How to autowire a class with non-empty constructor?

Question

I'd like to @Autowired a class that has a non-empty constructor. Just the the following as an example, it does not necessairly have to be a view/service. Could be whatever component you like, having non-default constructor:

@Component
class MyViewService {
  //the "datasource" to show in the view
  private List<String> companies companies;
  private MyObject obj;

  public MyViewService(List<String> companies, MyObject obj) {
    this.companies = companies;
    this.obj = obj;
  }
}

Of course I cannot just write

@Autowired
private MyViewService viewService;

as I'd like to use the constructor with the list. But how?

Are there better approaches than refactoring these sort of constructors to setters? I wouldn't like this approach as ideally the constructor forces other classes to provide all objects that are needed within the service. If I use setters, one could easily forget to set certain objects.

Well you can add @Autowire to the constructor. Works similar to setter injecton. — Icewind
– Icewind, Commented Jun 18, 2014 at 13:16
public(List<String> companies, MyObject obj) is nonsense. do you actually mean public MyViewService(List<String> companies, MyObject obj)? — DwB
– DwB, Commented Jun 18, 2014 at 13:19
How would Spring known what to pass as constructor parameters? — sp00m
– sp00m, Commented Jun 18, 2014 at 13:19
@sp00m no I'd like to pass the objects myself. These are not objects that can be autowired. — membersound
– membersound, Commented Jun 18, 2014 at 13:23

Paul · Accepted Answer · 2014-06-18 13:33:39Z

2

If you want Spring to manage MyViewService you have to tell Spring how to create an instance of it. If you're using XML configuration:

<bean id="myViewService" class="org.membersound.MyViewService">
  <constructor-arg index="0" ref="ref_to_list" />
  <constructor-arg index="1" ref="ref_to_object" />
</bean>

If you're using Java configuration then you'd call the constructor yourself in your @Beanannotated method.

Check out the Spring docs on this topic. To address a comment you made to another answer, you can create a List bean in XML as shown in the Spring docs. If the list data isn't fixed (which it's probably not) then you want to use an instance factory method to instantiate the bean.

In short, the answers you seek are all in the Spring docs :)

edited Jun 18, 2014 at 13:33

answered Jun 18, 2014 at 13:26

Paul

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3 Comments

membersound Over a year ago

OK I see. But one question remains taking eg my example: is it good practice to make data-supplying lists a spring-managed bean? I mean, this list would be probably only used for this particular service, and only in this constructor.

Paul Over a year ago

It's not a bad practice - it just depends on the problem you're trying to solve. If the elements of the list are known in advance then it's better to put them into a properties file and have Spring inject the values from the file.

flup Over a year ago

You can actually use list tags to specify the list in-place! See stackoverflow.com/questions/2416056/…

DwB · Accepted Answer · 2014-06-18 13:25:13Z

0

If a component has a non-default constructor then you need to configure the constructor in the bean configuration.

If you are using XML, it might look like this (example from the spring reference document):

<beans>
    <bean id="foo" class="x.y.Foo">
        <constructor-arg ref="bar"/>
        <constructor-arg ref="baz"/>
    </bean>

    <bean id="bar" class="x.y.Bar"/>
    <bean id="baz" class="x.y.Baz"/>
</beans>

The key here is constructor wiring of the bean that will be used for the @AutoWire. The way you use the bean has no impact.

answered Jun 18, 2014 at 13:25

DwB

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2 Comments

membersound Over a year ago

Taking my example, how would I make a List<String> companies a spring managed bean? Does it make sense at all to have a list of objects managed?

DwB Over a year ago

If you are using XML, configure a util:list bean that contains the list of companies. the Paul answer demonstrates this.

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