一、背景

Android 中的大文件下载需要使用分段下载，下载通常是在线程中进行的，假如有5段，那同时5个线程去执行下载，请求http返回文件流后，需要将多个文件流同时写进同一个文件，这里用到

RandomAccessFile。

分段上传的话，只需要根据每段文件阀值，例如,50M为一段，将文件按照设置的阀值，分段上传即可

二、相关代码

2.1 分段上传关键代码

忽略网络请求和状态码，每个人接口定义的的请求参数和返回code不一样

       private val DEFAULT_BLOCK_SIZE: Long = 50 * 1024 * 1024 //50MB
        val blockSize=DEFAULT_BLOCK_SIZE

        val randomAccessFile = RandomAccessFile(filePath, "r")
        val fileLen = randomAccessFile.length()

        //超过设定的单个文件大小,需要分块上传
            val blockFileNum = Math.ceil((fileLen / blockSize.toDouble())).toInt()
            XLogUtil.d("${TAG}blockFileNum:$blockFileNum,,,,fileLen:$fileLen,,,blockSize:$blockSize,,,requestId:$requestId")

            var offSet = 0L
            var successNum = 0
            var isSendResult = true

            for (i in 0 until blockFileNum) {
                val startOffset = i * blockSize
                val blockFileLen = Math.min(blockSize, fileLen - startOffset)
                val fileData = getFileData(filePath, offSet, blockFileLen.toInt())

                // 创建文件名请求体
                val requestBody = RequestBody.create(null, fileData)

                val call = RetrofitClient.getUploadFileService(
                    token,
                    requestId,
                    offSet.toString(),
                    uploadType
                ).uploadFile(file.name, requestBody)
                XLogUtil.d("${TAG}upload 第${i + 1}块 block file,offSet:$offSet,,,blockFileLen:$blockFileLen,,,blockFileNum:$blockFileNum,,,fileLen:$fileLen,,,filePath:$filePath,,,fileData size:${fileData?.size},,,requestId:$requestId")

                offSet += blockFileLen
                call.enqueue(object : Callback<ResponseBody?> {
                    override fun onResponse(
                        call: Call<ResponseBody?>,
                        response: Response<ResponseBody?>
                    ) {
                        val code = response.code()
                        XLogUtil.d("${TAG}upload 第${i + 1}块 block file result code:$code,,,requestId:$requestId")
                        if (code == 201) {
                            //处理成功响应
                            successNum++
                            if (successNum == blockFileNum) {
                                XLogUtil.d("${TAG}upload all block file success,blockFileNum:$blockFileNum,,,requestId:$requestId")
                                listener?.apply {
                                    onSuccess(Constant.SUCCESS, requestId)
                                }

                                //上传完
                            
                            }
                        } else {
                            //处理失败响应
          
                        }
                    }

                    override fun onFailure(call: Call<ResponseBody?>, t: Throwable) {
                        // 处理请求失败
                        XLogUtil.d("${TAG}upload 第${i + 1}块 block file onFailure message:${t.printStackTrace()}")
      
                    }
                })
            }











    /**
     * 根据偏移量获取分块文件数据
     */
    fun getFileData(filePath: String, offset: Long, length: Int): ByteArray? {
        // 使用RandomAccessFile随机访问文件
        var randomAccessFile: RandomAccessFile? = null
        try {
            randomAccessFile = RandomAccessFile(filePath, "r")
            val fileChannel = randomAccessFile.channel
            // 将文件的部分区域映射为内存区域
            val mappedByteBuffer =
                fileChannel.map(FileChannel.MapMode.READ_ONLY, offset, length.toLong())
            val data = ByteArray(length)
            // 从映射区域中读取数据
            mappedByteBuffer[data]
            return data
        } catch (e: Exception) {
            e.printStackTrace()
            return null
        } finally {
            if (randomAccessFile != null) {
                try {
                    randomAccessFile.close()
                } catch (e: Exception) {
                    e.printStackTrace()
                }
            }
        }
    }

2.2、分段下载关键代码

                val folder = File(DOWNLOAD_FOLDER_PATH)
                            if (!folder.exists()) {
                                folder.mkdirs()
                            }

                            val fileName = getFileNameWithPath(filePath)
                            val downFile = File(DOWNLOAD_FOLDER_PATH + File.separator + fileName)
                            if (!downFile.exists()) {
                                downFile.createNewFile()
                            }


                            // 使用输入流保存响应体到文件,这里通常是通过http请求，返回的文件流，替换即可
                            val inputStream = body.byteStream()

                            val rw = RandomAccessFile(downFile, "rw")
                            rw.seek(startPosition)//文件写入的初始位置
                            var hasReads = 0
                            var readLenght: Long = 0

                            val bytes = ByteArray(4096)
                            while ((inputStream.read(bytes).also { hasReads = it }) > 0) {
                                rw.write(bytes, 0, hasReads)
                                readLenght += hasReads
                                // val l = (readLenght * 100 / contentLength) as Int 单块文件写入进度
                            }


                            // 关闭文件输出流和输入流
                            inputStream.close()
                            rw.close()







    /**
     * 根据文件路径获取文件名
     */
    fun getFileNameWithPath(path: String): String {
        if (TextUtils.isEmpty(path)) {
            return ""
        }
        val start = path.lastIndexOf("/")
        return if (start != -1) {
            path.substring(start + 1)
        } else {
            "DEFAULT_NAME"
        }
    }