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This chapter contains the following topics:
Note: This chapter contains excerpts from the Java IDL document published by Sun Microsystems, Inc. and distributed with the JDK 1.2.
The idltojava
and m3idltojava
tools read an OMG IDL interface and translate it, or map it, to a Java interface. The m3idltojava
tool also creates stub, skeleton, helper, holder, and other files as necessary. While the idltojava
tool creates stub, skeleton, helper, holder, and other files, the skeleton files it produces cannot be used with the WLE system. When compiling the OMG IDL files to build server skeletons to be used with the WLE system, be sure to use the m3idltojava
tool.
These .java
files are generated from the OMG IDL file according to the mapping specified in the OMG document IDL/Java Language Mapping (available from the OMG Web site at http://www.omg.org
). We cross-reference the following four chapters of that document here for your convenience:
IDL to Java Overview
A summary of the IDL to Java language mapping follows.
CORBA objects are defined in OMG IDL. Before they can be used by a Java programmer, their interfaces must be mapped to Java classes and interfaces. Sun Microsystems, Inc. provides the idltojava
tool, and the WLE system includes the m3idltojava
tool, which performs this mapping automatically.
This overview shows the correspondence between OMG IDL constructs and Java constructs. Note that OMG IDL, as its name implies, defines interfaces. Like Java interfaces, IDL interfaces contain no implementations for their operations (methods in Java). In other words, IDL interfaces define only the signature for an operation (the name of the operation, the datatype of its return value, the datatypes of the parameters that it takes, and any exceptions that it raises). The implementations for these operations need to be supplied in Java classes written by a Java programmer.
The following table lists the main constructs of IDL and the corresponding constructs in Java.
IDL Construct |
Java Construct |
|---|---|
Note: When a CORBA operation takes a type that corresponds to a Java object type (a String , for example), it is illegal to pass a Java null as the parameter value. Instead, pass an empty version of the designated object type (for example, an empty String or an empty array). A Java null can be passed as a parameter only when the type of the parameter is a CORBA object reference, in which case the null is interpreted as a nil CORBA object reference.
Operations in an IDL interface may take out or inout parameters, as well as in parameters. The Java programming language only passes parameters by value and thus does not have out or inout parameters; therefore, these are mapped to what are called Holder classes. In place of the IDL out parameter, the Java programming language method will take an instance of the Holder class of the appropriate type. The result that was assigned to the out or inout parameter in the IDL interface is assigned to the value field of the Holder class.
The package org.omg.CORBA contains a Holder class for each of the basic types (BooleanHolder , LongHolder , StringHolder , and so on). It also has Holder classes for each generated class (such as TypeCodeHolder ), but these are used transparently by the ORB, and the programmer usually does not see them.
The Holder classes defined in the package org.omg.CORBA are:
AnyHolder
BooleanHolder
ByteHolder
CharHolder
DoubleHolder
FloatHolder
IntHolder
LongHolder
ObjectHolder
PrincipalHolder
ShortHolder
StringHolder
TypeCodeHolder
CORBA has two types of exceptions: standard system exceptions, which are fully specified by OMG, and user exceptions, which are defined by the individual application programmer. CORBA exceptions are a little different from Java exception objects, but those differences are largely handled in the mapping from IDL to Java.
Topics in this section include:
To specify an exception in IDL, the interface designer uses the raises
keyword. This is similar to the throws
specification in Java. When you use the exception keyword in IDL, you create a user-defined exception. The standard system exceptions cannot be specified this way.
CORBA defines a set of standard system exceptions, which are generally raised by the ORB libraries to signal systemic error conditions like:
Differences Between CORBA and Java Exceptions
System Exceptions
All IDL operations can throw system exceptions when invoked. The interface designer need not specify anything to enable operations in the interface to throw system exceptions -- the capability is automatic.
This makes sense because no matter how trivial an operation's implementation is, the potential of an operation invocation coming from a client that is in another process, and perhaps (likely) on another machine, means that a whole range of errors is possible.
Therefore, a CORBA client should always catch CORBA system exceptions. Moreover, developers cannot rely on the Java compiler to notify them of a system exception they should catch, because CORBA system exceptions are descendants of java.lang.RuntimeException
.
All CORBA system exceptions have the same structure:
exception <SystemExceptionName> { // descriptive of error System exceptions are subtypes of java.lang.RuntimeException
through org.omg.CORBA.SystemException
:
java.lang.Exception All CORBA system exceptions have a minor code field, which contains a number that provides additional information about the nature of the failure that caused the exception. Minor code meanings are not specified by the OMG; each ORB vendor specifies appropriate minor codes for that implementation. For the meaning of minor codes thrown by the Java ORB, see the section "Minor Code Meanings" on page 14-7.
All CORBA system exceptions have a completion status field, which indicates the status of the operation that threw the exception. The completion codes are:
System Exception Structure
unsigned long minor; // more detail about error
CompletionStatus completed; // yes, no, maybe
}
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+--java.lang.RuntimeException
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+--org.omg.CORBA.SystemException
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+--BAD_PARAM
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+--//etc. Minor Codes
Completion Status
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The object implementation has completed processing prior to the exception being raised.
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The object implementation was not invoked prior to the exception being raised.
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CORBA user exceptions are subtypes of java.lang.Exception through org.omg.CORBA.UserException :
java.lang.Exception
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+--org.omg.CORBA.UserException
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+-- Stocks.BadSymbol
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+--//etc.
Each user-defined exception specified in IDL results in a generated Java exception class. These exceptions are entirely defined and implemented by the programmer.
System exceptions all have a field minor that allows CORBA vendors to provide additional information about the cause of the exception. As stated in the CORBA 2.2 specification (13.4.2 Reply Message), the high order 20 bits of minor code value contain a 20-bit "vendor minor codeset ID" (VMCID); the low order 12 bits contain a minor code. BEA's VMCID is 0x54555000. Further, Sun defines single or double digit minor codes for its Java IDL ORB and BEA defines its minor code starting from 1,000. Thus, a condition common to either ORB uses the Java IDL minor code (and VMCID 0), and the BEA ORB unique minor code is 1,000 or greater.
For Sun Microsystems, Inc. minor codes, see the Java IDL documentation. For BEA's minor codes, see the Release Notes.
Table 14-2 Name Server Minor Codes and Their Meanings
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Copyright © 1999 BEA Systems, Inc. All rights reserved.
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