How do I read and write to a file using threads in

问题:

I'm writing an application where I need to read blocks in from a single file, each block is roughly 512 bytes. I am also needing to write blocks simultaneously.

One of the ideas I had was BlockReader implements Runnable and BlockWriter implements Runnable and BlockManager manages both the reader and writer.

The problem that I am seeing with most examples that I have found was locking problems and potential deadlock situations. Any ideas how to implement this?

回答1:

I would recommend the book java Concurrency in Practice, in this case section 5.3 (Producer-consumer pattern).

your solution could look something like:

BlockingQueue<Data> queue = 
    new  LinkedBlockingQueue<Data>(MAX_BLOCKS_IN_QUEUE_UNTIL_BLOCK );

for (int i=0; i < MAX_DATA_PRODUCERS; i++ ) {
   new Thread( new DataProducer( queue ) ).start();
}

new Thread(DataWriter( queue )).start

Obviously DataProducer and DataWriter are runnables.

 class DataProducer implements Runnable {
    ...
    queue.put(data); // blocks if MAX_BLOCKS_IN_QUEUE_UNTIL_BLOCK 
                     // are waiting to be written
                     // This prevents an OutOfMemoryException
    ...
 }

 class DataConsumer implements Runnable {
   ...
   try {
       while(true) {
           writeData(queue.take()); // blocks until there is a block of data
       }
   } catch (InteruptedException e) {
       Thread.currentThread().interrupt();
   }
   ...
 }

回答2:

You can have an array of locks e.g. 32 and use the index of the block as a hash to determine which lock to obtain. This way you can have concurrent read/writes (most of the time) and still ensure you won't be read/writing the same block in multiple threads.