selenium: What if user close the browser or webdriver? how can I detect if the browser is closed?

You can perform any action on driver object, if it is throwing UnreachableBrowserException, then there is problem to communicate with the browser.

The most common causes for this exception are:

1. The provided server address to RemoteWebDriver is invalid, so the connection could not be established.
2. The browser has died mid-test.

And you can call the following method to verify the browser is closed or not.

public boolean isBrowserClosed(WebDriver driver){    boolean isClosed = false;    try {        driver.getTitle();    } catch(UnreachableBrowserException ubex) {        isClosed = true;    }    return isClosed;}

Let me answer your questions one by one :

• What if user close the browser or webdriver : First of all Automated Test Execution shouldn't be interuptted by Manual Intervention. It's against all the Best Practices. If you forcefully close the Web Browser then WebDriver will throw org.openqa.selenium.WebDriverException as follows :

  Exception in thread "main" org.openqa.selenium.WebDriverException: Process unexpectedly closed with status: 0Build info: version: '3.8.1', revision: '6e95a6684b', time: '2017-12-01T19:05:32.194Z'System info: host: 'foo', ip: 'foo', os.name: 'foo', os.arch: 'foo', os.version: 'foo', java.version: 'foo'Driver info: driver.version: FirefoxDriverremote stacktrace: stack backtrace:   0:           0x47e934 - <no info>   1:           0x47f0a3 - <no info>   2:           0x442649 - <no info>   3:           0x449cc3 - <no info>   4:           0x42a890 - <no info>   5:           0x406f5e - <no info>   6:           0x40cfc9 - <no info>   7:           0x6bef19 - <no info>   8:           0x420756 - <no info>   9:           0x6b96e0 - <no info>  10:      0x7fa0fb01842 - BaseThreadInitThunk

• How can I detect if the browser is closed? : If the Automation Script handles proper initiation and closure of Web Browser you don't need to explicitly validate if Browser is Closed or not. Cross checking dead Web Browser and Web Browser session / chores would be a pure Overhead. So a better practice would be to write Clean Code

• Restart the webdriver if there is no browser : Should be handled by the Automation Script through WebDriver and Web Browser initialization.
• User close the browser, webdriver is not destroyed : As you WebDriver and Web Browser was started through you Automation Script an user closing the Web Browser would be a Malpractice. Web Browser closure must be handled by the the Automation Script.

• I want restart the webdriver : Neither you can connect to the previous instance of the WebDriver nor to the previous instance of the Web Browser. You have to reinitialize again as follows :

  WebDriver driver =  new ChromeDriver();

See the discussion How can I reconnect to the browser opened by webdriver with selenium? for details.