Java BufferedImage JPG 压缩而不写入文件

Question

我已经看到several examples 通过写入文件从 Java BufferedImage 对象制作压缩的 JPG 图像，但是是否可以在不写入文件的情况下执行 JPG 压缩？也许通过写信给ByteArrayOutputStream，比如this？

ImageWriter jpgWriter = ImageIO.getImageWritersByFormatName("jpg").next();
ImageWriteParam jpgWriteParam = jpgWriter.getDefaultWriteParam();
jpgWriteParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
jpgWriteParam.setCompressionQuality(0.7f);

ImageOutputStream outputStream = createOutputStream();
jpgWriter.setOutput(outputStream);
IIOImage outputImage = new IIOImage(image, null, null);

// in this example, the JPG is written to file...
// jpgWriter.write(null, outputImage, jpgWriteParam);
// jpgWriter.dispose();

// ...but I want to compress without saving, such as
ByteArrayOutputStream compressed = ???

Answer 1

只需将您的ByteArrayOutputStream 传递给ImageIO.createImageOutputStream(...)，如下所示：

// The important part: Create in-memory stream
ByteArrayOutputStream compressed = new ByteArrayOutputStream();

try (ImageOutputStream outputStream = ImageIO.createImageOutputStream(compressed)) {

    // NOTE: The rest of the code is just a cleaned up version of your code

    // Obtain writer for JPEG format
    ImageWriter jpgWriter = ImageIO.getImageWritersByFormatName("JPEG").next();

    // Configure JPEG compression: 70% quality
    ImageWriteParam jpgWriteParam = jpgWriter.getDefaultWriteParam();
    jpgWriteParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
    jpgWriteParam.setCompressionQuality(0.7f);

    // Set your in-memory stream as the output
    jpgWriter.setOutput(outputStream);

    // Write image as JPEG w/configured settings to the in-memory stream
    // (the IIOImage is just an aggregator object, allowing you to associate
    // thumbnails and metadata to the image, it "does" nothing)
    jpgWriter.write(null, new IIOImage(image, null, null), jpgWriteParam);

    // Dispose the writer to free resources
    jpgWriter.dispose();
}

// Get data for further processing...
byte[] jpegData = compressed.toByteArray();

PS：默认情况下，ImageIO 在创建ImageOutputStream 时会使用磁盘缓存。这可能会减慢您的内存流写入速度。要禁用它，请使用ImageIO.setCache(false)（全局禁用磁盘缓存）或显式创建MemoryCacheImageOutputStream（本地），如下所示：

ImageOutputStream outputStream = new MemoryCacheImageOutputStream(compressed);