Client proxies

The reference to a bean injected into an injection point, or obtained by programmatic lookup, is usually not a direct reference to an instance of a bean, unless the injected bean is of @Dependent scope.

Instead of the actual bean instance, Weld injects a client proxy that is responsible for ensuring only the bean instance associated with the current context has a method invoked on it. That might sound confusing, but it will become clearer with an example.

@RequestScoped
public class RequestBean {
  ...
}

@ApplicationScoped
public class ApplicationBean {
  @Inject
  RequestBean bean;
}

Given the two beans we just defined, we would not want the same @RequestScoped bean to be used by all requests to our application, as there is only one instance of the @ApplicationScoped bean. The client proxy is injected into the @ApplicationScoped bean instead of an instance of the @RequestScoped bean and is responsible for retrieving the bean instance from the current request scope whenever a method is called. Through the client proxy, Weld is able to have two different requests using the same @ApplicationScoped bean, while calling methods on their respective @RequestScoped bean instances without us needing to do any special wiring or configuration.

A client proxy is also beneficial in situations where we have a bean in a scope that can be serialized to disk, such as @SessionScoped, and it has references to beans in a scope that can be retrieved at will, such as @ApplicationScoped. It certainly does not provide any benefit to serialize a bean instance that can be retrieved whenever it's needed, so the client proxy is serialized in its place.