Welcome back! Today's focus will be on downloading files from an API using Java. 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 a hypothetical To-Do list API. This API supports handling text files, such as notes, which can be downloaded or uploaded through the /notes endpoint. 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 using Java and its HttpClient!

GET requests are fundamental for retrieving files from an API. When you send a GET request using Java's HttpClient, 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.

This code sends a GET request and writes the response content to a local file. This method works well for small files but can strain memory for larger files.

When dealing with large files, downloading them all at once can be inefficient and strain memory. To address this, Java's HttpClient allows handling the response as a stream using InputStream, which optimizes memory usage.

Below is a detailed code example demonstrating how to download the same file using streaming:

• FileOutputStream is used to write data to a file on disk.
• The try-with-resources block (try (...) {...}) automatically closes the streams after the operation is complete, preventing resource leaks.
• A buffer (byte[] buffer = new byte[1024]) is created to store chunks of data.
• inputStream.read(buffer) reads up to 1024 bytes at a time from the response body.
• If bytesRead == -1, it means end of file (EOF) is reached, and we stop reading.
• fileOutputStream.write(buffer, 0, bytesRead); writes the read bytes into the file.

By utilizing streaming, even large files are downloaded efficiently. This approach is especially useful when file sizes increase.

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 using Java's file reading mechanisms:

• FileReader(noteName): Opens the file for reading character-by-character.
• BufferedReader(reader): Wraps FileReader to improve performance by reading in chunks (lines) instead of single characters.
• try-with-resources: Ensures that BufferedReader closes automatically when done, preventing resource leaks.
• reader.readLine(): reads a single line from the file.
• If the file is missing or unreadable, an IOException is thrown. This step is essential for data verification, using try-catch blocks to gracefully handle any issues during the download and verification process.

In this lesson, you explored two methods for downloading files from an API using Java's HttpClient: a straightforward approach for smaller files and a more efficient streaming method for larger files. 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 with Java. Keep experimenting with different files and settings to further enhance your understanding and proficiency. Exciting topics await, such as file uploads and handling paginated responses. Your journey in mastering API interactions continues!