How to add locks to read and write from file methods

Question

I have to add locks to my readFromFile() and writeToFile() methods for school. Please see the method code below and advise me where in the code I would place the locks? Would I use ReentrantLock or ReadWriteReentrantLock? We have to use locks.

Thank you.

Read method

public static void readFromFile(List<Person> people) { FileReader fr = new FileReader("names"); Scanner sc = new Scanner(fr); while (sc.hasNext()) { String name = sc.nextLine(); int age = Integer.parseInt(sc.nextLine()); people.add(new Person(name, age)); } sc.close(); }

Write method

public static void writeToFile(List<Person> people) { File outputFile = new File (List<Person> people) PrintWriter pw = new PrintWriter(outputFile); for (Person p: people) { pw.println(p.name); } pw.close(); }

probably easiest to synchronize on an Object

Scary Wombat
– Scary Wombat

2017-10-11 08:02:31 +00:00
Commented Oct 11, 2017 at 8:02 — Scary Wombat
– Scary Wombat, Commented Oct 11, 2017 at 8:02

mlecz · Accepted Answer · 2017-10-11 08:06:55Z

Just guard whole method body

 static ReadWriteLock rwl = new ReentrantReadWriteLock(); public static void readFromFile(List<Person> people) { rwl.readLock().lock(); try { ... //method body is not changed } finally { rwl.readLock().unlock(); } } public static void writeToFile(List<Person> people) { rwl.writeLock().lock(); try { ... //method body is not changed } finally { rwl.writeLock().unlock(); } }

Sorontur · Accepted Answer · 2017-10-11 08:18:35Z

The difference between ReentrantLock and ReentrantReadWriteLock is a more fine granular control on the throughput.

A ReentrantLock has the same power as the java monitor locking with synchronized. It is just technically different.

If you have many reads and less writes it is worth to use a ReadWrite lock, because it allowes reads in parallel as long as there is no write.

If you use ReentrantLocks, always follow the try-finally pattern to have the lock free in any case. That is not needed with synchronized.

here a suggestion for the read in your example with readWrite. The write method is the same just use the writeLock().

private static final ReentrantReadWriteLock readWriteLock = new readWriteLock; public static void readFromFile(List<Person> people) { readWriteLock.readLock().lock(); try { // method body } finally { readWriteLock.readLock().unlock(); } }

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