file thread synchronization

Question

How to perform read and write operation by using thread synchronization.

Condition: If one file exists where writers may write information to, only one writer may write at a time. Confusion may arise if a reader is trying read at the same as a writer is writing. Since readers only look at the data, but do not modify the data, we can allow more than one reader to read at the same time.

//reader thread
    class Read extends Thread {
        static File Reader fr1 = null;
        static Buffered Reader br1 = null;

        static synchronized void reader() throws IO Exception {

            String path ="C:/Users/teja/Documents/file1.txt";

            fr1 = new File Reader(path);
            br1 = new Buffered Reader(fr);
            int i;
            while ((i = br. read()) != -1)
                System .out. print((char) i);
            System .out. print ln();
        }

        public void run() {
            try {
                reader();
            } catch (IO Exception e) {
                e. print Stack Trace();
            }
        }
    }
    //writer code
    class Writer extends Thread {
        static Buffered Writer bw1 = null;
        static File Writer fw1 = null;

        static synchronized void writer() throws IO Exception {
            Scanner scanner = new Scanner(System.in);
            System .out .print ln("enter data to be added:");
            String data = scanner. nextLine();

            String path = "C:/Users/vt/Documents/file1.txt";
            fw1 = new File Writer(path, true);

            bw1 = new Buffered Writer(fw1);
            bw1.newLine();
            bw1.write(data);
            bw1.flush();

            scanner. close();
            System. out . println("data added");
        }

        public void run() {
            try {
                writer();
            } catch (IO Exception e) {
                e. print Stack Trace();
            }
        }
    }
    //main method
    public class File Read Write {

        public static void main(String[] args) {
           Read rd1 =new Read();
           Read rd2=new Read();
           Writer wt1=new Writer();

           rd1.run();
           rd2.run();
           wt1.run();
           rd1. run();
            }           
    }

I am new to files and threading in java. I know this is not correct approach. Guide me.

If one file exists where writers may write information to, only one writer may write at a time. Confusion may arise if a reader is trying read at the same as a writer is writing. Since readers only look at the data, but do not modify the data, we can allow more than one reader to read at the same time.

Answer 1

There are two approaches to this.

(1) Either lock the resource and have the readers wait until the writer has completed the writing operation (or likewise, have a writer waits until all readers are done). This approach guarantees consistency, but can be slow if a lot of writers/readers are working on the resource at the same time (see Lock in java.util.concurrent.locks package).

(2) Keep an in-memory-version of the contents of the file that is served to readers only. When a change is made, this in-memory version is updated. Here, you'll have more speed, but you lose consistency and you'll need more memory.

Answer 2

The condition you want to avoid is generally referred as race condition and what you want to avoid it is a synchronization method between threads. There are more choices available but the most suitable for your case are mutex and read-write lock.

A mutex basically just lock the resource before any operation on the shared resource is performed, independently from the type of operation and free it after the operation is terminated. So a read will block the resource and any other operation, read or write will be blocked. A write will block the resource too so again no other read or write operation can be performed before the action is terminated and mutex unlocked. So basically a mutex has 2 states: locked and unlocked.

read-write lock gives you more freedom based on the fact that read only operations do not result in inconsistencies. A read-write lock has 3 states: unlocked, read lock, write lock. A write lock works as a regular mutex blocking any other operation. A read lock on the contrary blocks only write operations.

I am not a Java expert but from this answer mutex in Java can be used as the following:

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

private final Lock lock = new ReentrantLock(true);
lock.lock()
/*open file and do stuff*/
try {
  // do something
} catch (Exception e) {
  // handle the exception
} finally {
  lock.unlock();
}

Instead here you can find a description of a read-write lock class.

From the implementation point of view you can create an instance of one of the two synchronization method and have your read/write thread instances keeping a reference to it, as an instance variable.