Java, Junit - Capture the standard input / Output for use in a unit test [duplicate]

Answer 1

Here is the solution in place of ByteArrayOutputStream. It does not add anything to the idea of System.setOut. Rather, I want to share the implementation that is better than capturing everything into ByteArrayOutputStream. I prefer to capture only selected information and let all log messages to appear in the console as they are logged rather than capturing everything into a balckbox (of which size?) for later processing.

/**
 * Once started, std output is redirected to this thread. 
 * Thread redirects all data to the former system.out and
 * captures some strings.*/
static abstract class OutputCaputre extends Thread {

    // overrdie these methods for System.err
    PrintStream getDownstream() { return System.out;}
    void restoreDownstream() { System.setOut(downstream);}

    // will be called for every line in the log
    protected abstract void userFilter(String line);

    final PrintStream downstream;
    public final PipedInputStream pis;
    private final PipedOutputStream pos;
    OutputCaputre() throws IOException {
        downstream = getDownstream();

        pos = new PipedOutputStream();
        pis = new PipedInputStream(pos);
        System.setOut(new PrintStream(pos));

        start();
    }

    public void run() {
        try {
            BufferedReader br = new BufferedReader(new InputStreamReader(pis));

            // once output is resotred, we must terminate
            while (true) {
                String line = br.readLine();
                if (line == null) {
                    return;
                }
                downstream.println(line);
                userFilter(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void terminate() throws InterruptedException, IOException {
        restoreDownstream(); // switch back to std
        pos.close(); // there will be no more data - signal that
        join(); // and wait until capture completes
    }
};

Here is an example of using the class:

OutputCaputre outputCapture = new OutputCaputre() {
    protected void userFilter(String line) {
        downstream.println("Capture: " + line);
    }       
};
System.out.println("do you see me captured?");
// here is your test    
outputCapture.terminate(); // finally, stop capturing

Answer 2

Use System.setOut() (and System.setErr()) to redirect the output to an arbitrary printstream - which can be one that you read from programmatically.

For example:

final ByteArrayOutputStream myOut = new ByteArrayOutputStream();
System.setOut(new PrintStream(myOut));

// test stuff here...

final String standardOutput = myOut.toString();

Answer 3

The System class has methods setIn(), setOut() and setErr() that allow you to set the standard input, output and error streams, e.g. to a ByteArrayOutputStream that you can inspect at will.