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POCustom blocking queue locking issue
 

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  1. POCustom blocking queue locking issue
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    <p>I am trying to do some custom implementation of blocking queue with fixed length array of byte arrays. I am not removing polled elements, therefore I adjusted put method to return byte array so that it can be written directly (producer thread uses MappedByteBuffer to write directly to this byte array). I added "commitPut()" method to simply increase counters and set "lengths" arrays. (if multiple threads would be writing this could be the concurrency problems, but I know that only one thread is writing).</p> <p>Below is what I currently have. It works if I debug through step by step, but if I "run" it looks like it encounters some locking problems. I copied, stripped down and adjusted ArrayBlockingQueue code. Can someone with better knowledge please look at the class and tell me what I am doing wrong, or how to do it better (like write directy to buffer and set lengths array and counters at the same step)?</p> <pre><code>public class ByteArrayBlockingQueue { private final int[] lens; // array to valid lengths private final byte[][] items; // array of byte arrays private int takeIndex = 0; private int putIndex = 0; private int count = 0; public volatile int polledLen = 0; // lenght of last polled byte array private final ReentrantLock lock; private final Condition notEmpty; private final Condition notFull; final int inc(int i) { return (++i == items.length)? 0 : i; } public ByteArrayBlockingQueue(int capacity, int size, boolean fair) { if (capacity &lt;= 0) throw new IllegalArgumentException(); this.items = new byte[capacity][size]; this.lens = new int[capacity]; lock = new ReentrantLock(fair); notEmpty = lock.newCondition(); notFull = lock.newCondition(); } public byte[] put() throws InterruptedException { final byte[][] items = this.items; final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { try { while (count == items.length) notFull.await(); } catch (InterruptedException ie) { notFull.signal(); // propagate to non-interrupted thread throw ie; } //insert(e, len); return items[putIndex]; } finally { lock.unlock(); } } public void commitPut(int lenBuf) throws InterruptedException { final ReentrantLock lock = this.lock; lock.lockInterruptibly(); try { lens[putIndex] = lenBuf; putIndex = inc(putIndex); ++count; notEmpty.signal(); } finally { lock.unlock(); } } public byte[] poll() { final ReentrantLock lock = this.lock; lock.lock(); try { if (count == 0) return null; final byte[][] items = this.items; final int[] lens = this.lens; byte[] e = items[takeIndex]; this.polledLen = lens[takeIndex]; //items[takeIndex] = null; takeIndex = inc(takeIndex); --count; notFull.signal(); return e; } finally { lock.unlock(); } } } </code></pre>
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    1. COCan you describe the "locking problems" you are encountering?
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    2. COSeems like "producer thread" locks "consumer thread". I don't quite get these locks, although they seem good to me (almost the same principle as other build-in blocking queues). If I step through with a debugger, setting break points in "poll" and "put" - it works. I can see bitmaps (bytes in byte array) decode properly. As soon as I "run through" it fails (bytes cannot be decoded) - if I try to debug polled value, expression watcher fails with some "child update" error, so I assume it is a locking problem.
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