请问如何把 pdf 转换成 png 图片？

可以使用apache的pdfbox包进行转化，操作代码如下：

引入依赖：

org.apache.pdfbox
pdfbox
2.0.27

调用：

private static void toImage(String fileName) throws IOException {
File file = FileUtil.file("./" + fileName + ".pdf");
String outPath = "./out";
if (!FileUtil.exist(outPath)) {
FileUtil.mkdir(outPath);
}
outPath = FileUtil.file(outPath).getAbsolutePath();
try (PDDocument doc = PDDocument.load(file)) {
PDFRenderer renderer = new PDFRenderer(doc);
int pages = doc.getNumberOfPages();
for (int i = 0; i < pages; i++) {
BufferedImage image = renderer.renderImageWithDPI(i, DEFAULT_DPI);
ImageIO.write(image, "JPEG", new File(outPath + "/" + fileName + "-" + i + ".jpeg"));
}
}
}