Axum Testing Best Practices

Comprehensive guide for unit testing and integration testing Axum web applications using idiomatic Rust patterns.

Test Dependencies

Add these to your Cargo.toml for testing:

toml

[dev-dependencies]
tokio = { version = "1", features = ["full", "test-util"] }
tower = { version = "0.5", features = ["util"] }
http-body-util = "0.1"
serde_json = "1"

# Optional but recommended
axum-test = "16"  # High-level test client (alternative to manual tower::ServiceExt)

Testing Approaches

Axum supports two primary testing strategies:

Approach	Use Case	Complexity	Speed
Handler Unit Tests	Test business logic in isolation	Low	Fast
Integration Tests	Test full request/response cycle	Medium	Medium

Handler Unit Tests

Test handlers directly by calling them as async functions. Best for testing business logic without HTTP overhead.

Basic Handler Test

rust

use axum::{extract::State, Json};
use serde::{Deserialize, Serialize};

#[derive(Clone)]
struct AppState {
    db: MockDb,
}

#[derive(Serialize)]
struct User {
    id: u64,
    name: String,
}

async fn get_user(
    State(state): State<AppState>,
    axum::extract::Path(id): axum::extract::Path<u64>,
) -> Result<Json<User>, StatusCode> {
    state.db.find_user(id)
        .await
        .map(Json)
        .ok_or(StatusCode::NOT_FOUND)
}

#[cfg(test)]
mod tests {
    use super::*;
    use axum::extract::Path;

    #[tokio::test]
    async fn test_get_user_success() {
        // Arrange
        let mock_db = MockDb::with_user(User { id: 1, name: "Alice".into() });
        let state = AppState { db: mock_db };

        // Act
        let result = get_user(
            State(state),
            Path(1),
        ).await;

        // Assert
        let Json(user) = result.unwrap();
        assert_eq!(user.name, "Alice");
    }

    #[tokio::test]
    async fn test_get_user_not_found() {
        let state = AppState { db: MockDb::empty() };

        let result = get_user(State(state), Path(999)).await;

        assert_eq!(result.unwrap_err(), StatusCode::NOT_FOUND);
    }
}

Testing Handlers with JSON Body

rust

use axum::Json;

#[derive(Deserialize)]
struct CreateUser {
    name: String,
    email: String,
}

async fn create_user(Json(payload): Json<CreateUser>) -> impl IntoResponse {
    (StatusCode::CREATED, Json(json!({"id": 1, "name": payload.name})))
}

#[tokio::test]
async fn test_create_user() {
    let payload = CreateUser {
        name: "Bob".into(),
        email: "bob@example.com".into(),
    };

    let (status, Json(body)) = create_user(Json(payload)).await;

    assert_eq!(status, StatusCode::CREATED);
    assert_eq!(body["name"], "Bob");
}

Integration Tests with `tower::ServiceExt`

Test the full HTTP request/response cycle using tower::ServiceExt::oneshot. This is the recommended approach for most Axum tests.

Basic Integration Test

rust

use axum::{
    body::Body,
    http::{Request, StatusCode},
    routing::get,
    Router,
};
use http_body_util::BodyExt;
use tower::ServiceExt;  // Provides `oneshot` method

fn create_app() -> Router {
    Router::new()
        .route("/health", get(|| async { "OK" }))
        .route("/users/{id}", get(get_user))
}

#[tokio::test]
async fn test_health_check() {
    let app = create_app();

    let response = app
        .oneshot(
            Request::builder()
                .uri("/health")
                .body(Body::empty())
                .unwrap()
        )
        .await
        .unwrap();

    assert_eq!(response.status(), StatusCode::OK);

    let body = response.into_body().collect().await.unwrap().to_bytes();
    assert_eq!(&body[..], b"OK");
}

Testing POST Requests with JSON

rust

use axum::{
    body::Body,
    http::{header, Request, StatusCode},
    routing::post,
    Router,
};
use serde_json::json;

#[tokio::test]
async fn test_create_user_integration() {
    let app = Router::new().route("/users", post(create_user));

    let payload = json!({
        "name": "Alice",
        "email": "alice@example.com"
    });

    let response = app
        .oneshot(
            Request::builder()
                .method("POST")
                .uri("/users")
                .header(header::CONTENT_TYPE, "application/json")
                .body(Body::from(serde_json::to_string(&payload).unwrap()))
                .unwrap()
        )
        .await
        .unwrap();

    assert_eq!(response.status(), StatusCode::CREATED);
}

Testing with Application State

rust

use axum::extract::State;

#[derive(Clone)]
struct AppState {
    db_pool: PgPool,
    cache: Arc<RwLock<HashMap<String, String>>>,
}

fn create_test_app(state: AppState) -> Router {
    Router::new()
        .route("/users", get(list_users))
        .route("/users/{id}", get(get_user))
        .with_state(state)
}

#[tokio::test]
async fn test_with_state() {
    // Create test state with mocked dependencies
    let state = AppState {
        db_pool: create_test_pool().await,
        cache: Arc::new(RwLock::new(HashMap::new())),
    };

    let app = create_test_app(state);

    let response = app
        .oneshot(Request::builder().uri("/users").body(Body::empty()).unwrap())
        .await
        .unwrap();

    assert_eq!(response.status(), StatusCode::OK);
}

Testing with TestClient

For more ergonomic testing, use axum::test_helpers::TestClient (internal) or the axum-test crate:

Using axum-test Crate (Recommended)

rust

use axum_test::TestServer;

#[tokio::test]
async fn test_with_test_server() {
    let app = create_app();
    let server = TestServer::new(app).unwrap();

    // GET request
    let response = server.get("/health").await;
    response.assert_status_ok();
    response.assert_text("OK");

    // POST with JSON
    let response = server
        .post("/users")
        .json(&json!({"name": "Alice"}))
        .await;
    response.assert_status(StatusCode::CREATED);
}

Manual TestClient Pattern

rust

use reqwest::Client;
use std::net::SocketAddr;
use tokio::net::TcpListener;

async fn spawn_test_server(app: Router) -> SocketAddr {
    let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
    let addr = listener.local_addr().unwrap();

    tokio::spawn(async move {
        axum::serve(listener, app).await.unwrap();
    });

    addr
}

#[tokio::test]
async fn test_full_server() {
    let app = create_app();
    let addr = spawn_test_server(app).await;

    let client = Client::new();
    let response = client
        .get(format!("http://{}/health", addr))
        .send()
        .await
        .unwrap();

    assert_eq!(response.status(), 200);
}

Mocking Dependencies

Trait-Based Mocking

Define traits for dependencies to enable mocking:

rust

use async_trait::async_trait;

#[async_trait]
pub trait UserRepository: Send + Sync {
    async fn find_by_id(&self, id: u64) -> Option<User>;
    async fn create(&self, user: CreateUser) -> Result<User, DbError>;
}

// Production implementation
pub struct PgUserRepository {
    pool: PgPool,
}

#[async_trait]
impl UserRepository for PgUserRepository {
    async fn find_by_id(&self, id: u64) -> Option<User> {
        sqlx::query_as!(User, "SELECT * FROM users WHERE id = $1", id as i64)
            .fetch_optional(&self.pool)
            .await
            .ok()
            .flatten()
    }

    async fn create(&self, user: CreateUser) -> Result<User, DbError> {
        // ... implementation
    }
}

// Mock implementation for tests
#[cfg(test)]
pub struct MockUserRepository {
    users: std::sync::Mutex<Vec<User>>,
}

#[cfg(test)]
impl MockUserRepository {
    pub fn new() -> Self {
        Self { users: std::sync::Mutex::new(vec![]) }
    }

    pub fn with_users(users: Vec<User>) -> Self {
        Self { users: std::sync::Mutex::new(users) }
    }
}

#[cfg(test)]
#[async_trait]
impl UserRepository for MockUserRepository {
    async fn find_by_id(&self, id: u64) -> Option<User> {
        self.users.lock().unwrap().iter().find(|u| u.id == id).cloned()
    }

    async fn create(&self, user: CreateUser) -> Result<User, DbError> {
        let new_user = User { id: 1, name: user.name, email: user.email };
        self.users.lock().unwrap().push(new_user.clone());
        Ok(new_user)
    }
}

Using State with Trait Objects

rust

#[derive(Clone)]
pub struct AppState {
    pub users: Arc<dyn UserRepository>,
}

fn create_app(state: AppState) -> Router {
    Router::new()
        .route("/users/{id}", get(get_user))
        .with_state(state)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[tokio::test]
    async fn test_with_mock_repository() {
        let mock_repo = MockUserRepository::with_users(vec![
            User { id: 1, name: "Alice".into(), email: "alice@test.com".into() }
        ]);

        let state = AppState {
            users: Arc::new(mock_repo),
        };

        let app = create_app(state);

        let response = app
            .oneshot(Request::builder().uri("/users/1").body(Body::empty()).unwrap())
            .await
            .unwrap();

        assert_eq!(response.status(), StatusCode::OK);
    }
}

Testing Custom Extractors

Testing FromRequestParts Extractors

rust

use axum::{
    async_trait,
    extract::FromRequestParts,
    http::{request::Parts, StatusCode},
};

pub struct CurrentUser(pub User);

#[async_trait]
impl<S> FromRequestParts<S> for CurrentUser
where
    S: Send + Sync,
{
    type Rejection = StatusCode;

    async fn from_request_parts(parts: &mut Parts, _state: &S) -> Result<Self, Self::Rejection> {
        let auth_header = parts
            .headers
            .get("Authorization")
            .and_then(|v| v.to_str().ok())
            .ok_or(StatusCode::UNAUTHORIZED)?;

        // Validate token and get user...
        Ok(CurrentUser(User { id: 1, name: "Test".into() }))
    }
}

#[tokio::test]
async fn test_current_user_extractor() {
    async fn handler(CurrentUser(user): CurrentUser) -> String {
        user.name
    }

    let app = Router::new().route("/me", get(handler));

    // Test with valid auth
    let response = app.clone()
        .oneshot(
            Request::builder()
                .uri("/me")
                .header("Authorization", "Bearer valid-token")
                .body(Body::empty())
                .unwrap()
        )
        .await
        .unwrap();

    assert_eq!(response.status(), StatusCode::OK);

    // Test without auth
    let response = app
        .oneshot(Request::builder().uri("/me").body(Body::empty()).unwrap())
        .await
        .unwrap();

    assert_eq!(response.status(), StatusCode::UNAUTHORIZED);
}

Testing Middleware

Testing Layer Middleware

rust

use axum::middleware::{self, Next};
use axum::response::Response;
use axum::http::Request;

async fn auth_middleware(
    req: Request<Body>,
    next: Next,
) -> Result<Response, StatusCode> {
    let auth_header = req.headers()
        .get("Authorization")
        .and_then(|v| v.to_str().ok());

    match auth_header {
        Some(token) if token.starts_with("Bearer ") => Ok(next.run(req).await),
        _ => Err(StatusCode::UNAUTHORIZED),
    }
}

#[tokio::test]
async fn test_auth_middleware() {
    let app = Router::new()
        .route("/protected", get(|| async { "secret" }))
        .layer(middleware::from_fn(auth_middleware));

    // Without auth header
    let response = app.clone()
        .oneshot(Request::builder().uri("/protected").body(Body::empty()).unwrap())
        .await
        .unwrap();
    assert_eq!(response.status(), StatusCode::UNAUTHORIZED);

    // With valid auth header
    let response = app
        .oneshot(
            Request::builder()
                .uri("/protected")
                .header("Authorization", "Bearer valid-token")
                .body(Body::empty())
                .unwrap()
        )
        .await
        .unwrap();
    assert_eq!(response.status(), StatusCode::OK);
}

Testing Error Responses

rust

use axum::response::{IntoResponse, Response};
use axum::Json;
use serde::Serialize;

#[derive(Debug, Serialize)]
struct ErrorResponse {
    error: String,
    message: String,
}

enum AppError {
    NotFound,
    BadRequest(String),
    Internal(anyhow::Error),
}

impl IntoResponse for AppError {
    fn into_response(self) -> Response {
        let (status, error_response) = match self {
            AppError::NotFound => (
                StatusCode::NOT_FOUND,
                ErrorResponse { error: "not_found".into(), message: "Resource not found".into() }
            ),
            AppError::BadRequest(msg) => (
                StatusCode::BAD_REQUEST,
                ErrorResponse { error: "bad_request".into(), message: msg }
            ),
            AppError::Internal(_) => (
                StatusCode::INTERNAL_SERVER_ERROR,
                ErrorResponse { error: "internal_error".into(), message: "An error occurred".into() }
            ),
        };
        (status, Json(error_response)).into_response()
    }
}

#[tokio::test]
async fn test_error_responses() {
    async fn failing_handler() -> Result<(), AppError> {
        Err(AppError::NotFound)
    }

    let app = Router::new().route("/fail", get(failing_handler));

    let response = app
        .oneshot(Request::builder().uri("/fail").body(Body::empty()).unwrap())
        .await
        .unwrap();

    assert_eq!(response.status(), StatusCode::NOT_FOUND);

    let body = response.into_body().collect().await.unwrap().to_bytes();
    let error: ErrorResponse = serde_json::from_slice(&body).unwrap();
    assert_eq!(error.error, "not_found");
}

Testing WebSockets

rust

use axum::extract::ws::{Message, WebSocket, WebSocketUpgrade};
use tokio_tungstenite::connect_async;

async fn ws_handler(ws: WebSocketUpgrade) -> impl IntoResponse {
    ws.on_upgrade(handle_socket)
}

async fn handle_socket(mut socket: WebSocket) {
    while let Some(Ok(msg)) = socket.recv().await {
        if let Message::Text(text) = msg {
            socket.send(Message::Text(format!("Echo: {}", text).into())).await.ok();
        }
    }
}

#[tokio::test]
async fn test_websocket() {
    let app = Router::new().route("/ws", get(ws_handler));

    let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
    let addr = listener.local_addr().unwrap();

    tokio::spawn(async move {
        axum::serve(listener, app).await.unwrap();
    });

    let (mut ws_stream, _) = connect_async(format!("ws://{}/ws", addr))
        .await
        .unwrap();

    ws_stream.send(Message::Text("Hello".into())).await.unwrap();

    let msg = ws_stream.next().await.unwrap().unwrap();
    assert_eq!(msg, Message::Text("Echo: Hello".into()));
}

Test Organization

Recommended Project Structure

code

src/
├── handlers/
│   ├── mod.rs
│   └── users.rs       # Handler functions
├── routes/
│   └── mod.rs         # Router configuration
├── state.rs           # AppState definition
└── lib.rs

tests/
├── common/
│   └── mod.rs         # Shared test utilities
├── handlers/
│   └── users_test.rs  # Unit tests for handlers
└── integration/
    └── api_test.rs    # Full integration tests

Shared Test Utilities

rust

// tests/common/mod.rs
use crate::AppState;

pub async fn create_test_state() -> AppState {
    AppState {
        db: MockDb::new(),
        cache: Arc::new(RwLock::new(HashMap::new())),
    }
}

pub fn create_test_app() -> Router {
    // Create app with test configuration
}

pub async fn extract_body_json<T: DeserializeOwned>(response: Response<Body>) -> T {
    let body = response.into_body().collect().await.unwrap().to_bytes();
    serde_json::from_slice(&body).unwrap()
}

Best Practices

DO

•Use tower::ServiceExt::oneshot for integration tests (no server startup overhead)
•Create trait abstractions for external dependencies (databases, APIs)
•Test error paths and edge cases, not just happy paths
•Use #[tokio::test] for all async tests
•Keep test state isolated - each test should set up its own state
•Test middleware in isolation before testing with routes

DON'T

•Don't start a real server for every test (use oneshot instead)
•Don't share mutable state between tests without proper synchronization
•Don't test private implementation details - test public API behavior
•Don't forget to test authentication/authorization edge cases
•Don't ignore response body validation - assert on both status AND body

Testing Checklist

• Handler returns correct status codes
• Handler returns correct response body structure
• Handler handles invalid input gracefully
• Handler handles missing/unauthorized requests
• Middleware correctly modifies requests/responses
• Custom extractors reject invalid data
• Error responses follow consistent format
• Database operations are properly mocked
• Concurrent requests are handled correctly

Axum Testing Best Practices

Test Dependencies

Testing Approaches

Handler Unit Tests

Basic Handler Test

Testing Handlers with JSON Body

Integration Tests with tower::ServiceExt

Basic Integration Test

Testing POST Requests with JSON

Testing with Application State

Testing with TestClient

Using axum-test Crate (Recommended)

Manual TestClient Pattern

Mocking Dependencies

Trait-Based Mocking

Using State with Trait Objects

Testing Custom Extractors

Testing FromRequestParts Extractors

Testing Middleware

Testing Layer Middleware

Testing Error Responses

Testing WebSockets

Test Organization

Recommended Project Structure

Shared Test Utilities

Best Practices

DO

DON'T

Testing Checklist

Integration Tests with `tower::ServiceExt`