从ServletOutputStream获取file

可以通过以下步骤从`ServletOutputStream`获取文件：

1. 使用`ServletOutputStream`创建一个新的`FileOutputStream`对象。

ServletOutputStream outStream = response.getOutputStream();
FileOutputStream fileOutStream = new FileOutputStream("path/to/file");

2. 从`ServletOutputStream`中读取数据，并将其写入`FileOutputStream`中。

byte[] buffer = new byte[4096];
int bytesRead = -1;
while ((bytesRead = outStream.read(buffer)) != -1) {
    fileOutStream.write(buffer, 0, bytesRead);
}

3. 关闭`ServletOutputStream`和`FileOutputStream`。

outStream.flush();
outStream.close();
fileOutStream.close();

完整代码示例：

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // 设置响应头
    response.setContentType("application/octet-stream");
    response.setHeader("Content-Disposition", "attachment;filename=file.txt");

    // 从ServletOutputStream获取文件
    ServletOutputStream outStream = response.getOutputStream();
    FileOutputStream fileOutStream = new FileOutputStream("path/to/file");
    byte[] buffer = new byte[4096];
    int bytesRead = -1;
    while ((bytesRead = outStream.read(buffer)) != -1) {
        fileOutStream.write(buffer, 0, bytesRead);
    }
    outStream.flush();
    outStream.close();
    fileOutStream.close();
}