使用 Junit 5 测试模拟函数

Question

我需要测试这个函数，因为错误"" Width (0) and height (0) cannot be <= 0 "。函数是这样的：

public class StreamImage {
    private static final int MAX_WIDTH = 100 ;
    private static final int MAX_HEIGHT = 100;

    private AmazonS3 s3Client;

    public StreamImage(AmazonS3 s3Client) {
        this.s3Client = s3Client;
    }


    public InputStream getStreamImage(String srcBucket, String srcKey){
        S3Object s3Object = s3Client.getObject(new GetObjectRequest(
                srcBucket, srcKey));
        return s3Object.getObjectContent();
    }

    public Contenitore rielaborationImmage(String srcBucket, String srcKey, String imageType) throws IOException {
        InputStream objectData = getStreamImage(srcBucket, srcKey);
        BufferedImage srcImage = ImageIO.read(objectData);
        int srcHeight = srcImage.getHeight();
        int srcWidth = srcImage.getWidth();
        float scalingFactor = Math.min(MAX_WIDTH / srcWidth, MAX_HEIGHT / srcHeight);
        int width = (int) (scalingFactor * srcWidth);
        int height = (int) (scalingFactor * srcHeight);

        BufferedImage resizedImage = new BufferedImage(width, height,
                BufferedImage.TYPE_INT_RGB);
        Graphics2D g = resizedImage.createGraphics();
        // Fill with white before applying semi-transparent (alpha) images
        g.setPaint(Color.white);
        g.fillRect(0, 0, width, height);
        // Simple bilinear resize
        g.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                RenderingHints.VALUE_INTERPOLATION_BILINEAR);
        g.drawImage(srcImage, 0, 0, width, height, null);
        g.dispose();

        // Re-encode image to target format
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ImageIO.write(resizedImage, imageType, os);
        InputStream is = new ByteArrayInputStream(os.toByteArray());
        // Set Content-Length and Content-Type
        ObjectMetadata meta = new ObjectMetadata();
        meta.setContentLength(os.size());
        if ("jpeg".equals(imageType)) {
            meta.setContentType("image/jpeg");
        }
        if ("png".equals(imageType)) {
            meta.setContentType("image/png");
        }
        Contenitore is_n= new Contenitore( is, meta);
        return is_n;
    }
}

我想用 Junit 测试它，但我必须模拟函数“getStreamImage”，我该怎么做？还有其他方法可以测试吗？