OkHttp interceptors in Android Kotlin

Introduction

OkHttp interceptors let you watch and change network requests and responses. They help you add headers, log data, or handle errors easily.

Add a token or API key to every network request automatically.

Log request and response details for debugging your app.

Retry a request if it fails due to network issues.

Modify response data before your app uses it.

Block or redirect certain requests based on conditions.

Syntax

Android Kotlin

class MyInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val request = chain.request()
    // Modify request if needed
    val response = chain.proceed(request)
    // Modify response if needed
    return response
  }
}

val client = OkHttpClient.Builder()
  .addInterceptor(MyInterceptor())
  .build()

The intercept function runs for every request.

Use chain.proceed(request) to continue the request and get the response.

Examples

This interceptor adds an Authorization header to every request.

Android Kotlin

class HeaderInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val newRequest = chain.request().newBuilder()
      .addHeader("Authorization", "Bearer mytoken")
      .build()
    return chain.proceed(newRequest)
  }
}

This interceptor prints request URL and response code to the console.

Android Kotlin

class LoggingInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val request = chain.request()
    println("Sending request to ${request.url}")
    val response = chain.proceed(request)
    println("Received response with code ${response.code}")
    return response
  }
}

Sample App

This program creates an OkHttp client with a logging interceptor. It sends a GET request to https://httpbin.org/get. The interceptor prints the request URL and response code. Finally, it prints the length of the response body.

Android Kotlin

import okhttp3.Interceptor
import okhttp3.OkHttpClient
import okhttp3.Request
import okhttp3.Response

class SimpleLoggingInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val request = chain.request()
    println("Request URL: ${request.url}")
    val response = chain.proceed(request)
    println("Response code: ${response.code}")
    return response
  }
}

fun main() {
  val client = OkHttpClient.Builder()
    .addInterceptor(SimpleLoggingInterceptor())
    .build()

  val request = Request.Builder()
    .url("https://httpbin.org/get")
    .build()

  val response = client.newCall(request).execute()
  println("Response body length: ${response.body?.string()?.length}")
}

Important Notes

Interceptors run on a background thread, so avoid UI work inside them.

Use application interceptors for general tasks like adding headers.

Use network interceptors if you need to work with data after caching or retries.

Summary

OkHttp interceptors let you watch and change network requests and responses.

They are useful for adding headers, logging, retrying, or modifying data.

Create an interceptor by implementing the Interceptor interface and overriding intercept().

Practice

1. What is the main purpose of an OkHttp interceptor in Android Kotlin?

easy

A. To manage database transactions

B. To observe and modify network requests and responses

C. To create UI components dynamically

D. To handle user input events

Solution

1. Step 1: Understand the role of interceptors

   OkHttp interceptors allow you to watch and change network requests and responses before they are sent or after they are received.

2. Step 2: Compare with other options

   The other options relate to managing database transactions, creating UI components dynamically, and handling user input events, which are unrelated to network interception.

3. Final Answer:

   To observe and modify network requests and responses -> Option B

4. Quick Check:

   Interceptor purpose = Observe and modify network calls [OK]

Hint: Interceptors always deal with network requests/responses [OK]

Common Mistakes:

• Confusing interceptors with UI or database components
• Thinking interceptors handle user input
• Assuming interceptors manage app navigation

2. Which of the following is the correct way to implement an OkHttp interceptor in Kotlin?

easy

A. fun intercept(chain: Chain): Response {
return chain.proceed(chain.request())
}

B. class MyInterceptor : Runnable {
override fun run() {
// code
}
}

C. class MyInterceptor : Interceptor {
override fun intercept(chain: Interceptor.Chain): Response {
return chain.proceed(chain.request())
}
}

D. class MyInterceptor {
fun intercept(chain: Interceptor.Chain) {
// code
}
}

Solution

1. Step 1: Check the interface implementation

   OkHttp interceptors implement the Interceptor interface and override the intercept() method with the correct signature.

2. Step 2: Validate method signature and return type

   The intercept method must take Interceptor.Chain and return a Response object, as shown in class MyInterceptor : Interceptor { override fun intercept(chain: Interceptor.Chain): Response { return chain.proceed(chain.request()) } }.

3. Final Answer:

   Correct interceptor implementation with override and proper method signature -> Option C

4. Quick Check:

   Interceptor class implements Interceptor interface [OK]

Hint: Interceptor must override intercept(chain: Interceptor.Chain): Response [OK]

Common Mistakes:

• Not implementing Interceptor interface
• Wrong method signature or missing return type
• Using unrelated interfaces like Runnable

3. Given this interceptor code snippet, what will be the value of the "User-Agent" header in the outgoing request?

class UserAgentInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val newRequest = chain.request().newBuilder()
      .header("User-Agent", "MyAppAgent")
      .build()
    return chain.proceed(newRequest)
  }
}

medium

A. The request will have "User-Agent" header set to "MyAppAgent"

B. The request will have no "User-Agent" header

C. The request will have "User-Agent" header set to the default OkHttp value

D. The code will cause a compile error

Solution

1. Step 1: Analyze header modification

   The interceptor creates a new request builder from the original request and sets the "User-Agent" header explicitly to "MyAppAgent".

2. Step 2: Understand header effect

   Setting the header with .header() replaces any existing "User-Agent" header with the new value.

3. Final Answer:

   "User-Agent" header is set to "MyAppAgent" in the outgoing request -> Option A

4. Quick Check:

   Header set by interceptor = "MyAppAgent" [OK]

Hint: .header() replaces existing header values [OK]

Common Mistakes:

• Assuming header is appended instead of replaced
• Thinking default headers remain unchanged
• Confusing compile errors with runtime behavior

4. Identify the error in this interceptor code snippet:

class LoggingInterceptor : Interceptor {
  override fun intercept(chain: Interceptor.Chain): Response {
    val request = chain.request()
    println("Request URL: ${request.url}")
    val response = chain.proceed(request)
    println("Response code: ${response.code}")
  }

medium

A. Missing return statement for the Response object

B. Incorrect method name; should be interceptRequest

C. Cannot use println inside intercept method

D. Request object is not created properly

Solution

1. Step 1: Check method return type

   The intercept method must return a Response object, but this code does not return anything.

2. Step 2: Verify other code parts

   Method name and usage of println are valid; request creation is correct.

3. Final Answer:

   Missing return statement for the Response object -> Option A

4. Quick Check:

   intercept() must return Response [OK]

Hint: Always return the Response from intercept() [OK]

Common Mistakes:

• Forgetting to return the response
• Renaming intercept method incorrectly
• Thinking println is disallowed in intercept

5. You want to create an OkHttp interceptor that retries a request up to 3 times if the response code is 500 (server error). Which approach correctly implements this behavior?

hard

A. Use a separate thread to retry the request after intercept() returns

B. Throw an exception inside intercept() when response code is 500 to trigger retry

C. Modify the request headers to include "Retry: 3" and proceed once

D. Override intercept() to loop up to 3 times calling chain.proceed(request) until response.code != 500 or max retries reached

Solution

1. Step 1: Understand retry logic in intercept()

   Retries must happen inside intercept() by calling chain.proceed(request) multiple times if needed.

2. Step 2: Evaluate options for retry implementation

   Override intercept() to loop up to 3 times calling chain.proceed(request) until response.code != 500 or max retries reached correctly loops and retries up to 3 times checking response code. The other options do not implement retry correctly or rely on unsupported mechanisms.

3. Final Answer:

   Loop inside intercept() to retry request up to 3 times on 500 response -> Option D

4. Quick Check:

   Retry logic = loop in intercept() calling chain.proceed() [OK]

Hint: Retries must be done inside intercept() by repeating chain.proceed() [OK]

Common Mistakes:

• Trying to retry outside intercept() method
• Assuming headers control retry automatically
• Throwing exceptions to trigger retry without handling