Is it possible in Spring to create beans using annotations if they have properties set to different bean lists?

Question

Assume I have the classes A,B,C,D,E:

@Component
public class A {
}

@Component
public class B {
}

@Component
public class D {
}

@Component
public class E {
}

public class C {
private List myList;

public List getMyList() {
    return myList;
}

public void setMyList(List myList) {
    this.myList = myList;
}

}

This is my application context:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:util="http://www.springframework.org/schema/util"
xsi:schemaLocation="http://www.springframework.org/schema/beans 
http://www.springframework.org/schema/beans/spring-beans.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd
    http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-3.2.xsd">


    <util:list id="myList1">
        <ref bean="a"/>
        <ref bean="b"/>
    </util:list>

    <util:list id="myList2">
        <ref bean="d"/>
        <ref bean="e"/>
    </util:list>

    <bean id="myFirstC" class="com.app.C">
        <property name="myList" ref="myList1"></property>
    </bean>

    <bean id="mySecondC" class="com.app.C">
        <property name="myList" ref="myList2"></property>
    </bean>

    <context:component-scan base-package="com.app"></context:component-scan>
    </beans>

Is it possible to create the beans "myFirstC" and "mySecondC" using annotations inside C class? I have tried to autowire the lists inside the C class and then use @Bean annotation to create beans and set the lists, but get errors.

So, I tried to do it in this way:

@Component
@Configuration
public class C {

private List myList;

@Autowired
@Qualifier("myList1")
private List myList1;

@Autowired
@Qualifier("myList2")
private List myList2;

@Bean
public C getNewCBean1() {
    C c = new C();
    c.setMyList(myList1);
    return c;
}

@Bean
public C getNewCBean2() {
    C c = new C();
    c.setMyList(myList2);
    return c;
}

But get error like this:

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'c': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.util.List com.app.C.myList1; nested exception is org.springframework.beans.FatalBeanException: No element type declared for collection [java.util.List]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:288)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1116)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:626)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:932)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:479)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at com.app.Start.main(Start.java:9)
Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private java.util.List com.app.C.myList1; nested exception is org.springframework.beans.FatalBeanException: No element type declared for collection [java.util.List]
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:514)
    at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:285)
    ... 13 more
Caused by: org.springframework.beans.FatalBeanException: No element type declared for collection [java.util.List]
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:807)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:768)
    at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486)
    ... 15 more

I could not find a solution to this in documentations and forums. Any help will be appreciated. Thank you

Post what you tried, and post the errors you got. And please, generics exist since Java 5, 14 years ago. Stop using raw types. — JB Nizet, Nov 17 '18 at 15:07
All right, I have edited the question and added what I tried — FalseScience, Nov 17 '18 at 15:30

score 0 · Accepted Answer · answered Nov 17 '18 at 15:39

0

Your Java code doesn't do the same thing as your xml code at all:

<util:list id="myList1">
    <ref bean="a"/>
    <ref bean="b"/>
</util:list>

<util:list id="myList2">
    <ref bean="d"/>
    <ref bean="e"/>
</util:list>

<bean id="myFirstC" class="com.app.C">
    <property name="myList" ref="myList1"></property>
</bean>

<bean id="mySecondC" class="com.app.C">
    <property name="myList" ref="myList2"></property>
</bean>

This defines two beans myFirstC and mySecondC, containing the beans a, b and d, e respectively. So, in Java, you need two @Bean methods creating the same C beans:

 @Configuration
 public class Config {
     @Bean
     public C myFirstC(A a, B b) {
         return new C(Arrays.asList(a, b));
     }


     @Bean
     public C mySecondC(D d, E e) {
         return new C(Arrays.asList(d, e));
     }
 }

answered Nov 17 '18 at 15:39

JB Nizet

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This solves the problem, but is it possible to use "myList1" and "myList2" instead of (A a, B b) and (D d, E e)? – FalseScience Nov 17 '18 at 16:38
You could define two beans of type List, and inject them into your C beans, but why would you want that? – JB Nizet Nov 17 '18 at 16:39
Could you also give an example in that way? Thank you. This is actually a part of some youtube tutorial exercise for Spring(without solution), which consists of converting the context application to annotation based one. I am trying to learn the annotation based approach. – FalseScience Nov 17 '18 at 16:55
I tried to define the list like this: `@Bean public List myList2() { D d = new D(); E e = new E(); return Arrays.asList(d, e); }` and inject as argument like this: `public C mySecondC(@Qualifier("myList2") List list)` and get error : No element type declared for collection – FalseScience Nov 17 '18 at 17:10
1

Again, generics were introduced in Java 5, 14 years ago. And you're still using raw types. Why? https://stackoverflow.com/questions/2770321/what-is-a-raw-type-and-why-shouldnt-we-use-it – JB Nizet Nov 17 '18 at 17:11
Strange, never thought that those warnings could lead to errors – FalseScience Nov 17 '18 at 17:26
Still, after reading about raw types and changing List to List – FalseScience Nov 17 '18 at 21:46
Solved by using @Value("#{myList2}" instead of @Qualifier("myList2") – FalseScience Nov 17 '18 at 23:18

Is it possible in Spring to create beans using annotations if they have properties set to different bean lists?

1 Answers1