Java.lang.Process.exec()

Question

I have read a article about the "java.lang.Process.exec()" (url:http://www.javaworld.com/javaworld/jw-12-2000/jw-1229-traps.html)
Having some questions with the example in it.
The example code:
import java.util.*;
import java.io.*;
public class BadExecJavac
    public static void main(String args[])
        try
            Runtime rt = Runtime.getRuntime();
            Process proc = rt.exec("javac");
            //int exitVal = proc.exitValue();
            int exitVal = proc.waitFor();
            System.out.println("Process exitValue: " + exitVal);
        } catch (Throwable t)
            t.printStackTrace();
}The process can run but never complete.
Why? Just because when invoke the javac.exe without any argument,it will product a set of usage statements that describe how to run the program and the meaning of all the available program options.So the process is in deadlock.
The question is, when i change the exec("javac") to exec("javac a.java"), in which the "a.java" is not exist, so the jvm should product error:
error: cannot read: a.java
1 error
But after i changed the code and run the class, at this time the process can run and complete.
The two codes both product some statements,but why the first one never complete,but the second did. Why?

Answer 1

import java.util.*;
import java.io.*;
public class A
    public static void main(String args[])
        try
            Runtime rt = Runtime.getRuntime();
            Process proc = rt.exec("javac");
            InputStream is = proc.getErrorStream();
            int i=0;
            while ((i = is.read()) != -1)
                System.out.print((char)i);
            // int exitVal = proc.exitValue();
            // int exitVal = proc.waitFor();
            // System.out.println("Process exitValue: " + exitVal);
        } catch (Throwable t)
            t.printStackTrace();
}usng this modification, i could see some error messages.
because exec(cmd) executes the command in a separate process, it will not display the results on the current console. If you want see the results, you should use getInputStream() and getErrorStream().
good luch ^^