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
58 ### Scope of the `serialize` annotation
60 `serialize` can annotate class definitions, modules and attributes:
62 * The annotation on a class applies only to the class definition,
63 only attributes declared locally will be serialized.
64 However, each definition of a class (a refinement or specialization) can be annotated with `serialize`.
66 * A module declaration annotated with `serialize` states that all its class definitions
67 and locally declared attributes are serializable.
70 module shared_between_clients is serialize
73 * Attribute annotated with `serialize` states that it is to be serialized, when the rest of the class does not.
74 The class will become subclass to `Serializable` but its attributes are not to be serialized by default.
75 Only the attributes with the `serialize` annotation will be serialized.
78 # Only serialize the `name`
80 var name: String is serialize
81 var avatar_path: String = "/somepath/"+name is lazy
85 ## The `noserialize` annotation
87 The `noserialize` annotation mark an exception in a `serialize` module or class definition.
89 * By default a module is `noserialize`. There is no need to declare it as such.
91 * A class definition annotated with `noserialize` within a `serialize` module will not be made serializable.
93 * A `noserialize` attribute within a class or module annotated with `serialize` will not serialize this attribute.
94 The class will still be made subclass of `Serializable` and it won't affect the other attributes.
95 The `noserialize` attribute will not be set at deserialization.
96 Usually, it will also be annotated with `lazy` to get its value by another mean after the object has been deserialized.
99 # Once again, only serialize the `name`
100 class UserCredentials
104 var avatar_path: String = "/somepath/"+name is noserialize, lazy
108 ## The `serialize_as` annotation
110 By default, an attribute is identified in the serialization format by its Nit name.
111 The `serialize_as` attribute changes this behavior and sets the name of an attribute in the serialization format.
113 This annotation can be useful to change the name of an attribute to what is expected by a remote service.
114 Or to use identifiers in the serialization format that are reserved keywords in Nit (like `class` and `type`).
117 class UserCredentials
120 # Rename to "username" in JSON for compatibility with remote service
121 var name: String is serialize_as "username"
123 # Rename to a shorter "ap" for a smaller JSON file
124 var avatar_path: String = "/somepath/"+name is lazy, serialize_as "ap"
128 ## Custom serializable classes
130 The annotation `serialize` should be enough for most cases,
131 but in some cases you need more control over the serialization process.
133 For more control, create a subclass to `Serializable` and redefine `core_serialize_to`.
134 This method should use `Serializer::serialize_attribute` to serialize its components.
135 `serialize_attribute` works as a dictionary and organize attributes with a key.
137 You will also need to redefine `Deserializer::deserialize_class` to support this specific class.
138 The method should only act on known class names, and call super otherwise.
140 ### Example: the User class
142 The following example cannot use the `serialize` annotations
143 because some of the arguments to the `User` class need special treatment:
145 * The `name` attribute is perfectly normal, it can be serialized and deserialized
148 * The `password` attribute must be encrypted before being serialized,
149 and unencrypted on deserialization.
151 * The `avatar` attributes is kept as ASCII art in memory.
152 It could be serialized as such but it is cleaner to only
153 serialize the path to its source on the file system.
154 The data is reloaded on deserialization.
156 For this customization, the following code snippet implements
157 two serialization services: `User::core_serialize_to` and
158 `Deserializer::deserialize_class`.
161 module user_credentials
163 # User credentials for a website
170 # Clear text password
173 # User's avatar image as data blob
176 redef fun core_serialize_to(serializer: Serializer)
178 # This is the normal serialization process
179 serializer.serialize_attribute("name", name)
181 # Serialized an encrypted version of the password
183 # Obviously, `rot(13)` is not a good encrption
184 serializer.serialize_attribute("pass", password.rot(13))
186 # Do not serialize the image, only its path
187 serializer.serialize_attribute("avatar_path", avatar.path)
191 redef class Deserializer
192 redef fun deserialize_class(name)
194 if name == "User" then
195 # Deserialize normally
196 var user = deserialize_attribute("name")
199 var pass = deserialize_attribute("pass").rot(-13)
201 # Deserialize the path and load the avatar from the file system
202 var avatar_path = deserialize_attribute("avatar_path")
203 var avatar = new Image(avatar_path)
205 return new User(user, pass, avatar)
212 # An image loaded in memory as ASCII art
214 # Not really useful for this example, provided for consistency only.
216 # Path on the filesystem for `self`
219 # ASCII art composing this image
220 var ascii_art: String = path.read_all is lazy
225 See the documentation of the module `serialization::serialization` for more
226 information on the services to redefine.
228 ## Serialization services
230 The `serialize` annotation and the `Serializable` class are used on
231 classes specific to the business domain.
232 To write (and read) instances of these classes to a persistent format
233 you must use implementations of `Serializer` and `Deserializer`.
235 The main implementations of these services are `JsonSerializer` and `JsonDeserializer`,
236 from the `json_serialization` module.
240 import user_credentials
242 # Data to be serialized and deserialized
243 var couple = new Partnership(
244 new Person("Alice", 1985, new Image("alice.png")),
245 new Person("Bob", null, new Image("bob.png")))
247 var path = "serialized_data.json"
248 var writer = new FileWriter(path)
249 var serializer = new JsonSerializer(writer)
250 serializer.serialize couple
253 var reader = new FileReader(path)
254 var deserializer = new JsonDeserializer(reader.to_s)
255 var deserialized_couple = deserializer.deserialize
258 assert couple == deserialize_couple
261 ## Limitations and TODO
263 The serialization has some limitations:
265 * A limitation of the JSON parser prevents deserializing from files
266 with more than one object.
267 This could be improved in the future, but for now you should
268 serialize a single object to each files and use different instances of
269 serializer and deserializer each time.
271 * The serialization uses only the short name of a class, not its qualified name.
272 This will cause problem when different classes using the same name.
273 This could be solved partially in the compiler and the library.
274 A special attention must be given to the consistency of the name across
275 the different programs sharing the serialized data.
277 * The serialization support in the compiler need some help to
278 deal with generic types. A solution is to use `nitserial`,
279 the next section explores this subject.
281 ## Dealing with generic types
283 One limitation of the serialization support in the compiler is with generic types.
284 For example, the `Array` class is generic and serializable.
285 However, the runtime types of Array instances are parameterized and are unknown to the compiler.
286 So the compiler won't support serializing instances of `Array[MySerializable]`.
288 The tool `nitserial` solves this problem at the level of user modules.
289 It does so by parsing a Nit module, group or project to find all known
290 parameterized types of generic classes.
291 It will then generating a Nit module to handle deserialization of these types.
293 Usage steps to serialize parameterized types:
295 * Write your program, let's call it `my_prog.nit`,
296 it must use some parameterized serializable types.
297 Let's say that you use `Array[MySerializable]`.
299 * Run nitserial using `nitserial my_prog.nit` to
300 generate the file `my_prog_serial.nit`.
302 * Compile your program by mixing in the generated module with:
303 `nitc my_prog.nit -m my_prog_serial.nit`
305 This was a simple example, in practical cases you may need
306 to use more than one generated file.
307 For example, on a client/server system, an instance can be created
308 server-side, serialized and the used client-side.
309 In this case, two files will be generated by nitserial,
310 one for the server and one for the client.
311 Both the files should be compiled with both the client and the server.