Process.waitFor() a thread

Question

While running an external script, I want to read the ErrorStream and OutputStream of this script both simultaneously and separately and then process them further. Therefore, I start a Thread for one of the streams. Unfortunately, the Process doesn't seem to waitFor the Thread to be terminated, but return after the non-threaded stream has no further input.

In a nutshell, here is what I am doing:

ProcessBuilder pb = new ProcessBuilder(script);  
final Process p = pb.start();  

new Thread(new Runnable() {  
  public void run() {
    BufferedReader br = new BufferedReader(new InputStreamReader(p.getErrorStream()));
    ...read lines and process them...
  }
}).start();

BufferedReader br = new BufferedReader(new InputStreamReader(p.getInputStream()));
...read lines and process them...

int exitValue = p.waitFor();
p.getOutputStream().close();
return exitValue;

Is there any possibility to waitFor the Thread to be terminated?

Answer 1

Here's general code for doing what you want to do. In this case there is both input and output: I am piping someFile into the process and piping the output to System.out. Files.copy() and ByteStreams.copy() are just Guava convenience methods to hook up an InputStream to an OutputStream. We then wait for the command to finish.

final Process pr = Runtime.getRuntime().exec(cmd);

new Thread() {
    public void run() {
        try (OutputStream stdin = pr.getOutputStream()) {
            Files.copy(someFile, stdin);
        } 
        catch (IOException e) { e.printStackTrace(); }
    }
}.start();

new Thread() {
    public void run() {
        try (InputStream stdout = pr.getInputStream()) {
            ByteStreams.copy(stdout, System.out);
        } 
        catch (IOException e) { e.printStackTrace(); }  
    }
}.start();              

int exitVal = pr.waitFor();
if( exitVal == 0 )
    System.out.println("Command succeeded!");
else    
    System.out.println("Exited with error code " + exitVal);

A more verbose version if you are running prior to Java 7 with the try-with-resources block:

final Process pr = Runtime.getRuntime().exec(cmd);

new Thread() {
    public void run() {
        OutputStream stdin = null;
        try {
            Files.copy(someFile, stdin = pr.getOutputStream());
        } 
        catch (IOException e) { e.printStackTrace(); }
        finally {
            if( stdin != null ) {
                try { stdin.close(); } 
                catch (IOException e) { e.printStackTrace(); }
            }
        }               
    }
}.start();

new Thread() {
    public void run() {
        InputStream stdout = null;
        try {
            ByteStreams.copy(stdout = pr.getInputStream(), System.out);
        } 
        catch (IOException e) { e.printStackTrace(); }
        finally {
            if( stdout != null ) {
                try { stdout.close(); } 
                catch (IOException e) { e.printStackTrace(); }
            }
        }               
    }
}.start();              

int exitVal = pr.waitFor();
if( exitVal == 0 )
    System.out.println("Command succeeded!");
else    
    System.out.println("Exited with error code " + exitVal);

Answer 2

You can use Thread.join(...) to wait for a Thread to finish. Note that the call throws InterruptedException if the current thread receives an interrupt before the thread you are waiting for finishes.