Get spring bean via context using generic

Question

I have a bunch of repository beans that implement type Repository<T ? extends Node>. Now I can get a list of random nodes from the user and I want to get the appropriate repository for each node. Since Spring 4.0RC1 we can autowire repositories like this:

@Autowired Repository<SomeNode> someNodeRepository;

As documented here.

This works fine, but my question is how I can do this dynamically based on the generic type.

What I want to do is something like:

public <T extends Node> T saveNode(T node) {
    Repository<T> repository = ctx.getBean(Repository.class, node.getClass());
    return repository.save(node);
}

Where the second parameter is the generic type. This of course does not work, although it compiles.

I can't find any/the documentation on this.

There should be default name assigned to each bean (not sure how it builds). If you can debug and understand how default bean name builds for this case you can try to get bean by name — Stan, May 21 '15 at 14:18

score 34 · Accepted Answer · answered Jul 23 '17 at 09:38

34

You can do something like this:

String[] beanNamesForType = ctx.getBeanNamesForType(ResolvableType.forClassWithGenerics(Repository.class, node.getClass()));

// If you expect several beans of the same generic type then extract them as you wish. Otherwise, just take the first
Repository<T> repository = (Repository<T>) ctx.getBean(beanNamesForType[0]);

answered Jul 23 '17 at 09:38

jhas

587
6
8

I am doubting whether your code would compile. The argument type of `getBeanNamesForType` does not line with the return type of `forClassWithGenerics` as far as I can see. – Lodewijk Bogaards Jul 25 '17 at 20:47
3

I never paste code that I haven't checked. It compiles and works...Note that `getBeanNamesForType` has 3 versions, one of which accepts `ResolvableType` – jhas Jul 26 '17 at 12:42
You are right, I was looking at an old version of Spring. This seems like a legitimate answer! Have you tested this? – Lodewijk Bogaards Jul 26 '17 at 16:19
1

Yes I have, it works great. I also searched how to do this and ended up with this solution – jhas Jul 27 '17 at 06:37
1

Would be nice if the `getBean()` and `getBeansOfType()` methods also had overloads that accept `ResolvableType`. – jaco0646 Jan 02 '21 at 17:52
Your answer is greatly appreciated. Java's implementation of generic continues to astound and frustrate me. https://en.wikipedia.org/wiki/Criticism_of_Java#Generics – granadaCoder Apr 28 '21 at 09:52

score 3 · Answer 2 · answered Nov 02 '21 at 15:40

Since Spring 5.1 you can get a bean of type Repository<T> like that:

public static <T> Repository<T> getRepositoryFor(Class<T> clazz)
{
    ResolvableType type = ResolvableType.forClassWithGenerics(Repository.class, clazz);
    return (Repository<T>) context.getBeanProvider(type).getObject();
}

score 0 · Answer 3 · answered May 21 '15 at 15:23

If you could be sure that for every concrete subclass of Node (say SomeNode), every object of type SomeNode will be an actual SomeNode and not a subclass or a proxy, it would be easy. Just use a convention for the repository name (say SomeNodeRepository) and it would be trivial :

Repository<T> repository = ctx.getBean(node.getClass().getSimpleName()
        + "Repository", Repository.class);

But you know that there's a high risk of getting a subclass or proxy ...

So you can try to have each Node subclass to implement a nameForRepo method :

class Node {
    ...
    abstract String getNameForRepo();
}

and then in the subclasses

class SomeNode {
    static private final nameForRepo = "SomeNode";
    ...
    String getNameForRepo() {
        return nameForRepo;
    }
}

That way, even if you get a proxy or subclass, you will be able to do :

public <T extends Node> T saveNode(T node) {
    Repository<T> repository = ctx.getBean(node.getNameForRepository()
            + "Repository", Repository.class);
    return repository.save(node);
}

Alternatively, the method could directly return the repository name.

Thanks, not a bad work around really, but since @Autowire does the job at runtime, what I want is actually possible, so it is unsatisfactory. In the mean time I have my own work around in place, which is just a simple `Map, Class>>`. — Lodewijk Bogaards, May 21 '15 at 20:49

score 0 · Answer 4 · edited Jun 20 '23 at 18:16

If I understand well, you want to get an instance of a bean with a Repository class and a different generic type?

I'm afraid you don't have the dynamic way with spring, but I have a workaround solution:

Your generic type should be a field in your class, you must have a constructor in your Repository class for setting your generic type, your Repository class should be like this:
```
public class Repository<T>{
    Class<T> nodeClass;
    public Repository(Class<?> clazz){
        this.nodeClass = clazz;
    }
    // your codes...
}
```

declare a Repository bean for each Node, let's say you have Repository and Repository, if you are using xml configuration, you need to add:

<bean id="someNodeRep" class="your.package.Repository">
    <constructor-arg>
        <value>your.package.SomeNode</value>
    </constructor-arg>
</bean>
<bean id="otherNodeRep" class="your.package.Repository">
    <constructor-arg>
        <value>your.package.OtherNode</value>
    </constructor-arg>
</bean>

'autowire' your Repository in this way:

@Autowired
@Qualifier("someNodeRep")
Repository<SomeNode> someNodeRepository;

@Autowired
@Qualifier("otherNodeRep")
Repository<OtherNode> otherNodeRepository;

thanks for the solution, but I am not looking for a workaround currently. I have one already too. — Lodewijk Bogaards, May 22 '15 at 15:20

Get spring bean via context using generic

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