Spring - @Autowire a List with elements

Question

I have a controller that has this @Autowired:

@Autowired
Rules rules;

This Rules class is defined as follows::

@Service
public class Rules {

    @Autowired
    private List<RegistrationRule> allRules;

    public List<RegistrationRule> getAllRules() {
        return allRules;
    }
}

I want when doing the @Autowired in the list allRules it already comes with some standard items within it, as follows:

allRules.add( new EmployeePositionRule() );
allRules.add( new CostCenterRule() );

I tried to put in the Rules class builder in the following way:

public Rules() {
    allRules.add( new EmployeePositionRule() );
    allRules.add( new CostCenterRule() );
}

But if I do so an exception is released at the time of compiling the project:

org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'intRaptMecController': Unsatisfied dependency expressed through field 'rules'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'rules' defined in file [...validations\rules\Rules.class]: Instantiation of bean failed; nested exception is org.springframework.beans.BeanInstantiationException: Failed to instantiate [validations.rules.Rules]: Constructor threw exception; nested exception is java.lang.NullPointerException

What’s the right way to do it?

Answer 1

The reason is simple.

The builder of the class Rules is called before to inject the List<RegistrationRule> in the field allRules. Therefore, allRules will always be null.

This would be the current code, still with the problem:

@Service
public class Rules {

    @Autowired
    private List<RegistrationRule> allRules;

    public Rules() {
       allRules.add( new EmployeePositionRule() ); //allRules estará nulo!
       allRules.add( new CostCenterRule() );
    }

    public List<RegistrationRule> getAllRules() {
        return allRules;
    }
}

The correction may be to injection by manufacturer or by method set. In fact, avoid using @Autowired in private fields (the famous Field Injection), because this injection technique is the least recommended by several reasons, although it is (unfortunately) very common.

So try doing it this way:

@Service
public class Rules {

    private List<RegistrationRule> allRules;

    @Autowired
    public Rules(List<RegistrationRule> allRules) {
       allRules.add( new EmployeePositionRule() );
       allRules.add( new CostCenterRule() );
    }

    public List<RegistrationRule> getAllRules() {
        return allRules;
    }
}

Also confirm that your @Bean list of RegistrationRule was created correctly. It can be created within any class annotated with @Configuration (org.springframework.context.annotation.Configuration), in this way:

@Configuration
public class RulesConfiguration {

    @Bean
    public List<RegistrationRule> allRules() {
       List<RegistrationRule> rules = new ArrayList();
       return rules;
    }

}

Answer 2

As Leonardo Lima commented, the UnsatisfiedDependencyException happens because Spring didn’t find any Bean of the kind List<RegistrationRule>. For this, you can declare a method that returns the default values you need. Example:

@Bean
public List<RegistrationRule> myDefaultRules() {
  List<RegistrationRule> defaultRules = new ArrayList();
  defaultRules.add(new EmployeePositionRule());
  defaultRules.add(new CostCenterRule());
  return defaultRules;
}

Reappoint that the annotation @Bean serves to tell Spring that you are providing a dependency, in case your default list of RegistrationRule.

In case you wanted different values to be injected into your list allRules, you will need to declare other Beans and use the annotation @Qualifier to differentiate between which list of RegistrationRule you want to inject. Example:

@Bean(name = "defaultRules")
public List<RegistrationRule> myDefaultRules() {
  List<RegistrationRule> defaultRules = new ArrayList();
  defaultRules.add(new EmployeePositionRule());
  defaultRules.add(new CostCenterRule());
 return defaultRules;
}

@Bean(name = "otherRules")
public List<RegistrationRule> otherRules() {
  List<RegistrationRule> otherRules = new ArrayList();
  otherRules.add(new FooRule());
  otherRules.add(new MyOtherRule());
  return otherRules;
}

@Autowired
@Qualifier("defaultRules")
private List<RegistrationRule> defaultRules;

@Autowired
@Qualifier("otherRules")
private List<RegistrationRule> otherRules;

I hope I’ve helped!

EDIT 1:

Try this example.

@Configuration
public class RulesConfiguration {

    @Bean
    public List<RegistrationRule> allRules() {
        List<RegistrationRule> allRules = new ArrayList();
        allRules.add(new EmployeePositionRule());
        allRules.add(new CostCenterRule());
        return allRules;
    }

}

@Service
public class RuleService {

    @Autowired
    List<RegistrationRule> rules;

}