Getting Null value after bind method called in Reference object of OSGI

Question

In my project I have an OSGI bundle. In this bundle I have osgi references with bind and unbind methods .When I restart my service first unbind method is called ,set reference object as null and then bind method is called set value for reference object .

My problem is when in use that reference object ,it is Null.so that null pointer exception is thrown.

What did I do wrong here?

@Component (service = Service.class, immediate = true)
public class ServiceImpl implements Service{

@Reference (name = "CacheService", policy = ReferencePolicy.DYNAMIC, cardinality = 
ReferenceCardinality.MANDATORY, bind = "bindCacheService", unbind = "unbindCacheService")
private CacheService cacheService;

public void bindCacheService(CacheService cacheService)
{     
    this.cacheService= cacheService;
}

public void unbindCacheService(CacheService cacheService)
{     
    this.cacheService= null;
}

public TransformationPojo  getMetadata() throws Exception
{
  
    LOGGER.debug("TransformerIMPL TransformationMetadata getMetadata invoked {}", cacheService);
    TransformationPojo transformationPojo = cacheService.getTransCache();
    return transformationPojo.getMetadata();
}

}

While running this code I got null pointer exception while calling getMetadata() from another class.

2021-06-27T07:35:21.310+0200 | DEBUG | nt-Grouper-Task-10.216.200.229_6 | b.r.e.p.c.u.a.TransformerImpl | transformer.TransformerImpl  100 | 261 - com.common.utils.transformer - 1.30.0.SNAPSHOT | TransformerIMPL TransformationMetadata getMetadata invoked com.common.utils.cacheclient.metadatapojo.TransformationMetadata@765bea96
2021-06-27T07:35:21.310+0200 | DEBUG | nt-Grouper-Task-10.216.200.229_6 | b.r.e.p.c.u.a.TransformerImpl | transformer.TransformerImpl  100 | 261 - com.common.utils.transformer - 1.30.0.SNAPSHOT | TransformerIMPL TransformationMetadata getMetadata invoked null
2021-06-27T07:35:21.311+0200 | ERROR | nt-Grouper-Task-10.216.200.229_6 | .p.c.i.p.g.i.EventGroupingWorker | ice.internal.EventGroupingWorker  162 | 257 - com.common.integration.processors.groupingservice - 1.30.0.SNAPSHOT | Exception in querying the data for grouping. skipping the current query.
java.lang.NullPointerException: null
at com.common.utils.transformer.TransformerImpl.getMetadata(TransformerImpl.java:101)

Answer 1

After several trial and Error methods I got the solution for this Issue. If made CacheService cacheService as static Null pointer Exception didnt occur.

@Reference (name = "CacheService", policy = ReferencePolicy.DYNAMIC, cardinality = 
ReferenceCardinality.MANDATORY, bind = "bindCacheService", unbind = "unbindCacheService")
private static CacheService cacheService;

Answer 2

See Bound Service Replacement. When using dynamic policy, the new service is injected at the bind method before the old service is removed at the unbind method. So if you blindly set the field to null in the unbind method, you will remove the service previously injected.