Question

Is it possible to deserializable object from a file and then serializable only few of them into the same file? Of course numbers of object before and after must be the same. I don’t want to add new objects into file, but overwrite changed objects.
For example there are about 1000 objects which I get from file ‘file.ser’. I deserializable them, change only 3 and want to overwrite these into same file. Is it possible?

Answer 1

The short answer is no, you cannot just overwrite the changed objects “in-place” within the file.

When you create an ObjectOutputStream, even before you write an object into it, the stream writes some magic number marker bytes to the underlying stream. These bytes are automatically consumed by an ObjectInputStream when you create one. This is the stream header.

Furthermore, when you write an object using ObjectOutputStream.writeObject(), you’re not simply serialisaing that object is isolation. The stream keeps track of objects references that it has serialised previously (unless you call reset()), and instead of writing the object out in full, it will write a handle that points back to where the real object was written.

Finally, there is no block alignment or padding between the objects written to the stream — they’re written contiguously.

All of this means that if you change an object, the structural changes you’ve made (references to different objects etc) means that the handles could be different, and the length of the object could be different. Since there’s no block alignment occurring, you can’t write the object back to the same place because the changes you’ve made will (likely) affect the length and it won’t be the same, which means you’ll stomp over the top of another object.