TL;DR:始终使用显式等待.忘记隐式等待的存在.

TL;DR: Always use explicit wait. Forget that implicit wait exists.

以下是显式等待和隐式等待之间差异的简要概述:

Here is a quick rundown on the differences between explicit and implicit wait:

显式等待:

• 记录和定义的行为.
• 在 selenium 的本地部分运行(以您的代码语言).
• 适用于您能想到的任何条件.
• 返回成功或超时错误.
• 可以将缺少元素定义为成功条件.
• 可以自定义重试和要忽略的异常之间的延迟.

隐式等待:

• 未记录且几乎未定义的行为.
• 在 selenium 的远程部分(控制浏览器的部分)中运行.
• 仅适用于查找元素方法.
• 返回找到的元素或(超时后)未找到的元素.
• 如果检查元素的缺失必须一直等到超时.
• 除全局超时外无法自定义.

带有解释的代码示例.第一个隐式等待:

Code examples with explanation. First implicit wait:

WebDriver driver = new FirefoxDriver();
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
driver.get("http://somedomain/url_that_delays_loading");
WebElement myDynamicElement = driver.findElement(By.id("myDynamicElement"));

现在显式等待:

WebDriver driver = new FirefoxDriver();
driver.get("http://somedomain/url_that_delays_loading");
WebDriverWait wait = new WebDriverWait(driver, 10);
WebElement myDynamicElement = wait.until(
  ExpectedConditions.presenceOfElementLocated(By.id("myDynamicElement")));

两个代码示例都做同样的事情.找到某个元素并在 10 秒后未找到则放弃.隐式等待只能做到这一点.它只能尝试找到超时的元素.显式等待的优势在于它可以等待各种条件.还可以自定义超时并忽略某些异常.

Both code examples do the same thing. Find a certain element and give up if not found after 10 seconds. The implicit wait can do only this. It can only try to find an element with a timeout. The strength of explicit wait is that it can wait for all kinds of conditions. Also customize timeout and ignore certain exceptions.

可能的条件示例:elementToBeClickable、numberOfElementsToBeMoreThan 或 invisibilityOf.以下是内置预期条件的列表:https://seleniumhq.github.io/selenium/docs/api/java/org/openqa/selenium/support/ui/ExpectedConditions.html

Example of possible conditions: elementToBeClickable, numberOfElementsToBeMoreThan or invisibilityOf. Here is a list of the built in expected conditions: https://seleniumhq.github.io/selenium/docs/api/java/org/openqa/selenium/support/ui/ExpectedConditions.html

更多解释:

隐式等待超时仅对 findElement* 方法有效.如果设置，则所有 findElement* 将在声明无法找到元素之前等待"设置的时间.

The implicit wait timeout has effect only on findElement* methods. If set then all findElement* will "wait" for the set time before declaring that the element cannot be found.

findElement* 将如何等待尚未定义.这取决于浏览器或操作系统或 selenium 版本.可能的实现是:

How a findElement* will wait is not defined. It depends on browser or operating system or version of selenium. Possible implementations are:

• 反复尝试查找元素直到超时.找到元素后立即返回.
• 尝试查找元素.等到超时.再试一次.
• 等到超时.尝试查找元素.

这个列表是通过观察和阅读错误报告以及对 selenium 源代码的粗略阅读收集的.

This list is gathered from observations and reading bug reports and cursory reading of selenium source code.

我的结论:隐式等待是不好的.能力有限.该行为未记录在案且依赖于实现.

My conclusion: Implicit wait is bad. The capabilities are limited. The behaviour is undocumented and implementation dependent.

显式等待可以完成隐式等待可以做的一切，甚至更多.显式等待的唯一缺点是代码更冗长.但是这种冗长使代码变得明确.显式优于隐式.对吗?

Explicit wait can do everything implicit wait can and more. The only disadvantage of explicit wait is a bit more verbose code. But that verbosity makes the code explicit. And explicit is better that implicit. Right?

进一步阅读:

• 官方文档(并没有真正解释除了混合隐式和显式等待发出警告之外，还有其他问题).
• 回答 Jim Evans 提出的相关问题.Jim Evans 是 selenium 的维护者.总结:不要混用隐式等待和显式等待.
• 两篇博文详细解释了隐式和显式等待:
  • http://vnrtech.blogspot.de/2013/04/selenium-implicit-wait.html
  • http://vnrtech.blogspot.de/2013/04/selenium-explicit-wait.html
  • Official documentation (does not really explain the problematic other than warning from mixing implicit and explicit wait).
  • Answer on a related question from Jim Evans. Jim Evans is a maintainer of selenium. Summary: don't mix implicit and explicit wait.
  • Two blog posts explaining implicit and explicit wait in great detail:
    • http://vnrtech.blogspot.de/2013/04/selenium-implicit-wait.html
    • http://vnrtech.blogspot.de/2013/04/selenium-explicit-wait.html
    • http://code.google.com/p/selenium/问题/详细信息?id=2934
    • http://code.google.com/p/selenium/问题/详细信息?id=4471
    • http://code.google.com/p/selenium/问题/详细信息?id=7972
    • 显式等待
    • 隐式等待