java 如何了解运行时是否存在 bean？
声明：本页面是StackOverFlow热门问题的中英对照翻译，遵循CC BY-SA 4.0协议，如果您需要使用它，必须同样遵循CC BY-SA许可，注明原文地址和作者信息，同时你必须将它归于原作者（不是我）：StackOverFlow
原文地址: http://stackoverflow.com/questions/14723984/
Warning: these are provided under cc-by-sa 4.0 license. You are free to use/share it, But you must attribute it to the original authors (not me):
StackOverFlow
How can I understand whether a bean exists in runtime?
提问by Mary Ryllo
For example, I have a class
例如，我有一个类
public class Car{
private Motor motor;
public void setMotor(Motor motor){
this.motor = motor;
}
}
My bean looks like
我的豆子看起来像
<bean id="car" class="Car">
<property name="motor" ref="${motorProvider.getAvailableMotor()}"/>
</bean>
This method: motorProvider.getAvailableMotor()returns a bean name(string), of which motor I should use.
这个方法： motorProvider.getAvailableMotor()返回一个bean名称（字符串），我应该使用哪个电机。
But there can be a situation when such bean(with such name) is not created. How can I check it?
但是可能存在未创建此类 bean（具有此类名称）的情况。我该如何检查？
回答by Aaron Digulla
There are several patterns how to do this. Here is one I often use:
有几种模式可以做到这一点。这是我经常使用的一种：
public class Car{
private Motor motor;
@Autowired
private ApplicationContext applicationContext;
@PostConstruct
public void init() {
try {
motor = applicationContext.getBean( Motor.class );
} catch( NoSuchBeanDefinitionException e ) {
motor = new DefaultMotor();
}
}
}
Note you could also call applicationContext.containsBeanDefinition(name)but that would search your context twice (once in containsBeanDefinition()and then second time when you call getBean()) so catching the exception is usually faster.
请注意，您也可以调用，applicationContext.containsBeanDefinition(name)但这会搜索您的上下文两次（一次containsBeanDefinition()，然后在您调用时第二次getBean()），因此捕获异常通常会更快。
Since we catch a specific exception that says "bean doesn't exist", using if/elsehas almost no advantage anymore.
由于我们捕获了一个说“bean 不存在”的特定异常，因此使用if/else几乎没有任何优势了。
回答by user1121883
SPeL; something like:
标语；就像是：
<property name="motor" value="#(if(${motorProvider} != null) ${motorProvider.getAvailableMotor()})"/>
I think it was discussed also here: Spring - set a property only if the value is not null. As they said before for more information see: http://static.springsource.org/spring/docs/3.0.5.RELEASE/reference/expressions.html
我认为这里也讨论过：Spring - set a property only if the value is not null。正如他们之前所说的更多信息，请参见：http: //static.springsource.org/spring/docs/3.0.5.RELEASE/reference/expressions.html
