java - Runtime.getRuntime().exec(cmd) hanging

Question

I am executing a command which returns me the Revision number of a file; 'fileName'. But if there is some problem executing the command, then the application hangs up. What can I do to avoid that condition? Please find below my code.

String cmd= "cmd /C si viewhistory --fields=revision --project="+fileName; 
Process p = Runtime.getRuntime().exec(cmd) ;  
BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));  
String line = null; 
while ((line = in.readLine()) != null) {  
System.out.println(line);  
} 

} catch (Exception e) {  
e.printStackTrace();  
 }

Answer 1

I guess the issue is that you are only reading InputStream and not reading ErrorStream. You also have to take care that both the streams are read in parallel. It may so happen that currently the data piped from the output stream fills up the OS buffer, your exec command will be automatically be suspended to give your reader a chance to empty the buffer. But the program will still be waiting for the output to process. Hence, the hang occurs.

You can create a separate class to handle both the Input and Error Stream as follows,

public class ReadStream implements Runnable {
    String name;
    InputStream is;
    Thread thread;      
    public ReadStream(String name, InputStream is) {
        this.name = name;
        this.is = is;
    }       
    public void start () {
        thread = new Thread (this);
        thread.start ();
    }       
    public void run () {
        try {
            InputStreamReader isr = new InputStreamReader (is);
            BufferedReader br = new BufferedReader (isr);   
            while (true) {
                String s = br.readLine ();
                if (s == null) break;
                System.out.println ("[" + name + "] " + s);
            }
            is.close ();    
        } catch (Exception ex) {
            System.out.println ("Problem reading stream " + name + "... :" + ex);
            ex.printStackTrace ();
        }
    }
}

The way you use it is as follows,

String cmd= "cmd /C si viewhistory --fields=revision --project="+fileName; 
Process p = Runtime.getRuntime().exec(cmd) ;  
s1 = new ReadStream("stdin", p.getInputStream ());
s2 = new ReadStream("stderr", p.getErrorStream ());
s1.start ();
s2.start ();
p.waitFor();        
} catch (Exception e) {  
e.printStackTrace();  
} finally {
    if(p != null)
        p.destroy();
}