How to dynamically call a method on a qualified service?

Question

I have an interface as following:

public interface Solver {
    void execute(final String definition);
}

I have several solver implementations. For example:

@Qualifier("naive")
@Service
public class NaiveSolver implements Solver {
    @Override
    public void execute(final String definition) {
    }
}

@Qualifier("optimal")
@Service
public class OptimalSolver implements Solver {
    @Override
    public void execute(final String definition) {
    }
}

Now, in my database I have the data for these solvers as following:

type:       textual - examples: simple, naive
definition: textual

When fetching this data, I need to map the type column to the actual service instance, so it can solve the provided definition.

So, my question boils down to this: how can I, given a type string, get the instance of the corresponding Solver service instance of which the qualifier is equal to that type? I believe that @Autowired cannot be used here, as I need to find beans dynamically at runtime.

Nikolai Shevchenko · Accepted Answer · 2020-04-02T11:35:24.107

2

Since Spring 4 you can autowire multiple implemetations into a Map where bean qualifier is a key, and the bean itself is a value

@Autowired
private Map<String, Solver> solvers;

void doStuff() {
    String type = ... // obtain type
    Solver solver = solvers.get(type);
    solver.execute(...)
}

Update

Correct way of naming a bean is not

@Qualifier("naive")
@Service

but

@Service("naive")

@Qualifier is used along with @Autowired to ensure the correct bean is injected

@Autowired
@Qualifier("naive")
private Solver naiveSolver;

edited Apr 02 '20 at 11:35

answered Apr 02 '20 at 11:19

Nikolai Shevchenko

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This works, this is an awesome feature! Thank you for showing it. – Hazzo Apr 02 '20 at 11:36

score 1 · Answer 2 · answered Apr 02 '20 at 11:10

1

You can create configuration which will just hold mapping to your solvers:

@Autowired
@Qualifier("optimal")
private Solver naiveSolver;

@Bean
public Map<String, Solver> mapSolver() {
   Map<String, Solver> map = new HashMap<>();
   map.put("naive", naiveSolver);
   return map;
}

Or even going further you can follow factory pattern which will provide you different instances of solvers.

Another way you can dynamically get those beans from application context.

answered Apr 02 '20 at 11:10

Igor Konoplyanko

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Thank you. I believe the application context already holds all beans, with their possible qualifiers. So while I could manage my own map, I believe that's re-inventing what the application context already is doing. So in other words: I am interested in how to find the qualified service from the application context. Could you please show me how this can be done? – Hazzo Apr 02 '20 at 11:12

score 0 · Answer 3 · answered Apr 02 '20 at 11:16

You could merge the following solutions:

How to inject dependencies into a self-instantiated object in Spring?

Creating an instance from String in Java

private @Autowired AutowireCapableBeanFactory beanFactory;
public void doStuff() {
   Class c= Class.forName(className);
   MyBean obj  = c.newInstance();

   beanFactory.autowireBean(obj);
   // obj will now have its dependencies autowired.
}

How to dynamically call a method on a qualified service?

3 Answers3