SELENIUM是如何啟動瀏覽器的

來自專欄 測試圈TC

前幾天有同學問到selenium是怎麼樣啟動瀏覽器的(selenium啟動瀏覽器的原理)，當時稍微講解了一下，不過自我感覺不夠具體，現在特地把啟動原理通過代碼和一系列操作給串聯起來，希望可以幫助大家更好的理解。

以chrome瀏覽器為例，selenium啟動chrome瀏覽器的代碼如下：

源碼

def __init__(self, executable_path="chromedriver", port=0, options=None, service_args=None, desired_capabilities=None, service_log_path=None, chrome_options=None): """ Creates a new instance of the chrome driver. Starts the service and then creates new instance of chrome driver. :Args: - executable_path - path to the executable. If the default is used it assumes the executable is in the $PATH - port - port you would like the service to run, if left as 0, a free port will be found. - desired_capabilities: Dictionary object with non-browser specific capabilities only, such as "proxy" or "loggingPref". - options: this takes an instance of ChromeOptions """ if chrome_options: warnings.warn(use options instead of chrome_options, DeprecationWarning) options = chrome_options if options is None: # desired_capabilities stays as passed in if desired_capabilities is None: desired_capabilities = self.create_options().to_capabilities() else: if desired_capabilities is None: desired_capabilities = options.to_capabilities() else: desired_capabilities.update(options.to_capabilities()) self.service = Service( executable_path, port=port, service_args=service_args, log_path=service_log_path) self.service.start() try: RemoteWebDriver.__init__( self, command_executor=ChromeRemoteConnection( remote_server_addr=self.service.service_url), desired_capabilities=desired_capabilities) except Exception: self.quit() raise self._is_remote = False

其中跟瀏覽器啟動密切相關的是這幾句

self.service = Service( executable_path, port=port, service_args=service_args, log_path=service_log_path)self.service.start()

通過查看跟Service相關的代碼復盤得到啟動邏輯: 調用chromedriver可執行文件運行chromedirver。這也是為什麼我們需要把chromedriver放到系統PATH里的原因。

所以selenium先啟動了chromedriver。當然，我們可以手工啟動chromedriver來模擬這個啟動過程。

在命令行中運行下面的命令chromedirver

你應該可以看來類似的結果

Starting ChromeDriver 2.38.552518 (183d19265345f54ce39cbb94cf81ba5f15905011) on port 9515Only local connections are allowed.

這樣我們就手工啟動了chromedriver。driver監聽的埠是9515.

啟動了driver之後，我們需要告訴driver打開瀏覽器。selenium的源碼里這一過程如下

def start_session(self, capabilities, browser_profile=None): """ Creates a new session with the desired capabilities. :Args: - browser_name - The name of the browser to request. - version - Which browser version to request. - platform - Which platform to request the browser on. - javascript_enabled - Whether the new session should support JavaScript. - browser_profile - A selenium.webdriver.firefox.firefox_profile.FirefoxProfile object. Only used if Firefox is requested. """ if not isinstance(capabilities, dict): raise InvalidArgumentException("Capabilities must be a dictionary") if browser_profile: if "moz:firefoxOptions" in capabilities: capabilities["moz:firefoxOptions"]["profile"] = browser_profile.encoded else: capabilities.update({firefox_profile: browser_profile.encoded}) w3c_caps = _make_w3c_caps(capabilities) parameters = {"capabilities": w3c_caps, "desiredCapabilities": capabilities} response = self.execute(Command.NEW_SESSION, parameters) if sessionId not in response: response = response[value] self.session_id = response[sessionId] self.capabilities = response.get(value) # if capabilities is none we are probably speaking to # a W3C endpoint if self.capabilities is None: self.capabilities = response.get(capabilities) # Double check to see if we have a W3C Compliant browser self.w3c = response.get(status) is None self.command_executor.w3c = self.w3c

這一過程的核心就是就是向localhost:9515/session發送1個POST請求，並發送1個json對象，默認情況下，這個對象應該是下面這個樣子。

{ "capabilities": { "alwaysMatch": { "browserName": "chrome", "goog:chromeOptions": { "args": [], "extensions": [] }, "platformName": "any" }, "firstMatch": [ {} ] }, "desiredCapabilities": { "browserName": "chrome", "goog:chromeOptions": { "args": [], "extensions": [] }, "platform": "ANY", "version": "" }}

簡單理解就是告訴remote driver打開什麼瀏覽器，上面的例子里我們打開的是chrome瀏覽器。

我們可以手工還原這個過程。

確保chromedriver是在運行中的，然後打開postman，構造1個POST請求，路徑是localhost:9515/session。在Body里選擇raw和JSON(application/json), 貼入上面的json字元串，如下圖所示

點擊send，幾秒之後chrome瀏覽器應該可以正常啟動，並且postman的response里會有大致如下的返回值

{ "sessionId": "ad4407e133cfd5f3f49bff4c2f1f087a", "status": 0, "value": { "acceptInsecureCerts": false, "acceptSslCerts": false, "applicationCacheEnabled": false, "browserConnectionEnabled": false, "browserName": "chrome", "chrome": { "chromedriverVersion": "2.38.552518 (183d19265345f54ce39cbb94cf81ba5f15905011)", "userDataDir": "/var/folders/s6/f2_brc114wv2g8w0qggk_m2c0000gn/T/.org.chromium.Chromium.NMsAKJ" }, "cssSelectorsEnabled": true, "databaseEnabled": false, "handlesAlerts": true, "hasTouchScreen": false, "javascriptEnabled": true, "locationContextEnabled": true, "mobileEmulationEnabled": false, "nativeEvents": true, "networkConnectionEnabled": false, "pageLoadStrategy": "normal", "platform": "Mac OS X", "rotatable": false, "setWindowRect": true, "takesHeapSnapshot": true, "takesScreenshot": true, "unexpectedAlertBehaviour": "", "version": "66.0.3359.181", "webStorageEnabled": true }}

上面的返回里最重要的就是sessionId，因為後面所有跟瀏覽器的交互都是基於該id進行的。

總結

• selenium里，selenium client先打開chromedriver
• chromedirver創建session時打開了瀏覽器，所以瀏覽器的打開跟selenium無關，完全是chromedriver的能力

更多

其實上面的例子里我們手工調用了webdriver協議里的new session協議，創建了webdriver session。具體的細節大家可以參考協議了解更多。