如何直接获取到HttpServletResponse中的文件，给出可以直接使用的工具类

可以使用以下工具类来获取HttpServletResponse中的文件：

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

public class HttpResponseUtils {

    private static final int DEFAULT_BUFFER_SIZE = 1024 * 4;

    public static void write(HttpServletResponse response, byte[] data, String fileName, String contentType) throws IOException {
        response.reset();
        response.setHeader("Content-Disposition", "attachment;filename=" + fileName);
        response.setContentType(contentType);
        response.setContentLength(data.length);
        response.getOutputStream().write(data);
        response.getOutputStream().flush();
    }

    public static void write(HttpServletResponse response, byte[] data, String fileName) throws IOException {
        write(response, data, fileName, "application/octet-stream");
    }

    public static void write(HttpServletResponse response, String text, String contentType) throws IOException {
        response.reset();
        response.setContentType(contentType);
        response.setCharacterEncoding("UTF-8");
        response.getWriter().write(text);
        response.getWriter().flush();
    }

    public static void write(HttpServletResponse response, String text) throws IOException {
        write(response, text, "text/plain;charset=UTF-8");
    }

}

使用方法如下：

byte[] fileBytes = getFileBytes(); // 获取文件字节数组
String fileName = "example.txt";
HttpServletResponse response = getHttpServletResponse(); // 获取HttpServletResponse对象
HttpResponseUtils.write(response, fileBytes, fileName); // 写入响应

其中，`getFileBytes()`方法是获取文件字节数组的方法，`getHttpServletResponse()`方法是获取HttpServletResponse对象的方法。你需要根据具体情况自行实现这两个方法。