How do you test HTTP handlers in Go without starting a real server?

Go's net/http/httptest package provides httptest.NewRecorder() (a fake ResponseWriter) and httptest.NewServer() (a real TCP server on a random port) for testing HTTP code without network overhead.

import (
    "net/http/httptest"
    "testing"
)

// Unit test: test handler in isolation with mock dependencies
func TestGetUserHandler(t *testing.T) {
    // Setup mock repository
    mockRepo := &MockUserRepo{
        FindByIDFunc: func(ctx context.Context, id int) (*User, error) {
            return &User{ID: id, Name: "Alice"}, nil
        },
    }
    svc := NewUserService(mockRepo)
    h   := NewUserHandler(svc)

    // Build a request
    req := httptest.NewRequest(http.MethodGet, "/users/1", nil)
    req.SetPathValue("id", "1") // Go 1.22: set path params

    // Capture the response
    rec := httptest.NewRecorder()
    h.GetUser(rec, req)

    // Assert
    res := rec.Result()
    if res.StatusCode != http.StatusOK {
        t.Errorf("expected 200, got %d", res.StatusCode)
    }
    if ct := res.Header.Get("Content-Type"); ct != "application/json" {
        t.Errorf("expected application/json, got %s", ct)
    }
    var user User
    json.NewDecoder(res.Body).Decode(&user)
    if user.Name != "Alice" {
        t.Errorf("expected Alice, got %s", user.Name)
    }
}

// Integration test: use httptest.NewServer for real HTTP round-trip
func TestAPIIntegration(t *testing.T) {
    srv := httptest.NewServer(buildRouter())
    defer srv.Close() // frees the port

    resp, err := http.Get(srv.URL + "/health")
    if err != nil {
        t.Fatal(err)
    }
    defer resp.Body.Close()
    if resp.StatusCode != http.StatusOK {
        t.Errorf("expected 200, got %d", resp.StatusCode)
    }
}

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