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Basic Java Mapping for Classes
A Slice class is mapped to a Java class with the same name. The generated class contains a public data member for each Slice data member (just as for structures and exceptions), and a member function for each operation. Consider the following class definition:
class TimeOfDay {
short hour; // 0 - 23
short minute; // 0 - 59
short second; // 0 - 59
string format(); // Return time as hh:mm:ss
};
The Slice compiler generates the following code for this definition:
public interface _TimeOfDayOperations {
String format(Ice.Current current);
}
public interface _TimeOfDayOperationsNC {
String format();
}
public abstract class TimeOfDay extends Ice.ObjectImpl
implements _TimeOfDayOperations, _TimeOfDayOperationsNC {
public short hour;
public short minute;
public short second;
public TimeOfDay();
public TimeOfDay(short hour, short minute, short second);
// ...
}
There are a number of things to note about the generated code:
- The compiler generates "operations interfaces" called
_TimeOfDayOperationsand_TimeOfDayOperationsNC. These interfaces contain a method for each Slice operation of the class. - The generated class
TimeOfDayinherits (indirectly) fromIce.Object. This means that all classes implicitly inherit fromIce.Object, which is the ultimate ancestor of all classes. Note thatIce.Objectis not the same asIce.ObjectPrx. In other words, you cannot pass a class where a proxy is expected and vice versa.
If a class has only data members, but no operations, the compiler generates a non-abstract class. - The generated class contains a public member for each Slice data member.
- The generated class inherits member functions for each Slice operation from the operations interfaces.
- The generated class contains two constructors.
There is quite a bit to discuss here, so we will look at each item in turn.
Operations Interfaces in Java
The methods in the _<interface-name>Operations interface have an additional trailing parameter of type Ice.Current, whereas the methods in the _<interface-name>OperationsNC interface lack this additional trailing parameter. The methods without the Current parameter simply forward to the methods with a Current parameter, supplying a default Current. For now, you can ignore this parameter and pretend it does not exist.
If a class has only data members, but no operations, the compiler omits generating the <_interface-name>Operations and _<interface-name>OperationsNC interfaces.
Inheritance from Ice.Object in Java
Like interfaces, classes implicitly inherit from a common base class, Ice.Object. However, as shown in the illustration below, classes inherit from Ice.Object instead of Ice.ObjectPrx (which is at the base of the inheritance hierarchy for proxies). As a result, you cannot pass a class where a proxy is expected (and vice versa) because the base types for classes and proxies are not compatible.
Inheritance from Ice.ObjectPrx and Ice.Object.
Ice.Object contains a number of member functions:
package Ice;
public interface Object
{
boolean ice_isA(String s);
boolean ice_isA(String s, Current current);
void ice_ping();
void ice_ping(Current current);
String[] ice_ids();
String[] ice_ids(Current current);
String ice_id();
String ice_id(Current current);
void ice_preMarshal();
void ice_postUnmarshal();
DispatchStatus ice_dispatch(Request request, DispatchInterceptorAsyncCallback cb);
}
The member functions of Ice.Object behave as follows:
ice_isA
This function returnstrueif the object supports the given type ID, andfalseotherwise.
ice_ping
As for interfaces,ice_pingprovides a basic reachability test for the class.
ice_ids
This function returns a string sequence representing all of the type IDs supported by this object, including::Ice::Object.
ice_id
This function returns the actual run-time type ID for a class. If you callice_idthrough a reference to a base instance, the returned type id is the actual (possibly more derived) type ID of the instance.
ice_preMarshal
The Ice run time invokes this function prior to marshaling the object's state, providing the opportunity for a subclass to validate its declared data members.
ice_postUnmarshal
The Ice run time invokes this function after unmarshaling an object's state. A subclass typically overrides this function when it needs to perform additional initialization using the values of its declared data members.
ice_dispatch
This function dispatches an incoming request to a servant. It is used in the implementation of dispatch interceptors.
Note that the generated class does not override hashCode and equals. This means that classes are compared using shallow reference equality, not value equality (as is used for structures).
All Slice classes derive from Ice.Object via the Ice.ObjectImpl abstract base class. ObjectImpl implements the java.io.Serializable interface to support Java's serialization facility. ObjectImpl also supplies an implementation of clone that returns a shallow memberwise copy.
Class Data Members in Java
By default, data members of classes are mapped exactly as for structures and exceptions: for each data member in the Slice definition, the generated class contains a corresponding public data member.
If you wish to restrict access to a data member, you can modify its visibility using the protected metadata directive. The presence of this directive causes the Slice compiler to generate the data member with protected visibility. As a result, the member can be accessed only by the class itself or by one of its subclasses. For example, the TimeOfDay class shown below has the protected metadata directive applied to each of its data members:
class TimeOfDay {
["protected"] short hour; // 0 - 23
["protected"] short minute; // 0 - 59
["protected"] short second; // 0 - 59
string format(); // Return time as hh:mm:ss
};
The Slice compiler produces the following generated code for this definition:
public abstract class TimeOfDay extends Ice.ObjectImpl
implements _TimeOfDayOperations,
_TimeOfDayOperationsNC {
protected short hour;
protected short minute;
protected short second;
public TimeOfDay();
public TimeOfDay(short hour, short minute, short second);
// ...
}
For a class in which all of the data members are protected, the metadata directive can be applied to the class itself rather than to each member individually. For example, we can rewrite the TimeOfDay class as follows:
["protected"] class TimeOfDay {
short hour; // 0 - 23
short minute; // 0 - 59
short second; // 0 - 59
string format(); // Return time as hh:mm:ss
};
Note that you can optionally customize the mapping for data members to use getters and setters instead.
Class Operations in Java
Operations of classes are mapped to abstract member functions in the generated class. This means that, if a class contains operations (such as the format operation of our TimeOfDay class), you must provide an implementation of the operation in a class that is derived from the generated class. For example:
public class TimeOfDayI extends TimeOfDay {
public String format(Ice.Current current) {
DecimalFormat df = (DecimalFormat)DecimalFormat.getInstance();
df.setMinimumIntegerDigits(2);
return new String(df.format(hour) + ":" + df.format(minute) + ":" +
df.format(second));
}
}
Class Factories in Java
Having created a class such as TimeOfDayI, we have an implementation and we can instantiate the TimeOfDayI class, but we cannot receive it as the return value or as an out-parameter from an operation invocation. To see why, consider the following simple interface:
interface Time {
TimeOfDay get();
};
When a client invokes the get operation, the Ice run time must instantiate and return an instance of the TimeOfDay class. However, TimeOfDay is an abstract class that cannot be instantiated. Unless we tell it, the Ice run time cannot magically know that we have created a TimeOfDayI class that implements the abstract format operation of the TimeOfDay abstract class. In other words, we must provide the Ice run time with a factory that knows that the TimeOfDay abstract class has a TimeOfDayI concrete implementation. The Ice::Communicator interface provides us with the necessary operations:
module Ice {
local interface ObjectFactory {
Object create(string type);
void destroy();
};
local interface Communicator {
void addObjectFactory(ObjectFactory factory, string id);
ObjectFactory findObjectFactory(string id);
// ...
};
};
To supply the Ice run time with a factory for our TimeOfDayI class, we must implement the ObjectFactory interface:
class ObjectFactory implements Ice.ObjectFactory {
public Ice.Object create(String type) {
if (type.equals(M.TimeOfDay.ice_staticId())) {
return new TimeOfDayI();
}
assert(false);
return null;
}
public void destroy() {
// Nothing to do
}
}
The object factory's create method is called by the Ice run time when it needs to instantiate a TimeOfDay class. The factory's destroy method is called by the Ice run time when its communicator is destroyed.
The create method is passed the type ID of the class to instantiate. For our TimeOfDay class, the type ID is "::M::TimeOfDay". Our implementation of create checks the type ID: if it matches, the method instantiates and returns a TimeOfDayI object. For other type IDs, the method asserts because it does not know how to instantiate other types of objects.
Note that we used the ice_staticId method to obtain the type ID rather than embedding a literal string. Using a literal type ID string in your code is discouraged because it can lead to errors that are only detected at run time. For example, if a Slice class or one of its enclosing modules is renamed and the literal string is not changed accordingly, a receiver will fail to unmarshal the object and the Ice run time will raise NoObjectFactoryException. By using ice_staticId instead, we avoid any risk of a misspelled or obsolete type ID, and we can discover at compile time if a Slice class or module has been renamed.
Given a factory implementation, such as our ObjectFactory, we must inform the Ice run time of the existence of the factory:
Ice.Communicator ic = ...; ic.addObjectFactory(new ObjectFactory(), M.TimeOfDay.ice_staticId());
Now, whenever the Ice run time needs to instantiate a class with the type ID "::M::TimeOfDay", it calls the create method of the registered ObjectFactory instance.
The destroy operation of the object factory is invoked by the Ice run time when the communicator is destroyed. This gives you a chance to clean up any resources that may be used by your factory. Do not call destroy on the factory while it is registered with the communicator — if you do, the Ice run time has no idea that this has happened and, depending on what your destroy implementation is doing, may cause undefined behavior when the Ice run time tries to next use the factory.
The run time guarantees that destroy will be the last call made on the factory, that is, create will not be called concurrently with destroy, and create will not be called once destroy has been called. However, calls to create can be made concurrently.
Note that you cannot register a factory for the same type ID twice: if you call addObjectFactory with a type ID for which a factory is registered, the Ice run time throws an AlreadyRegisteredException.
Finally, keep in mind that if a class has only data members, but no operations, you need not create and register an object factory to transmit instances of such a class. Only if a class has operations do you have to define and register an object factory.
Class Constructors in Java
Classes have a default constructor that default-constructs each data member. This means members of primitive type are initialized to the equivalent of zero, and members of reference type are initialized to null. Note that applications must always explicitly initialize members of structure and enumerated types because the Ice run time does not accept null as a legal value for these types.
If you wish to ensure that data members of primitive and enumerated types are initialized to specific values, you can declare default values in your Slice definition. The default constructor initializes each of these data members to its declared value.
The generated class also contains a second constructor that accepts one argument for each member of the class. This allows you to create and initialize a class in a single statement, for example:
TimeOfDayI tod = new TimeOfDayI(14, 45, 00); // 14:45pm
For derived classes, the constructor requires an argument for every member of the class, including inherited members. For example, consider the the definition from Class Inheritance once more:
class TimeOfDay {
short hour; // 0 - 23
short minute; // 0 - 59
short second; // 0 - 59
};
class DateTime extends TimeOfDay {
short day; // 1 - 31
short month; // 1 - 12
short year; // 1753 onwards
};
The constructors for the generated classes are as follows:
public class TimeOfDay extends Ice.ObjectImpl {
public TimeOfDay() {}
public TimeOfDay(short hour, short minute, short second)
{
this.hour = hour;
this.minute = minute;
this.second = second;
}
// ...
}
public class DateTime extends TimeOfDay
{
public DateTime()
{
super();
}
public DateTime(short hour, short minute, short second,
short day, short month, short year)
{
super(hour, minute, second);
this.day = day;
this.month = month;
this.year = year;
}
// ...
}
If you want to instantiate and initialize a DateTime instance, you must either use the default constructor or provide values for all of the data members of the instance, including data members of any base classes.