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POIs a read lock on a ReentrantReadWriteLock sufficient for concurrent reading of a RandomAccessFile
 

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  1. POIs a read lock on a ReentrantReadWriteLock sufficient for concurrent reading of a RandomAccessFile
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    <p>I'm writing something to handle concurrent read/write requests to a database file.</p> <p><a href="http://java.sun.com/javase/6/docs/api/java/util/concurrent/locks/ReentrantReadWriteLock.html" rel="nofollow noreferrer">ReentrantReadWriteLock</a> looks like a good match. If all threads access a shared <a href="http://java.sun.com/javase/6/docs/api/java/io/RandomAccessFile.html" rel="nofollow noreferrer">RandomAccessFile</a> object, do I need to worry about the file pointer with concurrent readers? Consider this example:</p> <pre><code>import java.io.FileNotFoundException; import java.io.IOException; import java.io.RandomAccessFile; import java.util.concurrent.locks.ReentrantReadWriteLock; public class Database { private static final int RECORD_SIZE = 50; private static Database instance = null; private ReentrantReadWriteLock lock; private RandomAccessFile database; private Database() { lock = new ReentrantReadWriteLock(); try { database = new RandomAccessFile("foo.db", "rwd"); } catch (FileNotFoundException e) { e.printStackTrace(); } }; public static synchronized Database getInstance() { if(instance == null) { instance = new Database(); } return instance; } public byte[] getRecord(int n) { byte[] data = new byte[RECORD_SIZE]; try { // Begin critical section lock.readLock().lock(); database.seek(RECORD_SIZE*n); database.readFully(data); lock.readLock().unlock(); // End critical section } catch (IOException e) { e.printStackTrace(); } return data; } } </code></pre> <p>In the getRecord() method, is the following interleaving possible with multiple concurrent readers?</p> <blockquote> <p>Thread 1 -> getRecord(0)<br> Thread 2 -> getRecord(1)<br> Thread 1 -> acquires shared lock<br> Thread 2 -> acquires shared lock<br> Thread 1 -> seeks to record 0<br> Thread 2 -> seeks to record 1<br> Thread 1 -> reads record at file pointer (1)<br> Thread 2 -> reads record at file pointer (1) </p> </blockquote> <p>If there are indeed potential concurrency issues using ReentrantReadWriteLock and RandomAccessFile, what would an alternative be?</p>
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