Reading Objects from Random Access File

Question

I wrote a file using Java's FileChannel class that uses RandomAccessFiles. I wrote objects at various locations in the file. The objects were of variable sizes but all of the same class. I wrote the objects using the following idea :

ByteArrayOutputStream bos= new ByteArrayOutputStream();
        ObjectOutput out = new ObjectOutputStream(bos);
        out.writeObject(r);
        byte[] recordBytes= bos.toByteArray();

    ByteBuffer rbb= ByteBuffer.wrap(recordBytes);

    while(rbb.hasRemaining()) {
        fileChannel.write(rbb);
    }

Now I want to read from such a file. I dont want to have to specify the number of bytes to read. I want to be able to read the object directly using Object Input Stream. How to achieve this ?

I have to use Random Access Files because I need to write to different positions in file. I am also recording in a separate data structure, the locations where objects have been written.

Answer 1

I have to use Random Access Files because I need to write to different positions in file.

No, you don't. You can reposition a FileOutputStream or FileInputStream via its channel.

That would significantly simplify your writing code as well: you wouldn't need to use the buffer or channel, and depending on your needs you could omit the ByteArrayOutputStream as well. However, as you note in a comment, you won't know the size of the object in advance, and the ByteArrayOutputStream is a useful way to verify that you don't overrun your allotted space.

Object obj = // something

FileOutputStream fos = // an initialized stream

ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream oos = new ObjectOutputStream(bos);
oos.writeObject(obj);
oos.flush();

if (bos.size() > MAX_ALLOWED_SIZE)
   throw // or log, or whatever you want to do
else
{
    fos.getChannel().position(writeLocation);
    bos.writeTo(fos);
}

To read the objects, do the following:

FileInputStream fis = // an initialized stream

fis.getChannel().position(offsetOfSerializedObject);
ObjectInputStream iis = new ObjectInputStream(new BufferedInputStream(fis));
Object obj = iis.readObject();

One comment here: I wrapped the FileInputStream in a BufferedInputStream. In this specific case, where the file stream is repositioned before each use, that can provide a performance benefit. Be aware, however, that the buffered stream can read more bytes than are needed, and there are some situations using construct-as-needed object streams where it would be a really bad idea.

Answer 2

Why doesn't seek work for you? I believe you need to seek() to correct locations and then just read objects using your object stream. Also, if you store the correct locations of serialized objects, why don't you store their sizes? In this case you may apply ObjectInputStream against bytes you read from file.

Answer 3

The simplest solution that comes to mind is to write out the length of the array before writing out the array itself:

while(rbb.hasRemaining()) {
        fileChannel.writeLong(recordBytes.length);
        fileChannel.write(rbb);
    }

When reading the object, you first read the length. This'll tell you how many further bytes to read to get your object. Similarly to what you are already doing on the writing side, you could read the data into a byte[] and then use ByteArrayInputputStream and ObjectInputStream.

Answer 4

You could use a FileInputStream constructed on the RandomAccesFile's FileDescriptor object, like so:

FileDescriptor f = raf.getFD();
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(f));

Assuming that the RandomAccessFile is called raf.