CORBA - JavaTech: An Introduction to Scientific and Technical Computing with Java - page 534

input/output parameters with the keyword inout . Because Java always passes
parameters by value, the IDL-to-Java compiler must generate special “Holder”
classes to provide support for out and inout parameters.
Forexample, the demo() operation uses an inout parameter of type
CustomData . The IDL compiler generates a special support class named
CustomDataHolder. When the implementation class is built, the parameter
passed in the CustomData position must actually be a CustomDataHolder
object. Holder classes are created to “hold” an object of the root type. Thus
CustomDataHolder holds a CustomData object. In that way, a Java imple-
mentation can set the value of the CustomData object within the Holder class
for return to the caller. Both out and inout parameters are handled with Holder
classes. In fact, in a Java implementation, there is little difference between out
and inout parameter types. The only difference is that inout parameters are
expected to contain data upon entry to the method implementation while out
parameters should not. An attempt to retrieve the inner value of an out para-
meter results in a runtime failure, as does an attempt to pass an inout parameter
whose inner value has not been set.
Our IDL file also defines an exception called Cor19UserException .
Exceptions are not “thrown” in IDL terminology, but rather “raised,” as shown in
the example. However, the raises keyword maps into the Java throws keyword
in the output from the IDL compiler. There are a variety of built-in exception
types, but if a custom type is desired, it must be defined in the IDL file.
The Cor19Example interface declares that method1() returns void ,while
add() returns long . There is no int keyword in IDL, but when IDL types are
mapped to Java primitive types, an IDL long parameter becomes a Java int .
One of the first things a novice CORBA programmer must learn is to mentally
map everywhere that a Java int would be used to a long in the IDL file. When
the Java implementation is built, the parameters and return values identified as
long in the IDL file appear as int types in the Java implementation.
The parameter for method1() is an input ( in ) parameter of type string .
Note the lowercase “ s „ .Lowercase string is an IDL type that maps to a Java
String object. If one accidentally uses uppercase String in an IDL file, an
error is seen when the file is processed by the IDL compiler. This mistake is
common, but the solution is simple - just replace the erroneous String with
the correct string .
It should now be clear that the IDL file shown above defines a CORBA server
that provides the same services as the simple RMI example from Chapter 18 plus
afew more. Other features of IDL not yet described include the use of two kinds of
attributes and two kinds of constants (CORBA keyword const ). Attributes map
into instance variables in Java with accessor and mutator (also known as getter
and setter) methods (which must be implemented in the implementation class).
Attributes with the keyword readonly have only an accessor method. Read-
only attributes are values that may be changed within the server implementation,