Write and then read back a memory mapped file

MemoryMappedFileTest.java

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.TemporaryFolder;

import static org.junit.Assert.assertEquals;

public class MemoryMappedFileTest {
    @Rule
    public TemporaryFolder folder = new TemporaryFolder();

    /** Write some values to a memory mapped file and then read them back. */
    @Test
    public void writeAndReadBack() throws IOException {
        try (FileChannel fileChannel = new RandomAccessFile(folder.newFile(), "rw").getChannel()) {
            int itemCount = 512;
            int itemSize = 8;
            MappedByteBuffer buffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, itemCount * itemSize);
            for (long l = 0; l < itemCount; l++) {
                assertEquals((itemCount - l) * itemSize, buffer.remaining());
                buffer.putLong(l);
            }
            assertEquals(0, buffer.remaining());
            buffer.rewind();
            for (long l = 0; l < itemCount; l++) {
                assertEquals((itemCount - l) * itemSize, buffer.remaining());
                assertEquals(l, buffer.getLong());
            }
            assertEquals(0, buffer.remaining());
        }
    }

    /** Prove that the buffer is valid even after closing the channel. */
    @Test
    public void closeChannelBeforeUsingTheBuffer() throws IOException {
        int itemCount = 512;
        int itemSize = 8;

        File file = folder.newFile();
        FileChannel writeChannel = new RandomAccessFile(file, "rw").getChannel();
        MappedByteBuffer writeBuffer = writeChannel.map(FileChannel.MapMode.READ_WRITE, 0, itemCount * itemSize);
        writeChannel.close();

        for (long l = 0; l < itemCount; l++) {
            assertEquals((itemCount - l) * itemSize, writeBuffer.remaining());
            writeBuffer.putLong(l);
        }
        assertEquals(0, writeBuffer.remaining());

        FileChannel readChannel = new RandomAccessFile(file, "r").getChannel();
        assertEquals(itemCount * itemSize, readChannel.size());
        MappedByteBuffer readBuffer = readChannel.map(FileChannel.MapMode.READ_ONLY, 0, itemCount * itemSize);
        readChannel.close();

        for (long l = 0; l < itemCount; l++) {
            assertEquals((itemCount - l) * itemSize, readBuffer.remaining());
            assertEquals(l, readBuffer.getLong());
        }
        assertEquals(0, readBuffer.remaining());
    }

    /** Prove that the buffer is initialized with zeros */
    @Test
    public void readUninitializedData() throws IOException {
        try (FileChannel fileChannel = new RandomAccessFile(folder.newFile(), "rw").getChannel()) {
            MappedByteBuffer buffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 8);
            assertEquals(0, buffer.getLong());
        }
    }
}