搜索了selenium2 的 FAQ有提到过
Q: WebDriver fails to find elements / Does not block on page loads
This problem can manifest itself in various ways:
* Using WebDriver.findElement(...) throws ElementNotFoundException, but the element is clearly there - inspecting the DOM (using Firebug, etc) clearly shows it.
* Calling Driver.get returns once the HTML has been loaded - but Javascript code triggered by the onload event was not done, so the page is incomplete and some elements cannot be found.
* Clicking on an element / link triggers an operation that creates new element. However, calling findElement(s) after click returns does not find it. Isn't click supposed to be blocking?
* How do I know when a page has finished loading?
Explanation: WebDriver has a blocking API, generally. However, under some conditions it is possible for a get call to return before the page has finished loading. The classic example is Javascript starting to run after the page has loaded (triggered by onload). Browsers (e.g. Firefox) will notify WebDriver when the basic HTML content has been loaded, which is when WebDriver returns. It's difficult (if not impossible) to know when Javascript has finished executing, since JS code may schedule functions to be called in the future, depend on server response, etc. This is also true for clicking - when the platform supports native events (Windows, Linux) clicking is done by sending a mouse click event with the element's coordinates at the OS level - WebDriver cannot track the exact sequence of operations this click creates. For this reason, the blocking API is imperfect - WebDriver cannot wait for all conditions to be met before the test proceeds because it does not know them. Usually, the important matter is whether the element involved in the next interaction is present and ready.
Solution: Use the Wait class to wait for a specific element to appear. This class simply calls findElement over and over, discarding the NoSuchElementException each time, until the element is found (or a timeout has expired). Since this is the behaviour desired by default for many users, a mechanism for implicitly-waiting for elements to appear has been implemented. This is accessible through the WebDriver.manage().timeouts() call. (This was previously tracked on issue 26).
