This text is about developing RESTful web services using Java, Maven, Json and Spring 3. You can read instructions on how to setup a development environment for this kind of project here. The source code is available on GitHub.
[With Rest], requests and responses are built around the transfer of representations of resources. Resources are identified by global ID’s that typically use a uniform resource identifier (URI). Client applications use Http methods (such as GET, Post, PUT, or Delete) to manipulate the resource or collection of resources. Generally, a GET method is used to get or list the resource or collection of resources, Post is used to create, PUT is used to update or replace, and Delete is for removing the resource.
The Amazing Salume Application™
You work at a small business, a supplier company specialized in the finest salumes in the world, with a few clients, mostly bistros and restaurants (actually, there are only two clients, one bistro and one restaurant, but describing the company that way makes it looks larger). They both are good clients and the sales of the company is quite impressive. As the sales are expected to continue to grow, it is time to use a software application to aid the sale process. You decided to provide a RESTful web service to allow clients to manipulate their salumes orders through requests/responses in Json format.
Let’s build a parser to math expressions. It’s better start the job in very a simple way: process addition expressions like ‘1 + 1’.
Well… As the old Chinese saying goes, “a journey of one thousand miles begins with one tiny step”.
A Little Bit About Parsers
According to Grune and Jacobs:
“Parsing is the process of structuring a linear representation in accordance with a given grammar. [..] This ‘linear representation’ may be a sentence, a computer program, [..] in short any linear sequence in which the preceding elements in some way restrict the next element”[1, p. 3].
“To a computer scientist ‘1 + 2 * 3’ is a sentence in the language of ‘arithmetics on single digits’ [..], its structure can be shown, for instance, by inserting parentheses: (1 + (2 * 3)) and its semantics [i.e. its meaning] is probably 7”[1, p. 7].
We use a parser to build an expression tree. This tree contains the elements of a sentence according with the structure of a given grammar and in the correct order (afterwards we will need an interpreter that uses the tree to extract the semantic of the sentence).
We’ll start slowly, only dealing with the four binary basic operations: addition, subtraction, multiplication and division. Even though we defined a concise set of operations, I need to be even more humble in order to achieve our goals, and that’s why I’ll choose one of these operations to get started with the fun.
How about we begin with the addition operation? Sounds easy good to me. Doing that will lead the way to the remainder operations (I mean subtraction, multiplication and division operations). We can represent our math expression with a limited context-free grammar like this: