Java IDL: Exceptions - JDK 5 Documentation v1.4.1, Java 2 SDK 英文文档

Exceptions

CORBA has two types of exceptions: standard system exceptions which are fully specified by the 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:

Differences Between CORBA and Java Exceptions
System Exceptions
User Exceptions
Minor Code Meanings

Differences Between CORBA and Java Exceptions

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 need not (and cannot) be specified this way.

System Exceptions

CORBA defines a set of standard system exceptions, which are generally raised by the ORB libraries to signal systemic error conditions like:

Server-side system exceptions, such as resource exhaustion or activation failure.
Communication system exceptions, for example, losing contact with the object, host down, or cannot talk to ORB daemon (orbd).
Client-side system exceptions, such as invalid operand type or anything that occurs before a request is sent or after the result comes back.

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.

System Exception Structure

All CORBA system exceptions have the same structure:

exception <SystemExceptionName> { // descriptive of error
    unsigned long minor;          // more detail about error
    CompletionStatus completed;   // yes, no, maybe
}

System exceptions are subtypes of java.lang.RuntimeException through org.omg.CORBA.SystemException:

java.lang.Exception
 |
 +--java.lang.RuntimeException
     |
     +--org.omg.CORBA.SystemException
         |
         +--BAD_PARAM
         |
         +--//etc.

Minor Codes

All CORBA system exceptions have a minor code field, 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 Minor code meanings .

Completion Status

All CORBA system exceptions have a completion status field, indicating the status of the operation that threw the exception. The completion codes are:

COMPLETED_YES: The object implementation has completed processing prior to the exception being raised.
COMPLETED_NO: The object implementation was not invoked prior to the exception being raised.
COMPLETED_MAYBE: The status of the invocation is unknown.

User Exceptions

CORBA user exceptions are subtypes of java.lang.Exception through org.omg.CORBA.UserException:

java.lang.Exception
 |
 +--org.omg.CORBA.UserException
      |
      +-- Stocks.BadSymbol
      |
      +--//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

Minor Code Meanings

System exceptions all have a field minor that allows CORBA vendors to provide additional information about the cause of the exception. For a list of standard OMG minor code exceptions (OMGVMCID), refer to the OMG document at http://cgi.omg.org/docs/omg/02-06-01.txt. If you encounter a Sun minor code exception, email us for more information at javaidl@eng.sun.com. When requesting the meaning of a minor code please include the following information:

The platform on which you are running (e.g., Solaris, Linux, Win32)
The version of the JDK you are using (e.g., 1.4.0_01)
A stack trace

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