Inject spring bean dynamically

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:

[![enter image description here][1]][1]


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][2]).


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][3] 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?).

[1]:

[2]:

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[3]:

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You can use Spring @Conditional on a property value. Give both Beans the same name and it should work as only one Instance will be created.

Have a look here on how to use @Conditional on Services and Components:

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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.

This may be a duplicate question or at least very similar, anyway I answered this sort of question here:

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Pretty much when you want a different beans for a dependency at run-time you need to use a prototype scope. Then you can use a configuration to return different implementations of the prototype bean. You will need to handle the logic on which implementation to return yourself, (they could even be returning 2 different singleton beans it doesn't matter) But say you want new beans, and the logic for returning the implementation is in a bean called `SomeBeanWithLogic.isSomeBooleanExpression()`, then you can make a configuration:

@Configuration
public class SpringConfiguration
{

@Bean
@Autowired
@Scope("prototype")
public MyInterface createBean(SomeBeanWithLogic someBeanWithLogic )
{
if (someBeanWithLogic .isSomeBooleanExpression())
{
return new ImplA(); // I could be a singleton bean
}
else
{
return new ImplB(); // I could also be a singleton bean
}
}
}

There should never be a need to reload the context. If for instance, you want the implementation of a bean to change at run-time, use the above. If you really need to reload your application, because this bean was used in constructors of a singleton bean or something weird, then you need to re-think your design, and if these beans are really singleton beans. You shouldn't be reloading the context to re-create singleton beans to achieve different run-time behavior, that is not needed.

**Edit** The first part of this answer answered the question about dynamically injecting beans. As asked, but I think the question is more of one: 'how can I change the implementation of a singleton bean at run-time'. This could be done with a proxy design pattern.

interface MyInterface
{
public String doStuff();
}

@Component
public class Bean implements MyInterface
{
boolean todo = false; // change me as needed

// autowire implementations or create instances within this class as needed
@Qualifier("implA")
@Autowired
MyInterface implA;

@Qualifier("implB")
@Autowired
MyInterface implB;

public String doStuff()
{
if (todo)
{
return implA.doStuff();
}
else
{
return implB.doStuff();
}
}
}

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.

Be aware that - if interesting to know about - FileChangedReloadingStrategy makes your project highly dependent on the deployment conditions: the WAR/EAR should be exploded by container and your should have direct access to the file system, conditions that are not always met in all situations and environments.

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](

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) mechanism in combination with [ProxyFactoryBean](

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). 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](

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) 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.

You can use @Resource annotation for injection as originally answered [here][1]

e.g.

```
@Component("implA")
public class ImplA implements MyInterface {
...
}
```

```
@Component("implB")
public class ImplB implements MyInterface {
...
}
```
```
@Component
public class DependentClass {

@Resource(name = "\${myinterface.type}")
private MyInterface impl;

}
```

and then set the implementation type in properties file as -
```
myinterface.type=implA
```


[1]:

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public abstract class SystemService {

}

public class FooSystemService extends FileSystemService {

}

public class GoSystemService extends FileSystemService {

}


@Configuration
public class SystemServiceConf {


@Bean
@Conditional(SystemServiceCondition.class)
public SystemService systemService(@Value("${value.key}") value) {
switch (value) {
case A:
return new FooSystemService();
case B:
return new GoSystemService();
default:
throw new RuntimeException("unknown value ");
}
}
}

public class SystemServiceCondition implements Condition {


@Override
public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
return true;
}
}