Spring Boot项目以jar包形式启动，下载resources目录中文件为空的解决方案及原因分析

Spring Boot项目以jar包形式启动，下载resources目录中文件为空的解决方案及原因分析 背景

项目中有一个下载docx模板文件的功能。开发同学反馈：本地测试可以正常下载；部署到测试服务器后，下载的文件为空。

定位问题

开发环境和测试环境有哪些差异呢？

• 环境差异
  • 开发环境为windows
  • 测试环境为linux
  • 因java跨平台，这个差异基本可排除
• 运行方式差异
  • 开发环境直接从IDE中run
  • 测试环境是打包为jar后在run
  • 本地打包jar后run，可复现

验证问题

下载文件为空的代码

  
  @GetMapping("/downloadEmpty")
  public void downloadEmpty(HttpServletResponse res) {
    String path = "templates/demo.docx";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path);
        OutputStream outputStream = res.getOutputStream()) {
      res.addHeader("Content-Disposition", "attachment;filename=empty.docx");
      res.addHeader("Content-Length", String.valueOf(inputStream.available()));
      res.setContentType("application/octet-stream");
      byte[] bys = new byte[1024];
      int len;
      while ((len = inputStream.read(bys)) != -1) outputStream.write(bys, 0, len);
      outputStream.flush();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

以下代码输出docx文件的一些信息，经分析可知：读取到的inputStream正常，只是inputStream.available() == 0

  
  @GetMapping("/docx")
  public String doc() {
    StringBuilder sb = new StringBuilder();
    String path = "templates/demo.docx";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path)) {
      sb.append("path: ")
          .append(path)
          .append("
ClassLoader: ")
          .append(classLoader.getClass())
          .append("
InputStream: ")
          .append(inputStream.getClass())
          .append("
inputStream.available: ")
          .append(inputStream.available());
      byte[] bys = new byte[1024];
      int len = 0, total = 0;
      while ((len = inputStream.read(bys)) != -1) total += len;
      sb.append("
length: " + total);
    } catch (IOException e) {
      e.printStackTrace();
    }
    return sb.toString();
    
  }

解决方案

经测试及分析，注释掉res.addHeader(“Content-Length”, String.valueOf(inputStream.available()));即可；经验证，下载文件正常，代码如下：

  
  @GetMapping("/download")
  public void download(HttpServletResponse res) {
    String path = "templates/demo.docx";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path);
        OutputStream outputStream = res.getOutputStream()) {
      res.addHeader("Content-Disposition", "attachment;filename=demo.docx");
      //      res.addHeader("Content-Length", String.valueOf(inputStream.available()));
      res.setContentType("application/octet-stream");
      byte[] bys = new byte[1024];
      int len;
      while ((len = inputStream.read(bys)) != -1) outputStream.write(bys, 0, len);
      outputStream.flush();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

原理分析

经测试及分析，看似下载文件为空的问题，实质是获取到输入流inputStream.available() == 0的问题。为什么会返回0呢?
我们观察到读取docx文件时候，返回的inputStream是org.springframework.boot.loader.data.RandomAccessDataFile$DataInputStream；翻开源码看一下，DataInputStream继承了InputStream，而InputStream的available()方法直接返回了0。


在实际验证问题的过程中并没有这么一帆风顺。起初我使用了一个txt文件来验证此问题，但是无法重现，也就是说，打包jar后也可以通过available()方法获取到文件的大小。测试代码如下：

  
  @GetMapping("/txt")
  public String txt() {
    StringBuilder sb = new StringBuilder();
    String path = "templates/demo.txt";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path)) {
      sb.append("path: ")
          .append(path)
          .append("
ClassLoader: ")
          .append(classLoader.getClass())
          .append("
InputStream: ")
          .append(inputStream.getClass())
          .append("
inputStream.available: ")
          .append(inputStream.available());
    } catch (IOException e) {
      e.printStackTrace();
    }
    return sb.toString();
    
  }

查看源码时候，也得到了验证。txt文件属于压缩文件（DEFLATED），返回inputStream是org.springframework.boot.loader.jar.ZipInflaterInputStream，


而返回inputStream是org.springframework.boot.loader.data.RandomAccessDataFile$DataInputStream的文件属于非压缩文件（STORED）

总结

• Spring Boot打包为jar后运行时，通过class org.springframework.boot.loader.LaunchedURLClassLoader读取resources目录下文件。
• 分为2种类型文件
  • STORED类型
    • 读取到inputStream是org.springframework.boot.loader.data.RandomAccessDataFile$DataInputStream
    • input.available() == 0
  • DEFLATED
    • 读取到inputStream是org.springframework.boot.loader.jar.ZipInflaterInputStream
    • input.available() == size(文件大小)

以上给出了解决方案及原理分析，但并不建议将下载的文件放到resources目录下；可以放到分布式存储或其他文件系统中。

附

完整的测试代码

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;


@RestController
@RequestMapping(value = "/")
public class DemoController {
  
  @GetMapping("/docx")
  public String doc() {
    StringBuilder sb = new StringBuilder();
    String path = "templates/demo.docx";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path)) {
      sb.append("path: ")
          .append(path)
          .append("
ClassLoader: ")
          .append(classLoader.getClass())
          .append("
InputStream: ")
          .append(inputStream.getClass())
          .append("
inputStream.available: ")
          .append(inputStream.available());
      byte[] bys = new byte[1024];
      int len = 0, total = 0;
      while ((len = inputStream.read(bys)) != -1) total += len;
      sb.append("
length: " + total);
    } catch (IOException e) {
      e.printStackTrace();
    }
    return sb.toString();
    
  }
  
  @GetMapping("/txt")
  public String txt() {
    StringBuilder sb = new StringBuilder();
    String path = "templates/demo.txt";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path)) {
      sb.append("path: ")
          .append(path)
          .append("
ClassLoader: ")
          .append(classLoader.getClass())
          .append("
InputStream: ")
          .append(inputStream.getClass())
          .append("
inputStream.available: ")
          .append(inputStream.available());
    } catch (IOException e) {
      e.printStackTrace();
    }
    return sb.toString();
    
  }
  
  @GetMapping("/downloadEmpty")
  public void downloadEmpty(HttpServletResponse res) {
    String path = "templates/demo.docx";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path);
        OutputStream outputStream = res.getOutputStream()) {
      res.addHeader("Content-Disposition", "attachment;filename=empty.docx");
      res.addHeader("Content-Length", String.valueOf(inputStream.available()));
      res.setContentType("application/octet-stream");
      byte[] bys = new byte[1024];
      int len;
      while ((len = inputStream.read(bys)) != -1) outputStream.write(bys, 0, len);
      outputStream.flush();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
  
  @GetMapping("/download")
  public void download(HttpServletResponse res) {
    String path = "templates/demo.docx";
    ClassLoader classLoader = Demo3Application.class.getClassLoader();
    try (InputStream inputStream = classLoader.getResourceAsStream(path);
        OutputStream outputStream = res.getOutputStream()) {
      res.addHeader("Content-Disposition", "attachment;filename=demo.docx");
      //      res.addHeader("Content-Length", String.valueOf(inputStream.available()));
      res.setContentType("application/octet-stream");
      byte[] bys = new byte[1024];
      int len;
      while ((len = inputStream.read(bys)) != -1) outputStream.write(bys, 0, len);
      outputStream.flush();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}

Demo地址