you can use the `RemoteWebDriver` class and the `attachToSession` method

 To run Selenium in an already open browser window in Java, you can use the `RemoteWebDriver` class and the `attachToSession` method. Here's an example code that shows how to attach to an already open Chrome window:

```java

import org.openqa.selenium.WebDriver;

import org.openqa.selenium.chrome.ChromeDriver;

import org.openqa.selenium.chrome.ChromeOptions;

import org.openqa.selenium.remote.DesiredCapabilities;

import org.openqa.selenium.remote.RemoteWebDriver;

import java.net.URL;

public class AttachToChromeWindow {

    public static void main(String[] args) throws Exception {

        // get the session ID of the open Chrome window

        String chromeSessionId = "1234567890"; // replace with the actual session ID

        // attach to the session using RemoteWebDriver

        ChromeOptions chromeOptions = new ChromeOptions();

        chromeOptions.addArguments("--start-maximized");

        DesiredCapabilities capabilities = DesiredCapabilities.chrome();

        capabilities.setCapability(ChromeOptions.CAPABILITY, chromeOptions);

        WebDriver driver = new RemoteWebDriver(new URL("http://localhost:9515"), capabilities);

        ((RemoteWebDriver) driver).setSessionId(chromeSessionId);

        // navigate to a URL using the attached Chrome window

        driver.get("https://www.google.com");

    }

}

```

In this code, the `chromeSessionId` variable stores the session ID of the open Chrome window. You can obtain the session ID by running the command `chrome://version/` in the address bar of the open Chrome window and looking for the value of the `Remote debugging port` field.