1 # Abstract serialization services
3 The serialization services are based on the `serialize` and the `noserialize` annotations,
4 the `Serializable` interface and the implementations of `Serializer` and `Deserializer`.
6 ## The `serialize` annotation
8 A class annotated with `serialize` identifies it as a subclass of Serializable and
9 triggers the generation of customized serialization and deserialization services.
14 # Simple serializable class identifying a human
21 # Year of birth (`null` if unknown)
22 var birth: nullable Int
24 redef fun ==(o) do return o isa SELF and name == o.name and birth == o.birth
25 redef fun hash do return name.hash
29 The `Person` class also defines `==` and `hash`, this is optional but we will use it to make an important point.
30 By definition of a serializable class, an instance can be serialized to a stream, then deserialized.
31 The deserialized instance will not be the same instance, but they should be equal.
32 So, in this case, we can compare both instances with `==` to test their equality.
34 Some conditions applies to the classes that can be annotated as `serialize`.
35 All attributes of the class must be serializable, runtime errors will be
36 raised when trying to serialize non-serializable attributes.
38 In the class `Person`, all attributes are typed with classes the standards library.
39 These common types are defined defined as serializable by this project.
40 The attributes could also be typed with user-defined `serialize`
41 classes or any other subclass of `Serializable`.
44 # This `serialize` class is composed of two `serialize` attributes
51 redef fun ==(o) do return o isa SELF and partner_a == o.partner_a and partner_b == o.partner_b
52 redef fun hash do return partner_a.hash + 1024*partner_b.hash
56 ### Scope of the `serialize` annotation
58 `serialize` can annotate class definitions, modules and attributes:
60 * The annotation on a class applies only to the class definition,
61 only attributes declared locally will be serialized.
62 However, each definition of a class (a refinement or specialization) can be annotated with `serialize`.
64 * A module declaration annotated with `serialize` states that all its class definitions
65 and locally declared attributes are serializable.
68 module shared_between_clients is serialize
71 * Attribute annotated with `serialize` states that it is to be serialized, when the rest of the class does not.
72 The class will become subclass to `Serializable` but its attributes are not to be serialized by default.
73 Only the attributes with the `serialize` annotation will be serialized.
76 # Only serialize the `name`
78 var name: String is serialize
79 var avatar_path: String = "/somepath/"+name is lazy
83 ## The `noserialize` annotation
85 The `noserialize` annotation mark an exception in a `serialize` module or class definition.
87 * By default a module is `noserialize`. There is no need to declare it as such.
89 * A class definition annotated with `noserialize` within a `serialize` module will not be made serializable.
91 * A `noserialize` attribute within a class or module annotated with `serialize` will not serialize this attribute.
92 The class will still be made subclass of `Serializable` and it won't affect the other attributes.
93 The `noserialize` attribute will not be set at deserialization.
94 Usually, it will also be annotated with `lazy` to get its value by another mean after the object has been deserialized.
97 # Once again, only serialize the `name`
102 var avatar_path: String = "/somepath/"+name is noserialize, lazy
106 ## The `serialize_as` annotation
108 By default, an attribute is identified in the serialization format by its Nit name.
109 The `serialize_as` attribute changes this behavior and sets the name of an attribute in the serialization format.
111 This annotation can be useful to change the name of an attribute to what is expected by a remote service.
112 Or to use identifiers in the serialization format that are reserved keywords in Nit (like `class` and `type`).
115 class UserCredentials
118 # Rename to "username" in JSON for compatibility with remote service
119 var name: String is serialize_as "username"
121 # Rename to a shorter "ap" for a smaller JSON file
122 var avatar_path: String = "/somepath/"+name is lazy, serialize_as "ap"
126 ## Custom serializable classes
128 The annotation `serialize` should be enough for most cases,
129 but in some cases you need more control over the serialization process.
131 For more control, create a subclass to `Serializable` and redefine `core_serialize_to`.
132 This method should use `Serializer::serialize_attribute` to serialize its components.
133 `serialize_attribute` works as a dictionary and organize attributes with a key.
135 You will also need to redefine `Deserializer::deserialize_class` to support this specific class.
136 The method should only act on known class names, and call super otherwise.
138 ### Example: the User class
140 The following example cannot use the `serialize` annotations
141 because some of the arguments to the `User` class need special treatment:
143 * The `name` attribute is perfectly normal, it can be serialized and deserialized
146 * The `password` attribute must be encrypted before being serialized,
147 and unencrypted on deserialization.
149 * The `avatar` attributes is kept as ASCII art in memory.
150 It could be serialized as such but it is cleaner to only
151 serialize the path to its source on the file system.
152 The data is reloaded on deserialization.
154 For this customization, the following code snippet implements
155 two serialization services: `User::core_serialize_to` and
156 `Deserializer::deserialize_class`.
159 module user_credentials
161 # User credentials for a website
168 # Clear text password
171 # User's avatar image as data blob
174 redef fun core_serialize_to(serializer: Serializer)
176 # This is the normal serialization process
177 serializer.serialize_attribute("name", name)
179 # Serialized an encrypted version of the password
181 # Obviously, `rot(13)` is not a good encrption
182 serializer.serialize_attribute("pass", password.rot(13))
184 # Do not serialize the image, only its path
185 serializer.serialize_attribute("avatar_path", avatar.path)
189 redef class Deserializer
190 redef fun deserialize_class(name)
192 if name == "User" then
193 # Deserialize normally
194 var user = deserialize_attribute("name")
197 var pass = deserialize_attribute("pass").rot(-13)
199 # Deserialize the path and load the avatar from the file system
200 var avatar_path = deserialize_attribute("avatar_path")
201 var avatar = new Image(avatar_path)
203 return new User(user, pass, avatar)
210 # An image loaded in memory as ASCII art
212 # Not really useful for this example, provided for consistency only.
214 # Path on the filesystem for `self`
217 # ASCII art composing this image
218 var ascii_art: String = path.read_all is lazy
223 See the documentation of the module `serialization::serialization` for more
224 information on the services to redefine.
226 ## Serialization services
228 The `serialize` annotation and the `Serializable` class are used on
229 classes specific to the business domain.
230 To write (and read) instances of these classes to a persistent format
231 you must use implementations of `Serializer` and `Deserializer`.
233 The main implementations of these services are `JsonSerializer` and `JsonDeserializer`,
234 from the `json_serialization` module.
238 import user_credentials
240 # Data to be serialized and deserialized
241 var couple = new Partnership(
242 new Person("Alice", 1985, new Image("alice.png")),
243 new Person("Bob", null, new Image("bob.png")))
245 var path = "serialized_data.json"
246 var writer = new FileWriter(path)
247 var serializer = new JsonSerializer(writer)
248 serializer.serialize couple
251 var reader = new FileReader(path)
252 var deserializer = new JsonDeserializer(reader.to_s)
253 var deserialized_couple = deserializer.deserialize
256 assert couple == deserialize_couple
259 ## Limitations and TODO
261 The serialization has some limitations:
263 * A limitation of the JSON parser prevents deserializing from files
264 with more than one object.
265 This could be improved in the future, but for now you should
266 serialize a single object to each files and use different instances of
267 serializer and deserializer each time.
269 * The serialization uses only the short name of a class, not its qualified name.
270 This will cause problem when different classes using the same name.
271 This could be solved partially in the compiler and the library.
272 A special attention must be given to the consistency of the name across
273 the different programs sharing the serialized data.
275 * The serialization support in the compiler need some help to
276 deal with generic types. A solution is to use `nitserial`,
277 the next section explores this subject.
279 ## Dealing with generic types
281 One limitation of the serialization support in the compiler is with generic types.
282 For example, the `Array` class is generic and serializable.
283 However, the runtime types of Array instances are parameterized and are unknown to the compiler.
284 So the compiler won't support serializing instances of `Array[MySerializable]`.
286 The tool `nitserial` solves this problem at the level of user modules.
287 It does so by parsing a Nit module, group or project to find all known
288 parameterized types of generic classes.
289 It will then generating a Nit module to handle deserialization of these types.
291 Usage steps to serialize parameterized types:
293 * Write your program, let's call it `my_prog.nit`,
294 it must use some parameterized serializable types.
295 Let's say that you use `Array[MySerializable]`.
297 * Run nitserial using `nitserial my_prog.nit` to
298 generate the file `my_prog_serial.nit`.
300 * Compile your program by mixing in the generated module with:
301 `nitc my_prog.nit -m my_prog_serial.nit`
303 This was a simple example, in practical cases you may need
304 to use more than one generated file.
305 For example, on a client/server system, an instance can be created
306 server-side, serialized and the used client-side.
307 In this case, two files will be generated by nitserial,
308 one for the server and one for the client.
309 Both the files should be compiled with both the client and the server.