selenium 定位不到元素的几种情况

1.动态id定位不到元素
for example:
 

//WebElement xiexin_element = driver.findElement(By.id("_mail_component_82_82"));WebElement xiexin_element = driver.findElement(By.xpath("//span[contains(.,'写 信')]"));
xiexin_element.click();

  上面一段代码