如何转换清单 <String> Java中将字符串转换为BufferedImage

Question

I want to convert a Base64 String array to a BufferedImage . The approach I tried to accomplish:

static BufferedImage decodeToImage(List<String> imageStrings) {
    BufferedImage image = null;
    ByteBuffer imageByteBuffer = ByteBuffer.allocate(500000000);
    try {
        BASE64Decoder decoder = new BASE64Decoder();
        for (String imageString : imageStrings) {
            imageByteBuffer.put(imageString.getBytes());
        }

        ByteArrayInputStream bis = new ByteArrayInputStream(imageByteBuffer.array());
        image = ImageIO.read(bis);
        bis.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
    return image;
}

I want to return a BufferedImage but it returns null.

Answer 1

You can just use this:

public static BufferedImage decodeToImage(List<String> imageStrings) {
    try {
        byte[] bytes = Base64.getDecoder().decode(String.join("", imageStrings));
        return ImageIO.read(new ByteArrayInputStream(bytes));
    } catch (IOException e) {
        throw new UncheckedIOException(e);
    }
}

Answer 2

Have you looked at what are you getting in?

Cause this code

public static void main(String[] args) {
    String base64String = "data:image/png;base64,iVBORw0...=";
    String[] strings = base64String.split(",");
    List<String> param = new ArrayList<String>();
    param.add(strings[1]);
    BufferedImage bi = decodeToImage(param);
    System.out.println("BufferedImage: " + bi);
}

static BufferedImage decodeToImage(List<String> imageStrings) {
    BufferedImage image = null;
    ByteBuffer imageByteBuffer = ByteBuffer.allocate(500000000);
    try {
        for (String imageString : imageStrings) {
          imageByteBuffer.put(DatatypeConverter.parseBase64Binary(imageString));
        }
        ByteArrayInputStream bis = new ByteArrayInputStream(imageByteBuffer.array());
        image = ImageIO.read(bis);
        bis.close();
    } catch (Exception e) {
        System.out.println("error?");
        e.printStackTrace();
    }
    return image;
}

Will print

BufferedImage: BufferedImage@7eda2dbb: type = 6 ColorModel: #pixelBits = 32 numComponents = 4 color space = java.awt.color.ICC_ColorSpace@6576fe71 transparency = 3 has alpha = true isAlphaPre = false ByteInterleavedRaster: width = 302 height = 232 #numDataElements 4 dataOff[0] = 3

But if I just give it the whole base64 text, with the image info in front

 public static void main(String[] args) {
    String base64String = "data:image/png;base64,iVBORw0...=";
    String[] strings = base64String.split(",");
    List<String> param = new ArrayList<String>();
    param.add(base64String); //THIS IS THE DIFFERENT LINE
    BufferedImage bi = decodeToImage(param);
    System.out.println("BufferedImage: " + bi);
    }

It will print null

Replace base64String with a valid base 64 encoded png from

http://freeonlinetools24.com/base64-image

I cut it down in this example cause otherwise it becomes several thousand characters long.

Answer 3

Is the maximum length of a string in Java really a problem for your code? If not, I'd simplify your code like this (and fix the decoding bug):

static BufferedImage decodeToImage(String imageString) {
    try {
        BASE64Decoder decoder = new BASE64Decoder();    
        return ImageIO.read(new ByteArrayInputStream(decoder.decode(imageString)));
    } catch (IOException e) {
        e.printStackTrace();
    }

    return null;
}

Or, if you really have a problem with the length of the strings (Base64 does produce output longer than the input, so it may make sense in extreme cases, but verify this, as it otherwise complicates your code unnecessary), keep the list of strings. I'd simply skip the ByteBuffer and use a ByteArrayOutputStream instead, as it's much easier to work with.

static BufferedImage decodeToImage(List<String> imageStrings) {
    try {
        ByteArrayOuputStream buffer = new ByteArrayOuputStream();

        BASE64Decoder decoder = new BASE64Decoder();
        for (String imageString : imageStrings) {
            buffer.write(decoder.decode(imageString));
        }

        return ImageIO.read(new ByteArrayInputStream(buffer.toArray()));
    } catch (IOException e) {
        e.printStackTrace();
    }

    return null;
}

PS: While you should generally close streams, the close() method on ByteArrayInputStream and ByteArrayOutputStream are no-ops.

Answer 4

Buffers are little bit tricky to work with. Might be a good idea to take a look here: https://docs.oracle.com/javase/7/docs/api/java/nio/Buffer.html#flip()

As far as I can remember you have to prepare the buffer before you can read it; the position of the internal 'cursor' has to be set to the start position from where you then can read 'capacity' bytes. Well, as I said: As far as I can remember. It's more then twelve years ago I had to directly work with buffers.