Do I need to synchronize @Autowired method in Spring when I want to save multiple beans in collection?

Question

Let's imagine I have the next classes in the project based on Spring framework:

interface I {
    String getName()
}

@Component
class I1 implements I {
    @Override
    String getName() {return "I1"}
}

@Component
class I2 implements I {
    @Override
    String getName() {return "I1"}
}

And I want to gather them all in the map using the @Autowired method:

@Component
public class A {
    private Map<I> map = new HashMap<>()

    @Autowired
    public registerI(I i) {
        map.put(i.getName(), i)
    }
}

Should I make this method registerI synchronized? I mean, can Spring call this method in several threads simultaneously? Or this method will be called sequentially?

Thanks

you can just try it and find out. And why would it be called multiple times? — f1sh, Dec 17 '19 at 18:01
@Deadpool it doesn't matter in this case. I want to know - do I need to use synchronized collection here or not. — Dmitro Liakhov, Dec 17 '19 at 18:03
@f1sh I didn't reproduce any race condition error in that case, but I am not still sure that it is correct code. — Dmitro Liakhov, Dec 17 '19 at 18:04
Why not use a List as the parameter in the method? That will ensure only one method call. — earandap, Dec 17 '19 at 18:11
Or even better: use constructor injection and pass the List as argument to the constructor. — JB Nizet, Dec 17 '19 at 18:23
Or even better just inject `Map` and Spring will automatically inject all the beans into the map itself, just like it does for a collection. Although it will use the bean name and not the `getName` method. — M. Deinum, Dec 18 '19 at 07:15

Karol Dowbecki · Answer 1 · 2019-12-17T18:38:22.640

3

You don't have to use synchronized because Spring bean initialization is single-threaded and thread-safe. You can think of gotchas like thread-scoped or lazy beans but for regular singleton beans initialization happens in one thread.

You might want to use synchronized to make sure that after registerI() method is called your object is safely published, although auto-wired constructor with final field is more readable.

@Component
public class A {
    private final Map<String, I> map;

    public A(List<I> list) {
        map = list.stream().collect(Collectors.toMap(I::getName, i -> i));
    }

}

edited Dec 17 '19 at 18:38

answered Dec 17 '19 at 18:33

Karol Dowbecki

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Thanks, for the response. Can you provide a link on documentation, where is said, that spring bean initialization is single-threaded? – Dmitro Liakhov Dec 17 '19 at 20:19
There is [Parallel bean initialization during startup \[SPR-8767\] #13410](https://github.com/spring-projects/spring-framework/issues/13410) which suggested to implement the multi-threaded singleton initialization. It was discussed but never implemented. – Karol Dowbecki Dec 17 '19 at 20:46

score 1 · Answer 2 · answered Dec 17 '19 at 18:33

You will get an exception during app startup because Spring cannot determine the correct implementation of interface "I" what you want to inject. You should use @Qualifier.

If you want to accomplish that scenario, this should be enough.

@Component
    public static class A {
        private Map<String,I> map = new HashMap<>();

        public A(List<I> list) {
            //map = list.stream().collect(Collectors.toMap(I::getName, x -> x));
            for (I i : list) {
                map.put(i.getName(), i);
            }
        }
    }

You will end with only one value in the map.

The commented line works if there are not duplicate map keys.

score 0 · Answer 3 · answered Dec 17 '19 at 18:41

0

You can autowire context and get all the interested beans from it in a @PostConstruct method and create a hashmap with it.

Or

If you want that Map to be shared amongst multiple classes, make it a @Bean

@Configuration
class SomeConfig{

  @Autowire Context context;
  @Bean(name = "mapBean")
  public Map<String, MyCustomClassName1> mapBean() {
    Map<String, MyCustomClassName1> map = new HashMap<>();
    //populate the map here - from Context
    return map;
  }
}

answered Dec 17 '19 at 18:41

so-random-dude

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https://stackoverflow.com/questions/23630255/how-to-correct-inject-map-of-bean-in-spring-context – so-random-dude Dec 17 '19 at 18:47
https://stackoverflow.com/questions/54134333/how-to-create-a-simple-factory-pattern-with-autowired-beans-in-spring – so-random-dude Dec 17 '19 at 18:48
https://stackoverflow.com/questions/7446301/autowire-reference-beans-into-list-by-type – so-random-dude Dec 17 '19 at 18:48

score 0 · Answer 4 · edited Dec 17 '19 at 18:49

0

Spring fills List by your beans. After you can create map in postConstruct

@Component
public class A {

  @Autowired
  private List<I> list;

  @Autowired
  private Map<String, I> map;   

  @PostConstruct
  private void init(){
    map = list.stream()
            .collect(Collectors.toMap(I::getName, element->element);
  }
}

edited Dec 17 '19 at 18:49

so-random-dude

15,277
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answered Dec 17 '19 at 18:47

serhii

1
1

Do I need to synchronize @Autowired method in Spring when I want to save multiple beans in collection?

4 Answers4