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selenium是如何启动浏览器的

selenium是如何启动浏览器的

作者: 乙醇的简书 | 来源:发表于2018-07-01 08:06 被阅读2次

    前几天有同学问到selenium是怎么样启动浏览器的(selenium启动浏览器的原理),当时稍微讲解了一下,不过自我感觉不够具体,现在特地把启动原理通过代码和一系列操作给串联起来,希望可以帮助大家更好的理解。

    以chrome浏览器为例,selenium启动chrome浏览器的代码如下:

    源码

    
    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
    
    

    其中跟浏览器启动密切相关的是这几句

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

    通过查看跟Service相关的代码复盘得到启动逻辑: 调用chromedriver可执行文件运行chromedirver。这也是为什么我们需要把chromedriver放到系统PATH里的原因。

    所以selenium先启动了chromedriver。当然,我们可以手工启动chromedriver来模拟这个启动过程。

    在命令行中运行下面的命令chromedirver

    你应该可以看来类似的结果

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

    这样我们就手工启动了chromedriver。driver监听的端口是9515.

    启动了driver之后,我们需要告诉driver打开浏览器。selenium的源码里这一过程如下

    
    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
    

    这一过程的核心就是就是向localhost:9515/session发送1个POST请求,并发送1个json对象,默认情况下,这个对象应该是下面这个样子。

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

    简单理解就是告诉remote driver打开什么浏览器,上面的例子里我们打开的是chrome浏览器。

    我们可以手工还原这个过程。

    确保chromedriver是在运行中的,然后打开postman,构造1个POST请求,路径是localhost:9515/session。在Body里选择raw和JSON(application/json), 贴入上面的json字符串,如下图所示

    postman.png

    点击send,几秒之后chrome浏览器应该可以正常启动,并且postman的response里会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,因为后面所有跟浏览器的交互都是基于该id进行的。

    总结

    • selenium里,selenium client先打开chromedriver
    • chromedirver创建session时打开了浏览器,所以浏览器的打开跟selenium无关,完全是chromedriver的能力

    更多

    其实上面的例子里我们手工调用了webdriver协议里的new session协议,创建了webdriver session。具体的细节大家可以参考协议了解更多。

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