit the program name and the number of arguments to its constructor.

import java.io.BufferedReader;import java.io.IOException;import java.io.InputStreamReader;public class ProcessOutputExample{    public static void main(String[] arguments) throws IOException,            InterruptedException    {        System.out.println(getProcessOutput());    }    public static String getProcessOutput() throws IOException, InterruptedException    {        ProcessBuilder processBuilder = new ProcessBuilder("java",                "-version");        processBuilder.redirectErrorStream(true);        Process process = processBuilder.start();        StringBuilder processOutput = new StringBuilder();        try (BufferedReader processOutputReader = new BufferedReader(                new InputStreamReader(process.getInputStream()));)        {            String readLine;            while ((readLine = processOutputReader.readLine()) != null)            {                processOutput.append(readLine + System.lineSeparator());            }            process.waitFor();        }        return processOutput.toString().trim();    }}

Prints:

java version "1.8.0_45"Java(TM) SE Runtime Environment (build 1.8.0_45-b15)Java HotSpot(TM) 64-Bit Server VM (build 25.45-b02, mixed mode)

You already have the process-object (name pr). You can get the Input-, Output- and Errorstream. In your case you want pr.getInputStream(). Read from that, that is connected to the output of the process.