Spring 3：注入默认Bean，除非存在另一个Bean [英] Spring 3: Inject Default Bean Unless Another Bean Present
问题描述
我想通过XML配置Spring，如果存在特定的bean，它将被注入目标bean。如果它不存在，将注入一个不同的默认bean。
I would like to configure Spring via XML such that if a particular bean exists, it will be injected into the target bean. If it does not exist, a different, default bean, will be injected.
例如，如果我有这样的文件
For example if I have a file like this
<bean id="carDriver" class="Driver">
<property name="car" value="SOME EXPRESSION GOES HERE, SEE ATTEMPT BELOW"/>
</bean>
<bead id="defaultCar" class="Car">
<property name="name" value="Honda Accord"/>
</bean>
加载它，我想要 defaultCar 注入司机。但是，如果我还加载以下文件：
And load it, I would like the defaultCar injected into the driver. However, if I also load the following file:
<bean id="customCar" class="FlyingCar">
<property name="name" value="Rocket Car"/>
<property name="maxAltitude" value="80000"/>
</bean>
我希望 customCar bean成为用来代替 defaultCar bean。我最初的尝试不起作用，但我认为说明了我想要实现的目标：
I would want the customCar bean to be used instead of the defaultCar bean. My initial attempt does not work, but I think illustrates what I'm trying to achieve:
<bean id="carDriver" class="Driver">
<property name="car" value="#{ @customCar eq null ? 'defaultCar' : 'customCar' }"/>
</bean>
我知道如何使用 PropertyPlaceholderConfigurer ，但除了包含自定义bean的文件之外，我不想提供属性文件/ VM属性/环境变量/等。谢谢！
I know how to do this with a PropertyPlaceholderConfigurer, but I don't want to have to provide a property file / VM property / environment variable / etc. in addition to the file that contains the custom bean. Thanks!
更新：
基于使用一个工厂豆评论，我调查了这个，并提出了以下解决方案。首先，我创建了一个通用工厂bean，允许您指定默认bean名称和覆盖bean名称：
Based on the "use a factory bean" comments, I looked into this and came up with the following solution. First, I created a generic factory bean that allows you to specify a default bean name and an override bean name:
public class DefaultOverrideFactoryBean implements FactoryBean, BeanFactoryAware {
public Object getObject() throws Exception {
return beanFactory.containsBean(overrideBeanName) ?
beanFactory.getBean(overrideBeanName) :
beanFactory.getBean(defaultBeanName);
}
public Class<?> getObjectType() {
return Object.class;
}
public boolean isSingleton() {
return true;
}
public void setBeanFactory(BeanFactory beanFactory) throws BeansException {
this.beanFactory = beanFactory;
}
public void setDefaultBeanName(String defaultBeanName) {
this.defaultBeanName = defaultBeanName;
}
public void setOverrideBeanName(String overrideBeanName) {
this.overrideBeanName = overrideBeanName;
}
private String defaultBeanName;
private String overrideBeanName;
private BeanFactory beanFactory;
}
要配置我的示例汽车司机，你可以这样做：
To configure my example car driver, you would do this:
<bean id="carDriver" class="Driver">
<property name="car">
<bean class="DefaultOverrideFactoryBean">
<property name="defaultBeanName" value="defaultCar"/>
<property name="overrideBeanName" value="customCar"/>
</bean>
</property>
</bean>
我本来希望使用SpEL，但这样可行。也许添加一个自定义架构元素可以使它变得更干净。
I would have preferred to use SpEL, but this works. Perhaps adding a custom schema element woud make this cleaner.
赞赏其他评论。
推荐答案
使用FactoryBean是最简单的解决方案 - 您可以描述您想要的任何算法。
更多信息来自
Using FactoryBean is the simplest solution - you can describe any algorithm you want. More information is at
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