CODEWITHSUNDEEP
javamultithreadingconcurrencyioinputstream
MCL
MCL

Reputation: 4065

Block I/O in InputStream's read()

I'm trying to write an algorithm, that downloads a video live stream. Specifically, the respective stream I'm trying to fetch is based on a dynamic .m3u8 playlist file, which periodically provides URIs of new video files. The main goal is to combine those individual media files into one coherent InputStream.
I actually succeeded in getting it to work: I periodically check for new media files, that appear inside the playlist, and pass their HTTP streams to a custom InputStream implementation, namely InputStreamChain. Since it's a live stream, I assume it to be endless, at least for the time being. Ergo, I wanted my InputStreamChain's read() never to send the -1. Unfortunately, it did; every time when all queued media streams were consumed, the InputStreamChain ended. Instead, I wanted it to block I/O, until a new media file arrives. So, I came up with a working solution: I adjusted the read() method to loop until there's a new stream available (a TimerTask will provide the new files). In the loop, I built in a Thread.sleep(), in order to reduce the CPU load:

public int read() throws IOException {  
    int bit = current.read();  
    if (bit == -1 && streams.size() > 0) {  
        // left out due to lacking relevance
    } else if(bit == -1 && streams.size() == 0) {
        while(streams.size() == 0) {
            Thread.currentThread().sleep(50);
        }
        return read();
    }
    return bit;
}

Although it seems to work, I have a feeling, that I'm not doing it how I'm supposed to. I also tried using Lock together with Condition.await(), but when my TimerTask tried to trigger Condition.signal(), it just threw a IllegalMonitorStateException.
That's why I'm asking the question:

In what way should I delay/block an InputStream's read() method, especially in my scenario?

Edit:

For the sake of completeness, I'm going to provide my failed Lock approach, too:

private ReentrantLock ioLock;
private Condition ioCond;
private boolean waitingForStream = false;

public InputStreamChain() {
    ioLock = new ReentrantLock();
    ioCond = ioLock.newCondition();
}

public synchronized InputStreamChain addInputStream(final InputStream stream) {
    streams.addLast(stream);
    if (current == null) {
        current = streams.removeFirst();
    }
    if(waitingForStream) {
        ioCond.signal();
    }
    return this;
}

public int read() throws IOException {
    int bit = current.read();
    if (bit == -1 && streams.size() > 0) {
        // do stuff
    } else if(bit == -1) {
        waitingForStream = true;
        ioLock.lock();
        try {
            ioCond.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
            waitingForStream = false;
            ioLock.unlock();
        }
        return read();
    }
    return bit;
}

Upvotes: 0

Views: 560

Answers (1)

Valeri Atamaniouk
Valeri Atamaniouk

Reputation: 5163

Probably you are not using synchronized block. Here is an example:

class MyReader
{
     public int read() throws IOException {  
        int bit = current.read();  
        if (bit == -1 && streams.size() > 0) {  
            // left out due to lacking relevance
        } else if(bit == -1 && streams.size() == 0) {
            waitForNextStream();
            return read();
        }
        return bit;
    }

    private synchronized void waitForNextStream()
    {
        // TODO add close handling, set current here
        while (streams.isEmpty())
        {
            wait();
        }
    }


    public synchronized void addNextStream(InputStream is)
    {
        streams.add(is);
        notify();
    }
}

Upvotes: 1

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