Constructor injection using Spring annotation @Autowired does not work

Question

I have created 2 simple classes. Constructor of one class is annotated as @Autowired. It accepts the object of another class. But this code fails.

Classes :- 1) SimpleBean.java

@Configuration
public class SimpleBean {
  InnerBean prop1;

  public InnerBean getProp1() {
    return prop1;
  }

  public void setProp1( InnerBean prop1) {
    System.out.println("inside setProp1 input inner's property is "
        + prop1.getSimpleProp1());
    this.prop1 = prop1;
  }

  @Autowired(required=true)
  public SimpleBean(InnerBean prop1) {
    super();
    System.out.println("inside SimpleBean constructor inner's property is "
        + prop1.getSimpleProp1());
    this.prop1 = prop1;
  }
}

2) Inner.java

@Configuration
public class InnerBean {
  String simpleProp1;

  public String getSimpleProp1() {
    return simpleProp1;
  }

  public void setSimpleProp1(String simpleProp1) {
    this.simpleProp1 = simpleProp1;
  }

}

When I try to load ApplicationConext

ApplicationContext acnxt = new AnnotationConfigApplicationContext("com.domain");

It gives following error :-

Exception in thread "main" org.springframework.beans.factory.BeanCreationException:         Error creating bean with name 'simpleBean' defined in file [C:\Users\owner\Documents\Java Project\MyWorkSpace\springMVCSecond\WebContent\WEB-INF\classes\com\domain\SimpleBean.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.domain.SimpleBean$$EnhancerByCGLIB$$4bc418be]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.domain.SimpleBean$$EnhancerByCGLIB$$4bc418be.<init>()
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:965)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:911)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:485)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:293)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:290)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:192)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:585)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:895)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:425)
at org.springframework.context.annotation.AnnotationConfigApplicationContext.<init>(AnnotationConfigApplicationContext.java:75)
at com.test.SpringAnnotationTest.main(SpringAnnotationTest.java:12)
Caused by: org.springframework.beans.BeanInstantiationException: Could not instantiate bean class [com.domain.SimpleBean$$EnhancerByCGLIB$$4bc418be]: No default constructor found; nested exception is java.lang.NoSuchMethodException: com.domain.SimpleBean$$EnhancerByCGLIB$$4bc418be.<init>()
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:70)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateBean(AbstractAutowireCapableBeanFactory.java:958)
... 12 more
Caused by: java.lang.NoSuchMethodException: com.domain.SimpleBean$$EnhancerByCGLIB$$4bc418be.<init>()
at java.lang.Class.getConstructor0(Unknown Source)
at java.lang.Class.getDeclaredConstructor(Unknown Source)
at org.springframework.beans.factory.support.SimpleInstantiationStrategy.instantiate(SimpleInstantiationStrategy.java:65)
... 13 more

If I introduce no-arg constructor in SimpleBean class. It does not give error. But this does not give me pre-poulated object of SimpleBean ( as in XML configuration using < constructor-arg > ). So when using annotation, is it mandatory to have no-arg constructor ? What is the appropriate way out ?

Answer 1

From the javadoc of @Configuration:

 Configuration is meta-annotated as a {@link Component}, therefore Configuration
 classes are candidates for component-scanning and may also take advantage of
 {@link Autowired} at the field and method but not at the constructor level.

So you'll have to find some other way to do it, unfortunately.

Answer 2

I believe you are mixing up the @Configuration and @Component annotation. As per the spring docs, @Configuration is used to create beans using Java code (any methods annotated with @Bean create a bean, whereas classes annotated with @Component are automatically created..

I hope the following illustrates this:

InnerBean.java:

// this bean will be created by Config
public class InnerBean {
  String simpleProp1;

  public String getSimpleProp1() {
    return simpleProp1;
  }

  public void setSimpleProp1(String simpleProp1) {
    this.simpleProp1 = simpleProp1;
  }
}

SimpleBean.java:

// This bean will be created because of the @Component annotation, 
// using the constructor with the inner bean autowired in
@Component
public class SimpleBean {
  InnerBean prop1;

  public InnerBean getProp1() {
    return prop1;
  }

  @Autowired(required = true)
  public SimpleBean(InnerBean prop1) {
    this.prop1 = prop1;
  }
}

OuterBean.java

// this bean will be created by Config and have the SimpleBean autowired.
public class OuterBean {
  SimpleBean simpleBean;

  @Autowired
  public void setSimpleBean(SimpleBean simpleBean) {
    this.simpleBean = simpleBean;
  }

  public SimpleBean getSimpleBean() {
    return simpleBean;
  }
}

Config.java

// this class will create other beans
@Configuration
public class Config {
  @Bean
  public OuterBean outerBean() {
    return new OuterBean();
  }

  @Bean
  public InnerBean innerBean() {
    InnerBean innerBean = new InnerBean();
    innerBean.setSimpleProp1("test123");
    return innerBean;
  }
}

Main.java:

public class Main {
  public static void main(String[] args) {
    ApplicationContext ctx = new AnnotationConfigApplicationContext("com.acme");
    OuterBean outerBean = ctx.getBean("outerBean", OuterBean.class);
    System.out.println(outerBean.getSimpleBean().getProp1().getSimpleProp1());
  }
}

The main class uses the AnnotationConfigApplicationContext to scan for both @Configuration and @Component annotations and create the beans accordingly.