底層原理如下：
1.在自動化測試過程中，存在三部分組件：客戶端腳本+驅動+瀏覽器終端。
2.驅動文件，以geckodriver.exe為例，這個可執行的驅動文件啟動后，相當於一個暴露了一系列接口的服務器，監聽某一端口，例如：89890。
3.客戶端的操作（訪問頁面，定位元素，輸入數據，點擊按鈕等）都是封裝成了接口請求（eg：/session/xx/yy），然后提交到驅動服務器。
4.驅動服務器接收到客戶端的請求后，再跟終端瀏覽器交互。
5.終端瀏覽器做出相應操作。
下圖描述了整個交互過程：
以定位元素為例，定位搜索框，我們來看底下這行代碼在執行的時候底層到底經歷了些什么：
WebElement searchInputBox = driver.findElement(By.id("js_keyword"));
實際，底層請求時，每個請求會被封裝為一個command，然后根據不同的commannd封裝得到不同的HttpRequest對象：
根據此命令，得到接口地址：
拿到此接口地址封裝為一個HttpRequest請求。
client.execute（httpRequest，true），執行接口調用：
至於其他操作：往輸入框輸入數據，點擊按鈕等，都是對應一個接口地址，通過調用接口，請求驅動來處理，最后驅動同瀏覽器進行交互，瀏覽器按照指示做出對應操作。
Selenium有一個類AbstractHttpCommandCodec，此類中維護了眾多自動化操作對應的接口地址：
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，可能會有人好奇，驅動服務器是何時啟動的服務。其實是在執行下面這行代碼的時候啟動的，大家可執行去debug調試selenium的底層代碼：
//創建火狐驅動對象 WebDriver driver = new FirefoxDriver();
當上面這行代碼執行完，可以發現eclipse的控制台顯示了如下信息：
1531911173760 geckodriver INFO geckodriver 0.19.0 1531911173772 geckodriver INFO Listening on 127.0.0.1:21984
說明此驅動服務器成功啟動了，並且監聽了本機的21984端口，等待客戶端發起請求，並處理。
轉載於：https://www.cnblogs.com/nickjiang/p/9332480.html#4033469