底层原理如下： 1.在自动化测试过程中，存在三部分组件：客户端脚本+驱动+浏览器终端。 2.驱动文件，以geckodriver.exe为例，这个可执行的驱动文件启动后，相当于一个暴露了一系列接
底层原理如下：
1.在自动化测试过程中，存在三部分组件：客户端脚本+驱动+浏览器终端。
2.驱动文件，以geckodriver.exe为例，这个可执行的驱动文件启动后，相当于一个暴露了一系列接口的服务器，监听某一端口，例如：89890。
3.客户端的操作（访问页面，定位元素，输入数据，点击按钮等）都是封装成了接口请求（eg：/session/xx/yy），然后提交到驱动服务器。
4.驱动服务器接收到客户端的请求后，再跟终端浏览器交互。
5.终端浏览器做出相应操作。
下图描述了整个交互过程：
以定位元素为例，定位搜索框，我们来看底下这行代码在执行的时候底层到底经历了些什么：
WebElement searchInputBox = driver.findElement(By.id("js_keyword"));
实际，底层请求时，每个请求会被封装为一个command，然后根据不同的commannd封装得到不同的HttpRequest对象：
根据此命令，得到接口地址：
拿到此接口地址封装为一个HttpRequest请求。
client.execute（httpRequest，true），执行接口调用：
至于其他操作：往输入框输入数据，点击按钮等，都是对应一个接口地址，通过调用接口，请求驱动来处理，最后驱动同浏览器进行交互，浏览器按照指示做出对应操作。
Selenium有一个类AbstractHttpCommandCodec，此类中维护了众多自动化操作对应的接口地址：
public AbstractHttpCommandCodec() {
defineCommand(STATUS, get("/status"));
defineCommand(GET_ALL_SESSIONS, get("/sessions"));
defineCommand(NEW_SESSION, post("/session"));
defineCommand(GET_CAPABILITIES, get("/session/:sessionId"));
defineCommand(QUIT, delete("/session/:sessionId"));
defineCommand(GET_SESSION_LOGS, post("/logs"));
defineCommand(GET_LOG, post("/session/:sessionId/log"));
defineCommand(GET_AVAILABLE_LOG_TYPES, get("/session/:sessionId/log/types"));
defineCommand(SWITCH_TO_FRAME, post("/session/:sessionId/frame"));
defineCommand(SWITCH_TO_PARENT_FRAME, post("/session/:sessionId/frame/parent"));
defineCommand(CLOSE, delete("/session/:sessionId/window"));
defineCommand(SWITCH_TO_WINDOW, post("/session/:sessionId/window"));
defineCommand(FULLSCREEN_CURRENT_WINDOW, post("/session/:sessionId/window/fullscreen"));
defineCommand(GET_CURRENT_URL, get("/session/:sessionId/url"));
defineCommand(GET, post("/session/:sessionId/url"));
defineCommand(GO_BACK, post("/session/:sessionId/back"));
defineCommand(GO_FORWARD, post("/session/:sessionId/forward"));
defineCommand(REFRESH, post("/session/:sessionId/refresh"));
defineCommand(SET_ALERT_CREDENTIALS, post("/session/:sessionId/alert/credentials"));
defineCommand(UPLOAD_FILE, post("/session/:sessionId/file"));
defineCommand(SCREENSHOT, get("/session/:sessionId/screenshot"));
defineCommand(ELEMENT_SCREENSHOT, get("/session/:sessionId/screenshot/:id"));
defineCommand(GET_TITLE, get("/session/:sessionId/title"));
defineCommand(FIND_ELEMENT, post("/session/:sessionId/element"));
defineCommand(FIND_ELEMENTS, post("/session/:sessionId/elements"));
defineCommand(GET_ELEMENT_PROPERTY, get("/session/:sessionId/element/:id/property/:name"));
defineCommand(CLICK_ELEMENT, post("/session/:sessionId/element/:id/click"));
defineCommand(CLEAR_ELEMENT, post("/session/:sessionId/element/:id/clear"));
defineCommand(
GET_ELEMENT_VALUE_OF_CSS_PROPERTY,
get("/session/:sessionId/element/:id/css/:propertyName"));
defineCommand(FIND_CHILD_ELEMENT, post("/session/:sessionId/element/:id/element"));
defineCommand(FIND_CHILD_ELEMENTS, post("/session/:sessionId/element/:id/elements"));
defineCommand(IS_ELEMENT_ENABLED, get("/session/:sessionId/element/:id/enabled"));
defineCommand(ELEMENT_EQUALS, get("/session/:sessionId/element/:id/equals/:other"));
defineCommand(GET_ELEMENT_RECT, get("/session/:sessionId/element/:id/rect"));
defineCommand(GET_ELEMENT_LOCATION, get("/session/:sessionId/element/:id/location"));
defineCommand(GET_ELEMENT_TAG_NAME, get("/session/:sessionId/element/:id/name"));
defineCommand(IS_ELEMENT_SELECTED, get("/session/:sessionId/element/:id/selected"));
defineCommand(GET_ELEMENT_SIZE, get("/session/:sessionId/element/:id/size"));
defineCommand(GET_ELEMENT_TEXT, get("/session/:sessionId/element/:id/text"));
defineCommand(SEND_KEYS_TO_ELEMENT, post("/session/:sessionId/element/:id/value"));
defineCommand(GET_ALL_COOKIES, get("/session/:sessionId/cookie"));
defineCommand(GET_COOKIE, get("/session/:sessionId/cookie/:name"));
defineCommand(ADD_COOKIE, post("/session/:sessionId/cookie"));
defineCommand(DELETE_ALL_COOKIES, delete("/session/:sessionId/cookie"));
defineCommand(DELETE_COOKIE, delete("/session/:sessionId/cookie/:name"));
defineCommand(SET_TIMEOUT, post("/session/:sessionId/timeouts"));
defineCommand(SET_SCRIPT_TIMEOUT, post("/session/:sessionId/timeouts/async_script"));
defineCommand(IMPLICITLY_WAIT, post("/session/:sessionId/timeouts/implicit_wait"));
defineCommand(GET_APP_CACHE_STATUS, get("/session/:sessionId/application_cache/status"));
defineCommand(IS_BROWSER_ONLINE, get("/session/:sessionId/browser_connection"));
defineCommand(SET_BROWSER_ONLINE, post("/session/:sessionId/browser_connection"));
defineCommand(GET_LOCATION, get("/session/:sessionId/location"));
defineCommand(SET_LOCATION, post("/session/:sessionId/location"));
defineCommand(GET_SCREEN_ORIENTATION, get("/session/:sessionId/orientation"));
defineCommand(SET_SCREEN_ORIENTATION, post("/session/:sessionId/orientation"));
defineCommand(GET_SCREEN_ROTATION, get("/session/:sessionId/rotation"));
defineCommand(SET_SCREEN_ROTATION, post("/session/:sessionId/rotation"));
defineCommand(IME_GET_AVAILABLE_ENGINES, get("/session/:sessionId/ime/available_engines"));
defineCommand(IME_GET_ACTIVE_ENGINE, get("/session/:sessionId/ime/active_engine"));
defineCommand(IME_IS_ACTIVATED, get("/session/:sessionId/ime/activated"));
defineCommand(IME_DEACTIVATE, post("/session/:sessionId/ime/deactivate"));
defineCommand(IME_ACTIVATE_ENGINE, post("/session/:sessionId/ime/activate"));
// Mobile Spec
defineCommand(GET_NETWORK_CONNECTION, get("/session/:sessionId/network_connection"));
defineCommand(SET_NETWORK_CONNECTION, post("/session/:sessionId/network_connection"));
defineCommand(SWITCH_TO_CONTEXT, post("/session/:sessionId/context"));
defineCommand(GET_CURRENT_CONTEXT_HANDLE, get("/session/:sessionId/context"));
defineCommand(GET_CONTEXT_HANDLES, get("/session/:sessionId/contexts"));
}
另外，可能会有人好奇，驱动服务器是何时启动的服务。其实是在执行下面这行代码的时候启动的，大家可执行去debug调试selenium的底层代码：
//创建火狐驱动对象 WebDriver driver = new FirefoxDriver();
当上面这行代码执行完，可以发现eclipse的控制台显示了如下信息：
1531911173760 geckodriver INFO geckodriver 0.19.0 1531911173772 geckodriver INFO Listening on 127.0.0.1:21984
说明此驱动服务器成功启动了，并且监听了本机的21984端口，等待客户端发起请求，并处理。
转载于：https://www.cnblogs.com/nickjiang/p/9332480.html#4033469