How do I identify that I am at the last byte of a serialized Java object?

Question

Question

What is (if there is any) terminating characters/byte sequences in serialized java objects?

Background

I'm working on a small self-education project where I would like to serialize java objects and write them to a stream where there are read and then unserialized. Since, I will need to identify the borders between serialized objects and I can't be sure that the current object is not the last one, is there a terminating character that is always there that I can use as my identifier?

I noticed that there is a magic number ACED that allows me to identify the start of the object, so how do I identify the end?

EDIT: If there is no terminating character, is there any safe terminating characters/sequences that I can use (insert) to identify the end of the object?

Answer 1

In theory you should always be able to find the end of an object, in practice you cannot. I understand the problem is customised writeObject implementations that don't call either defaultReadObject or readFields have a non-standard representation.

I've played about with serialisation in the past. Including creating streams for use when I've been doing unusual things to the ObjectInputStream. It's not pleasant(!).

You can read the details in the spec, and the source is worth a read.

Answer 2

Have you considered applying a record-marking layer similar to HTTP Chunked encoding?

The Chunked encoding is intended to solve a generalization of this scenario: identifying the end of a message of indeterminate length that both itself contains no identifiable end, and is embedded in a longer stream without ending it.

Answer 3

there are none. AFAIK the only requirement is that the deserialiser know when to stop reading, when given a corresponding serialisation. subject to that, the serialiser can write whatever it wants -- in any position not just the last.

if you're old skool dump a 32-bit length field at the beginning a refuse to handle objects bigger than 4 gig.

nu scool, you just make sure your read and your write logic are consistent and don't care about the length.

Answer 4

You can add a terminating object to your object stream. e.g. null or a special String.

However, I suggest that you instead convert the ObjectsStream to a byte[] and write the byte length of the byte[] followed by its data. This way each ObjectStream is independent and you always know where it finishes.