Boost Java Productivity: 4 Must‑Use Open‑Source Libraries (MapStruct, Retrofit, Faker, Wiremock)

Today I share several Java open‑source libraries that are very useful.

1. MapStruct

What does MapStruct do?

MapStruct is a code generator that creates Java object mappers based on annotations.

It can convert an object of type A to type B by configuring field mappings.

Why use it in projects?

In multi‑layer web projects we often need to map DTOs to BOs, which is tedious and error‑prone.

DTO (Data Transfer Object): object transferred from Service layer. BO (Business Object): object encapsulating business logic output from Service.

Traditional conversion is like copying many fields manually, which is cumbersome.

public class CarMapper {
    CarDto carDoToCarDto(Car car) {
        CarDto carDto = new CarDto();
        carDto.setCarId(car.getCarId());
        carDto.setWheel(car.getWheel());
        carDto.setCarType(car.getCarType());
        carDto.setCarColor(car.getCarColor());
        ...
    }
}

Before MapStruct we used Apache BeanUtils or Spring BeanUtils, which have performance issues and runtime errors.

MapStruct generates mapper classes at compile time, avoiding reflection and providing fast, compile‑time‑checked mappings.

Example:

@Mapper
public interface CarMapper {
    CarMapper INSTANCE = Mappers.getMapper(CarMapper.class);

    @Mapping(target = "seatCount", source = "numberOfSeats")
    CarDto carBoToCarDto(Car car);
}

Since it works at compile time, type mismatches are caught early, reducing runtime failures.

Repository: https://github.com/mapstruct/mapstruct

2. Retrofit

What does Retrofit do?

Retrofit is an HTTP client library for accessing third‑party HTTP services.

Why use it?

It simplifies URL construction and asynchronous callbacks, especially in non‑Spring projects that need to call many external services.

Traditional HttpClient requires manual URL building and thread‑pool management.

URIBuilder uriBuilder = new URIBuilder(uriBase);
uriBuilder.setParameter("a", "valuea");
uriBuilder.setParameter("b", "valueb");
uriBuilder.setParameter("c", "valuec");
// ...

Retrofit handles parameter binding and provides built‑in asynchronous call handling.

Retrofit retrofit = new Retrofit.Builder()
        .baseUrl("http://xxx.example.com/")
        .build();

public interface BlogService {
    @GET("blog/{id}")
    Call<ResponseBody> getBlog(@Path("id") int id);
}

BlogService service = retrofit.create(BlogService.class);
Call<ResponseBody> call = service.getBlog(2);
call.enqueue(new Callback<ResponseBody>() {
    @Override
    public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
        try {
            System.out.println(response.body().string());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    public void onFailure(Call<ResponseBody> call, Throwable t) {
        t.printStackTrace();
    }
});

Repository: https://github.com/square/retrofit

3. Faker

What does Faker do?

Faker generates realistic fake data such as names, addresses, etc., useful for testing.

Why use it?

It ensures data follows correct formats and can produce large volumes of data quickly, supporting custom generation rules.

Faker faker = new Faker();

String name = faker.name().fullName(); // Miss Samanta Schmidt
String firstName = faker.name().firstName(); // Emory
String lastName = faker.name().lastName(); // Barton

String streetAddress = faker.address().streetAddress(); // 60018 Sawayn Brooks Suite 449

With Faker, developers can create test users with a single line of code.

Repository: https://github.com/DiUS/java-faker

4. Wiremock

What does Wiremock do?

Wiremock is a testing framework that simulates external services.

Why use it?

It removes the dependency on third‑party services during unit testing, avoiding IP limits, rate limits, or downtime.

WireMock.stubFor(get(urlPathMatching("/aliyun/.*"))
        .willReturn(aResponse()
                .withStatus(200)
                .withHeader("Content-Type", APPLICATION_JSON)
                .withBody("\"testing-library\": \"WireMock\"")));

CloseableHttpClient httpClient = HttpClients.createDefault();
HttpGet request = new HttpGet(String.format("http://localhost:%s/aliyun/wiremock", port));
HttpResponse httpResponse = httpClient.execute(request);
String stringResponse = convertHttpResponseToString(httpResponse);

verify(getRequestedFor(urlEqualTo(ALIYUN_WIREMOCK_PATH)));
assertEquals(200, httpResponse.getStatusLine().getStatusCode());
assertEquals(APPLICATION_JSON, httpResponse.getFirstHeader("Content-Type").getValue());
assertEquals("\"testing-library\": \"WireMock\"", stringResponse);

Repository: https://github.com/wiremock/wiremock

Conclusion

Java’s rich ecosystem provides many ready‑made tools that save time and avoid reinventing the wheel.

You’re not the first to face these problems; the community has already built solutions for you.