前言：前端使用uniapp中的uni.canvasToTempFilePath方法将画板中的内容保存为jpg上传至后端处理

uni.canvasToTempFilePath({canvasId: 'firstCanvas',sourceType: ['album'],fileType: "jpg",success: function (res1) {let signature_base64 = res1.tempFilePath;let signature_file = that.base64toFile(signature_base64);// 将签名存储到服务器uni.uploadFile({url: "convertJpgToPdf",name: "file",file: signature_file,formData: {busiId: 'ceshi12',token: token},success: function (res1) {console.log(res1);}})}});// 将base64转为filebase64toFile: function(base64String) {// 从base64字符串中解析文件类型var mimeType = base64String.match(/^data:(.*);base64,/)[1];// 生成随机文件名var randomName = Math.random().toString(36).substring(7);var filename = randomName + '.' + mimeType.split('/')[1];let arr = base64String.split(",");let bstr = atob(arr[1]);let n = bstr.length;let u8arr = new Uint8Array(n);while (n--) {u8arr[n] = bstr.charCodeAt(n);}return new File([u8arr], filename, { type: mimeType });},

java代码：
首先在pom.xml中安装依赖

<dependency><groupId>org.apache.pdfbox</groupId><artifactId>pdfbox</artifactId><version>2.0.8</version>
</dependency>

使用方法：

/*** 将jpg转为pdf文件* @param jpgFile* @return* @throws IOException*/public MultipartFile convertJpgToPdf(MultipartFile jpgFile) throws IOException {// 保存MultipartFile到临时文件File tempFile = new File(System.getProperty("java.io.tmpdir"), "temp.jpg");jpgFile.transferTo(tempFile);// 创建PDF文档PDDocument pdfDoc = new PDDocument();PDPage pdfPage = new PDPage();pdfDoc.addPage(pdfPage);// 从临时文件创建PDImageXObjectPDImageXObject pdImage = PDImageXObject.createFromFile(tempFile.getAbsolutePath(), pdfDoc);// 获取PDF页面的内容流以添加图像PDPageContentStream contentStream = new PDPageContentStream(pdfDoc, pdfPage);// 获取PDF页面的大小PDRectangle pageSize = pdfPage.getMediaBox();float pageWidth = pageSize.getWidth();float pageHeight = pageSize.getHeight();// 在PDF页面上绘制图像float originalImageWidth = pdImage.getWidth();float originalImageHeight = pdImage.getHeight();// 初始化缩放比例float scale = 1.0f;// 判断是否需要缩放图片if (originalImageWidth > pageWidth || originalImageHeight > pageHeight) {// 计算宽度和高度的缩放比例，取较小值float widthScale = pageWidth / originalImageWidth;float heightScale = pageHeight / originalImageHeight;scale = Math.min(widthScale, heightScale);}// 计算缩放后的图片宽度和高度float scaledImageWidth = originalImageWidth * scale;float scaledImageHeight = originalImageHeight * scale;// 计算图片在页面中居中绘制的位置float x = (pageWidth - scaledImageWidth) / 2;float y = (pageHeight - scaledImageHeight) / 2;// 绘制缩放后的图片到PDF页面contentStream.drawImage(pdImage, x, y, scaledImageWidth, scaledImageHeight);// 关闭内容流contentStream.close();// 将PDF文档写入到字节数组中ByteArrayOutputStream pdfOutputStream = new ByteArrayOutputStream();pdfDoc.save(pdfOutputStream);// 关闭PDF文档pdfDoc.close();// 删除临时文件tempFile.delete();// 创建代表PDF的MultipartFileMultipartFile pdfFile = new MockMultipartFile("converted12.pdf", "converted12.pdf", "application/pdf", pdfOutputStream.toByteArray());saveFileToLocalDisk(pdfFile, "D:/");return pdfFile;}/*** 将MultipartFile file文件保存到本地磁盘* @param file* @param directoryPath* @throws IOException*/public void saveFileToLocalDisk(MultipartFile file, String directoryPath) throws IOException {// 获取文件名String fileName = file.getOriginalFilename();// 创建目标文件File targetFile = new File(directoryPath + File.separator + fileName);// 将文件内容写入目标文件try (FileOutputStream outputStream = new FileOutputStream(targetFile)) {outputStream.write(file.getBytes());} catch (IOException e) {// 处理异常e.printStackTrace();throw e;}}

MockMultipartFile类

package com.jiuzhu.server.common;
import org.springframework.util.FileCopyUtils;
import org.springframework.web.multipart.MultipartFile;import java.io.ByteArrayInputStream;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;public class MockMultipartFile implements MultipartFile {private final String name;private String originalFilename;private String contentType;private final byte[] content;/*** Create a new MultipartFileDto with the given content.** @param name    the name of the file* @param content the content of the file*/public MockMultipartFile(String name, byte[] content) {this(name, "", null, content);}/*** Create a new MultipartFileDto with the given content.** @param name          the name of the file* @param contentStream the content of the file as stream* @throws IOException if reading from the stream failed*/public MockMultipartFile(String name, InputStream contentStream) throws IOException {this(name, "", null, FileCopyUtils.copyToByteArray(contentStream));}/*** Create a new MultipartFileDto with the given content.** @param name             the name of the file* @param originalFilename the original filename (as on the client's machine)* @param contentType      the content type (if known)* @param content          the content of the file*/public MockMultipartFile(String name, String originalFilename, String contentType, byte[] content) {this.name = name;this.originalFilename = (originalFilename != null ? originalFilename : "");this.contentType = contentType;this.content = (content != null ? content : new byte[0]);}/*** Create a new MultipartFileDto with the given content.** @param name             the name of the file* @param originalFilename the original filename (as on the client's machine)* @param contentType      the content type (if known)* @param contentStream    the content of the file as stream* @throws IOException if reading from the stream failed*/public MockMultipartFile(String name, String originalFilename, String contentType, InputStream contentStream)throws IOException {this(name, originalFilename, contentType, FileCopyUtils.copyToByteArray(contentStream));}@Overridepublic String getName() {return this.name;}@Overridepublic String getOriginalFilename() {return this.originalFilename;}@Overridepublic String getContentType() {return this.contentType;}@Overridepublic boolean isEmpty() {return (this.content.length == 0);}@Overridepublic long getSize() {return this.content.length;}@Overridepublic byte[] getBytes() throws IOException {return this.content;}@Overridepublic InputStream getInputStream() throws IOException {return new ByteArrayInputStream(this.content);}@Overridepublic void transferTo(File dest) throws IOException, IllegalStateException {FileCopyUtils.copy(this.content, dest);}
}