提供向 URI 标识的资源发送数据和从 URI 标识的资源接收数据的公共方法；支持以 http:、https:、ftp:、和 file: 方案标识符开头的 URI，获取页面内容或者文件-URI identifies the resources available to send data to and from URI identifies the resource to receive data on public ways　support for http:, https:, ftp:, and file: the program identifier at the beginning of the URI, access to the page content or files