Unit Testing REST APIs with WireMock

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>