写入CSV文件,然后在Appengine(Java)中将其压缩

Question

我目前正在开发一个用Java做的项目,谷歌上的appengine.

Appengine不允许存储文件,因此无法使用任何磁盘表示对象.其中一些包括File类.

我想写数据并将其导出到几个csv文件,然后将其压缩,并允许用户下载它.

如果不使用任何文件类,我怎么能这样做？我在文件处理方面不是很有经验,所以我希望你们能告诉我.

谢谢.

Answer 1

您可以创建一个zip文件,并在用户下载时添加到该文件中.如果您使用的是servlet,那么这是一个很好的方法:

protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    // ..... process request

    // ..... then respond
    response.setContentType("application/zip");
    response.setStatus(HttpServletResponse.SC_OK);

    // note : intentionally no content-length set, automatic chunked transfer if stream is larger than the internal buffer of the response
    ZipOutputStream zipOut = new ZipOutputStream(response.getOutputStream());

    byte[] buffer = new byte[1024 * 32];
    try {
            // case1: already have input stream, typically ByteArrayInputStream from a byte[] full of previoiusly prepared csv data

            InputStream in = new BufferedInputStream(getMyFirstInputStream());
            try {
                zipOut.putNextEntry(new ZipEntry("FirstName"));

                int length;
                while((length = in.read(buffer)) != -1) {
                    zipOut.write(buffer, 0, length);
                }

                zipOut.closeEntry();
            } finally {
                in.close();
            }

            // case 2: write directly to output stream, i.e. you have your raw data but need to create csv representation

            zipOut.putNextEntry(new ZipEntry("SecondName"));

            // example setup, key is to use the below outputstream 'zipOut' write methods
            Object mySerializer = new MySerializer(); // i.e. csv-writer
            Object myData = getMyData(); // the data to be processed by the serializer in order to make a csv file

            mySerizalier.setOutput(zipOut);

            // write whatever you have to the zipOut
            mySerializer.write(myData);

            zipOut.closeEntry();

            // repeat for the next file.. or make for-loop

        }
    } finally { 
        zipOut.close();
    }
}

除非存在内存限制,否则没有理由将数据存储在文件中.文件为您提供InputStream和OutputStream,两者都具有内存中的等价物.

请注意,创建一个csv编写器通常意味着执行类似这样的操作,其中一点是获取一段数据(数组列表或映射,无论你有什么)并将其转换为byte []部分.使用DataOutputStream(如果您愿意自己创建)或OutputStreamWriter等工具将byte []部分附加到OutputStream中.