app0Jfif为null怎么办

如果在设置DPI时，`app0Jfif`为null，可以尝试使用`javax_imageio_1.0`元数据标记来设置DPI。下面是修改后的代码：

import javax.imageio.ImageIO;
import javax.imageio.ImageWriteParam;
import javax.imageio.ImageWriter;
import javax.imageio.metadata.IIOMetadata;
import javax.imageio.metadata.IIOMetadataNode;
import javax.imageio.plugins.jpeg.JPEGImageWriteParam;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

public class ImageDPIExample {
    public static void main(String[] args) {
        // 加载图像
        BufferedImage image = null;
        try {
            image = ImageIO.read(new File("input.jpg"));
        } catch (IOException e) {
            e.printStackTrace();
        }

        // 创建一个新的图像，设置分辨率
        BufferedImage newImage = new BufferedImage(image.getWidth(), image.getHeight(), BufferedImage.TYPE_INT_RGB);
        Graphics2D g2d = newImage.createGraphics();
        g2d.drawImage(image, 0, 0, null);
        g2d.dispose();

        // 设置DPI
        String outputImagePath = "output.jpg";
        File outputFile = new File(outputImagePath);
        try {
            ImageWriter writer = ImageIO.getImageWritersByFormatName("jpg").next();
            ImageWriteParam writeParam = writer.getDefaultWriteParam();
            if (writeParam.canWriteCompressed()) {
                writeParam.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
                writeParam.setCompressionQuality(1f);
            }

            // 设置DPI
            IIOMetadata metadata = writer.getDefaultImageMetadata(ImageTypeSpecifier.createFromRenderedImage(newImage), writeParam);
            String metadataFormat = metadata.getNativeMetadataFormatName();
            IIOMetadataNode root = (IIOMetadataNode) metadata.getAsTree(metadataFormat);

            IIOMetadataNode jpegVariety = getChildNode(root, "JPEGvariety");
            IIOMetadataNode markerSequence = getChildNode(jpegVariety, "markerSequence");

            IIOMetadataNode app0JFIF = getChildNode(markerSequence, "app0JFIF");
            if (app0JFIF == null) {
                app0JFIF = new IIOMetadataNode("app0JFIF");
                app0JFIF.setAttribute("majorVersion", "1");
                app0JFIF.setAttribute("minorVersion", "2");
                app0JFIF.setAttribute("resUnits", "1");
                app0JFIF.setAttribute("Xdensity", "300");
                app0JFIF.setAttribute("Ydensity", "300");

                markerSequence.appendChild(app0JFIF);
            } else {
                app0JFIF.setAttribute("resUnits", "1");
                app0JFIF.setAttribute("Xdensity", "300");
                app0JFIF.setAttribute("Ydensity", "300");
            }

            metadata.mergeTree(metadataFormat, root);

            // 保存图像
            writer.setOutput(ImageIO.createImageOutputStream(outputFile));
            writer.write(metadata, new IIOImage(newImage, null, metadata), writeParam);

            writer.dispose();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static IIOMetadataNode getChildNode(IIOMetadataNode parentNode, String nodeName) {
        for (int i = 0; i < parentNode.getLength(); i++) {
            if (parentNode.item(i).getNodeName().equalsIgnoreCase(nodeName)) {
                return (IIOMetadataNode) parentNode.item(i);
            }
        }
        return null;
    }
}

上述代码中，我们使用`getChildNode()`方法来获取元数据节点，如果`app0JFIF`节点不存在，则创建一个新的节点，并设置相关属性。如果`app0JFIF`节点已存在，则直接设置属性值。

这样修改后的代码应该可以在没有`app0JFIF`节点的情况下设置DPI。请注意，上述示例代码仅适用于保存JPEG图像，并且设置的DPI为300。你可以根据需要进行修改和调整。