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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.

Note

The client proxy being serialized to disk instead of the actual bean instance has the added benefit of not recursively serializing to disk a potentially large tree of bean instances that have references to other beans.

Unproxyable bean types

Due to limitations of the Java language, there are some legal bean types that are not able to have a client proxy created for them by the container. If an injection point tries to inject a bean type that is unable to be proxied, and it is not declared in the @Dependent scope, the container will abort deployment with an appropriate error message.

The following bean types are unable to have a client proxy created for them:

  • Classes without a non-private constructor with no parameters, that is, a default constructor
  • Classes declared final or with final methods
  • Primitive types
  • Array types

Here are some tips on how to resolve an unproxyable dependency error, such as the ones just mentioned:

  • Add a default constructor to the bean type being injected
  • Create an interface that can be implemented by the bean being injected and change the injection point to use the interface instead of the implementation
  • If none of the previous work, we can set the scope to be @Dependent
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