Write/Read a series of different serialized objects to/from a file

Question

I have a collection of objects:

Map<BufferedImage, Map<ImageTransform, Set<Point>>> map

I want to write those to a file, and then be able to read them back in the same struct.

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I can't just write the collection as it is, because BufferedImage doesn't implement the Serializable (nor the Externalizable) interface. So I need to use the methods from the ImageIO class to write the image.

ImageTransform is a custom object that implements Serializable. So, I believe the value part of my map collection, should be writeable as it is.

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Here is what I do to write to the file:

    ObjectOutputStream out = new ObjectOutputStream(new FileOutputStream(file));
    for (BufferedImage image : map.keySet()) {
        ImageIO.write(image, "PNG", out);  // write the image to the stream
        out.writeObject(map.get(image));   // write the 'value' part of the map
    }

Here is what I do to read back from the file:

    ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
    while(true) {
        try {
            BufferedImage image = ImageIO.read(in);
            Map<ImageTransform, Set<Point>> value = 
                (Map<ImageTransform, Set<Point>>) in.readObject(); // marker
            map.put(image, value);
        } catch (IOException ioe) {
            break;
        }
    }

However, this doesn't work. I get a java.io.OptionalDataException at marker.

java.io.OptionalDataException
at java.io.ObjectInputStream.readObject0(ObjectInputStream.java:1300)
at java.io.ObjectInputStream.readObject(ObjectInputStream.java:368)

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My question is, firstly, is the writing concept correct ? is ImageIO#write good for this case, or should I think about using/storing the BufferedImage#getRgb int[] array ? is the array more compact (as in, takes up less space in the file) ?
Secondly, how should I be reading the object back from the file ? How do I know when the EOF is reached ? Why doesn't the above work ?

I hope the info provided is enough, if you need more info on something, please tell me.
Thanks in advance.

Answer 1

It's not working as ObjectOutputStream and ObjectInputStream write/expect a certain file format that is violated when you write an image out of order. To use ObjectStreams successfully you will need to observe the contract that is specifed by ObjectStreams.

To do this you will need to create a holding class, and use this class as the key to your map instead of BufferedImages. This holding class should implement Serializable and a three methods (not in any actual interface) that mark the Class as needing special handling during reading and writing. The method signatures must be exactly as specified or serialization won't work.

For more information have a look at the documentation on ObjectOutputStream.

public class ImageHolder implements Serializable {

    BufferedImage image;

    public ImageHolder(BufferedImage image) {
        this.image = image;
    }

    private void readObject(ObjectInputStream stream) 
            throws IOException, ClassNotFoundException {
        image = ImageIO.read(stream);
    }

    private void writeObject(ObjectOutputStream stream) 
            throws IOException {
        ImageIO.write(image, "PNG", stream);
    }

    private void readObjectNoData() throws ObjectStreamException {
        // leave image as null
    }

And then serialsation should be as simple as outputStream.writeObject(map). Though you will need to check that the implementing class of ImageTransform is serialisable too.