Web Scraping With Kotlin: A Comprehensive Guide

Web scraping is the process of automatically extracting data from websites. It allows you to retrieve large amounts of structured information from web pages and save it to a file or database for further analysis. While web scraping can be done manually by copying and pasting, it is usually performed programmatically using a script or tool.

Kotlin is an excellent language choice for web scraping. As a modern, concise and expressive language, Kotlin makes it simple to write clean and readable scraping code. Its extensive standard library and third-party scraping frameworks provide robust functionality for fetching web page content, parsing HTML, and extracting the desired data. Kotlin‘s built-in null safety and other conveniences also help avoid common pitfalls.

In this in-depth guide, we‘ll explore how to scrape websites effectively using Kotlin. We‘ll cover the best libraries and tools for the job, walk through building a real-world scraper step-by-step, and share tips and best practices. Let‘s get started!

Top Kotlin Web Scraping Libraries

To jumpstart your Kotlin web scraping project, consider leveraging one of these popular open-source libraries:

• skrape{it}: An HTML testing and web scraping library inspired by Groovy‘s Geb. It provides a concise DSL for analyzing and extracting data from HTML documents.

• ksoup: A Kotlin port of the Java jsoup library. It makes it easy to parse and manipulate HTML from a URL, file, or string.

• karate: An open-source web API testing and web automation framework. While primarily designed for API testing, its browser automation capabilities also make it useful for scraping.

For this tutorial, we‘ll use skrape{it}, as it offers an intuitive and expressive way to traverse and extract HTML page data. But the general concepts will apply regardless of your specific library choice.

Step-By-Step Kotlin Scraper Tutorial

Now we‘ll learn web scraping with Kotlin by building a step-by-step example. Our scraper will extract quotes and author names from a sample quotes website. Here is the basic process:

1. Set up a new Kotlin project and add the skrape{it} dependency
2. Inspect the target website‘s HTML structure to determine how to locate the desired data
3. Use skrape{it} to fetch the web page and parse its HTML
4. Extract the sought-after quote and author data from the parsed HTML
5. Output the scraped data to the console
6. Paginate through all the quote pages and scrape each one

Let‘s walk through each step in detail.

Step 1: Kotlin Project Setup

First, create a new Kotlin project in your preferred IDE. Make sure you have a recent version of Kotlin installed.

Next, add the skrape{it} dependency to your project. With Gradle, include this line in your build.gradle.kts file:

implementation("it.skrape:skrapeit:1.2.2")

For a Maven project, add this to your pom.xml dependencies:

<dependency>
    <groupId>it.skrape</groupId>
    <artifactId>skrapeit</artifactId>
    <version>1.2.2</version>
</dependency>

Step 2: Analyze Website HTML

Before we start coding, we need to inspect our target website‘s HTML to determine how to pinpoint the data we want to extract.

Open the quotes website in your browser. Right-click a quote and select "Inspect" to open the browser developer tools. This allows us to see the underlying HTML.

We can see that each quote is contained in an HTML element like this:

<div class="quote">
    <span class="text">"The world as we...</span>
    <span>
        by <small class="author">Albert Einstein</small>
    </span>
</div>

So to scrape the quotes, we‘ll need to:

1. Find all the <div> elements with the class quote
2. Within each, extract the text of the <span> with class text for the quote content
3. And extract the text of the <small> with class author for the author name

Step 3: Fetch and Parse Page HTML

Now that we know how the target data is structured, let‘s use skrape{it} to fetch the page HTML and parse it.

First, we‘ll import the required skrape{it} classes:

import it.skrape.core.htmlDocument
import it.skrape.fetcher.*

Then retrieve the page HTML using the skrape function:

val document = skrape(HttpFetcher) { 
    request {
        url = "http://quotes.toscrape.com/"
    }
}.htmlDocument { 
    relaxed = true
}    

This tells skrape{it} to:

1. Use its built-in HttpFetcher to retrieve the contents of the specified URL
2. Parse the fetched HTML into a htmlDocument
3. Use relaxed parsing mode to handle any malformed HTML

Step 4: Extract Quote Data

With the parsed HTML document in hand, we can now extract the quotes and authors.

To find each quote <div>, we use the findAll function with a CSS selector:

val quotes = document.findAll("div.quote")

Then we iterate through each quote element to extract its text and author:

val extractedQuotes = quotes.map {
    val quoteText = it.findFirst("span.text").text
    val quoteAuthor = it.findFirst("small.author").text
    QuoteData(quoteText, quoteAuthor)
}

Here we use findFirst to pinpoint the quote and author elements within each quote, and extract their text. The results are saved in a QuoteData object:

data class QuoteData(val text: String, val author: String) 

Step 5: Output Scraped Data

Finally, let‘s display our scraped quotes by printing them to the console:

extractedQuotes.forEach { 
    println("${it.text} - ${it.author}")
}

The output will look something like:

"The world as we have created..." - Albert Einstein
"It is our choices, ..." - J.K. Rowling
...

And with that, we‘ve successfully extracted quotes from a web page using Kotlin!

The full code for our simple scraper:

import it.skrape.core.*
import it.skrape.fetcher.*

data class QuoteData(val text: String, val author: String)

fun main() {
    val document = skrape(HttpFetcher) {
        request {
            url = "http://quotes.toscrape.com/"
        }
    }.htmlDocument {
        relaxed = true
    }

    val quotes = document.findAll("div.quote")

    val extractedQuotes = quotes.map {
        val quoteText = it.findFirst("span.text").text
        val quoteAuthor = it.findFirst("small.author").text
        QuoteData(quoteText, quoteAuthor)
    }

    extractedQuotes.forEach {
        println("${it.text} - ${it.author}") 
    }
}

Step 6: Scrape Multiple Pages

So far our scraper only fetches the first page of quotes. But the quotes are split across multiple pages. To scrape all the pages, we‘ll need to:

1. Check if there is a "Next" page link
2. If so, extract its URL and recursively scrape it
3. Repeat until no more "Next" link is found

Here is how we modify our code to handle pagination:

fun extractQuotes(url: String) {
    val document = skrape(HttpFetcher) {
        request { this.url = url }
    }.htmlDocument { relaxed = true }

    val quotes = document.findAll("div.quote")

    val extractedQuotes = quotes.map {
        val quoteText = it.findFirst("span.text").text
        val quoteAuthor = it.findFirst("small.author").text
        QuoteData(quoteText, quoteAuthor)
    }

    extractedQuotes.forEach {
        println("${it.text} - ${it.author}") 
    }

    val nextPageLink = document
        .findFirst(".pager .next a")
        ?.attribute("href")

    nextPageLink?.let { 
        val nextPageUrl = "http://quotes.toscrape.com$it"
        extractQuotes(nextPageUrl)
    }
}

fun main() {
    extractQuotes("http://quotes.toscrape.com/")
}

The new extractQuotes function:

1. Fetches and parses the specified URL
2. Scrapes the quote data from it
3. Checks if the page contains a "Next" link
4. If found, extracts the URL and recursively calls itself with the new URL

We also now have to pass the initial URL to kick off the pagination process.

With this recursive scraping logic, we can now extract quotes from all the pages on the site!

Exporting Scraped Data to CSV

Printing to the console is fine for testing, but usually we‘ll want to save our scraped data to a file for further processing and analysis. A common format is CSV.

To generate a CSV file from our extracted quotes, we can use Kotlin‘s built-in writeText function:

import java.io.File

...

fun writeQuotesToCsv(quotes: List<QuoteData>) {
    val csvHeader = "Quote, Author\n"
    val csvRows = quotes.joinToString("\n") { 
        "\"${it.text}\",\"${it.author}\""
    }

    val csvString = csvHeader + csvRows
    File("quotes.csv").writeText(csvString)
}

fun main() {
    val allQuotes = mutableListOf<QuoteData>()

    fun extractQuotes(url: String) {
        ...
        // collect all quote data in list
        allQuotes.addAll(extractedQuotes)
        ...
    }

    extractQuotes("http://quotes.toscrape.com/")
    writeQuotesToCsv(quotes)
}

This code:

1. Defines a CSV header row
2. Converts each quote object to a CSV row string
3. Joins all rows into a complete CSV string
4. Writes the CSV data to a "quotes.csv" file

To build up the complete list of quotes across pages, we use a top-level allQuotes list that accumulates the quotes from each page.

After the scraping is done, we pass the full quote list to writeQuotesToCsv to save it to disk.

We can now load our "quotes.csv" file into a spreadsheet tool for further exploration!

Using Proxies to Avoid Blocking

When scraping a website, it‘s important to be mindful of the server load we generate. Requesting pages too aggressively can burden the site‘s server, and also get our IP address blocked.

To avoid this, we can introduce delays between page requests. skrape{it} provides a delay function for this:

val document = skrape(HttpFetcher) {
    request { this.url = url }
    delay(3000)
}.htmlDocument { relaxed = true }

This tells skrape{it} to wait 3 seconds between each request, reducing the pace of our scraping.

Another approach is to route requests through a proxy server, which anonymizes your IP address. There are several free and paid Kotlin-compatible proxy services, such as:

• Bright Data
• Smartproxy
• IPRoyal

To use a proxy with skrape{it}, set the proxy field on your HTTP request:

val document = skrape(HttpFetcher) {
    request { 
        this.url = url
        proxy = ProxyBuilder().apply {
            host = "proxyhost.com"
            port = 1234
        }.build()
    }
}.htmlDocument { relaxed = true }

This routes the request through the specified proxy server, masking your true IP address from the target site.

Web Scraping Best Practices and Ethics

When scraping websites, it‘s crucial to do so ethically and legally. Some best practices include:

• Check the site‘s terms of service and robots.txt
  • Many websites prohibit scraping in their TOS
  • The robots.txt file specifies which pages are allowed or disallowed for scraping
  • If scraping is prohibited, do not proceed
• Don‘t overwhelm the server with requests
  • Send requests at a reasonable pace, with delays between them
  • Use caching to avoid repeated hits to the same pages
• Identify your scraper in the User-Agent header
  • Don‘t misrepresent your scraper as a browser
  • Provide a way for the site owner to contact you
• Respect data copyright and licensing terms
  • Don‘t republish copyrighted data without permission
  • Abide by the license terms for any data you scrape
• Use good judgment
  • Don‘t scrape sensitive personal information
  • Consider the impact your scraping might have on the site and its users

As long as you follow these guidelines and stay mindful of how your scraper affects others, web scraping can be a powerful tool to glean useful insights from data on the web.

Conclusion

In this guide, we learned how Kotlin can be a great fit for web scraping tasks. We walked through an example of building a complete Kotlin scraper using the skrape{it} library to extract quotes from a website.

Some key takeaways:

• Kotlin‘s concise syntax and rich ecosystem make it well-suited for scraping
• Libraries like skrape{it} provide intuitive DSLs for fetching and parsing HTML
• Inspect the HTML structure of your target site to pinpoint the data to scrape
• Paginate through all target pages to comprehensively scrape a whole site
• Save your scraped data in a structured format like CSV for further analysis
• Be mindful of the target server, using proxies and delays to avoid overload
• Always scrape ethically, respecting the site‘s terms of use and copyrights

With the foundations you‘ve learned here, you‘re ready to start gathering data from the web using the power and expressiveness of Kotlin. Happy scraping!