Having created a servant class such as the rudimentary NodeI class in
Section 12.4.2, 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 take the following steps:
Node servant = new NodeI("Fred");
This code creates a new NodeI instance and assigns its address to a reference of type
Node. This works because
NodeI is derived from
Node, so a
Node reference can refer to an instance of type
NodeI. However, if we want to invoke a member function of the
NodeI class at this point, we must use a
NodeI reference:
NodeI servant = new NodeI("Fred");
Whether you use a Node or a
NodeI reference depends purely on whether you want to invoke a member function of the
NodeI class: if not, a
Node reference works just as well as a
NodeI reference.
Each Ice object requires an identity. That identity must be unique for all servants using the same object adapter.
1 An Ice object identity is a structure with the following Slice definition:
module Ice {
struct Identity {
string name;
string category;
};
// ...
};
The full identity of an object is the combination of both the name and
category fields of the
Identity structure. For now, we will leave the
category field as the empty string and simply use the
name field. (See
Section 28.6 for a discussion of the
category field.)
Ice.Identity id = new Ice.Identity();
id.name = "Fred"; // Not unique, but good enough for now
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:
Assuming that the object adapter is in the active state (see Section 28.4), client requests are dispatched to the servant as soon as you call
add.
As we discussed in Section 2.5.1, the Ice object model assumes that object identities are globally unique. One way of ensuring that uniqueness is to use UUIDs (Universally Unique Identifiers)
[14] as identities. The
Ice.Util package contains a helper function to create such identities:
public class Example {
public static void
main(String[] args)
{
System.out.println(Ice.Util.generateUUID());
}
}
When executed, this program prints a unique string such as 5029a22c‑e333‑4f87‑86b1‑cd5e0fcce509. Each call to
generateUUID creates a string that differs from all previous ones.
2 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. Using this operation, we can create an identity and register a servant with that identity in a single step as follows:
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 in
Chapter 3. 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.
The add and
addWithUUID servant activation operations on the object adapter return a proxy for the corresponding Ice object. This means we can write:
NodePrx proxy = NodePrxHelper.uncheckedCast(
_adapter.addWithUUID(new NodeI("Fred")));
Here, 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
Ice.ObjectPrx.
module Ice {
local interface ObjectAdapter {
Object* createProxy(Identity id);
// ...
};
};
Note that 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:
Ice.Identity id = new Ice.Identity();
id.name = Ice.Util.generateUUID();
Ice.ObjectPrx o = _adapter.createProxy(id);
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
Chapter 31.)
