Using Runtime.getRuntime().exec in eclipse

Question

I'm using Runtime.getRuntime().exec in eclipse to run another java program from the current program. I've used the following code.

InputStreamReader isr=new InputStreamReader(System.in);
BufferedReader br=new BufferedReader(isr);
System.out.println("Enter the class name");
String s=br.readLine();    
String str="XYZ";
String[] cmd = {"java","-cp", "C:/Users/..../workspace/Testing/bin",s,str};         
Process pro=Runtime.getRuntime().exec(cmd);

I'm also passing a string "XYZ" to that program. That program just accepts the string and displays

Your string is XYZ

But by using the line

String[] cmd = {"java","-cp", "C:/Users/..../workspace/Testing/bin",s,str};

i'm able to run the program but it is not accepting any arguments. It is neither displaying the output nor showing any errors.

Where am i going wrong?

Consider the program to be called is

import java.io.*;
public class Test
{
public static void main(String[] args) throws IOException
{   
    InputStreamReader isr=new InputStreamReader(System.in);
    BufferedReader br=new BufferedReader(isr);
    System.out.println("Enter any string");
    String s=br.readLine();
    System.out.println("Your string is "+s);    
}
}

This program should accept the string XYZ and prints Your string is XYZ

Pressman

For the record if you manually execute the exe process outside of eclipse it works correctly? – Xeric

@TheCapn: Execution is working with and without eclipse but i'm not able to pass any arguments in both ways – Pressman

I suspect the issue is with the program you're calling. Do you have source code for that or is it more of a blackbox operation? – Xeric

I do have the source code but that is a part of my project. Anyways consider a basic program which accepts the argument "XYZ" from the above program and just prints Your string is XYZ – Pressman

My point is that if you're not able to execute the application properly outside of eclipse (i.e. through your terminal) then the issue must be with the program you're calling. Is there a misunderstanding between us? – Xeric

@TheCapn the calling program is perfect. If you have a look at the sample calling program above, i'm able to catch hold of the argument if the line 'String s=br.readLine();' is written as `String s=args[0];' – Pressman

Answer 1

You need to read the output (and error) streams from the Process using getInputStream() and getErrorStream(). You’ll need a separate thread for this if you want to wait for the process to complete.

String[] cmd = {"java", "-cp", "C:/Users/..../workspace/Testing/bin", s, str};
Process p = new ProcessBuilder(cmd).redirectErrorStream(true).start();
final InputStream pOut = p.getInputStream();
Thread outputDrainer = new Thread()
{
    public void run()
    {
        try
        {
            int c;
            do
            {
                c = pOut.read();
                if (c >= 0)
                    System.out.print((char)c);
            }
            while (c >= 0);
        }
        catch (IOException e)
        {
            e.printStackTrace();
        }
    }
};
outputDrainer.start();

p.waitFor();

If you are using Java 7 and want all output of the process to be redirected to the console, the code is considerably simpler:

String[] cmd = {"java", "-cp", "C:/Users/..../workspace/Testing/bin", s, str};
Process p = new ProcessBuilder(cmd).redirectError(Redirect.INHERIT)
                                   .redirectOutput(Redirect.INHERIT)
                                   .start();
p.waitFor();

The redirectError() and redirectOutput() methods with Redirect.INHERIT cause output to just be sent to the parent Java process.

Grandiloquence

even your code is not working. I've pasted a very simple code in my question (the program to be called) which should accept the string "XYZ" and display the output "Your string is XYZ" in our program – Pressman

To clarify: the java docs show the redirectError and redirectOutput as being supported since java 7 ("if you are using java7" is a little ambiguous and the edit queue is currently full so i cant tweak the language and add the citation myself) – Rail

Answer 2

You are executing javac, the language compiler. I believe you want to invoke the java runtime on the class file with your main method. Replace javac with java, and specify your main class.

Maxey

oh sorry. that is java over there. not javac. – Pressman

Answer 3

You are passing your data as an Argument but reading it from System.in. If you read the data from the arguments it'll work. So do what @prunge said to capture what the subprocess print and use this as your test and everything will work just fine.

import java.io.*;
public class Test
{
    public static void main(String[] args) throws IOException
    {   
        if(args.length==0)
            System.out.println("No String received");
        else
            System.out.println("Your string is " + args[0]);    
    }
}

Be sure that you check both the InputStream and the ErrorStream.

If you however want to pass the data via System.in then you have to change your call code to:

InputStreamReader isr=new InputStreamReader(System.in);
BufferedReader br=new BufferedReader(isr);
System.out.println("Enter the class name");
String s=br.readLine();    
String str="XYZ";
String[] cmd = {"java","-cp", "C:/Users/..../workspace/Testing/bin",s};         
Process pro=Runtime.getRuntime().exec(cmd);
PrintWriter output= new PrintWriter(pro.getOutputStream());
output.println(str);

Disenchant