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unit-test-wiremock-rest-api

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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