<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.2.5.RELEASE</version>
</parent>
<dependencies>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <!--SpringBoot文件上传-->
    <dependency>
      <groupId>commons-fileupload</groupId>
      <artifactId>commons-fileupload</artifactId>
      <version>1.3.1</version>
    </dependency>
    <dependency>
      <groupId>commons-io</groupId>
      <artifactId>commons-io</artifactId>
      <version>2.5</version>
    </dependency>
 </dependencies>

package com.whj.controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.*;
/**
 * @author 王恒杰
 * @date 2022/10/26 9:26
 * @Description:
 */
@RestController
@RequestMapping("/file")
public class FileController {
    @GetMapping("/downloadFile")
    public void downloadCheck(HttpServletRequest request, HttpServletResponse response) {
        try {
            File file = new File("D:\\Idea\\stamp\\Itext\\src\\main\\resources\\pdf\\EncryptPDF.pdf");
            BufferedInputStream bis = null;
            OutputStream os = null;
            FileInputStream fileInputStream = null;
            response.setCharacterEncoding("utf-8");
            response.setContentType("application/pdf");
            response.setHeader("Content-Disposition", "attachment; filename=EncryptPDF");
            try {
                fileInputStream = new FileInputStream(file);
                byte[] buff = new byte[1024];
                bis = new BufferedInputStream(fileInputStream);
                os = response.getOutputStream();
                int i = bis.read(buff);
                while (i != -1) {
                    os.write(buff, 0, buff.length);
                    i = bis.read(buff);
                    os.flush();
                }
                os.flush();
                os.close();
//                return SimpleResult.ok("导出成功",os);
            } catch (IOException e) {
                e.printStackTrace();
//                return SimpleResult.fail("导出失败",null);
            } finally {
                if (bis != null) {
                    try {
                        bis.close();
                        fileInputStream.close();
                    } catch (IOException e) {
                        e.printStackTrace();
//                        return SimpleResult.fail("导出失败",null);
                    }
                }
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    @GetMapping("/previewCheck")
    public void previewCheck( HttpServletRequest request, HttpServletResponse response) throws
            IOException {
        FileInputStream is = new FileInputStream(new File("D:\\Idea\\stamp\\Itext\\src\\main\\resources\\pdf\\EncryptPDF.pdf"));
        // 清空response
        response.reset();
        //2、设置文件下载方式
        response.setCharacterEncoding("utf-8");
        response.setContentType("application/pdf");
        OutputStream outputStream = response.getOutputStream();
        int count = 0;
        byte[] buffer = new byte[1024 * 1024];
        while ((count = is.read(buffer)) != -1) {
            outputStream.write(buffer, 0, count);
        }
        outputStream.flush();
    }
}