java Spring 3：除非另一个 Bean 存在，否则注入默认 Bean
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Spring 3: Inject Default Bean Unless Another Bean Present
提问by SingleShot
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.
我想通过 XML 配置 Spring，以便如果存在特定的 bean，它将被注入到目标 bean 中。如果它不存在，将注入一个不同的默认 bean。
For example if I have a file like this
例如，如果我有一个这样的文件
<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>
And load it, I would like the defaultCarinjected into the driver. However, if I also load the following file:
并加载它，我想defaultCar注入驱动程序。但是，如果我还加载以下文件：
<bean id="customCar" class="FlyingCar">
<property name="name" value="Rocket Car"/>
<property name="maxAltitude" value="80000"/>
</bean>
I would want the customCarbean to be used instead of the defaultCarbean. My initial attempt does not work, but I think illustrates what I'm trying to achieve:
我希望使用customCarbean 而不是defaultCarbean。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：
<bean id="carDriver" class="Driver">
<property name="car" value="#{ @customCar eq null ? 'defaultCar' : 'customCar' }"/>
</bean>
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!
我知道如何使用 . 来执行此操作PropertyPlaceholderConfigurer，但除了包含自定义 bean 的文件之外，我不想还必须提供属性文件/VM 属性/环境变量等。谢谢！
Update:
更新：
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:
基于“使用工厂 bean”的评论，我对此进行了调查并提出了以下解决方案。首先，我创建了一个通用工厂 bean，它允许您指定默认 bean 名称和覆盖 bean 名称：
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;
}
To configure my example car driver, you would do this:
要配置我的示例汽车驱动程序，您可以这样做：
<bean id="carDriver" class="Driver">
<property name="car">
<bean class="DefaultOverrideFactoryBean">
<property name="defaultBeanName" value="defaultCar"/>
<property name="overrideBeanName" value="customCar"/>
</bean>
</property>
</bean>
I would have preferred to use SpEL, but this works. Perhaps adding a custom schema element woud make this cleaner.
我更喜欢使用 SpEL，但这有效。也许添加一个自定义架构元素会让这更干净。
Additional comments appreciated.
补充意见表示赞赏。
采纳答案by Eugene Ryzhikov
Using FactoryBean is the simplest solution - you can describe any algorithm you want. More information is at
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。更多信息在
回答by EliuX
You may used @Qualifier to choose one version of Car (custom or default), but you shall know the specific name of what you gonna use, and you may want to use just:
您可以使用@Qualifier 来选择一个版本的 Car（自定义或默认），但您应该知道要使用的具体名称，并且您可能只想使用：
@Autowired
private Car car;
You may also use @Primary to solve this, but it just gives a preference to avoid ambiguity and it will be created the unwanted versions. So i would recomend to use the annotation
您也可以使用 @Primary 来解决这个问题，但它只是提供了一个避免歧义的偏好，并且会创建不需要的版本。所以我建议使用注释
org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean
So you will only instantate one bean if another is not created. Its specially usefull when the beans are declared in differents modules.
因此，如果未创建另一个 bean，您将仅实例化一个 bean。当 bean 在不同的模块中声明时，它特别有用。
//Core module creates a default Car
@Bean()
@ConditionalOnMissingBean(Car.class)
Car car()
{
return new DefaultCar();
}
and
和
//Car module creates the wanted prototype car
@Bean()
Car car()
{
return new Toyota();
}
回答by vdr
With Spring 3.0.7
使用 Spring 3.0.7
<bean id="carDriver" class="Driver">
<property name="car" value="#{ getBeanFactory().containsBean('customCar') ? getBeanFactory().getBean('customCar') : defaultCar }"/>
</bean>
回答by sourcedelica
Use JavaConfig:
使用 JavaConfig：
@Configuration
public class CarConfig {
@Autowired(required=false) @Qualifier("custom")
Car customCar;
@Autowired @Qualifier("default")
Car defaultCar;
@Bean
public Car car() {
return customCar != null ? customCar : defaultCar;
}
}
and
和
<bean id="defaultCar" class="Car">
<qualifier="default"/>
<property name="name" value="Honda Accord"/>
</bean>
<!-- customCar defined somewhere else -->
<bean id="carDriver" class="Driver">
<property name="car" ref="car"/>
</bean>
回答by Slava Semushin
I'm not sure but probably declaring custom bean with primary="true"might help you.
我不确定，但可能声明自定义 beanprimary="true"可能对您有所帮助。
回答by Juha Hanka
spring-boot-starter 1.4.0.RELEASE (spring-core 4.3.2.RELEASE)
or you could do like this:
spring-boot-starter 1.4.0.RELEASE (spring-core 4.3.2.RELEASE)
或者你可以这样做：
public interface SomeService {
}
------------------------------------------------------------------------
public interface CustomSomeService extends SomeService {
}
------------------------------------------------------------------------
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.stereotype.Service;
@Service
@ConditionalOnMissingBean(CustomSomeService.class)
public class DefaultSomeService implements SomeService {
}
------------------------------------------------------------------------
import org.springframework.stereotype.Service;
@Service
public class AdvancedSomeService implements CustomSomeService {
}
------------------------------------------------------------------------
class Application{
@Autowired
private SomeService someService;
/*
Now if ApplicationContext contains CustomSomeService implementation
'someService' use custom implementation. If CustomSomeService is
missing 'someService' contains DefaultSomeService implementation.
*/
}
------------------------------------------------------------------------
import static org.junit.Assert.assertTrue;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringRunner;
@RunWith(SpringRunner.class)
@ContextConfiguration(classes = { DefaultSomeService.class, AdvancedSomeService.class })
public class SomeServiceTest {
@Autowired
private SomeService someService;
@Test
public void test() {
assertTrue(AdvancedSomeService.class.isInstance(someService));
}
}
------------------------------------------------------------------------
import static org.junit.Assert.assertTrue;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.test.context.ContextConfiguration;
import org.springframework.test.context.junit4.SpringRunner;
@RunWith(SpringRunner.class)
@ContextConfiguration(classes = { DefaultSomeService.class})
public class SomeServiceTest {
@Autowired
private SomeService someService;
@Test
public void test() {
assertTrue(DefaultSomeService.class.isInstance(someService));
}
}
回答by omnomnom
With the newest Spring version you can use SpEL-based definition of your default value:
使用最新的 Spring 版本，您可以使用基于 SpEL 的默认值定义：
@Required
@Value("#{new com.my.company.DefaultStrategy()}")
public void setStrategy(final MyStrategy strategy) {
this.strategy = strategy;
}
If you set this property from Spring context, bean that you defined in context will be injected. Otherwise, container injects bean specified by @Valueannotation.
如果您从 Spring 上下文设置此属性，您在上下文中定义的 bean 将被注入。否则，容器注入@Value注解指定的 bean 。
