Springboot下载功能，附件超过8K不能显示下载弹窗，页面乱码问题

Springboot项目中遇到一个文件下载问题，当文件大小超过8K时，不会出现弹出窗，而是直接在页面显示乱码。

有问题的源码如下：

@RequestMapping(value = "/exportFile")
public void exportFile(HttpServletRequest request, HttpServletResponse response, String fileName) {
    String filePath = ReflectUtil.class.getClassLoader().getResource("").getPath();
    File exportfile = new File(filePath + "/export/" + fileName);
    try (FileInputStream is = new FileInputStream(exportfile ); OutputStream out = response.getOutputStream();) {
        if (is != null) {
            int b = 0;
            byte[] buffer = new byte[4096];
            while (b != -1) {
                b = is.read(buffer);
                if (b != -1) {
                    out.write(buffer, 0, b);
                }
            }
            response.setContentType("application/octet-stream");
            response.setHeader("Content-disposition", "attachment;filename=" + "exportFile.xls");
            response.flushBuffer();
            out.flush();
        }
    } catch (IOException e) {
        log.error("error is {}", e);
    }
} 

解决方案：将上面蓝色标识的二行放到方法最上面，问题解决。

@RequestMapping(value = "/exportFile")
public void exportFile(HttpServletRequest request, HttpServletResponse response, String fileName) {
    response.setContentType("application/octet-stream");
    response.setHeader("Content-disposition", "attachment;filename=" + "exportFile.xls");
    String filePath = ReflectUtil.class.getClassLoader().getResource("").getPath();
    File exportfile = new File(filePath + "/export/" + fileName);
    try (FileInputStream is = new FileInputStream(exportfile ); OutputStream out = response.getOutputStream();) {
        if (is != null) {
            int b = 0;
            byte[] buffer = new byte[4096];
            while (b != -1) {
                b = is.read(buffer);
                if (b != -1) {
                    out.write(buffer, 0, b);
                }
            }
            response.flushBuffer();
            out.flush();
        }
    } catch (IOException e) {
        log.error("error is {}", e);
    }
}