How to determine the exact state of a BufferedReader?

Question

I have a BufferedReader (generated by new BufferedReader(new InputStreamReader(process.getInputStream()))). I'm quite new to the concept of a BufferedReader but as I see it, it has three states:

1. A line is waiting to be read; calling bufferedReader.readLine will return this string instantly.
2. The stream is open, but there is no line waiting to be read; calling bufferedReader.readLine will hang the thread until a line becomes available.
3. The stream is closed; calling bufferedReader.readLine will return null.

Now I want to determine the state of the BufferedReader, so that I can determine whether I can safely read from it without hanging my application. The underlying process (see above) is notoriously unreliable and so might have hung; in this case, I don't want my host application to hang. Therefore I'm implementing a kind of timeout. I tried to do this first with threading but it got horribly complicated.

Calling BufferedReader.ready() will not distinguish between cases (2) and (3) above. In other words, if ready() returns false, it might be that the stream just closed (in other words, my underlying process closed gracefully) or it might be that the underlying process hung.

So my question is: how do I determine which of these three states my BufferedReader is in without actually calling readLine? Unfortunately I can't just call readLine to check this, as it opens my app up to a hang.

I am using JDK version 1.5.

Answer 1

Finally I found a solution to this. Most of the answers here rely on threads, but as I specified earlier, I am looking for a solution which doesn't require threads. However, my basis was the process. What I found was that processes seem to exit if both the output (called "input") and error streams are empty and closed. This makes sense if you think about it.

So I just polled the output and error streams and also tried to determine if the process had exited or not. Below is a rough copy of my solution.

public String readLineWithTimeout(Process process, long timeout) throws IOException, TimeoutException {
  BufferedReader output = new BufferedReader(new InputStreamReader(process.getInputStream()));
  BufferedReader error = new BufferedReader(new InputStreamReader(process.getErrorStream()));
  boolean finished = false;
  long startTime = 0;
  while (!finished) {
    if (output.ready()) {
      return output.readLine();
    } else if (error.ready()) {
      error.readLine();
    } else {
      try {
        process.exitValue();
        return null;
      } catch (IllegalThreadStateException ex) {
        //Expected behaviour
      }
    }
    if (startTime == 0) {
      startTime = System.currentTimeMills();
    } else if (System.currentTimeMillis() > startTime + timeout) {
      throw new TimeoutException();
    }
  }
}

Answer 2

There is a state where some data may be in the buffer, but not necessarily enough to fill a line. In this case, ready() would return true, but calling readLine() would block.

You should easily be able to build your own ready() and readLine() methods. Your ready() would actually try to build up a line, and only when it has done so successfully would it return true. Then your readLine() could return the fully-formed line.

Answer 3

This is a pretty fundamental issue with java's blocking I/O API.

I suspect you're going to want to pick one of:

(1) Re-visit the idea of using threading. This doesn't have to be complicated, done properly, and it would let your code escape a blocked I/O read fairly gracefully, for example:

final BufferedReader reader = ...
ExecutorService executor = // create an executor here, using the Executors factory class.
Callable<String> task = new Callable<String> {
   public String call() throws IOException {
      return reader.readLine();
   }
};
Future<String> futureResult = executor.submit(task);
String line = futureResult.get(timeout);  // throws a TimeoutException if the read doesn't return in time

(2) Use java.nio instead of java.io. This is a more complicated API, but it has non-blocking semantics.

Answer 4

Have you confirmed by experiment your assertion that ready() will return false even if the underlying stream is at end of file? Because I would not expect that assertion to be correct (although I haven't done the experiment).

Answer 5

You could use InputStream.available() to see if there is new output from the process. This should work the way you want it if the process outputs only full lines, but it's not really reliable.

A more reliable approach to the problem would be to have a seperate thread dedicated to reading from the process and pushing every line it reads to some queue or consumer.

Answer 6

In general, you have to implement this with multiple threads. There are special cases, like reading from a socket, where the underlying stream has a timeout facility built-in.

However, it shouldn't be horribly complicated to do this with multiple threads. This is a pattern I use:

private static final ExecutorService worker = 
  Executors.newSingleThreadExecutor();

private static class Timeout implements Callable<Void> {
  private final Closeable target;
  private Timeout(Closeable target) {
    this.target = target;
  }
  public Void call() throws Exception {
    target.close();
    return null;
  }
}

...

InputStream stream = process.getInputStream();
Future<?> task = worker.schedule(new Timeout(stream), 5, TimeUnit.SECONDS);
/* Use the stream as you wish. If it hangs for more than 5 seconds, 
   the underlying stream is closed, raising an IOException here. */
...
/* If you get here without timing out, cancel the asynchronous timeout 
  and close the stream explicitly. */
if(task.cancel(false))
  stream.close();

Answer 7

You could make your own wrapper around InputStream or InputStreamReader that works on a byte-by-byte level, for which ready() returns accurate values.

Your other options are threading which could be done simply (look into some of the concurrent data structures Java offers) and NIO, which is very complex and probably overkill.

Answer 8

If you just want the timeout then the other methods here are possibly better. If you want a non-blocking buffered reader, here's how I would do it, with threads: (please note I haven't tested this and at the very least it needs some exception handling added)

public class MyReader implements Runnable {
    private final BufferedReader reader;
    private ConcurrentLinkedQueue<String> queue = new ConcurrentLinkedQueue<String>();
    private boolean closed = false;

    public MyReader(BufferedReader reader) {
        this.reader = reader;
    }

    public void run() {
        String line;
        while((line = reader.readLine()) != null) {
            queue.add(line);
        }
        closed = true;
    }

    // Returns true iff there is at least one line on the queue
    public boolean ready() {
        return(queue.peek() != null);
    }

    // Returns true if the underlying connection has closed
    // Note that there may still be data on the queue!
    public boolean isClosed() {
        return closed;
    }

    // Get next line
    // Returns null if there is none
    // Never blocks
    public String readLine() {
        return(queue.poll());
    }
}

Here's how to use it:

BufferedReader b; // Initialise however you normally do
MyReader reader = new MyReader(b);
new Thread(reader).start();

// True if there is data to be read regardless of connection state
reader.ready();

// True if the connection is closed
reader.closed();

// Gets the next line, never blocks
// Returns null if there is no data
// This doesn't necessarily mean the connection is closed, it might be waiting!
String line = reader.readLine(); // Gets the next line

There are four possible states:

1. Connection is open, no data is available
2. Connection is open, data is available
3. Connection is closed, data is available
4. Connection is closed, no data is available

You can distinguish between them with the isClosed() and ready() methods.