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?

why did he have to add waitFor? – f.khantsis Apr 26 '20 at 08:46 — f.khantsis, Apr 26 '20 at 08:46

score 68 · Accepted Answer · edited Dec 13 '15 at 19:04

68

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);

edited Dec 13 '15 at 19:04

Clashsoft

11,553
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answered Nov 16 '11 at 10:23

KV Prajapati

93,659
19
148
186

2

ok, it is work. but strange read normal output of command from error stream. – maxormo Nov 16 '11 at 10:27
@max - Take a look at - http://download.oracle.com/javase/1.5.0/docs/api/java/lang/ProcessBuilder.html – KV Prajapati Nov 16 '11 at 10:31
@AVD there described how to merge error & standard outputs. but not describe why it marked as 'error'. It's really seems strange. – Alexiuscrow Dec 22 '16 at 12:09
@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. – Kervvv Apr 29 '18 at 22:25
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` – absin Jul 02 '18 at 10:09

score 11 · Answer 2 · edited Aug 25 '15 at 22:32

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)

score 2 · Answer 3 · answered Nov 16 '11 at 10:20

2

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.

answered Nov 16 '11 at 10:20

Mnementh

50,487
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score 0 · Answer 4 · edited Oct 01 '15 at 01:27

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]));
}

score 0 · Answer 5 · answered Sep 08 '19 at 07:14

0

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.

answered Sep 08 '19 at 07:14

Kideok Kim

185
1
10

read the output from java exec

5 Answers5

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