Java 下载支持断点续传服务端

private void downFile(HttpServletResponse response, HttpServletRequest request, String location){
    BufferedInputStream bis = null;
    try {
        File file = new File(location);
        if (file.exists()) {
            long p = 0L;
            long toLength = 0L;
            long contentLength = 0L;
            int rangeSwitch = 0; // 0,从头开始的全文下载；1,从某字节开始的下载（bytes=27000-）；2,从某字节开始到某字节结束的下载（bytes=27000-39000）
            long fileLength;
            String rangBytes = "";
            fileLength = file.length();
 
            // get file content
            InputStream ins = new FileInputStream(file);
            bis = new BufferedInputStream(ins);
 
            // tell the client to allow accept-ranges
            response.reset();
            response.setHeader("Accept-Ranges", "bytes");
 
            // client requests a file block download start byte
            String range = request.getHeader("Range");
            if (range != null && range.trim().length() > 0 && !"null".equals(range)) {
                response.setStatus(javax.servlet.http.HttpServletResponse.SC_PARTIAL_CONTENT);
                rangBytes = range.replaceAll("bytes=", "");
                if (rangBytes.endsWith("-")) {  // bytes=270000-
                    rangeSwitch = 1;
                    p = Long.parseLong(rangBytes.substring(0, rangBytes.indexOf("-")));
                    contentLength = fileLength - p;  // 客户端请求的是270000之后的字节（包括bytes下标索引为270000的字节）
                } else { // bytes=270000-320000
                    rangeSwitch = 2;
                    String temp1 = rangBytes.substring(0, rangBytes.indexOf("-"));
                    String temp2 = rangBytes.substring(rangBytes.indexOf("-") + 1, rangBytes.length());
                    p = Long.parseLong(temp1);
                    toLength = Long.parseLong(temp2);
                    contentLength = toLength - p + 1; // 客户端请求的是 270000-320000 之间的字节
                }
            } else {
                contentLength = fileLength;
            }
 
            // 如果设设置了Content-Length，则客户端会自动进行多线程下载。如果不希望支持多线程，则不要设置这个参数。
            // Content-Length: [文件的总大小] - [客户端请求的下载的文件块的开始字节]
            response.setHeader("Content-Length", new Long(contentLength).toString());
 
            // 断点开始
            // 响应的格式是:
            // Content-Range: bytes [文件块的开始字节]-[文件的总大小 - 1]/[文件的总大小]
            if (rangeSwitch == 1) {
                String contentRange = new StringBuffer("bytes ").append(new Long(p).toString()).append("-")
                        .append(new Long(fileLength - 1).toString()).append("/")
                        .append(new Long(fileLength).toString()).toString();
                response.setHeader("Content-Range", contentRange);
                bis.skip(p);
            } else if (rangeSwitch == 2) {
                String contentRange = range.replace("=", " ") + "/" + new Long(fileLength).toString();
                response.setHeader("Content-Range", contentRange);
                bis.skip(p);
            } else {
                String contentRange = new StringBuffer("bytes ").append("0-")
                        .append(fileLength - 1).append("/")
                        .append(fileLength).toString();
                response.setHeader("Content-Range", contentRange);
            }
 
            String fileName = file.getName();
            response.setContentType("application/octet-stream");
            response.addHeader("Content-Disposition", "attachment;filename=" + fileName);
 
            OutputStream out = response.getOutputStream();
            int n = 0;
            long readLength = 0;
            int bsize = 1024;
            byte[] bytes = new byte[bsize];
            if (rangeSwitch == 2) {
                // 针对 bytes=27000-39000 的请求，从27000开始写数据                    
                while (readLength <= contentLength - bsize) {
                    n = bis.read(bytes);
                    readLength += n;
                    out.write(bytes, 0, n);
                }
                if (readLength <= contentLength) {
                    n = bis.read(bytes, 0, (int) (contentLength - readLength));
                    out.write(bytes, 0, n);
                }                   
            } else {
                while ((n = bis.read(bytes)) != -1) {
                    out.write(bytes,0,n);                                                      
                }                   
            }
            out.flush();
            out.close();
            bis.close();
        } else {
            if (logger.isDebugEnabled()) {
                logger.debug("Error: file " + location + " not found.");
            }                
        }
    } catch (IOException ie) {
        // 忽略 ClientAbortException 之类的异常
    } catch (Exception e) {
        logger.error(e);
    }
}
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