HTTP supports multiple request methods.

Along with response, servers send HTTP Status code also. It is a numeric value used to know the response status. Following are the most commonly used HTTP Status Codes :

• 200 - OK : Successful request
• 404 - Not Found : Resource not found
• 500 - Internal Server Error

You can learn more about HTTP response status codes here.

Let us now learn how to send HTTP requests to Web APIs programmatically.

Let’s dig a bit deeper and understand how the above code functions internally. Following are the basic steps required to perform a HTTP GET request :

1. Create a URL object for required resource url

    val url = URL(/* url */)
    

2. Establish a connection using URL#openConnection() function

    val connection = url.openConnection() as HttpURLConnection
    

3. Create an InputStream instance for reading response

    val inputStream = connection.inputStream
    

4. Read the response as ByteArray using InputStream#readBytes() function and convert it to a string

    val response = String(inputStream.readBytes())
    

5. Check the response status

    // If not 'OK', throw error
   if (connection.responseCode != HttpURLConnection.HTTP_OK) {
       error("${connection.responseCode} : ${connection.responseMessage} : $response")
   }
    

Example :

 fun main() {
    val url = URL("http://numbersapi.com/7/math")
    val connection = url.openConnection() as HttpURLConnection
    val response: String = String(connection.inputStream.readBytes())
		if (connection.responseCode != HttpURLConnection.HTTP_OK) {
		    error("${connection.responseCode} : ${connection.responseMessage} : $response")
		}
    println("response = $response")
}
 

We can define a generic function for HTTP GET as :

 fun httpGet(urlString: String): String {
    val url = URL(urlString)
    val connection = url.openConnection() as HttpURLConnection
    val response = String(connection.inputStream.readBytes())
    if (connection.responseCode != HttpURLConnection.HTTP_OK) {
        error("${connection.responseCode} : ${connection.responseMessage} : $response")
    }
    return response
}

// Usage :
fun main() {
    val response = httpGet("http://numbersapi.com/7/math")
    println("response = $response")
}
 

Following steps need to be followed to send a HTTP POST request :

1. Create a URL object for required resource url

    val url = URL(/* url */)
    

2. Establish a connection using URL#openConnection() function

    val connection = url.openConnection()as HttpURLConnection
    

3. Create an OutputStream instance for sending payload

    val outputStream = connection.outputStream
    

4. Send payload as ByteArray using OutputStream#write() function and flush the stream at end

    outputStream.write(/* bytes */)
   outputStream.flush()
    

5. Create an InputStream instance for reading response

    val inputStream = connection.inputStream
    

6. Read the response as ByteArray using InputStream#readBytes() function and convert it to a string

    val response = String(inputStream.readBytes())
    

7. Check the response status

    // If not 'OK', throw error
   if (connection.responseCode != HttpURLConnection.HTTP_OK) {
       error("${connection.responseCode} : ${connection.responseMessage} : $response")
   }
    

PasteBin is a Plain text storage website. We can paste any text that we want to share and get a unique URL against it. PasteBin also allows creating new Paste using their API. Following is an example how to do so :

 fun createNewPaste(
    text: String,
    apiKey: String = "PASTE_YOUR_API_KEY_HERE"
): String {
    
    // Connect
    val url = "https://pastebin.com/api/api_post.php"
    val connection = URL(url).openConnection() as HttpURLConnection

    // Specify POST request
    connection.doOutput = true

    // Send Payload
    connection.setRequestProperty("Content-Type", "application/x-www-form-urlencoded")
    val payload = "api_dev_key=$apiKey&api_paste_code=$text&api_option=paste"
    connection.outputStream.run {
        write(payload.toByteArray())
        flush()
    }

    // Get response
    val inputStream = connection.inputStream
    val response = String(inputStream.readBytes())

    // Check status code
    if (connection.responseCode != HttpURLConnection.HTTP_OK) {
        error("${connection.responseCode} : ${connection.responseMessage} : $response")
    }

    // Return PasteBin URL response
    return response
}

// Usage :
fun main() {
    val pasteUrl = createNewPaste("This is secret code!")
    println("Paste created @ $pasteUrl")
}

/* Output :

Paste created @ https://pastebin.com/YYZbJ1B1
 */
 

The above code to send HTTP POST request might seem long. We can simplify and make the code more concise using HTTP Client libraries like OkHttp, Ktor etc. We will learn more about the Ktor library in Coroutines module.