序列化：将字节转换为字节？

Question

The object itself is a sequence of bytes and that is how does the machine understand all the data, whether it's object, text, images..etc. Could you clear this idea for me why we are converting a sequence of bytes (object) into another byte? Do we restructure the bytes when we do serialization, or make a template that holds this object to give it a special meaning when transmitted over the network? suppose a certain method, that takes the object from memory as it is, and put that object into an IP datagrams and send it through the network, what issue that may arise?

Answer 1

First: compression.

You must understand, that image file on disk and image file rendered from memory - are not the same. On disk they (usually, forget about BMP) are compressed. With current network throughput and hdd's capacities, compressing is essential.

Second: architecture.

Number in memory is just a sequence of bits, yes. But, what bit-count is counted as number? 8? 16? 32? 64? Any of them. There are bytes, words, integers, longs, floats (hell, floats!) and another couple of dozens of them. And bitorder also matters, so-called big-endian and little-endian. So 123456789 on one (x86) machine is not the same number on another machine (x64, for example).

Third: file (read: transmission) format != object-in-memory format.

Well, there is difference between data structure in file (or network packet), and when object loaded from that file in memory. And additionally, object-in-memory structure can differ from program version to version. Loaded-to-memory image in Win 3.1 and, fe, Vista is a hell big difference. Also, structures packing and 4-, 8-, 16-, 32-bit-boundary aligning etc, etc, etc.

Answer 2

The object itself includes many references , which are pointers to where another component of the object happens to exist in memory on this particular machine at this particular moment. The point of serialization is that it converts objects into bytes that can be read at some other time, possibly on some other machine.

Additionally, object representations in memory are optimized for fast access and modification, not necessarily taking the minimum number of bytes. Some serialization protocols, especially for use in RPCs or data storage, optimize for how many bytes have to be transmitted or stored using compression algorithms that make it more difficult to access or modify the properties of the object in exchange for using fewer bytes to do it.

Answer 3

The object itself is a sequence of bytes

No. The object itself isn't just a 'sequence of bytes', unless it contains nothing but primitive data. It can contain

• references to other objects
• those objects may already have been serialized, in which case a back-reference needs to be serialized, not the referenced object all over again
• those references may be null
• there may be no object at all, just primitive data

All these things increase the complexity of the task well beyond the naive notion of just serializing 'a sequence of bytes'.