Learn How to write a parser in Java

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How-to-write-a-parser-in-Java

How-to-write-a-parser-in-Java

Parser is an important component in any programming language. There are multiple open source parsers available in the market. So, the developer has to select the correct parser as per the requirement. In some situations, the correct parser may not be available freely, which means that the developer would have to develop their own custom parsers in different languages like Java Programming, C++, etc. The reason behind developing custom parsers can be performance issue, complexity, flaws in parsing, not matching the requirement, etc.

In this article we will try to explore how parsing is performed in Java and we will also have a look at different popular Java parsers.

What is parsing and what is a parser?
Before going into details, we must know the meaning of the terms ‘parsing’ and ‘parser’. Let’s have a look.

In simple words, parsing can be defined as the process of breaking down a block of data into smaller pieces based on some pre-defined set of rules. And then interpret, modify or manage the small pieces of data as per the requirement.

And, parser is a software program that is used to break the data into smaller chunks. A parser can be written in any languages based on the requirement.

What are the different types of parsers in Java?
Parsers can be categorized in different ways. In the simplest way, a parser can be either sequential or random. In a sequential parser, only the current parsed data is accessible. It cannot go back or forward. In a random parser, parsed data can be accessed randomly, so moving back and forth is possible. SAX and StAX parsers are examples of sequential parser and XML DOM is an example of a random parser.

In a different way, parsers can be classified as text parser or XML parser. A text parser parses textual data whereas XML parser parses XML/JSON data. In our discussion, we will focus on popular Java DOM and SAX parsers and their examples.

DOM parser and SAX parser
DOM (Document Object Model) defines an interface which can be used to manipulate XML documents. XML parsers are written by implementing this interface. DOM parsers are random parsers which are suitable when:

  • Information about the structure of the document is important
  • You need to move back and forth within the structure

DOM parser provides several Java interfaces and methods to work with the XML data. It returns a tree structure of all the elements in a XML document. And the tree can be traversed to work with the data.

SAX (Simple API for XML) is a sequential event-based parser. It parses the XML data in a sequential manner, starting from the root till the end. It does not form a tree structure to parse; rather it sends an event notification while parsing elements. SAX is suitable when

  • Linear and sequential processing is required
  • The XML document is too large
  • Complex nesting in XML is not there
  • Part of the XML document needs to be manipulated

SAX parser provides interfaces with call-back methods to get event notification during parsing.

How to implement a DOM parser in Java?
In this section, we will work with an XML document and a DOM parser. Following is a sample XML file containing employee’s data of a company. This is the input file to the parser.

Following is a XML document containing employee related data of a company. The root element is ‘company’, which is at the top of the document. After that, ‘employee’ is the next branch element. It contains employee data like name, salary, etc. Parsing will start from the root element onward.

Listing 1: Sample XML document for processing

Now, let us create a Java parser by using DOM parsing model. Following are the steps to be followed in the program to extract the data.

  • In the import section get all the XML related packages
  • Access input data file and create document builder
  • Extract the root element
  • Create node list containing ’employee’ node
  • Iterate through the node list and extract values

Listing 2: Implementing DOM parser

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Now, compile and run the Java program keeping the XML document in a proper location. The output of the application will be shown as below. It shows all the employee data found in the XML file.

Parser best practices
In case of parsers, best practices depend upon the situation and developer’s requirements. A text parser is suitable when you are parsing text input, and then tokenizing/splitting it and making use of the data. XML parsers are suitable when you receive XML/JSON data as an input. Following are some of the best practice rules followed in XML parsing.

DOM parser is best fit when the numbers of elements are under 1000 and you have a requirement of adding/deleting elements. But as DOM creates a tree structure before it starts processing, performance is an important parameter. So, for partial manipulation of an XML document, DOM is not recommended.

SAX is best fit for large XML files with linear structure and unique elements. It is light weight and suitable for shallow xml document parsing. As it does not make any tree structure, the performance is better than DOM parser.

Conclusion:
Parsing is an integral part of any programming languages. Java has its own method of parsing text, XML data. In this article, we have covered different areas of parsing as a generic concept. And then we have talked about specific areas of parsing and parsers like DOM and SAX. In the example section, we have covered DOM parser and its implementation details. At the end of the article we concluded with the best parsing practices in the industry.

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