read the output from java exec

Question

Hello i have some question about java. here is my code:

public static void main(String[] args) throws Exception {
 Process pr = Runtime.getRuntime().exec("java -version");
 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);
}

in that code i'am trying to get a java version command execute is ok, but i can't read the output it just return null. Why?

• f.khantsis: why did he have to add waitFor?

Answer 1

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 streams
ps.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);

• maxormo: ok, it is work. but strange read normal output of command from error stream.
• KV Prajapati: @max - Take a look at - download.oracle.com/javase/1.5.0/docs/api/java/lang/…
• Alexiuscrow: @AVD there described how to merge error & standard outputs. but not describe why it marked as 'error'. It's really seems strange.
• Kervvv: @adatapost what is the approach if I am using Process.waitFor() in conjunction with reading this stream? e.g. a download is occurring. Will the line have to complete printing, then I can hit my logic to forcibly destroy process (e.g. pr.destory()) I want to know if they can execute in parallel.
• absin: If you are doing something liek: new ProcessBuilder().command().inheritIO() remove the inheritIO() or the stream will get redirected to console and not to in

Answer 2

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)

Answer 3

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.

Answer 4

try this

public static final Pair<Integer,Integer> javaVersion(File file) throws IOException {
 final ProcessBuilder pb = new ProcessBuilder("java", "-version");
 pb.directory(new File(file.getCanonicalPath() + File.separator + "bin"));
 pb.redirectErrorStream(true);
 // Call the test target
 final Process process = pb.start();
 final InputStream in = process.getInputStream();
 final InputStream err = process.getErrorStream();
 BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(in,"UTF-8"));
 String s = bufferedReader.readLine();
 int start = s.indexOf('\"');
 int end = s.lastIndexOf('\"');
 String substring = s.substring(start + 1, end);
 String[] split = substring.split("\\.");
 return new Pair<>(Integer.parseInt(split[0]),Integer.parseInt(split[1]));
}

Answer 5

I also suffered this issue because I didn't set $JAVA_HOME correctly. (I forgot Contents/Home).
After I edit $JAVA_HOME, update Gradle JVM, and remove .idea directory to re-build with gradle, It works well.