Welcome back! Today's focus will be on downloading files from an API. Understanding how to retrieve files efficiently not only enhances your technical skills but also broadens your application's capabilities. In this lesson, we'll explore a practical scenario using our To-Do list API, which, in addition to managing tasks, supports handling text files such as notes. These notes can be downloaded or uploaded through the /notes endpoint, allowing functionality for storing supplementary information. For example, users might keep notes about a meeting or important reminders.

By understanding how to interact with this endpoint, you can effectively manage notes within your application. By the end of this lesson, you'll know how to request a file from an API, save it locally, and verify its contents.

Let's dive into downloading files with precision and confidence!

Before we dive into file downloads, let's briefly discuss PrintWriter. This class from the java.io package provides a convenient way to write formatted text to files. Unlike lower-level file writing mechanisms, PrintWriter handles character encoding automatically and provides methods for writing strings directly to files.

Here's how you can initialize and use a PrintWriter:

Scala
Copy to clipboard
1import java.io.{File, PrintWriter}
2
3// Basic initialization
4val writer = new PrintWriter(new File("example.txt"))
5
6// With character encoding specified
7val writerWithEncoding = new PrintWriter(new File("example.txt"), "UTF-8")
8
9// Using try-finally for proper resource management
10val pw = new PrintWriter(new File("example.txt"), "UTF-8")
11try 
12  pw.write("Hello, World!")
13finally 
14  pw.close()

It's particularly useful when dealing with text files as it manages the character encoding (like UTF-8) and provides automatic line separation handling.

GET requests are fundamental for retrieving files from an API. When you send a GET request using requests.get, your client communicates with the server at a specified URL, asking it to provide the file. The server responds with the file data, if available and permissible, along with an HTTP status code (like 200 OK).

Here's a basic example of downloading a file named welcome.txt from our API at http://localhost:8000/notes. This approach downloads the entire file at once, which is manageable for smaller files.

Scala
Copy to clipboard
1import requests.*
2import scala.util.{Try, Success, Failure}
3import java.io.{File, PrintWriter}
4
5def downloadFile(): Unit = 
6  val baseUrl = "http://localhost:8000"
7  val noteName = "welcome.txt"
8
9  val result = Try:
10    val response = requests.get(s"$baseUrl/notes/$noteName")
11
12    // Check the HTTP status code to ensure successful download
13    response.statusCode match
14      // Status codes 200-299 indicate success
15      case code if code >= 200 && code < 300 =>
16        // Create a new file with 'downloaded_' prefix
17        val file = File(s"downloaded_$noteName")
18        // Initialize PrintWriter with UTF-8 encoding for text writing
19        val pw = new PrintWriter(file, "UTF-8")
20        try 
21          // response.text() gets the response body as a String
22          pw.write(response.text()) 
23        finally 
24          // Ensure the writer is closed to prevent resource leaks
25          pw.close()
26        Success(())
27      case _ => 
28        // For any other status code, create a failure with error message
29        Failure(new Exception(s"Failed to download file: ${response.statusMessage}"))
30
31  // Pattern match on the result to handle success or failure
32  result match
33    case Success(_) => println(s"File $noteName downloaded successfully.")
34    case Failure(e) => println(s"An error occurred: ${e.getMessage}")

Let's break down the important components we're using:

When working with files, PrintWriter is our trusty tool for writing text data. The try-finally block ensures we always "put the cap back on the pen" by calling close(), even if something goes wrong while writing.

The response.text() method converts what we received from the server into a readable string format. It's particularly useful for text files like our notes, handling all the conversion work for us.

Finally, we use Try to wrap our operations in a safety net. If anything goes wrong - maybe the server is down, or the file doesn't exist - Try catches these issues and turns them into manageable Success or Failure cases. This way, our application can respond gracefully to problems instead of crashing.

Once you've downloaded a file, it's imperative to verify its contents to ensure a successful transfer. In our example, after downloading, you can open the file and print its content to confirm data integrity:

Scala
Copy to clipboard
1import scala.util.{Try, Success, Failure}
2import scala.io.Source
3
4def verifyFileContent(): Unit = 
5  val noteName = "welcome.txt"
6  
7  val result = Try:
8    val source = Source.fromFile(s"downloaded_$noteName")
9    val content = source.getLines().mkString("\n")
10    source.close()
11    println(content)
12
13  result match
14    case Success(_) => println("File content verified successfully.")
15    case Failure(e) => println(s"File error occurred: ${e.getMessage}")

If everything is functioning correctly, you should see an output similar to:

Plain text
Copy to clipboard
1Welcome to Your Notes! 📝
2
3This is a sample note that comes with the application.

This step is essential for data verification. The familiar error-handling techniques come into play once more, using Try to gracefully address any issues during the download and verification process.

In this lesson, you explored a straightforward approach for downloading files from an API. You've practiced verifying file integrity by reading its contents post-download and reinforced your knowledge of error management. As you proceed to the practice exercises, you'll have the opportunity to apply these skills, reinforcing your ability to manage API interactions effectively.

Keep experimenting with different files and settings, as this will further enhance your understanding and proficiency. Exciting topics await, such as file uploads and handling paginated responses. Your journey in mastering API interactions continues!