Having created a servant class such as the rudimentary
NodeI class, you can instantiate the class to create a concrete servant that can receive invocations from a client. However, merely instantiating a servant class is insufficient to incarnate an object. Specifically, to provide an implementation of an Ice object, you must follow these steps:
- Instantiate a servant class.
- Create an identity for the Ice object incarnated by the servant.
- Inform the Ice run time of the existence of the servant.
- Pass a proxy for the object to a client so the client can reach it.
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Instantiating an Objective-C Servant
Instantiating a servant means to allocate an instance on the heap:
This code creates a new
NodeI instance. For this example, we used the convenience constructor we saw earlier. Of course, you are not obliged to define such a constructor but, if you do not, you must explicitly call
autorelease on the servant.
For a delegate servant, instantiation would look as follows:
Creating an Identity in Objective-C
Each Ice object requires an identity. That identity must be unique for all servants using the same object adapter.
The Ice object model assumes that all objects (regardless of their adapter) have a globally unique identity.
An Ice object identity is a structure with the following Slice definition:
The full identity of an object is the combination of both the
category fields of the
Identity structure. For now, we will leave the
category field as the empty string and simply use the
name field. (The
category field is most often used in conjunction with servant locators.)
To create an identity, we simply assign a key that identifies the servant to the
name field of the
Activating an Objective-C Servant
Merely creating a servant instance does nothing: the Ice run time becomes aware of the existence of a servant only once you explicitly tell the object adapter about the servant. To activate a servant, you invoke the
add operation on the object adapter. Assuming that we have access to the object adapter in the
adapter variable, we can write:
Note the two arguments to
add: the servant and the object identity. Calling
add on the object adapter adds the servant and the servant's identity to the adapter's servant map and links the proxy for an Ice object to the correct servant instance in the server's memory as follows:
- The proxy for an Ice object, apart from addressing information, contains the identity of the Ice object. When a client invokes an operation, the object identity is sent with the request to the server.
- The object adapter receives the request, retrieves the identity, and uses the identity as an index into the servant map.
- If a servant with that identity is active, the object adapter retrieves the servant pointer from the servant map and dispatches the incoming request into the correct method on the servant.
Assuming that the object adapter is in the active state, client requests are dispatched to the servant as soon as you call
Putting the preceding points together, we can write a simple method that instantiates and activates one of our
NodeI servants. For this example, we use a simple method on our servant called
activate that activates a servant in an object adapter with the passed identity:
UUIDs as Identities in Objective-C
The Ice object model assumes that object identities are globally unique. One way of ensuring that uniqueness is to use UUIDs (Universally Unique Identifiers) as identities. The
ICEUtil class contains a helper function to create such identities:
When executed, this method returns a unique string such as
5029a22c-e333-4f87-86b1-cd5e0fcce509. Each call to
generateUUID creates a string that differs from all previous ones. You can use a UUID such as this to create object identities. For convenience, the object adapter has an operation
addWithUUID that generates a UUID and adds a servant to the servant map in a single step:
Note that the operation returns the proxy for the servant just activated.
Creating Proxies in Objective-C
Once we have activated a servant for an Ice object, the server can process incoming client requests for that object. However, clients can only access the object once they hold a proxy for the object. If a client knows the server's address details and the object identity, it can create a proxy from a string, as we saw in our first example. However, creation of proxies by the client in this manner is usually only done to allow the client access to initial objects for bootstrapping. Once the client has an initial proxy, it typically obtains further proxies by invoking operations.
The object adapter contains all the details that make up the information in a proxy: the addressing and protocol information, and the object identity. The Ice run time offers a number of ways to create proxies. Once created, you can pass a proxy to the client as the return value or as an out-parameter of an operation invocation.
Proxies and Servant Activation in Objective-C
addWithUUID servant activation operations on the object adapter return a proxy for the corresponding Ice object, as we saw earlier. This means we can write:
addWithUUID both activates the servant and returns a proxy for the Ice object incarnated by that servant in a single step.
Note that we need to use an
uncheckedCast here because
addWithUUID returns a proxy of type
Direct Proxy Creation in Objective-C
The object adapter offers an operation to create a proxy for a given identity:
createProxy creates a proxy for a given identity whether a servant is activated with that identity or not. In other words, proxies have a life cycle that is quite independent from the life cycle of servants:
This creates a proxy for an Ice object with the identity returned by
generateUUID. Obviously, no servant yet exists for that object so, if we return the proxy to a client and the client invokes an operation on the proxy, the client will receive an
ObjectNotExistException. (We examine these life cycle issues in more detail in Object Life Cycle.)