Spring依赖注入问题

Question

I have a bean I am trying to configure in Spring context using Constructor injection. When I pass subclass for one of the constructor arguments, the bean is instantiated by Spring container only if I do not specify the "type" attribute. Would anybody have any idea what's wrong? Below are more specifics.

class MyClass{
   public MyClass(SomeAbstractBase absObject){
      //do stuff
   }
}

class ConcreteClass extends SomeAbstractBase{
   //
} 

Spring configs (First and second do not work but the third one using type attribute works)- Config I-

<bean id="concreteclass" 
         class="ConcreteClass"/>
<bean id="myclass" 
        class="MyClass">
        <constructor-arg type="ConcreteClass" ref="concreteclass"/>
</bean>

Config II-

<bean id="concreteclass" 
         class="ConcreteClass"/>
<bean id="myclass" 
        class="MyClass">
        <constructor-arg type="SomeAbstractBase" ref="concreteclass"/>
</bean>

Config III-

 <bean id="concreteclass" 
             class="ConcreteClass"/>
    <bean id="myclass" 
            class="MyClass">
            <constructor-arg ref="concreteclass"/>
    </bean>

I get the following exception at initialization-

Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'jedispool' defined in class path resource [cache-spring-config.xml]: Could not resolve matching constructor (hint: specify index/type/name arguments for simple parameters to avoid type ambiguities)

Why would neither of the first or second config work?

Thank you

Answer 1

Type argument accept only the full qualified type (because if not spring cannot determine exactly the package and the type will not match your class):

So you need to use the canonical name of your class to be a type

ConcreteClass => com.your.app.ConcreteClass

A class is not just a name, is a package + name (too be short)

Answer 2

I believe the error you had appeared because you had static nested classes. Here's an SSCCE

package test;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class Test {
    public static void main(String[] args) throws Exception {
        System.out.println(MyClass.class.getName());
        System.out.println(ConcreteClass.class.getName());
        System.out.println(SomeAbstractBase.class.getName());
        ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("spring.xml");  
    }
    static class MyClass {
        public MyClass(SomeAbstractBase absObject) {
        }
    }
    static class ConcreteClass extends SomeAbstractBase {
        //
    }
    static abstract class SomeAbstractBase {
    }
}

With spring.xml containing

<bean id="concreteclass" class="test.Test.ConcreteClass" />
<bean id="myclass" class="test.Test.MyClass">
    <constructor-arg  type="test.Test.SomeAbstractBase" ref="concreteclass" />
</bean>

The above fails with a UnsatisfiedDependencyException . In this specific example, the class attribute isn't used correctly. The fully qualified class name for ConcreteClass is test.Test$ConcreteClass , not test.Test.ConcreteClass as I've specified. The same applies to MyClass and SomeAbstractBase .

However, specifying test.Test.ConcreteClass in the class attribute for the concreteclass bean doesn't fail because at some point during processing of bean declarations, Spring tries to resolve the class String into a Class object using Class.forName(String) . It will call ClassUtils.forName(String, ClassLoader) to do this. Initially it will fail because there is no such class test.Test.ConcreteClass . However, this is done in a try-catch which on ClassNotFoundException will transform the String class name from test.Test.ConcreteClass to test.Test$ConcreteClass and try again. It will work and correctly create a bean of type test.Test$ConcreteClass for your concreteclass bean.

When it tries to create your myclass bean, however, it does not apply such logic for resolving which constructor to use and therefore cannot understand that with the type attribute value of test.Test.SomeAbstractBase , you actually meant test.Test$SomeAbstractBase , so it fails saying the type is ambiguous.

Change your bean declarations to correct types

<bean id="concreteclass" class="test.Test$ConcreteClass" />
<bean id="myclass" class="test.Test$MyClass">
    <constructor-arg  type="test.Test$SomeAbstractBase" ref="concreteclass" />
</bean>

and it will work.

Take a look at kakawait's answer , you need to specify the fully qualified class name for the bean you are trying to instantiate.