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
• WireMock Stubbing Guide

• references/advanced-examples.md

  - Error scenarios, body verification, timeouts