Inject spring bean dynamically

Question

In a java-spring web-app I would like to be able to dynamically inject beans. For example I have an interface with 2 different implementations:

In my app I'm using some properties file to configure injections:

#Determines the interface type the app uses. Possible values: implA, implB
myinterface.type=implA

My injections actually loaded conditionally relaying on the properties values in the properties file. For example in this case myinterface.type=implA wherever I inject MyInterface the implementation that will be injected will be ImplA (I accomplished that by extending the Conditional annotation).

I would like that during runtime - once the properties are changed the following will happen (without server restart):

1. The right implementation will be injected. For example when setting myinterface.type=implB ImplB will be injected where-ever MyInterface is used
2. Spring Environment should be refreshed with the new values and re-injected as well to beans.

I thought of refreshing my context but that creates problems. I thought maybe to use setters for injection and re-use those setters once properties are re-configured. Is there a working practice for such a requirement?

Any ideas?

UPDATE

As some suggested I can use a factory/registry that holds both implementations (ImplA and ImplB) and returns the right one by querying the relevant property. If I do that I still have the second challenge - the environment. for example if my registry looks like this:

@Service
public class MyRegistry {

private String configurationValue;
private final MyInterface implA;
private final MyInterface implB;

@Inject
public MyRegistry(Environmant env, MyInterface implA, MyInterface ImplB) {
        this.implA = implA;
        this.implB = implB;
        this.configurationValue = env.getProperty("myinterface.type");
}

public MyInterface getMyInterface() {
        switch(configurationValue) {
        case "implA":
                return implA;
        case "implB":
                return implB;
        }
}
}

Once property has changed I should re-inject my environment. any suggestions for that?

I know I can query that env inside the method instead of constructor but this is a performance reduction and also I would like to think of an ider for re-injecting environment (again, maybe using a setter injection?).

Answer 1

I would keep this task as simple as possible. Instead of conditionally load one implementation of the MyInterface interface at startup and then fire an event that triggers dynamic loading of another implementation of the same interface, I would tackle this problem in a different way, that is much simpler to implement and maintain.

First of all, I'd just load all possible implementations:

@Component
public class MyInterfaceImplementationsHolder {

    @Autowired
    private Map<String, MyInterface> implementations;

    public MyInterface get(String impl) {
        return this.implementations.get(impl);
    }
}

This bean is just a holder for all implementations of the MyInterface interface. Nothing magic here, just common Spring autowiring behavior.

Now, wherever you need to inject a specific implementation of MyInterface, you could do it with the help of an interface:

public interface MyInterfaceReloader {

    void changeImplementation(MyInterface impl);
}

Then, for every class that needs to be notified of a change of the implementation, just make it implement the MyInterfaceReloader interface. For instance:

@Component
public class SomeBean implements MyInterfaceReloader {

    // Do not autowire
    private MyInterface myInterface;

    @Override
    public void changeImplementation(MyInterface impl) {
        this.myInterface = impl;
    }
}

Finally, you need a bean that actually changes the implementation in every bean that has MyInterface as an attribute:

@Component
public class MyInterfaceImplementationUpdater {

    @Autowired
    private Map<String, MyInterfaceReloader> reloaders;

    @Autowired
    private MyInterfaceImplementationsHolder holder;

    public void updateImplementations(String implBeanName) {
        this.reloaders.forEach((k, v) -> 
            v.changeImplementation(this.holder.get(implBeanName)));
    }
}

This simply autowires all beans that implement the MyInterfaceReloader interface and updates each one of them with the new implementation, which is retrieved from the holder and passed as an argument. Again, common Spring autowiring rules.

Whenever you want the implementation to be changed, you should just invoke the updateImplementations method with the name of the bean of the new implementation, which is the lower camel case simple name of the class, i.e. myImplA or myImplB for classes MyImplA and MyImplB.

You should also invoke this method at startup, so that an initial implementation is set on every bean that implements the MyInterfaceReloader interface.

Answer 2

I solved a similar issue by using org.apache.commons.configuration.PropertiesConfiguration and org.springframework.beans.factory.config.ServiceLocatorFactoryBean:

Let VehicleRepairService be an interface:

public interface VehicleRepairService {
    void repair();
}

and CarRepairService and TruckRepairService two classes that implements it:

public class CarRepairService implements VehicleRepairService {
    @Override
    public void repair() {
        System.out.println("repair a car");
    }
}

public class TruckRepairService implements VehicleRepairService {
    @Override
    public void repair() {
        System.out.println("repair a truck");
    }
}

I create an interface for a service factory:

public interface VehicleRepairServiceFactory {
    VehicleRepairService getRepairService(String serviceType);
}

Let use Config as configuration class:

@Configuration()
@ComponentScan(basePackages = "config.test")
public class Config {
    @Bean 
    public PropertiesConfiguration configuration(){
        try {
            PropertiesConfiguration configuration = new PropertiesConfiguration("example.properties");
            configuration
                    .setReloadingStrategy(new FileChangedReloadingStrategy());
            return configuration;
        } catch (ConfigurationException e) {
            throw new IllegalStateException(e);
        }
    }

    @Bean
    public ServiceLocatorFactoryBean serviceLocatorFactoryBean() {
        ServiceLocatorFactoryBean serviceLocatorFactoryBean = new ServiceLocatorFactoryBean();
        serviceLocatorFactoryBean
                .setServiceLocatorInterface(VehicleRepairServiceFactory.class);
        return serviceLocatorFactoryBean;
    }

    @Bean
    public CarRepairService carRepairService() {
        return new CarRepairService();
    }

    @Bean
    public TruckRepairService truckRepairService() {
        return new TruckRepairService();
    }

    @Bean
    public SomeService someService(){
        return new SomeService();
    }
}

By using FileChangedReloadingStrategy your configuration be reload when you change the property file.

service=truckRepairService
#service=carRepairService

Having the configuration and the factory in your service, let you can get the appropriate service from the factory using the current value of the property.

@Service
public class SomeService  {

    @Autowired
    private VehicleRepairServiceFactory factory;

    @Autowired 
    private PropertiesConfiguration configuration;


    public void doSomething() {
        String service = configuration.getString("service");

        VehicleRepairService vehicleRepairService = factory.getRepairService(service);
        vehicleRepairService.repair();
    }
}

Hope it helps.

Answer 3

If I understand you correctly then the goal is not to replace injected object instances but to use different implementations during interface method call depends on some condition at run time.

If it is so then you can try to look at the Sring TargetSource mechanism in combination with ProxyFactoryBean. The point is that proxy objects will be injected to beans that uses your interface, and all the interface method calls will be sent to TargetSource target.

Let's call this "Polymorphic Proxy".

Have a look at example below:

ConditionalTargetSource.java

@Component
public class ConditionalTargetSource implements TargetSource {

    @Autowired
    private MyRegistry registry;

    @Override
    public Class<?> getTargetClass() {
        return MyInterface.class;
    }

    @Override
    public boolean isStatic() {
        return false;
    }

    @Override
    public Object getTarget() throws Exception {
        return registry.getMyInterface();
    }

    @Override
    public void releaseTarget(Object target) throws Exception {
        //Do some staff here if you want to release something related to interface instances that was created with MyRegistry.
    }

}

applicationContext.xml

<bean id="myInterfaceFactoryBean" class="org.springframework.aop.framework.ProxyFactoryBean">
    <property name="proxyInterfaces" value="MyInterface"/>
    <property name="targetSource" ref="conditionalTargetSource"/>
</bean>
<bean name="conditionalTargetSource" class="ConditionalTargetSource"/>

SomeService.java

@Service
public class SomeService {

  @Autowired
  private MyInterface myInterfaceBean;

  public void foo(){
      //Here we have `myInterfaceBean` proxy that will do `conditionalTargetSource.getTarget().bar()`
      myInterfaceBean.bar();
  }

}

Also if you want to have both MyInterface implementations to be Spring beans, and the Spring context could not contains both instances at the same time then you can try to use ServiceLocatorFactoryBean with prototype target beans scope and Conditional annotation on target implementation classes. This approach can be used instead of MyRegistry.

P.S. Probably Application Context refresh operation also can do what you want but it can cause other problems such as performance overheads.