Now that we understand what it means for an Ice object to exist, we can look at what is involved in creating an Ice object. Fundamentally, there is only one way for an Ice object to come into being: the server must instantiate a servant for the object and add an entry for that servant to the ASM (or, alternatively, arrange for a servant locator to return a servant from its
locate operation—see
Section 32.7).
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DirectoryIPtr root = new DirectoryI("/", 0);
adapter‑>addWithUUID(root); // Ice object exists now
The servant exists as soon as the call to new completes, and the Ice object exists as soon as the code adds the servant to the ASM: at that point, the Ice object becomes reachable to clients who hold a proxy to it.
This is the way we created Ice objects for our file system application in earlier chapters. However, doing so is not all that interesting because the only files and directories that exist are those that the server decides to create when it starts up. What we really want is a way for
clients to create and destroy directories and files.
The canonical way to create an object is to use the factory pattern [2]. The factory pattern, in a nutshell, says that objects are created by invoking an operation (usually called
create) on an object factory:
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interface PhoneEntry {
idempotent string name();
idempotent string getNumber();
idempotent void setNumber(string phNum);
};
exception PhoneEntryExists {
string name;
string phNum;
};
interface PhoneEntryFactory {
PhoneEntry* create(string name, string phNum)
throws PhoneEntryExists;
};
The entries in the phone book consist of simple name–number pairs. The interface to each entry is called
PhoneEntry and provides operations to read the name and to read and write the phone number. (For a real application, the objects would likely be more complex and encapsulate more state. However, these simple objects will do for the purposes of this discussion.)
To create a new entry, a client calls the create operation on a
PhoneEntryFactory object. (The factory is a singleton object
[2], that is, only one instance of that interface exists in the server.) It is the job of
create to create a new
PhoneEntry object, using the supplied name as the object identity.
An immediate consequence of using the name as the object identity is that create can raise a
PhoneEntryExists exception: presumably, if a client attempts to create an entry with the same name as an already-existing entry, we need to let the client know about this. (Whether this is an appropriate design is something we examine more closely in
Section 35.9.)
create returns a proxy to the newly-created object, so the client can use that proxy to invoke operations. However, this is by convention only. For example,
create could be a
void operation if the client has some other way to eventually get a proxy to the new object (such as creating the proxy from a string, or locating the proxy via a search operation). Alternatively, you could define "bulk" creation operations that allow clients to create several new objects with a single RPC. As far as the Ice run time is concerned, there is nothing special about a factory operation: a factory operation is just like any other operation; it just so happens that a factory operation creates a new Ice object as a side effect of being called, that is, the
implementation of the operation is what creates the object, not the Ice run time.
Also note that create accepts a
name and a
phNum parameter, so it can initialize the new object. This is not compulsory, but generally a good idea. An alternate factory operation could be:
interface PhoneEntryFactory {
PhoneEntry* create(string name)
throws PhoneEntryExists;
};
With this design, the assumption is that the client will call setNumber after it has created the object. However, in general, allowing objects that are not fully initialized is a bad idea: it all too easily happens that a client either forgets to complete the initialization, or happens to crash or get disconnected before it can complete the initialization. Either way, we end up with a partially-initialized object in the system that can cause surprises later.
3
Similarly, so-called generic factories are also something to be avoided:
dictionary<string, string> Params;
exception CannotCreateException {
string reason;
};
interface GenericFactory {
Object* create(Params p)
throws CannotCreateException;
};
The intent here is that a GenericFactory can be used to create any kind of object; the
Params dictionary allows an arbitrary number of parameters to be passed to the
create operation in the form of name–value pairs, for example:
GenericFactoryPrx factory = ...;
Ice::ObjectPrx obj;
Params p;
// Make a car.
//
p["Make"] = "Ford";
p["Model"] = "Falcon";
obj = factory‑>create(p);
CarPrx car = CarPrx::checkedCast(obj);
// Make a horse.
//
p.clear();
p["Breed"] = "Clydesdale";
p["Sex"] = "Male";
obj = factory‑>create(p);
HorsePrx horse = HorsePrx::checkedCast(obj);
We strongly discourage you from creating factory interfaces such as this, unless you have a good overriding reason: generic factories undermine type safety and are much more error-prone than strongly-typed factories.
PhoneEntryPrx
PhoneEntryFactory::create(const string& name,
const string& phNum,
const Current& c)
{
try {
CommunicatorPtr comm = c.adapter.getCommunicator();
PhoneEntryPtr servant = new PhoneEntryI(name, phNum);
return PhoneEntryPrx::uncheckedCast(
c.adapter‑>add(servant,
comm‑>stringToIdentity(name)));
} catch (const Ice::AlreadyRegisteredException&) {
throw PhoneEntryExists(name, phNum);
}
}
The create function instantiates a new
PhoneEntryI object (which is the servant for the new
PhoneEntry object), adds the servant to the ASM, and returns the proxy for the new object. Adding the servant to the ASM is what creates the new Ice object, and client requests are dispatched to the new object as soon as that entry appears in the ASM (assuming the object adapter is active).
Note that, even though this code contains no explicit lock, it is thread-safe. The
add operation on the object adapter is atomic: if two clients concurrently add a servant with the same identity, exactly one thread succeeds in adding the entry to the ASM; the other thread receives an
AlreadyRegisteredException. Similarly, if two clients concurrently call
create for different entries, the two calls execute concurrently in the server (if the server is multi-threaded); the implementation of
add in the Ice run time uses appropriate locks to ensure that concurrent updates to the ASM cannot corrupt anything.
