For a precise list of supported sections of official CORBA specifications with which the Java[TM] Platform, Standard Edition 6 complies, see Official Specifications for CORBA support in Java[TM] SE 6.
The classes and interfaces described in this section can be put into four groups: ORB classes, Exceptions, Helper classes, and Holder classes.
An ORB handles (or brokers) method invocations between a client and the method's implementation on a server. Because the client and server may be anywhere on a network, and because the invocation and implementation may be written in different programming languages, an ORB does a great deal of work behind the scenes to accomplish this communication.
Most of what an ORB does is completely transparent to the user, and a major portion of the CORBA package consists of classes used by the ORB behind the scenes. The result is that most programmers will use only a small part of this package directly. In fact, most programmers will use only a few methods from the ORB class, some exceptions, and occasionally, a holder class.
Before an application can enter the CORBA environment, it must first:
The following operations are provided to initialize applications and obtain the appropriate object references:
When an application requires a CORBA environment it needs a mechanism to get an ORB object reference and possibly an OA object reference (such as the root POA). This serves two purposes. First, it initializes an application into the ORB and OA environments. Second, it returns the ORB object reference and the OA object reference to the application for use in future ORB and OA operations.
In order to obtain an ORB object reference, applications call the ORB.init operation. The parameters to the call can comprise an identifier for the ORB for which the object reference is required, and an arg_list, which is used to allow environment-specific data to be passed into the call.
These are the ORB methods that provide access to the ORB:
Using the init() method without parameters initiates a singleton ORB, which can only give typecode creation anys needed in code generated in Helper classes by idlj.
Applications require a portable means by which to obtain their initial object references. References are required for the root POA, POA Current, Interface Repository, and various Object Services instances. The functionality required by the application is similar to that provided by the Naming Service. However, the OMG does not want to mandate that the Naming Service be made available to all applications in order that they may be portably initialized. Consequently, the operations shown in this section provide a simplified, local version of the Naming Service that applications can use to obtain a small, defined set of object references which are essential to its operation. Because only a small well-defined set of objects are expected with this mechanism, the naming context can be flattened to be a single-level name space. This simplification results in only two operations being defined to achieve the functionality required.
Initial references are obtained via two operations provided in the ORB object interface, providing facilities to list and resolve initial object references. These are:
An example that uses some of these methods is Getting Started with Java IDL.
The documentation on Java IDL exceptions has more information and explains the difference between system exceptions and user-defined exceptions.
The following is a list of the system exceptions (which are unchecked exceptions inheriting through org.omg.CORBA.SystemException from java.lang.RuntimeException) that are defined in the package org.omg.CORBA:
BAD_CONTEXT
BAD_INV_ORDER
BAD_OPERATION
BAD_PARAM
BAD_TYPECODE
COMM_FAILURE
DATA_CONVERSION
FREE_MEM
IMP_LIMIT
INITIALIZE
INTERNAL
INTF_REPOS
INVALID_TRANSACTION
INV_FLAG
INV_IDENT
INV_OBJREF
INV_POLICY
MARSHAL
NO_IMPLEMENT
NO_MEMORY
NO_PERMISSION
NO_RESOURCES
NO_RESPONSE
OBJECT_NOT_EXIST
OBJ_ADAPTER
PERSIST_STORE
TRANSACTION_REQUIRED
TRANSACTION_ROLLEDBACK
TRANSIENT
UNKNOWN
The following is a list of user-defined exceptions defined in the package org.omg.CORBA.
Bounds
UnknownUserException
WrongTransaction
PolicyError
For example, the package org.omg.CORBA.TypeCodePackage contains two exceptions thrown by methods in the class TypeCode. These exceptions are:
Another package that is a subpackage of CORBA is the portable package. It provides a set of ORB APIs that makes it possible for code generated by one vendor's IDL compiler to run on another vendor's ORB.
Support for out and inout parameter passing modes requires the use of additional holder classes. Because the Java programming language does not support out or inout parameters, holder classes are needed as a means of passing a parameter that can be modified. To support portable stubs and skeletons, holder classes also implement the org.omg.CORBA.portable.Streamable interface.
Holder classes are named by appending "Holder" to the name of the type. The name of the type refers to its name in the Java programming language. For example, a holder class for the interface named Account in the Java programming language would be named AccountHolder.
Holder classes are available for all of the basic IDL datatypes in the org.omg.CORBA package. So, for example, there are already-defined classes for LongHolder, ShortHolder, FloatHolder, and so on. Classes are also generated for all named user-defined IDL types except those defined by typedefs. (Note that in this context user defined includes types that are defined in OMG specifications such as those for the Interface Repository, and other OMG services.)
Each holder class has:
The default constructor sets the value field to the default value for the type as defined by the Java language:
As an example, if the interface Account, defined in OMG IDL,
were mapped to the Java programming language, the following holder class
would be generated:
public final class AccountHolder implements
org.omg.CORBA.portable.Streamable
{
// field that holds an Account object
public Account value = null;
// default constructor
public AccountHolder ()
{
}
// creates a new AccountHolder from initialValue
public AccountHolder (Account initialValue)
{
value = initialValue;
}
// reads the contents of i and assigns the contents to value
public void _read (org.omg.CORBA.portable.InputStream i)
{
value = AccountHelper.read (i);
}
// writes value to o
public void _write (org.omg.CORBA.portable.OutputStream o)
{
AccountHelper.write (o, value);
}
// returns the typecode for Account
public org.omg.CORBA.TypeCode _type ()
{
return AccountHelper.type ();
}
}
For more information on Holder classes, see Chapter 1.4, Mapping for Basic Types in the OMG IDL to Java Language Mapping. The Holder classes defined in the package org.omg.CORBA are:
AnyHolder AnySeqHolder BooleanHolder BooleanSeqHolder ByteHolder CharHolder CharSeqHolder CurrentHolder DoubleHolder DoubleSeqHolder FixedHolder FloatHolder FloatSeqHolder IntHolder LongHolder LongLongSeqHolder LongSeqHolder ObjectHolder OctetSeqHolder ParameterModeHolder PolicyErrorHolder PolicyListHolder PrincipalHolder ServiceInformationHolder ShortHolder ShortSeqHolder StringHolder StringSeqHolder TypeCodeHolder ULongLongSeqHolder ULongSeqHolder UnknownUserExceptionHolder UShortSeqHolder ValueBaseHolder WCharSeqHolder WrongTransactionHolder WStringSeqHolder
Helper files supply several static methods needed to manipulate the type. These include:
ValueHelper interface (if it is a user-defined
value type)
The helper class for a mapped IDL interface or abstract interface
also include narrow operation(s). The static narrow method allows
an org.omg.CORBA.Object to be narrowed to the object reference
of a more specific type. The IDL exception CORBA.BAD_PARAM
is thrown if the narrow fails because the object reference does not
support the requested type. A different system exception is raised
to indicate other kinds of errors. Trying to narrow a null will always
succeed with a return value of null. Generally, the only helper method an application programmer uses is
the narrow method. The other methods are normally used behind
the scenes and are transparent to the programmer.
Helper classes fall into two broad categories, helpers for value types and helpers for non value types. Because all of the helper classes in one category provide the same methods, one generic explanation of each category of helper classes is presented here.
When OMG IDL is mapped to the Java programming language,
a "helper" class is generated for each user-defined type.
This generated class will have the name of the user-defined type with
the suffix Helper appended. For example, if the
interface Account is defined in OMG IDL, the
idlj compiler will automatically generate a class named
AccountHelper. The AccountHelper class
will contain the static methods needed for manipulating instances of the type,
in this case, Account objects.
narrow Methodorg.omg.CORBA.Object object
or a java.lang.Object object. This object must be cast to its
more specific type before it can be operated on. For example, an
Account object will be returned as a generic object and must
be narrowed to an Account object so that Account
methods may be called on it.
The narrow method has two forms, one that takes an
org.omg.CORBA.Object object and one that takes a
java.lang.Object object. Whether the interface is abstract or
not determines which narrow method its helper class will provide.
The helper class for an interface
that is not abstract will have a narrow method that takes a CORBA
object, whereas the narrow method for an interface that is abstract
will
take an object in the Java programming language. The helper class for a
non-abstract interface that has at least one abstract base interface will provide
both versions of the narrow method.
The Hello World tutorial uses a narrow method that looks like this:
// create and initialize the ORB
ORB orb = ORB.init(args, null);
// get the root naming context
org.omg.CORBA.Object objRef =
orb.resolve_initial_references("NameService");
// Use NamingContextExt instead of NamingContext. This is
// part of latest Inter-Operable naming Service.
NamingContextExt ncRef = NamingContextExtHelper.narrow(objRef);
// resolve the Object Reference in Naming
String name = "Hello";
helloImpl = HelloHelper.narrow(ncRef.resolve_str(name));
narrow
method if the type defined in OMG IDL maps to an interface in the Java
programming language. Types that are not value types will have a basic
helper class generated for them.
For example, assuming that the interface Account is not a
value type IDL type and is also not an abstract interface and has no
abstract base interfaces, its AccountHelper class will look
like this:
abstract public class AccountHelper
{
private static String _id = "IDL:Account:1.0";
// inserts an Account object into an Any object
public static void insert (org.omg.CORBA.Any a, Account that)
{
org.omg.CORBA.portable.OutputStream out = a.create_output_stream ();
a.type (type ());
write (out, that);
a.read_value (out.create_input_stream (), type ());
}
// extracts an Account object from an Any object
public static Account extract (org.omg.CORBA.Any a)
{
return read (a.create_input_stream ());
}
private static org.omg.CORBA.TypeCode __typeCode = null;
// gets the typecode for this type
synchronized public static org.omg.CORBA.TypeCode type ()
{
if (__typeCode == null)
{
__typeCode = org.omg.CORBA.ORB.init ().create_interface_tc (AccountHelper.id (), "Account");
}
return __typeCode;
}
// gets the repository id for this type
public static String id ()
{
return _id;
}
// reads an Account object from an input stream
public static Account read (org.omg.CORBA.portable.InputStream istream)
{
return narrow (istream.read_Object (_AccountStub.class));
}
// writes an Account object to an outputstream
public static void write (org.omg.CORBA.portable.OutputStream ostream, Account value)
{
ostream.write_Object ((org.omg.CORBA.Object) value);
}
// converts (narrows) an Object to an Account object
public static Account narrow (org.omg.CORBA.Object obj)
{
if (obj == null)
return null;
else if (obj instanceof Account)
return (Account)obj;
else if (!obj._is_a (id ()))
throw new org.omg.CORBA.BAD_PARAM ();
else
{
org.omg.CORBA.portable.Delegate delegate = ((org.omg.CORBA.portable.ObjectImpl)obj)._get_delegate ();
_AccountStub stub = new _AccountStub ();
stub._set_delegate(delegate);
return stub;
}
}
}
Assuming that Address is a value type, the
AddressHelper class will look like this:
abstract public class AddressHelper
{
private static String _id = "IDL:Address:1.0";
// same as for non-value type
public static void insert (org.omg.CORBA.Any a, Address that)
{
org.omg.CORBA.portable.OutputStream out = a.create_output_stream ();
a.type (type ());
write (out, that);
a.read_value (out.create_input_stream (), type ());
}
// same as for non-value type
public static Address extract (org.omg.CORBA.Any a)
{
return read (a.create_input_stream ());
}
private static org.omg.CORBA.TypeCode __typeCode = null;
private static boolean __active = false;
// getting the typecode for the type
synchronized public static org.omg.CORBA.TypeCode type ()
{
if (__typeCode == null)
{
synchronized (org.omg.CORBA.TypeCode.class)
{
if (__typeCode == null)
{
if (__active)
{
return org.omg.CORBA.ORB.init().create_recursive_tc ( _id );
}
__active = true;
org.omg.CORBA.ValueMember[] _members0 = new org.omg.CORBA.ValueMember[0];
org.omg.CORBA.TypeCode _tcOf_members0 = null;
__typeCode = org.omg.CORBA.ORB.init ().create_value_tc (_id, "Address", org.omg.CORBA.VM_NONE.value, null, _members0);
__active = false;
}
}
}
return __typeCode;
}
// same as for non-value type
public static String id ()
{
return _id;
}
// reads a serializable instance of Address from the given input stream
public static Address read (org.omg.CORBA.portable.InputStream istream)
{
return (Address)((org.omg.CORBA_2_3.portable.InputStream) istream).read_value (id ());
}
// writes a serializable instance of Address to the given output stream
public static void write (org.omg.CORBA.portable.OutputStream ostream, Address value)
{
((org.omg.CORBA_2_3.portable.OutputStream) ostream).write_value (value, id ());
}
}
The Helper classes defined in the package org.omg.CORBA are:
AnySeqHelper
BooleanSeqHelper
CharSeqHelper
CompletionStatusHelper
CurrentHelper
DefinitionKindHelper
DoubleSeqHelper
FieldNameHelper
FloatSeqHelper
IdentifierHelper
IDLTypeHelper
LongLongSeqHelper
LongSeqHelper
NameValuePairHelper
ObjectHelper
OctetSeqHelper
ParameterModeHelper
PolicyErrorCodeHelper
PolicyErrorHelper
PolicyHelper
PolicyListHelper
PolicyTypeHelper
RepositoryIdHelper
ServiceDetailHelper
ServiceInformationHelper
SetOverrideTypeHelper
ShortSeqHelper
StringSeqHelper
StringValueHelper
StructMemberHelper
ULongLongSeqHelper
ULongSeqHelper
UnionMemberHelper
UnknownUserExceptionHelper
UShortSeqHelper
ValueBaseHelper
ValueMemberHelper
VersionSpecHelper
VisibilityHelper
WCharSeqHelper
WrongTransactionHelper
WStringSeqHelper
WStringValueHelper
This is why several interfaces in the org.omg.CORBA package consist of a single field, value, which is a short. This field is a constant used for such things as an error code or value modifier. For example, the value field of the interface BAD_POLICY is one of the possible reasons for the exception PolicyError to be thrown. To specify this error code, you would use BAD_POLICY.value.
The exception PolicyError uses the value field of the following interfaces as its possible error codes.
ValueMember object's
access method to denote the visibility of the ValueMember object.
An ORB does not require that there be an interface repository, and Java
IDL does not include one. Even though this release does not include an
implementation of an interface repository, the following IR classes and
interfaces have been included for the purpose of creating typecodes (see
create_value_tc, create_struct_tc, create_union_tc and create_exception_tc
methods in interface org.omg.CORBA.ORB):
&nbs
Some of the API included in org.omg subpackages is provided for conformance with the current OMG CORBA specification but is not implemented in Sun's release of the JDKTM. This enables other JDK licensees to provide implementations of this API in standard extensions and products.
Some of the API included in org.omg subpackages throw NO_IMPLEMENT exceptions for various reasons. Among these reasons are: