serialization - 

Abstract serialization services

The serialization services are based on the serialize and the noserialize annotations, the Serializable interface and the implementations of Serializer and Deserializer.

The serialize annotation

A class annotated with serialize identifies it as a subclass of Serializable and triggers the generation of customized serialization and deserialization services.

import serialization

# Simple serializable class identifying a human
class Person
    serialize

    # First and last name
    var name: String

    # Year of birth (`null` if unknown)
    var birth: nullable Int

    redef fun ==(o) do return o isa SELF and name == o.name and birth == o.birth
    redef fun hash do return name.hash
end

The Person class also defines == and hash, this is optional but we will use it to make an important point. By definition of a serializable class, an instance can be serialized to a stream, then deserialized. The deserialized instance will not be the same instance, but they should be equal. So, in this case, we can compare both instances with == to test their equality.

Some conditions applies to the classes that can be annotated as serialize. All attributes of the class must be serializable, runtime errors will be raised when trying to serialize non-serializable attributes.

In the class Person, all attributes are typed with classes the standards library. These common types are defined defined as serializable by this project. The attributes could also be typed with user-defined serialize classes or any other subclass of Serializable.

# This `serialize` class is composed of two `serialize` attributes
class Partnership
    serialize

    var partner_a: Person
    var partner_b: Person

    redef fun ==(o) do return o isa SELF and partner_a == o.partner_a and partner_b == o.partner_b
    redef fun hash do return partner_a.hash + 1024*partner_b.hash
end

Scope of the serialize annotation

serialize can annotate class definitions, modules and attributes:

  • The annotation on a class applies only to the class definition, only attributes declared locally will be serialized. However, each definition of a class (a refinement or specialization) can be annotated with serialize.

  • A module declaration annotated with serialize states that all its class definitions and locally declared attributes are serializable.

    module shared_between_clients is serialize
  • Attribute annotated with serialize states that it is to be serialized, when the rest of the class does not. The class will become subclass to Serializable but its attributes are not to be serialized by default. Only the attributes with the serialize annotation will be serialized.

    # Only serialize the `name`
    class UserCredentials
      var name: String is serialize
      var avatar_path: String = "/somepath/"+name is lazy
    end

The noserialize annotation

The noserialize annotation mark an exception in a serialize module or class definition.

  • By default a module is noserialize. There is no need to declare it as such.

  • A class definition annotated with noserialize within a serialize module will not be made serializable.

  • A noserialize attribute within a class or module annotated with serialize will not serialize this attribute. The class will still be made subclass of Serializable and it won't affect the other attributes. The noserialize attribute will not be set at deserialization. Usually, it will also be annotated with lazy to get its value by another mean after the object has been deserialized.

    # Once again, only serialize the `name`
    class UserCredentials
      serialize
    
      var name: String
      var avatar_path: String = "/somepath/"+name is noserialize, lazy
    end

The serialize_as annotation

By default, an attribute is identified in the serialization format by its Nit name. The serialize_as attribute changes this behavior and sets the name of an attribute in the serialization format.

This annotation can be useful to change the name of an attribute to what is expected by a remote service. Or to use identifiers in the serialization format that are reserved keywords in Nit (like class and type).

class UserCredentials
    serialize

    # Rename to "username" in JSON for compatibility with remote service
    var name: String is serialize_as "username"

    # Rename to a shorter "ap" for a smaller JSON file
    var avatar_path: String = "/somepath/"+name is lazy, serialize_as "ap"
end

Custom serializable classes

The annotation serialize should be enough for most cases, but in some cases you need more control over the serialization process.

For more control, create a subclass to Serializable and redefine core_serialize_to. This method should use Serializer::serialize_attribute to serialize its components. serialize_attribute works as a dictionary and organize attributes with a key.

You will also need to redefine Deserializer::deserialize_class to support this specific class. The method should only act on known class names, and call super otherwise.

Example: the User class

The following example cannot use the serialize annotations because some of the arguments to the User class need special treatment:

  • The name attribute is perfectly normal, it can be serialized and deserialized directly.

  • The password attribute must be encrypted before being serialized, and unencrypted on deserialization.

  • The avatar attributes is kept as ASCII art in memory. It could be serialized as such but it is cleaner to only serialize the path to its source on the file system. The data is reloaded on deserialization.

For this customization, the following code snippet implements two serialization services: User::core_serialize_to and Deserializer::deserialize_class.

module user_credentials

# User credentials for a website
class User
    super Serializable

    # User name
    var name: String

    # Clear text password
    var password: String

    # User's avatar image as data blob
    var avatar: Image

    redef fun core_serialize_to(serializer: Serializer)
    do
        # This is the normal serialization process
        serializer.serialize_attribute("name", name)

        # Serialized an encrypted version of the password
        #
        # Obviously, `rot(13)` is not a good encrption
        serializer.serialize_attribute("pass", password.rot(13))

        # Do not serialize the image, only its path
        serializer.serialize_attribute("avatar_path", avatar.path)
    end
end

redef class Deserializer
    redef fun deserialize_class(name)
    do
        if name == "User" then
            # Deserialize normally
            var user = deserialize_attribute("name")

            # Decrypt password
            var pass = deserialize_attribute("pass").rot(-13)

            # Deserialize the path and load the avatar from the file system
            var avatar_path = deserialize_attribute("avatar_path")
            var avatar = new Image(avatar_path)

            return new User(user, pass, avatar)
        end

        return super
    end
end

# An image loaded in memory as ASCII art
#
# Not really useful for this example, provided for consistency only.
class Image
    # Path on the filesystem for `self`
    var path: String

    # ASCII art composing this image
    var ascii_art: String = path.read_all is lazy
end

See the documentation of the module serialization::serialization for more information on the services to redefine.

Serialization services

The serialize annotation and the Serializable class are used on classes specific to the business domain. To write (and read) instances of these classes to a persistent format you must use implementations of Serializer and Deserializer.

The main implementations of these services are JsonSerializer and JsonDeserializer, from the json_serialization module.

import json
import user_credentials

# Data to be serialized and deserialized
var couple = new Partnership(
    new Person("Alice", 1985, new Image("alice.png")),
    new Person("Bob", null, new Image("bob.png")))

var path = "serialized_data.json"
var writer = new FileWriter(path)
var serializer = new JsonSerializer(writer)
serializer.serialize couple
writer.close

var reader = new FileReader(path)
var deserializer = new JsonDeserializer(reader.to_s)
var deserialized_couple = deserializer.deserialize
reader.close

assert couple == deserialize_couple

Limitations and TODO

The serialization has some limitations:

  • A limitation of the JSON parser prevents deserializing from files with more than one object. This could be improved in the future, but for now you should serialize a single object to each files and use different instances of serializer and deserializer each time.

  • The serialization uses only the short name of a class, not its qualified name. This will cause problem when different classes using the same name. This could be solved partially in the compiler and the library. A special attention must be given to the consistency of the name across the different programs sharing the serialized data.

  • The serialization support in the compiler need some help to deal with generic types. A solution is to use nitserial, the next section explores this subject.

Dealing with generic types

One limitation of the serialization support in the compiler is with generic types. For example, the Array class is generic and serializable. However, the runtime types of Array instances are parameterized and are unknown to the compiler. So the compiler won't support serializing instances of Array[MySerializable].

The tool nitserial solves this problem at the level of user modules. It does so by parsing a Nit module, group or project to find all known parameterized types of generic classes. It will then generating a Nit module to handle deserialization of these types.

Usage steps to serialize parameterized types:

  • Write your program, let's call it my_prog.nit, it must use some parameterized serializable types. Let's say that you use Array[MySerializable].

  • Run nitserial using nitserial my_prog.nit to generate the file my_prog_serial.nit.

  • Compile your program by mixing in the generated module with: nitc my_prog.nit -m my_prog_serial.nit

This was a simple example, in practical cases you may need to use more than one generated file. For example, on a client/server system, an instance can be created server-side, serialized and the used client-side. In this case, two files will be generated by nitserial, one for the server and one for the client. Both the files should be compiled with both the client and the server.

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