Question

我熟悉working with images。我retrieve/read an image from a URL，其中URL没有文件扩展名。然后我希望write/save the image到本地存储，但我必须指定图像文件扩展名（即JPG，PNG等），我无法通过BufferedImage检索其扩展名。

有人可以指出它是如何完成的吗？任何其他方法都可以。

Answer 1

使用ImageReader.getFormatName()

您可以使用ImageIO.getImageReaders(Object input)获取文件的图像阅读器。

我自己没有测试过，但你可以尝试一下：

ImageInputStream iis = ImageIO.createImageInputStream(file);

Iterator<ImageReader> imageReaders = ImageIO.getImageReaders(iis);

while (imageReaders.hasNext()) {
    ImageReader reader = (ImageReader) imageReaders.next();
    System.out.printf("formatName: %s%n", reader.getFormatName());
}

Answer 2

如果对象是URL，则使用ImageIO.createImageInputStream（obj）的建议将不起作用。

另一种方法是使用URLConnection.guessContentTypeFromStream（InputStream流）方法。此方法通过检查流的前12个字节来猜测内容类型。

使用此方法的一个复杂因素是它需要标记支持给定的流参数，并且不支持java url.openStream（）返回的流。

此外，如果你想确定内容类型并将图像下载到BufferedImage，那么最好是解决方案只下载一次内容（而不是两次通过，一次确定内容类型和第二次下载图片。）

一种解决方案是使用PushbackInputStream。 PushbackInputStream可用于下载第一个初始字节以确定内容类型。然后可以在流上推回字节，以便ImageIO.read（流）可以完整地读取流。

可能的解决方案：

// URLConnection.guessContentTypeFromStream only needs the first 12 bytes, but
// just to be safe from future java api enhancements, we'll use a larger number
int pushbackLimit = 100;
InputStream urlStream = url.openStream();
PushbackInputStream pushUrlStream = new PushbackInputStream(urlStream, pushbackLimit);
byte [] firstBytes = new byte[pushbackLimit];
// download the first initial bytes into a byte array, which we will later pass to 
// URLConnection.guessContentTypeFromStream  
pushUrlStream.read(firstBytes);
// push the bytes back onto the PushbackInputStream so that the stream can be read 
// by ImageIO reader in its entirety
pushUrlStream.unread(firstBytes);

String imageType = null;
// Pass the initial bytes to URLConnection.guessContentTypeFromStream in the form of a
// ByteArrayInputStream, which is mark supported.
ByteArrayInputStream bais = new ByteArrayInputStream(firstBytes);
String mimeType = URLConnection.guessContentTypeFromStream(bais);
if (mimeType.startsWith("image/"))
    imageType = mimeType.substring("image/".length());
// else handle failure here

// read in image
BufferedImage inputImage = ImageIO.read(pushUrlStream);

Answer 3

如果您从网址获取图片，则表示您可以通过InputStream访问该图片。您可以使用ImageIO来获取图像类型（格式），并使用以下代码同时创建BufferedImage。

public static BufferedImageWrapper getImageAndTypeFromInputStream(InputStream is) {

    String format = null;
    BufferedImage bufferedimage = null;
    try (ImageInputStream iis = ImageIO.createImageInputStream(is);) {

      Iterator<ImageReader> readers = ImageIO.getImageReaders(iis);

      if (readers.hasNext()) {

        ImageReader reader = readers.next();
        format = reader.getFormatName();
        reader.setInput(iis);
        bufferedimage = reader.read(0);
      }
    } catch (IOException e) {
      logger.error("ERROR DETERMINING IMAGE TYPE!!!", e);
    }

    return new BufferedImageWrapper(format, bufferedimage);
  }

  public static class BufferedImageWrapper {

    private final String imageType;
    private final BufferedImage bufferedimage;

    /**
     * Constructor
     *
     * @param imageType
     * @param bufferedimage
     */
    public BufferedImageWrapper(String imageType, BufferedImage bufferedimage) {
      this.imageType = imageType;
      this.bufferedimage = bufferedimage;
    }

    public String getImageType() {

      return imageType;
    }

    public BufferedImage getBufferedimage() {

      return bufferedimage;
    }

  }

Answer 4

这可以通过输入URL对象（即图像）并返回文件扩展名

来实现

它需要初始下载到java tmp目录，然后在ImageReader尝试获取图像类型后删除它

public String getImageFileExtFromUrl(URL urlObject) throws URISyntaxException, IOException{
    System.out.println("IN DOWNLOAD FILE FROM URL METHOD");
    String tmpFolder = System.getProperty("java.io.tmpdir");
    String tmpFileStr = tmpFolder + "/" + new Date().getTime();
    Files.copy(urlObject.openStream(), Paths.get(tmpFileStr), StandardCopyOption.REPLACE_EXISTING);
    File download = new File(tmpFileStr);
    System.out.println("FILE DOWNLOAD EXISTS: " + download.exists() );
    try{
        ImageInputStream iis = ImageIO.createImageInputStream(download);
        Iterator<ImageReader> iter = ImageIO.getImageReaders(iis);

        ImageReader reader = iter.next();
        String formatName = reader.getFormatName();
        System.out.println("FOUND IMAGE FORMAT :" + formatName);
        iis.close();
        return formatName;
    }catch(Exception e){
        e.printStackTrace();
    }finally{
        Files.delete(Paths.get(tmpFileStr));
    }
    return null;



}

Java使用URL中的BufferedImage获取图像扩展名/类型

4 个答案:

这可以通过输入URL对象（即图像）并返回文件扩展名