exec command not working

[govind bhardwaj]

The following code was working fine in a separate program, but when I inserted into my Servlet code, it always returns a null. Can anyone please tell where am I going wrong.

PrintWriter out=res.getWriter();
String command = "java "+function+" "+argument1+" "+argument2;
Process child = Runtime.getRuntime().exec(command);
//                       child.waitFor();
BufferedReader buf = new BufferedReader(new InputStreamReader(child.getInputStream()));
String answer;
while((answer=buf.readLine())!=null)
{
    answer = buf.readLine();
    out.write(answer);
}
out.close();

[Maneesh Godbole]

Welcome to the Ranch.

In future, while posting code, please UseCodeTags

[Campbell Ritchie]

Where does it return null? There aren't any return statements in what you posted.

[govind bhardwaj]

the thing inside the while loop doesn't seem to work.
So i wrote down

String answer;
answer=buf.readLine();
out.write(command+answer);

and what i at the other end was
"java add 2 3null"

[William Brogden]

If the process failed at the start and only wrote to the stderr stream, then you would get null from the buf.readLine()

Does the Java program you are attempting to run have some assumption about the "current directory"? Remember, in the servlet environment you have no control over the "current directory."

Bill

[Guido Granobles]

If you are using Tomcat as a servlet container then the Java classes that you want execute should be in the ROOT directory. That is to say in something like $TOMCA_HOME/webapps/yourWebApp/ROOT. If your classes are packaged then you should have the directory in there: ROOT/namePackage1/myClass

[Wouter Oet]

It may not completely apply to this topic however this is a good article about using Runtime.exec().