Ajax方式文件下载(后台返回为json或文件流)

在应用开发中，经常需要下载文件(如导出Excel)，调用后台接口时，如果后台报错需要弹出错误信息，如果没有报错正常下载文件。本文主要介绍前台及后台(基于java)的处理方法，文中使用到的软件版本：Spring 4.3.9、Java 1.8.0_191、Jquery 1.12.4、Chrome 85.0.4183.121。

1、实现思路

方法一：通过原生Ajax响应头来区分是文本还是流，然后通过blob来处理返回数据

方法二：分成两个接口，先调用生成文件的接口，如果文件成功生成则再调用文件下载的接口；如果文件生成失败则弹出失败信息

2、方法一(Ajax响应头区分文本还是流)

2.1、后台

/**
 * 根据sql导成csv；成功返回文件流，失败返回错误信息
 * @param sql 导出sql
 * @param col 字段信息，以逗号分隔
 * @param title 文件头信息，以逗号分隔
 * @param separator csv文件的分隔符
 * @param request
 * @param response
 */
@RequestMapping("exportForCsv.do")
@ResponseBody
public CallResult<String> exportForCsv(String sql, String col, String title, String separator, String fileName, HttpServletRequest request, HttpServletResponse response) {
    logger.info("sql={}", sql);
    logger.info("title={}", title);
    logger.info("separator={}", separator);
    logger.info("fileName={}", fileName);
    logger.info("baseServiceName={}", baseServiceName);
    CallResult<String> result = null;
    ByteArrayOutputStream baos = null;
    InputStream in = null;
    try {
        if (Util.isNull(separator)) {
            separator = ",";
        }
        if (Util.isNull(fileName)) {
            fileName = "data.csv";
        }
        baos = new ByteArrayOutputStream();
        getService().exportForCsv(baos, sql, col, title, separator);//这边调用service把文件写入输入流，请自行实现
        in = new ByteArrayInputStream(baos.toByteArray());
        writeStream(in, fileName, request, response);
    } catch(Exception e) {
        result = new CallResult<>(-1, "发生异常");
        e.printStackTrace();
    } finally {
        FileUtil.close(baos);
        FileUtil.close(in);
    }
    return result;
}
private void writeStream(InputStream in, String fileName, HttpServletRequest request, HttpServletResponse response) throws IOException {
    OutputStream out = null;
    try {
        String urlFileName = "";
        if (request.getHeader("User-Agent").toLowerCase().indexOf("firefox") > 0) {
            urlFileName = new String(fileName.getBytes("UTF-8"), "ISO8859-1");
        } else {
            urlFileName = java.net.URLEncoder.encode(fileName, "UTF-8");
        }

        response.reset();
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename="" + urlFileName + """);
        response.setHeader("Connection", "close");

        out = new BufferedOutputStream(response.getOutputStream(), BUFFER_SIZE);
        byte buf[] = new byte[BUFFER_SIZE];
        int len;
        while ((len = in.read(buf)) != -1) {
            out.write(buf, 0, len);
        }
    } finally {
        FileUtil.close(out);
    }
}

2.2、前台

2.2.1、拼接参数

function expForCsv() {
    let sql = "";//请自行设置
    let col = "";//请自行设置
    let title = "";//请自行设置
    let fileName = "";//请自行设置

　　 let xhr = new XMLHttpRequest();
    xhr.open("post", "${pageContext.request.contextPath}/common/export/exportForCsv.do", true);
    xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhr.responseType = "blob";
    xhr.onload = function() {
        if (xhr.getResponseHeader("Content-type") == 'application/octet-stream') {
            let url = window.URL.createObjectURL(xhr.response);
            let a = document.createElement("a");
            a.href = url;
            a.style.display = 'none'
            a.download = fileName;
            a.click();
            window.URL.revokeObjectURL(url);
            a.remove();
        } else {
            let reader = new FileReader();
            reader.addEventListener('loadend', function (e) {
                let data = JSON.parse(e.target.result);
                alert('提示', "系统出错，错误信息为：" + data.description + ",请将该信息提供给代维人员寻求帮助");
            });
            reader.readAsText(xhr.response);
        }
    };
    xhr.send('sql=' + sql + '&col=' + col + '&title=' + title + '&separator=|&t=' + new Date().getTime());
}

2.2.2、使用FormData

function expForCsv(type) {
    let sql = "";//请自行设置
    let col = "";//请自行设置
    let title = "";//请自行设置
    let fileName = ";//请自行设置

    let xhr = new XMLHttpRequest();
    xhr.open("post", "${pageContext.request.contextPath}/common/export/exportForCsv.do", true);
    //xhr.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
    xhr.responseType = "blob";
    xhr.onload = function() {
        $.messager.progress('close');
        if (xhr.getResponseHeader("Content-type") == 'application/octet-stream') {
            let url = window.URL.createObjectURL(xhr.response);
            let a = document.createElement("a");
            a.href = url;
            a.style.display = 'none'
            a.download = fileName;
            a.click();
            window.URL.revokeObjectURL(url);
            a.remove();
        } else {
            let reader = new FileReader();
            reader.addEventListener('loadend', function (e) {
                let data = JSON.parse(e.target.result);
                alert('提示', "系统出错，错误信息为：" + data.description + ",请将该信息提供给代维人员寻求帮助");
            });
            reader.readAsText(xhr.response);
        }
    };
    let data = new FormData();
    data.append('sql', getCsql(type));
    data.append('col', col);
    data.append('title', title);
    data.append('separator', '|');
    data.append('t', new Date().getTime() + "");
    xhr.send(data);
}

3、方法二(两个接口)

3.1、后台

3.1.1、生成文件接口

/**
 * 根据sql导成csv；成功返回生成的文件名称，失败返回错误信息
 * @param sql 导出sql
 * @param col 字段信息，以逗号分隔
 * @param title 文件头信息，以逗号分隔
 * @param separator csv文件分隔符
 * @param baseServiceName
 * @return
 */
@RequestMapping("exportForCsv2")
@ResponseBody
public CallResult<String> exportForCsv2(String sql, String col, String title, String separator) {
    logger.info("sql={}", sql);
    logger.info("col={}", col);
    logger.info("title={}", title);
    logger.info("separator={}", separator);
    logger.info("baseServiceName={}", baseServiceName);
    OutputStream os = null;
    CallResult<String> result = new CallResult<>();
    try {
        String fileName = "数据信息(" + DateUtil.getCurrentDateString("yyyyMMddHHmmsss") + ").xlsx";
        os = new FileOutputStream(new File(WebconfigFactory.getTempFolder() + "数据信息(" + DateUtil.getCurrentDateString("yyyyMMddHHmmsss") + ").xlsx"));

        getService().exportForCsv(os, sql, col, title, separator);//调用service生成文件内容，请自行实现

        result.setResult(fileName);
    } catch(Exception e) {
        result = new CallResult<>(-1, "发生异常");
        e.printStackTrace();
    } finally {
        FileUtil.close(os);
    }

    return result;
}

3.1.2、下载文件接口

/**
 * 下载方法
 * 1.点击页面下载，需传参数：
 *         fileName：磁盘上文件的名称，必需
 *         filePath或pathType或webPath：
 *             文件路径，传其中之一参数即可;
 *             filePath是磁盘的绝对路径；
 *             pathType是路径类型，对应webconfig.properties中的key
 *             webPath是相对web应用的路径，如/spa
 *         fileRealName：下载保存后的文件名称，可选；如果为空则与fileName一样
 *   
 * 2.后台导出文件下载，需传属性：
 *         fileName：磁盘上文件的名称，必需
 *         fileRealName：下载保存后的文件名称，可选；如果为空则与fileName一样
 */
@RequestMapping("download")
public void download(HttpServletRequest request, HttpServletResponse response) {
    String filePath = "";
    String fileName = request.getParameter("fileName");
    String fileRealName = "";
    if (!Util.isNull(fileName)) {
        filePath = request.getParameter("filePath");
        if (Util.isNull(filePath)) {
            String webPath = request.getParameter("webPath");
            if (Util.isNull(webPath)) {
                String pathType = request.getParameter("pathType");
                filePath = WebconfigFactory.getValue(pathType);
            } else {
                //filePath = request.getRealPath(webPath) + System.getProperty("file.separator");
                filePath = request.getServletContext().getRealPath(webPath) + System.getProperty("file.separator");
            }
        }
        fileRealName = request.getParameter("fileRealName");
    } else {//后台导出文件下载
        fileName = (String) request.getAttribute("fileName");
        filePath = WebconfigFactory.getTempFolder();
        fileRealName = (String) request.getAttribute("fileRealName");
    }
    
    if (Util.isNull(fileRealName)) {
        fileRealName = fileName;
    }
    
    BufferedInputStream bis = null;
    OutputStream out = null;
    try {
        String userInfoHeader = "User-Agent";
        String firefoxName = "firefox";
        String urlFileName = "";
        if (request.getHeader(userInfoHeader).toLowerCase().indexOf(firefoxName) > 0) {
            urlFileName = new String(fileRealName.getBytes("UTF-8"), "ISO8859-1");
        } else {
            urlFileName = java.net.URLEncoder.encode(fileRealName, "UTF-8");
        }
        
        response.reset();
        response.setContentType("application/octet-stream");
        response.setHeader("Content-Disposition", "attachment; filename="" + urlFileName + """);
        response.setHeader("Connection", "close");
        
        bis = new BufferedInputStream(new FileInputStream(filePath + fileName), BUFFER_SIZE);
        out = new BufferedOutputStream(response.getOutputStream(), BUFFER_SIZE);
        byte[] buf = new byte[BUFFER_SIZE];
        int len;
        while ((len = bis.read(buf)) != -1) {
            out.write(buf, 0, len);
        }
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        FileUtil.close(bis);
        FileUtil.close(out);
    }
}

3.2、前台

function exp() {
    let sql = "";//请自行设置
    let col = "";//请自行设置
    let title = "";//请自行设置
    $.messager.progress({title : '处理中...'});//easyui控件，可用其他控件
    $.getJSON(
        "${pageContext.request.contextPath}/common/export/exportForCsv.do",
        {
            sql : sql,
            col : col,
            title : title,
            separator: "|",
            t : new Date().getTime()
        },
        function(data) {
            $.messager.progress('close');//easyui控件，可用其他控件
            if (data.returnCode == 0) {
                location.href = "${pageContext.request.contextPath}/common/upDownload/download.do?fileName=" + encodeURI(data.result) + "&fileRealName=" + encodeURI("数据.csv") + "&pathType=path.temp.folder";
            } else {
                $.messager.alert('提示', "系统出错，错误信息为：" + data.description + ",请将该信息提供给代维人员寻求帮助");
            }
        }
    );
}

注意例子里sql是从台传到后台，这样容易引起SQL注入攻击；严谨的做法应该是前台传参数，后台拼sql，并使用PrepareStatement来执行sql。