inputStream对象的available()方法
???? /** * Gets the content length in bytes specified by the response header field * {@code content-length} or {@code -1} if this field is not set. * * @return the value of the response header field {@code content-length}. * @since Android 1.0 */ public int getContentLength() { return getHeaderFieldInt("Content-Length", -1); //$NON-NLS-1$ }

?

关键：getHeaderFieldInt("Content-Length", -1);

意思是从http预解析头中获取“Content-length”字段的值

其实也是类似从文件描述符中获取文件的总大小