Adapter Pattern in Java: Quick Example

^{Published on: 2025-03-05}

The adapter pattern promotes decoupling by abstracting away how classes work together.

Example

We want to retrieve stored images through our service and output them to the HttpServletResponse.getOutputStream in our controller.

@RestController
@RequestMapping("/image")
public class ImagesController {
    @GetMapping(value = "/image/{id}", produces = MediaType.IMAGE_PNG_VALUE)
    public void fetchImage(@PathVariable String id, HttpServletResponse response) {
        this.imageService.fetch(id, new HttpOutputAdapter(response));
    }
}

@Service
public class ImageService {
  public void fetch(String id, IOutputAdapter outputAdapter) {
    try {
      try (OutputStream out = outputAdapter.getOutputStream();
           InputStream in = imageRepo.fetch(id).getInputStream()) {
        byte[] buffer = new byte[1024];
        int readLen = 0;

        while ((readLen = in.read(buffer)) != -1) {
          out.write(buffer, 0, readLen);
        }
      }
    } catch (IOException e) {
      throw new ServiceException(String.format("Failed to retrieve the image with id %s.", id), e);
    }
  }
}

This is what the adapter looks like:

public interface IOutputAdapter {
  OutputStream getOutputStream() throws IOException;
}

public class HttpOutputAdapter implements IOutputAdapter {
  private final HttpServletResponse response;

  private HttpOutputAdapter(HttpServletResponse response) {
    this.response = response;
  }

  @Override
  public OutputStream getOutputStream() throws IOException {
    return response.getOutputStream();
  }
}