User guide

jocelyn edited this page Nov 17, 2014 · 6 revisions

PLEASE REFER TO SOURCE CODE https://github.com/eiffelhub/json/blob/master/doc/user_guide.mediawiki

Table of Contents

Preface

This document is a living document! As always read and try out the code to understand what's really going on.

About the project

The eJSON project was started by Javier Velilla in 2008. The aim was simply to provide JSON support to Eiffel programmers. A couple of other people have been involved to various extent since the start; Berend de Boer, Jocelyn Fiat and Manu Stapf. In 2009 Paul Cohen joined the project as an active developer and later Jocelyn Fiat.

The current active maintainers:

  • Javier Velilla
  • Jocelyn Fiat
The formal name of the project is “eJSON”.

For questions regarding eJSON please contact

Current version and status

The latest release is 0.6.0. eJSON has been improved and cleaned. The converters are not obsolete.

Introduction

What is JSON?

JSON (JavaScript Object Notation) is a lightweight computer data interchange format. It is a text-based, human-readable format for representing simple data structures and associative arrays (called objects). See the Wikipedia article on JSON, www.json.org and www.json.com for more information.

The JSON format is specified in IETF RFC 4627 by Douglas Crockford. The official Internet MIME media type for JSON is "application/json". The recommended file name extension for JSON files is ".json".

Advantages

1. Lightweight data-interchange format. 2. Easy for humans to read and write. 3. Enables easy integration with AJAX/JavaScript web applications. See the article Speeding Up AJAX with JSON for a good short discussion on this subject. 4. JSON data structures translate with ease into the native data structures universal to almost all programming languages used today.

Use in Eiffel applications

JSON can be used as a general serialization format for Eiffel objects. As such it could be used as a:

  • Data representation format in REST-based web service applications written in Eiffel.
  • Serialization format for Eiffel objects in persistence solutions.
  • File format for configuration files in Eiffel systems.

Prerequisites

eJSON works today with EiffelStudio 13.11 There is an optional extension that requires the latest snapshot of the Gobo Eiffel libraries (a working snapshot is distributed with EiffelStudio). The depencencies on Gobo are on Gobo's unicode and regex libraries and for some of the extra features in eJSON, on Gobos structure classes DS_HASH_TABLE and DS_LINKED_LIST.

eJSON is intended to work with all ECMA compliant Eiffel compilers.

Installation

You can either download a given release and install on your machine or you can get the latest snapshot of the code. To download go to the download page. To get the latest snapshot of the code do:

$ git clone https://github.com/eiffelhub/json.git json
Note that the latest json release is also delivered with EiffelStudio installation under $ISE_LIBRARY/contrib/library/text/parser/json.

Cluster and directory layout

 	json/
 	  library/ (Root directory for eJSON library classes)
 		kernel/ (All classes in this cluster should eventually only depend on ECMA Eiffel and FreeELKS).
 		  json_array.e
 		  json_boolean.e
 		  json_null.e
 		  json_number.e
 		  json_object.e
 		  json_string.e
 		  json_value.e

parser/ json_parser.e json_parser_access.e json_reader.e json_tokens.e utility/ file/ json_file_reader.e visitor/ json_visitor.e json_iterator.e json_pretty_string_visitor.e print_json_visitor.e converters/ (JSON core converter classes !OBSOLETE!)

 		  json_converter.e
 		  json_hash_table_converter.e
 		  json_list_converter.e
 		  json_linked_list_converter.e
 		  json_arrayed_list_converter.e

support/ ejson.e shared_ejson.e gobo_converters/ (JSON core converter classes support for GOBO !OBSOLETE!) converters/

 			json_ds_hash_table_converter.e
 			json_ds_linked_list_converter.e

shared_gobo_ejson.e

 	  test/ (Contains autotest suite)
 		autotest/ (AutoTest based unit test).
 	  examples/ (Example code)

Future development

Here is a list of suggestions for future development of eJSON.

  • Ongoing: Provide a JSON_FACTORY class for easy conversion between arbitrary JSON and Eiffel values.
  • Ongoing: Provide a mechanism for users to add custom converters between JSON values and user space Eiffel classes.
  • Ongoing: Implement a full test framework for eJSON.
  • Suggestion: Investigate performance and improve it if neccessary.
  • Suggestion: Support JSON references. See JSON Referencing Proposal and Library and JSON referencing in Dojo for more information.
  • Suggestion: Support JSON path. See JSONPath - XPath for JSON for more information.
  • Suggestion: Support JSON schema validation. See JSON Schema Proposal for more information.
  • Suggestion: Support RDF JSON serialization. See RDF JSON Specification for more information.
  • Suggestion: Add support to JSON classes for conversion from Eiffel manifest values. So one can write things like:

A simple example

There are two basic approaches to using eJSON; either you use the basic JSON_VALUE classes, converting to and from JSON values to corresponding Eiffel instances or you use the high level eJSON interface class SHARED_EJSON. Of course you can use a mix of both approaches if you find it appropriate!

Here is an example of how to create a JSON number value from an INTEGER and then obtain the JSON representation for that value.

 	simple_example is
 		local
 			i: INTEGER
 			jn: JSON_NUMBER
 			s: STRING
 		do
 			i := 42
 			create jn.make_integer (i)
 			s := jn.representation -- s.is_equal ("42")
 		end

Mapping of JSON values to Eiffel values

JSON number

JSON numbers are represented by the class JSON_NUMBER. JSON number values can be converted to/from NATURAL_*, INTEGER_* and REAL_64 values. For floating point values REAL_* is used. The complete mapping is as follows:

JSON number -> Eiffel:

  • -128 <= n <= +127 -> INTEGER_8
  • n can't be represented by INTEGER_8 and -32768 <= n <= +32767 -> INTEGER_16
  • n can't be represented by INTEGER_16 and -2147483648 <= n <= +2147483647 -> INTEGER_32
  • n can't be represented by INTEGER_32 and -9223372036854775808 <= n <= +9223372036854775807 -> INTEGER_64
  • n can't be represented by INTEGER_64 and 9223372036854775808 <= n <= 18446744073709551615 -> NATURAL_64
  • n has fractional dot '.' -> REAL_64.
  • n -> eJSON exception if number can't be represented by a INTEGER_64, NATURAL_64 or REAL_64.
Eiffel -> JSON number:
  • NATURAL_8, NATURAL_16, NATURAL_32, NATURAL_64, NATURAL -> JSON number
  • INTEGER_8, INTEGER_16, INTEGER_32, INTEGER_64, INTEGER -> JSON number
  • REAL_32, REAL_64, REAL -> JSON number
You can use the following creation routines to create JSON_NUMBER instances:
  • JSON_NUMBER.make_integer
  • JSON_NUMBER.make_real
  • JSON_NUMBER.make_natural
 	eiffel_to_json_number_representation is
 		local
 			i: INTEGER
 			r: REAL 
 			jn: JSON_NUMBER
 		do
 			print ("JSON representation of Eiffel INTEGER: '")
 			i := 123
 			create jn.make_integer (i)
 			print (jn.representation)
 			print ("'%N")
 			print ("JSON representation of Eiffel REAL: '")
 			r := 12.3
 			create jn.make_real (r)
 			print (jn.representation)
 			print ("'%N")
 		end

The output of the above code will be:

 	JSON representation of Eiffel INTEGER: '123'
 	JSON representation of Eiffel REAL: '12.300000190734863'

JSON boolean

JSON boolean values are represented by the class JSON_BOOLEAN. The JSON boolean value "true" is converted to/from the BOOLEAN value "True" and the JSON boolean value "false is converted to/from the BOOLEAN value "False".

 	eiffel_to_json_boolean_representation is
 		local
 			b: BOOLEAN
 			jb: JSON_BOOLEAN
 		do
 			print ("JSON representation of Eiffel BOOLEAN: '")
 			b := True
 			create jb.make (b)
 			print (jb.representation)
 			print ("'%N")
 			print("JSON representation of Eiffel BOOLEAN: '")
 			b := False
 			create jb.make (b)
 			print (jb.representation)
 			print ("'%N")
 		end

The output of the above code will be:

 	JSON representation of Eiffel BOOLEAN: 'true'
 	JSON representation of Eiffel BOOLEAN: 'false'

JSON string

JSON strings are represented by the class JSON_STRING. JSON string values can be converted to/from STRING_32, STRING and CHARACTER values. The complete mapping is as follows:

JSON string -> Eiffel:

  • All JSON strings -> STRING or STRING_32
Eiffel -> JSON string:
  • STRING_32 -> JSON string
  • STRING -> JSON string
  • CHARACTER -> JSON string
 	eiffel_to_json_string_representation is
 		local
 			s: STRING
 			js: JSON_STRING
 		do
 			print ("JSON representation of Eiffel STRING: '")
 			s := "JSON rocks!"
 			create js.make_from_string (s)
 			print (js.representation)
 			print ("'%N")
 		end

The output of the above code will be:

 	JSON representation of Eiffel STRING: '"JSON rocks!"'

Note: JSON escape unicode characters, as well a other specific characters, to get the unescaped string value, use either 'unescaped_string_8' or 'unescaped_string_32'.

JSON null

The JSON null value is represented by the class JSON_NULL. The JSON null value can be converted to/from Void.

 	eiffel_to_json_null_representation is
 		local
 			a: ANY
 			jn: JSON_NULL
 		do
 			create jn
 			print ("JSON representation for JSON null value: '")
 			print (jn.representation)
 			print ("'%N")
 			a := Void
 			if attached {JSON_NULL} json.value (a) as l_jn then -- json from SHARED_EJSON!
 				print ("JSON representation of Eiffel Void reference: '")
 				print (l_jn.representation)
 				print ("'%N")
 			end
 		end

The output of the above code will be:

 	JSON representation for JSON null value: 'null'
 	JSON representation of Eiffel Void reference: 'null'

JSON array

JSON array is represented by the class JSON_ARRAY.

JSON object

JSON object is represented by the class JSON_OBJECT.

The eJSON visitor pattern

TBD. See examples.

The eJSON file reader class

TBD.

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