12.4 Mapping for Interfaces

The server-side mapping for interfaces provides an up-call API for the Ice run time: by implementing member functions in a servant class, you provide the hook that gets the thread of control from the Ice server-side run time into your applica­tion code.

12.4.1 Skeleton Classes

On the client side, interfaces map to proxy classes (see Section 5.12). On the server side, interfaces map to skeleton classes. A skeleton is a class that has a pure virtual member function for each operation on the corresponding interface. For example, consider the Slice definition for the Node interface we defined in Chapter 5 once more:

module Filesystem {
    interface Node {
        idempotent string name();
    };
    // ...
};

The Slice compiler generates the following definition for this interface:

package Filesystem;

public interface _NodeOperations
{
    String name(Ice.Current current);
}

public interface _NodeOperationsNC
{
    String name();
}

public interface Node extends Ice.Object,
                              _NodeOperations,
                              _NodeOperationsNC {}

public abstract class _NodeDisp extends Ice.ObjectImpl
                                implements Node
{
    // Mapping‑internal code here...
}

The important points to note here are:

• As for the client side, Slice modules are mapped to Java packages with the same name, so the skeleton class definitions are part of the Filesystem package.

• For each Slice interface <interface‑name>, the compiler generates Java interfaces _<interface‑name>Operations and
_<interface‑name>OperationsNC (_NodeOperations and _NodeOperationsNC in this example). These interfaces contains a member function for each operation in the Slice interface. (You can ignore the Ice.Current parameter for the time being—we discuss it in detail in Section 32.6.)

• For each Slice interface <interface‑name>, the compiler generates a Java interface <interface‑name> (Node in this example). That interface extends Ice.Object and the two operations interfaces.

• For each Slice interface <interface‑name>, the compiler generates an abstract class _<interface‑name>Disp (_NodeDisp in this example). This abstract class is the actual skeleton class; it is the base class from which you derive your servant class.

12.4.2 Servant Classes

In order to provide an implementation for an Ice object, you must create a servant class that inherits from the corresponding skeleton class. For example, to create a servant for the Node interface, you could write:

package Filesystem;

public final class NodeI extends _NodeDisp {

    public NodeI(String name)
    {
        _name = name;
    }

    public String name(Ice.Current current)
    {
        return _name;
    }

    private String _name;
}

By convention, servant classes have the name of their interface with an I‑suffix, so the servant class for the Node interface is called NodeI. (This is a convention only: as far as the Ice run time is concerned, you can choose any name you prefer for your servant classes.) Note that NodeI extends _NodeDisp, that is, it derives from its skeleton class.

As far as Ice is concerned, the NodeI class must implement only a single method: the name method that it inherits from its skeleton. This makes the servant class a concrete class that can be instantiated. You can add other member func­tions and data members as you see fit to support your implementation. For example, in the preceding definition, we added a _name member and a constructor. (Obviously, the constructor initializes the _name member and the name function returns its value.)

Normal and idempotent Operations

Whether an operation is an ordinary operation or an idempotent operation has no influence on the way the operation is mapped. To illustrate this, consider the following interface:

interface Example {
               void   normalOp();
    idempotent void   idempotentOp();
    idempotent string readonlyOp();
};

The operations class for this interface looks like this:

public interface _ExampleOperations
{
    void normalOp(Ice.Current current);
    void idempotentOp(Ice.Current current);
    String readonlyOp(Ice.Current current);
}

Note that the signatures of the member functions are unaffected by the idempotent qualifier.