/unit-test-wiremock-rest-api

name: unit-test-wiremock-rest-api
description: Provides patterns for unit testing external REST APIs using WireMock. Stubs API responses, verifies request details, simulates failures (timeouts, 4xx/5xx errors), and validates HTTP client behavior without real network calls. Use when testing service integrations with external APIs or mocking HTTP endpoints.
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Unit Testing REST APIs with WireMock

Overview

Patterns for testing external REST API integrations with WireMock: stubbing responses, verifying requests, error scenarios, and fast tests without network dependencies.

When to Use

• Testing services calling external REST APIs
• Stubbing HTTP responses for predictable test behavior
• Testing error scenarios (timeouts, 5xx errors, malformed responses)
• Verifying request details (headers, query params, request body)

Instructions

1. **Add dependency**: WireMock in test scope (Maven/Gradle) 2. **Register extension**: `@RegisterExtension WireMockExtension` with `dynamicPort()` 3. **Configure client**: Use `wireMock.getRuntimeInfo().getHttpBaseUrl()` as base URL 4. **Stub responses**: `stubFor()` with request matching (URL, headers, body) 5. **Execute and assert**: Call service methods, validate results with AssertJ 6. **Verify requests**: `verify()` to ensure correct API usage

**If stub not matching**: Check URL encoding, header names, use `urlEqualTo` for query params.

**If tests hanging**: Configure connection timeouts in HTTP client; use `withFixedDelay()` for timeout simulation.

**If port conflicts**: Always use `wireMockConfig().dynamicPort()`.

Examples

Maven Dependencies

<dependency>
  <groupId>org.wiremock</groupId>
  <artifactId>wiremock</artifactId>
  <version>3.4.1</version>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.assertj</groupId>
  <artifactId>assertj-core</artifactId>
  <scope>test</scope>
</dependency>

Basic Stubbing and Verification

import com.github.tomakehurst.wiremock.junit5.WireMockExtension;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.RegisterExtension;
import static com.github.tomakehurst.wiremock.client.WireMock.*;
import static org.assertj.core.api.Assertions.assertThat;

class ExternalWeatherServiceTest {

  @RegisterExtension
  static WireMockExtension wireMock = WireMockExtension.newInstance()
    .options(wireMockConfig().dynamicPort())
    .build();

  @Test
  void shouldFetchWeatherDataFromExternalApi() {
    wireMock.stubFor(get(urlEqualTo("/weather?city=London"))
      .withHeader("Accept", containing("application/json"))
      .willReturn(aResponse()
        .withStatus(200)
        .withHeader("Content-Type", "application/json")
        .withBody("{\"city\":\"London\",\"temperature\":15,\"condition\":\"Cloudy\"}")));

    String baseUrl = wireMock.getRuntimeInfo().getHttpBaseUrl();
    WeatherApiClient client = new WeatherApiClient(baseUrl);
    WeatherData weather = client.getWeather("London");

    assertThat(weather.getCity()).isEqualTo("London");
    assertThat(weather.getTemperature()).isEqualTo(15);

    wireMock.verify(getRequestedFor(urlEqualTo("/weather?city=London"))
      .withHeader("Accept", containing("application/json")));
  }
}

See `references/advanced-examples.md` for error scenarios, body verification, timeout simulation, and stateful testing.

Best Practices

• **Dynamic port**: Prevents conflicts in parallel test execution
• **Verify requests**: Ensures correct API usage by the client
• **Test errors**: Cover timeouts, 4xx, 5xx scenarios
• **Focused stubs**: One concern per test
• **Auto-reset**: `@RegisterExtension` resets WireMock between tests
• **Never call real APIs**: Always stub third-party endpoints

Constraints and Warnings

• **Dynamic ports required**: Fixed ports cause parallel execution conflicts
• **HTTPS testing**: Configure WireMock TLS settings if testing TLS connections
• **Stub precedence**: More specific stubs take priority over general ones
• **Performance**: WireMock adds overhead; mock at client layer for faster tests
• **API changes**: Keep stubs synchronized with actual API contracts

References

• [WireMock Documentation](https://wiremock.org/)
• [WireMock Stubbing Guide](https://wiremock.org/docs/stubbing/)
• `references/advanced-examples.md` - Error scenarios, body verification, timeouts