Tuesday, June 18, 2013

Creating Spring Bean dynamically in the Runtime

In my training someone asked me whether it is possible to create an object (a Spring Bean) dynamically so you can choose which implementation you want to have in the runtime. So at the compile time you don't know what object actually should be created yet. The application should decide what object to be created based on a property file.

1. We create an annotation so we can mark the method which should be able to create the object dynamically:

...
package your.package;
...
@Retention(RetentionPolicy.RUNTIME)
public @interface InjectDynamicObject {
}
...

2. Use the new created annotation in your method which should be able to create the object dynamically:

...
@Named("customerBo")
public class CustomerBoImpl implements CustomerBo {
...
    @Override
  @InjectDynamicObject
  public Customer getDynamicCustomer() {
        return this.dynamicCustomer;
}
...

3. Write an aspect with Pointcut and Advise which change the object returned by the method in the step 2:

...
@Named
@Aspect
public class DynamicObjectAspect {

    // This comes from the property file 
@Value("#{objects.object}")
private String object;

@Inject
private ApplicationContext applicationContext;

@Pointcut("execution(@your.package.InjectDynamicObject * *(..))")
public void beanAnnotatedWithInjectDynamicObject() {
}

@Around("beanAnnotatedWithInjectDynamicObject()")
public Object adviceBeanAnnotatedWithInjectDynamicObject(
ProceedingJoinPoint pjp) throws Throwable {
Object returnResult = pjp.proceed();
       // Create the bean or object depends on the property file
Object createdObject = applicationContext.getBean(object);
return createdObject;
}
}
...

4. Write your unit test to test the method:

...
    @Test
public void testCustomerOnlineOrOffline() {
// Dynamic object creation
System.out.println("DYNAMIC CUSTOMER: "
+ customerBo.getDynamicCustomer().getName());
}
...

OK, there is another easier way to do this ;-) Without Aspects and AspectJ, just pure Spring:

Just inject all your component implementations into a Map and get the correct implementation out of it. Just like what we have done in eXTra Client application. Please take a look at our implementation of PluginsLocatorManager as an example: http://goo.gl/itpcb. Spring injects the Map with Bean name as String and the Bean itself automagically. "... Even typed Maps can be autowired as long as the expected key type is String. The Map values will contain all beans of the expected type, and the keys will contain the corresponding bean names" (see Spring documentation for details).

...
@Named("customerBo")
public class CustomerBoImpl implements CustomerBo {
...
    // We inject the customer implementations into a Map
    @Inject
    private Map<String, Customer> customerDynamicMap;


    // This comes from the property file as a key for the Map
@Value("#{objects.object}")
private String object;
    
    @Override
  public Customer getDynamicCustomer() {
        return this.customerDynamicMap.get(object);
}
...

Have fun!
Lofi

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