- @Primary:首选bean
①当一个类型的类有多个时,@Autowired指定bean就会抛出异常->
org.springframework.beans.factory.NoUniqueBeanDefinitionException
比如同一个Dessert接口,有三个实现类:Cake,Cookies,IceCream
public interface Dessert {
}
@Component
public class Cake implements Dessert {
}
@Component
public class Cookies implements Dessert {
}
@Component
public class IceCream implements Dessert {
}
下面的代码就会出错:
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes=AmbiguousConfig.class)
public class AmbiguousTest {
@Autowired
private Dessert dessert;
@Test
public void test() {
System.out.println(dessert);
System.out.println("--");
}
}
@Autowired是按照类型注入的,现在容器中Dessert 类型的bean有三个: cake,cookies,iceCream
Caused by: org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type 'com.Chapter3.AutowiredAmbiguous.Dessert' available: expected single matching bean but found 3: cake,cookies,iceCream
at org.springframework.beans.factory.config.DependencyDescriptor.resolveNotUnique(DependencyDescriptor.java:215)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1116)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1065)
at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:584)
... 28 more
@Primary可以指定当有多个相同类型的bean时,优先装载,或者选择一个来注入,就像你最喜欢蛋糕cake,不管什么情况下你都想注入Cake类型的:
@Component
@Primary
public class Cake implements Dessert {
}
AmbiguousTest运行之后打印出:
com.Chapter3.AutowiredAmbiguous.Cake@30ee2816
--
说明注入的是Cake类型的。
- 自定义限定符注解来限定bean
自定义限定符注解可以不断缩小bean的范围,比如上面的例子使用自定义限定符注解来实现:
//先定义一个自定义限定符注解
@Target({ElementType.CONSTRUCTOR,ElementType.FIELD,ElementType.METHOD,ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
//自定义限定符注解要标记@Qualifier注解
@Qualifier
public @interface MyBean {
}
然后在Cake类上标注:
@Component
//使用自定义限定符注解来标注
@MyBean
public class Cake implements Dessert {
}
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(classes=AmbiguousConfig.class)
public class AmbiguousTest {
@Autowired
//表示要注入的是被自定义限定符@MyBean标注的类型
@MyBean
private Dessert dessert;
@Test
public void test() {
System.out.println(dessert);
System.out.println("--");
}
}