步骤 2 - 使用正则表达式捕获Range消息头
有了这个HTTP服务器做基础，我们现在就可以用如下代码处理Range消息头了. 我们使用正则表达式将消息头分割，以获取开始和结束字符串。然后使用 parseInt() 方法将它们转换成整形数. 如果返回值是 NaN (非数字not a number), 那么这个字符串就是没有在这个消息头中的. 参数totalLength展示了当前文件的总字节数. 我们将使用它计算开始和结束位置.
function readRangeHeader(range, totalLength) {
/*
* Example of the method 'split' with regular expression.
*
* Input: bytes=100-200
* Output: [null, 100, 200, null]
*
* Input: bytes=-200
* Output: [null, null, 200, null]
*/
if (range == null || range.length == 0)
return null;
var array = range.split(/bytes=([0-9]*)-([0-9]*)/);
var start = parseInt(array[1]);
var end = parseInt(array[2]);
var result = {
Start: isNaN(start) ? 0 : start,
End: isNaN(end) ? (totalLength - 1) : end
};
if (!isNaN(start) && isNaN(end)) {
result.Start = start;
result.End = totalLength - 1;
}
if (isNaN(start) && !isNaN(end)) {
result.Start = totalLength - end;
result.End = totalLength - 1;
}
return result;
}
回到函数 httpListener(), 在HTTP方法通过之后，现在我们来检查请求的数据范围是否可用. 如果浏览器没有发送 Range 消息头过来, 请求就会直接被当做一般的请求对待. 服务器会返回整个文件，HTTP状态将会是 200 OK. 另外我们还会看看开始和结束位置是否比文件长度更大或者相等. 只要有一个是这种情况，请求的数据范围就是不能被满足的. 返回的状态就将会是 416 Requested Range Not Satisfiable 而 Content-Range 也会被发送.
var responseHeaders = {};
var stat = fs.statSync(filename);
var rangeRequest = readRangeHeader(request.headers['range'], stat.size);
// If 'Range' header exists, we will parse it with Regular Expression.
if (rangeRequest == null) {
responseHeaders['Content-Type'] = getMimeNameFromExt(path.extname(filename));
responseHeaders['Content-Length'] = stat.size; // File size.
responseHeaders['Accept-Ranges'] = 'bytes';
// If not, will return file directly.
sendResponse(response, 200, responseHeaders, fs.createReadStream(filename));
return null;
}
var start = rangeRequest.Start;
var end = rangeRequest.End;
// If the range can't be fulfilled.
if (start >= stat.size || end >= stat.size) {
// Indicate the acceptable range.
responseHeaders['Content-Range'] = 'bytes */' + stat.size; // File size.
// Return the 416 'Requested Range Not Satisfiable'.
sendResponse(response, 416, responseHeaders, null);
return null;
}
步骤 4 - 满足请求
最后使人迷惑的一块来了。对于状态 216 Partial Content, 我们有另外一种格式的 Content-Range 消息头，包括开始，结束位置以及当前文件的总字节数. 我们也还有 Content-Length 消息头，其值就等于开始和结束位置之间的差。在最后一句代码中，我们调用了 createReadStream() 并将开始和结束位置的值给了第二个参数选项的对象, 这意味着返回的流将只包含从开始到结束位置的只读数据.
// Indicate the current range.
responseHeaders['Content-Range'] = 'bytes ' + start + '-' + end + '/' + stat.size;
responseHeaders['Content-Length'] = start == end ? 0 : (end - start + 1);
responseHeaders['Content-Type'] = getMimeNameFromExt(path.extname(filename));
responseHeaders['Accept-Ranges'] = 'bytes';
responseHeaders['Cache-Control'] = 'no-cache';
// Return the 206 'Partial Content'.
sendResponse(response, 206,
responseHeaders, fs.createReadStream(filename, { start: start, end: end }));
下面是完整的 httpListener() 回调函数.
function httpListener(request, response) {
// We will only accept 'GET' method. Otherwise will return 405 'Method Not Allowed'.
if (request.method != 'GET') {
sendResponse(response, 405, { 'Allow': 'GET' }, null);
return null;
}
var filename =
initFolder + url.parse(request.url, true, true).pathname.split('/').join(path.sep);
// Check if file exists. If not, will return the 404 'Not Found'.
if (!fs.existsSync(filename)) {
sendResponse(response, 404, null, null);
return null;
}
var responseHeaders = {};
var stat = fs.statSync(filename);
var rangeRequest = readRangeHeader(request.headers['range'], stat.size);
// If 'Range' header exists, we will parse it with Regular Expression.
if (rangeRequest == null) {
responseHeaders['Content-Type'] = getMimeNameFromExt(path.extname(filename));
responseHeaders['Content-Length'] = stat.size; // File size.
responseHeaders['Accept-Ranges'] = 'bytes';
// If not, will return file directly.
sendResponse(response, 200, responseHeaders, fs.createReadStream(filename));
return null;
}
var start = rangeRequest.Start;
var end = rangeRequest.End;
// If the range can't be fulfilled.
if (start >= stat.size || end >= stat.size) {
// Indicate the acceptable range.
responseHeaders['Content-Range'] = 'bytes */' + stat.size; // File size.
// Return the 416 'Requested Range Not Satisfiable'.
sendResponse(response, 416, responseHeaders, null);
return null;
}
// Indicate the current range.
responseHeaders['Content-Range'] = 'bytes ' + start + '-' + end + '/' + stat.size;
responseHeaders['Content-Length'] = start == end ? 0 : (end - start + 1);
responseHeaders['Content-Type'] = getMimeNameFromExt(path.extname(filename));
responseHeaders['Accept-Ranges'] = 'bytes';
responseHeaders['Cache-Control'] = 'no-cache';
// Return the 206 'Partial Content'.
sendResponse(response, 206,
responseHeaders, fs.createReadStream(filename, { start: start, end: end }));
}