import java.nio.ByteBuffer;

import java.nio.IntBuffer;

import java.nio.channels.FileChannel;

import java.nio.channels.FileLock;

import java.io.IOException;

import java.io.RandomAccessFile;

public class FileLockDemo{

private static final int MAX_QUERIES = 150000;

private static final int MAX_UPDATES = 150000;

private static final int REC_LENGTH = 16;

static ByteBuffer buffer = ByteBuffer.allocate(REC_LENGTH);

static IntBuffer intBuffer = buffer.asIntBuffer();

static int counter = 1;

public static void main(String[] args) throws IOException{

boolean isWriter = false;

if(args.length != 0){

isWriter = true;

}

RandomAccessFile raf = new RandomAccessFile("FileLockDemo.temp", isWriter ? "rw" : "r");

FileChannel fc = raf.getChannel();

if(isWriter){

update(fc);

}else{

query(fc);

}

}

static void query(FileChannel fc) throws IOException{

for(int i=0; i<MAX_QUERIES; i++){

System.out.println("acquiring shared lock");

FileLock lock = fc.lock(0, REC_LENGTH, true); // Lock a record region of the file.

try{

buffer.clear();

fc.read(buffer, 0); // From file position 0.

int a = intBuffer.get(0);

int b = intBuffer.get(1);

int c = intBuffer.get(2);

int d = intBuffer.get(3);

System.out.println("Reading: " + a + ", " + b + ", " + c + ", " +d);

if(a*2!=b || a*3!=c || a*4!=d){

System.err.println("Error: record corruption.");

return;

}

}finally{

lock.release();

}

}

}

static void update(FileChannel fc) throws IOException{

for(int i=0;i<MAX_UPDATES;i++){

System.out.println("acquiring exclusive lock.");

FileLock lock = fc.lock(0, REC_LENGTH, false);

try{

intBuffer.clear();

int a = counter;

int b = counter*2;

int c = counter*3;

int d = counter*4;

System.out.println("Writing: " + a + ", " + b + ", " + c + ", " + d);

intBuffer.put(a).put(b).put(c).put(d);

counter++;

buffer.rewind();

fc.write(buffer, 0); // write into file beginning from position 0.

}finally{

lock.release();

}

}

}

}