Draining Standard Error in Java

Question

When launching a process from Java, both stderr and stdout can block on output if I don't read from the pipes. Currently I have a thread that pro-actively reads from one and the main thread blocks on the other.

Is there an easy way to join the two streams or otherwise cause the subprocess to continue while not losing the data in stderr?

Answer 1

Just have two threads, one reading from stdout, one from stderr?

Answer 2

Set the redirectErrorStream property on ProcessBuilder to send stderr output to stdout:

ProcessBuilder builder = new ProcessBuilder(command);
builder.redirectErrorStream(true);

You should then create a thread to deal with the process stream, something like the following:

Process p = builder.start();

InputHandler outHandler = new InputHandler(p.getInputStream());

Where InputHandler is defined as:

private static class InputHandler extends Thread {

    private final InputStream is;

    private final ByteArrayOutputStream os;

    public InputHandler(InputStream input) {
        this.is = input;
        this.os = new ByteArrayOutputStream();
    }

    public void run() {
        try {
            int c;
            while ((c = is.read()) != -1) {
                os.write(c);
            }
        } catch (Throwable t) {
            throw new IllegalStateException(t);
        }
    }

    public String getOutput() {
        try {
        os.flush();
        } catch (Throwable t) {
            throw new IllegalStateException(t);
        }
        return os.toString();
    }

}

Alternatively, just create two InputHandlers for the InputStream and ErrorStream. Knowing that the program will block if you don't read them is 90% of the battle :)