Unsatisfied dependency expressed through constructor argument with index 0 of type

“Unsatisfied dependency expressed through constructor argument with index 0 of type” is a common error message that occurs when working with the Spring framework in Java. It typically means that the Spring container is unable to find a suitable bean to inject into a constructor argument at index 0. In this blog post, we will take a look at what causes this error and how to fix it.



Root Cause

The error message is caused when the Spring container is trying to instantiate a bean but is unable to find a suitable bean to inject into the constructor argument at index 0. This can happen for a few different reasons:

  1. T
  2. he bean that is being injected into the constructor argument is not defined in the Spring context. To fix this, you will need to add the missing bean to your Spring configuration file.
  3. The bean that is being injected into the constructor argument is defined in the Spring context, but it is defined with the wrong type. To fix this, you will need to ensure that the type of the bean being injected matches the type of the constructor argument.
  4. The bean that is being injected into the constructor argument is defined in the Spring context, but it is defined with the wrong name. To fix this, you will need to ensure that the name of the bean being injected matches the name of the constructor argument.


How to reproduce the error

Here is an example of a class that would cause the “Unsatisfied dependency expressed through constructor argument with index 0 of type” error:

@Service
public class MyService {
 
    private MyDependency myDependency;
 
    @Autowired
    public MyService(MyDependency myDependency) {
        this.myDependency = myDependency;
    }
 
    // ...
}


@Service
public class MyDependency {
 
    // ...
}


Solution 1

In this example, if the MyDependency bean is not defined or not defined with the same name in the Spring context, you would see the error message “Unsatisfied dependency expressed through constructor argument with index 0 of type.” To fix this, you would need to ensure that the MyDependency bean is defined and that it is defined with the same name as the constructor argument.

@Service
public class MyService {
 
    private MyDependency myDependency;
 
    @Autowired
    public MyService(@Qualifier("myDependency")MyDependency myDependency) {
        this.myDependency = myDependency;
    }
 
    // ...
}

In this example, we use the @Qualifier annotation to specify the name of the bean we want to inject. This way Spring knows which bean to inject into the constructor.



Solution 2

Another way to fix this issue is by using the @Primary annotation. You can use this annotation on the bean that you want to be the primary candidate for injection. For example, in the following code snippet, we have two beans of the same type MyDependency with different names.

@Service
@Primary
public class MyDependency1 implements MyDependency { 
   // ... 
}

@Service
public class MyDependency2 implements MyDependency { 
   // ... 
}

Here, MyDependency1 is marked as primary and will be injected by default, unless we explicitly specify otherwise.

@Service
public class MyService {
 
    private MyDependency myDependency;
 
    @Autowired
    public MyService(MyDependency myDependency) {
        this.myDependency = myDependency;
    }
 
    // ...
}


Solution 3

The @Primary annotation can also be used on a method level. For example, if you want to define a MyDependency bean through a factory method, you can use the @Bean annotation along with @Primary on the factory method.

@Configuration
public class MyConfig {

    @Bean
    @Primary
    public MyDependency myDependency() {
        return new MyDependencyImpl();
    }

}

In this example, the myDependency() method is annotated with @Bean and @Primary to indicate that it is the primary bean of the type MyDependency.

In addition to this, You can also use @Primary annotation on the @Autowired constructor parameter or on the @Autowired setter method.



Conclusion

In conclusion, when you encounter the “Unsatisfied dependency expressed through constructor argument with index 0 of type” error, there are several ways to fix it. You can ensure that the bean being injected is defined in the Spring context, that it is defined with the correct type, and that it is defined with the correct name. You can also use the @Qualifier annotation to specify the name of the bean you want to inject. Additionally, you can use the @Primary annotation to define a primary candidate for injection.



Leave a Reply