25. Serialization


Serialization is the process of writing objects to an output stream, and reading them back from an input stream. Serialization provides a simple mechanism to persist objects to a file or to pass objects over a network. Serialization is also used with actors as a safe way to pass messages between actors. Fantom serialization uses a human friendly text format which looks a lot just like Fantom source code (in fact it is a subset of the source grammar).

Data Model

Serialization in Java is graph based - it will handle an arbitrary number of references to a particular object. Fantom serialization is strictly tree based, it will not attempt to keep track of object references - it is up to you design your data models as a tree. If you need to cross reference objects in your tree, then you should use a Uri or some other identifier.

Each object in the tree is classified as a literal, simple, or complex. Most of the standard Fantom literals such as Bool, Int, Str are supported as well as the collections types List and Map. Simples are leaf nodes serialized via a string representation. Complexes are an aggregate node defined by their fields which store nested objects (either literals, simples, or other complexes). You can also mark any complex type as a collection.


The Serializable facet is used to mark types which are serializable. By default a serializable object will serialize all of its non-static fields. You can use the Transient facet to annotate a field which should not be serialized. A contrived example:

class Rectangle
  Int x; Int y
  Int w; Int h
  @Transient Int area

A serializable object must support a make constructor which either takes no parameters or takes an it-block. If the constructor takes an it-block then the field values are passed in using Field.makeSetFunc and the object is given a chance to perform validation. For example:

const class Rec
  new make(|This| f)
    if (id <= 0) throw ArgErr("id <= 0")
  const Int id

"Rec { id = 3 }".in.readObj  // ok
"Rec {}".in.readObj          // throws ArgErr


Set the Serializable.simple field to mark a type which is serialized atomically to and from a string representation. The Obj.toStr method must return a suitable string representation of the object. Each simple type must also declare a constructor called fromStr which takes one or more parameters where the first parameter is a Str and returns an instance of the object. An example:

@Serializable { simple = true }
class Point
  static new fromStr(Str s) { t := s.split(','); return make(t[0].toInt, t[1].toInt) }
  new make(Int x, Int y) { this.x = x; this.y = y }
  override Str toStr() { return "$x,$y" }
  Int x := 0
  Int y := 0


Set the Serializable.collection field to mark a type as a container of child objects. Collections provide a concise syntax for nesting zero or more children items in the same scope as any serialized fields. This allows you to nest configuration items and children inside one set of curly braces. Every collection type must support an add and each method which are used by readObj and writeObj respectively. See the example code.


The following methods are available for serialization:

Serializing objects to and from streams is a piece of cake:

// write an object to an output stream

// read an object from an input stream
obj := in.readObj

Both Buf and File have convenience methods. For example to serialize to and from a file:

obj := [true, 5, "hi", `file.txt`]
f := File(`test.txt`)
obj2 := f.readObj

By default writeObj will optimize for performance. But if you are generating a file which should look pretty for humans to read and edit, you can control the output using options. For example to indent each level of the output tree by 2 spaces and skip fields at their default values:

out.writeObj(obj, ["indent":2, "skipDefaults":true])


The Fantom serialization syntax is designed to be easy to read and write by a human, but also efficient for machine processing. The syntax is based on the Fantom programming language itself, although it is purely declarative (no expressions or behavior). An object is defined as one of:

  • Literal: one of the standard Fantom literals using the exact same representation as you would use in your source code (this includes List and Map)
  • Simple: the string representation of a simple type
  • Complex: a type and its list of field name/values pairs

The Fantom programming language is a complete superset of the serialization syntax - you can paste any serialized object into a source file as an expression.


You can include zero or more using statements at the top of a serialized object document. Using statements allow unqualified type names to be used:

// qualified names
["red":fwt::Color("#f00"), "blue":fwt::Color("#0f0")]

// unqualified names
using fwt
["red":Color("#f00"), "blue":Color("#0f0")]

You can use any of the standard using statements in your serialization documents:

using pod                  =>  import all types in pod
using pod::name            =>  import single type
using pod::name as rename  =>  import type with new name

Note that unlike normal source code, the sys pod is not imported automatically. If you wish to use unqualified type names for the sys pod, then you need to explicitly import via using sys.


Most of the standard Fantom literals are serialized using the same representation as defined by the Fantom programming language:

NOTE: the special Float values NaN, INF, and -INF must be represented using the simple syntax: sys::Float("NaN") sys::Float("INF") sys::Float("-INF")


A simple is serialized as: <type>("<toStr>"). When writing the object, the Obj.toStr method is called to obtain the string representation. When reading the object the fromStr method is used to decode the string back into an object. Examples:

sys::Depend("foo 1.2-3.4")

You may use this syntax directly in source code via the simple expression.


A complex is serialized as a list of field name/value pairs separated by a newline or a semicolon (just like Fantom statements). Any field can be omitted, in which case the field's default is used. The syntax for a complex is:

  <field1> = <value1>
  <field2> = <value2>

An example of a serializable class and an serialized instance:

class Person
  Str name
  Int age
  Str[] children
  Str address

  name = "Homer Simson"
  age = 39
  children = ["Bart", "Lisa", "Maggie"]

You may use this syntax directly in source code via it-blocks.


Collections are serialized just like a complex - all the fields are serialized as name/value pairs. After the fields are serialized, all the child items iterated by the each method are serialized with a comma separator. During deserialization, the children are added back via the add method.

We can rewrite the Person example above as a collection:

@Serializable { collection = true }
class Person
  Void add(Person kid) { kids.add(kid) }
  Void each(|Person kid| f) { kids.each(f) }
  Str name
  @transient private Person[] kids := Person[,]

  name = "Homer Simson"
  acme::Person { name = "Bart" },
  acme::Person { name = "Lisa" },
  acme::Person { name = "Maggie" },

This syntax is also supported directly in source code via with-blocks.


The formal grammar of the Fantom serialization formats:

// .fog file format for single object
objDoc    := header obj

header    := [using]*
using     := usingPod | usingType | usingAs
usingPod  := "using" id eos
usingType := "using" id "::" id eos
usingAs   := "using" id "::" id "as" id eos
obj       := literal | simple | complex
literal   := bool | int | float | decimal | str | duration | uri |
             typeLiteral | slotLiteral | list | map
simple    := type "(" str ")"
complex   := type ["{" [children] "}"]
children  := child [(eos|",") child]*
child     := field | item
eos       := ";" | newline
field     := id "=" obj
item      := obj

The literal, using, and type productions use the same grammar as the Fantom programming language. However the type production can never be a function type.