从命令行读取输出

Question

I'm really stuck here, I've read number of articles and answers on stackoverflow, but nothing solved my problem.

I've got the method which runs Selenium Server Hub from cmd.exe using batch file:

   public static boolean startGrid() throws IOException {
        ProcessBuilder builder = new ProcessBuilder("cmd", "/c", "start", START_GRID_BAT_PATH);
        builder.redirectErrorStream(true);
        Process process = builder.start();
        String out = getCmdOutput(process);
        return out.contains("Selenium Grid hub is up and running");
    }

The server is started successfully and is running in open cmd. To make sure that server is up I use the following method to get the output from the cmd:

   protected static String getCmdOutput(Process proc) throws java.io.IOException {
        String res = "";
        BufferedReader stdInput = new BufferedReader(new
                InputStreamReader(proc.getInputStream()));
        String s = "";
        while ((s = stdInput.readLine()) != null) {
            res += s;
        }
        return res;
    }

And here is where the problem starts - method hangs at the the line s = stdInput.readLine()) != null It seems that it can't read any line from the cmd, although I can see that there is number of lines in the output produced by running server.

Any ideas what is wrong?

Answer 1

Maybe the output produced by the server doesn't contain any \\r or \\n characters but is simply line-wrapped.

Anyhow, you shouldn't rely on the line feed characters when reading the output.

The following example shows how to read binary data from the InputStream of the process non-blocking with java.nio, convert the bytes to chars and write the result to a StringWriter - which is preferrable to using string concatenation.

protected static String getCmdOutput(Process proc, Consumer<String> consumer) 
  final InputStream source = proc.getInputStream();
  final StringWriter sink = new StringWriter();

  final ByteBuffer buf = ByteBuffer.allocate(1024);
  final CharBuffer chars = CharBuffer.allocate(1024);
  final CharsetDecoder decoder = Charset.defaultCharset().newDecoder();

  try(final ReadableByteChannel from = Channels.newChannel(source)) {
    while (from.read(buf) != -1) {
      buf.flip();
      decoder.decode(buf, chars, false);
      chars.flip();
      sink.write(chars.array(), chars.position(), chars.remaining());
      buf.compact();
      chars.clear();
    }
  } catch (IOException e) {
    e.printStackTrace();
  }
  return sink.toString();
}

Edit: As you are starting a process that is continuously producing output, the loop won't finish as long as the process is running. So to continue your parent process, you have to start a thread that is capturing and processing the output of the process continuously

For example

Consumer<String> consumer = ...;
Thread t = new Thread(() -> {

  final InputStream source = proc.getInputStream();
  final StringWriter sink = new StringWriter();

  final ByteBuffer buf = ByteBuffer.allocate(1024);
  final CharBuffer chars = CharBuffer.allocate(1024);
  final CharsetDecoder decoder = Charset.defaultCharset().newDecoder();

  try(final ReadableByteChannel from = Channels.newChannel(source)) {
    while (from.read(buf) != -1) {
      buf.flip();
      decoder.decode(buf, chars, false);
      chars.flip();
      sink.write(chars.array(), chars.position(), chars.remaining());
      forward(sink, consumer); //forward the captured content to a consumer
      buf.compact();
      chars.clear();
    }
  } catch (IOException e) {
    e.printStackTrace();
  }
});
t.start();

private void forward(StringWriter sink, Consumer<String> consumer) {
  StringBuffer buf = sink.getBuffer();
  int pos = buf.lastIndexOf("\n"); 
  //you may use other options for triggering the process here,
  //i.e. on every invocation, on fixed length, etc
  if(pos != -1){
    consumer.accept(buf.substring(0, pos + 1));
    buf.delete(0, pos + 1);
  }
  //or to send the entire buffer
  /*
  consumer.accept(sink.toString());
  sink.getBuffer().delete(0, sink.getBuffer().length());
  */
}

Depending on your use case, you may not necessarily need a separate thread. It might be ok to process the child-process' output in the main thread and do what you want with the output in the consumer.

Answer 2

It doesn't hang, it just blocks waiting for a line to be read. And after the line has been read, it waits for the next. And so on.

Getting the output stream of a child process should be made in a separate thread, because the child process could in fact hang, than your main thread will hang, too.

An example:

given the instance methods

private static final long WAITTIME_FOR_CHILD = Duration.ofMinutes(5).toNanos();
private static final String START_SEQUENCE = "whatever";

private final Lock lock = new ReentrantLock();
private final Condition waitForStart = lock.newCondition();
private boolean started;

the code that starts the child process

// prepare child process
lock.lock();
try {
  Process process = processBuilder.start();
  // both standard and error output streams get redirected
  pipe(process.getInputStream(), System.out, START_SEQUENCE);
  pipe(process.getErrorStream(), System.err, START_SEQUENCE);
  // loop because of 'spurious wakeups' - should happen only on Linux,
  // but better take care of it
  // wait until WAITTIME_FOR_CHILD (default 5 minutes, see above)
  long waitTime = WAITTIME_FOR_CHILD;
  while (!started) {
    // we wait hier until the child process starts or timeout happens
    // value <= 0 means timeout
    waitTime = waitForStart.awaitNanos(waitTime);
    if (waitTime <= 0) {
      process.destroyForcibly();
      throw new IOException("Prozess xxx couldn't be started");
    }
  }
} catch (IOException | InterruptedException e) {
  throw new RuntimeException("Error during start of xxx", e);
} finally {
  lock.unlock();
}

private void getCmdOutput(InputStream in, PrintStream out, final String startSeq) {
  startDaemon(() -> {
    try (Scanner sc = new Scanner(in)) {
      while (sc.hasNextLine()) {
        String line = sc.nextLine();
        // check your know output sequence
        if (line.contains(startSeq)) {
          lock.lock();
          try {
            if (!started) {
              started = true;
              // waiting for process start finished
              waitForStart.signal();
            }
          } finally {
            lock.unlock();
          }
        }
        out.println(line);
      }
    }
  });
}

private void startDaemon(Runnable task) {
  Thread thread = new Thread(task);
  thread.setDaemon(true);
  thread.start();
}

Answer 3

尝试更换while条件是这样的：

while ((s = stdInput.readLine()) != null && !s.equals("")) {

Answer 4

 ProcessBuilder pb =
   new ProcessBuilder("myCommand", "myArg1", "myArg2");
 Map<String, String> env = pb.environment();
 env.put("VAR1", "myValue");
 env.remove("OTHERVAR");
 env.put("VAR2", env.get("VAR1") + "suffix");
 pb.directory(new File("myDir"));
 File log = new File("log");
 pb.redirectErrorStream(true);
 pb.redirectOutput(Redirect.appendTo(log));
 Process p = pb.start();
 assert pb.redirectInput() == Redirect.PIPE;
 assert pb.redirectOutput().file() == log;
 assert p.getInputStream().read() == -1;

You can redirect output to a file as oracle documentation offers in basic example:

https://docs.oracle.com/javase/7/docs/api/java/lang/ProcessBuilder.html