I am using a subroutine (FORM, ENDFORM) where there is a class-method calling that raises an exception. I am catching the exception from the class-method calling, but how can I pass this exception to where I call the subroutine (PERFORM)?
There's a documentation section for Class-Based Exceptions in Procedures .
If a class-based exception is not handled in a procedure, the system attempts to propagate it to the caller of the procedure. The exceptions that can be propagated from a procedure must be declared in its interface. The caller then knows which exceptions to expect from the procedure. Class-based exceptions are divided into three categories, which determine whether the declaration must be explicit and how it is checked.
In methods of local classes and subroutines, the addition RAISING of the statements METHODS and FORM is used for the declaration
Undeclared exceptions cannot leave a procedure and violate the interface if they are not handled within the procedure. A violation of the interface raises an exception of the predefined class CX_SY_NO_HANDLER, whose exception object contains a reference to the original exception in the attribute PREVIOUS.
See also documentation for FORM RAISING.
START-OF-SELECTION.
TRY.
PERFORM test.
CATCH lcx_some_exception INTO DATA(lx_ex).
ENDTRY.
FORM test RAISING lcx_some_exception.
DATA(lo_test) = NEW lcl_test( ).
lo_test->test( ).
ENDFORM.
Related
I'm not sure what is the correct way of handling exceptions in RHS of rules.
I've a statefullSession (drools version 5.5) that keeps firing until halt gets called i.e.
Runnable firingTask = new Runnable() {
public void run() {
ksession.fireUntilHalt();
};
taskExecutor.execute(firingTask);
The problem is that some parts of my code that gets called as a part of consequence section might throw an exception. If that happens no furher activations happen
My question is:
Is there any correct way how application exception should be propagated back to drools? Or is it that any RHS code has to be run inside try/catch block and these exceptions should be propagated as some additional facts into session?
The general strategies for exception handling apply here as well: just consider a consequence as being a static method that is being called due to the fireUntilHalt.
If the exception should or must be handled in the context of the RHS code, you'll have to use a try-catch statement. Otherwise, the exception is propagated and the remainder of the RHS code will not be executed. (This may include the omission of some essential statement such as a modify or insert or retract, which may affect the progress of the session.)
If you catch the exception org.drools.runtime.rule.ConsequenceException in a try around the fireUntilHalt, you can call fireUntilHalt again. The remarks from the previous paragraph apply here as well.
How you log an exception is completely up to you, but inserting Exception objects as facts (and writing rules to reason over them?) seems rather unconventional.
I want to apply some roles dynamically to a moose class. I am writing that logic inside BUILD function. If there are any failures while applying role, I want to return FALSE. What will be the impact of that return value. How can I handle failure in the code where I am creating the object of this class?
Moose ignores the return value of BUILD; it is called for side-effects only. I'd expect "failures while applying role" to be caused by bugs and those failures to be evidenced by runtime exceptions. If you don't want the program to crash because of runtime exceptions, appropriately wrap your call to new() with eval.
Trying to understand more about RPCs to answer a homework question: Are exceptions handle the same way for the caller? Are the details of how exceptions are raised on the server any different? Are there any additional differences if you have to rethrow?
OR, can someone just explain what the main differences are between local and remote exceptions? And maybe give an idea of what things to look out for if I wanted to implement remote exceptions.
Here's a simple version of an RPC server / client library:
Server:
try
receive message
deserialize arguments
invoke appropriate method
serialize result
transmit result
catch any Exeption
serialize Exception
transmit Exception
Client (Library code, not the caller):
try
serialize arguments
make remote call
receive "something"
deserialize "something" (could be serialized exception or result)
catch Timeout,Network,Other exceptions not from server
handle whatever the library handles
if deserialized "something" is an exception
rethrow exception from server for caller to catch
else, good/expected results
return results
So, if you want exceptions to be caught by the caller, they possibly differ from regular exceptions in that they must be serialized and transmitted over the network to be re-thrown for the caller.
The caller need not do anything special if the Client library exists. If the client library doesn't exist, then the caller needs to also take the role of the client library. This means that the caller needs to distinguish between serialized results and serialized exceptions (at which point the programmer would probably implement an ad-hoc version of the client library code anyway just to avoid having ugly caller code).
Obviously you can't just use type signatures to distinguish between Exceptions and results (otherwise, what happens if the return type and the exception types for an RPC method were the same?). So there's a tiny bit of overhead in the serialization code for the server to label the different responses.
Suppose you have a property with copy semantics. What should you do in the setter if the copy method fails? (I presume this is a possibility, since a copy usually starts with an alloc/init combo, which can fail and return nil.) Apple recommends returning error codes rather than using exceptions, but a setter generally has a void return type. What is the recommended approach? How do you signal that an error has occurred?
The Apple recommendation is really that exceptions should be reserved for exceptional situations. This is sometimes a recommended programming practice anyway, but in the case of Objective-C is reinforced due to the higher cost of exception handling.
So you can throw an exception if you wish and it is appropriate, e.g. running out of memory (copy failed) is (hopefully!) exceptional.
That said, some programming practices also recommend that properties should not throw exceptions; usually on the basis that something that looks like assignment obj.property = value; would be confusing if exceptions were thrown (unlike [obj setProperty:value]).
So that get us to setting the property to the "zero" for the type (nil, 0, 0.0, NO etc.).
To return more details of the error record details of the error which can be queried after the "zero" has been detected. This is essentially the approach used by the underlying ("Unix") syscalls, and many library functions, were errno is set before a "zero" is returned.
There is no way to signal an error, other than that the property whose setter you called would be nil. You can check for nil after executing the setter, just as you would to confirm success after alloc/init'ing a new instance.
I am not able to understand the places where an error handling or where an exception handling should be used. I assume this, if it is an existing framework class there are delegate methods which will facilitate the programmer to send an error object reference and handle the error after that. Exception handling is for cases where an operation of a programmer using some framework classes throws an error and i cannot get an fix on the error object's reference.
Is this assumption valid ? or how should i understand them ?
You should use exceptions for errors that would never appear if the programmer would have checked the parameters to the method that throws the exception. E.g. divide by 0 or the well known "out of bounds"-exception you get from NSArrays.
NSErrors are for errors that the programmer could do nothing about. E.g. parsing a plist file. It would be a waste of resources if the program would check if the file is a valid plist before it tries to read its content. For the validity check the program must parse the whole file. And parsing a file to report that it is valid so you can parse it again would be a total waste. So the method returns a NSError (or just nil, which tells you that something went wrong) if the file can't be parsed.
The parsing for validity is the "programmer should have checked the parameters" part. It's not applicable for this type of errors, so you don't throw a exception.
In theory you could replace the out of bounds exception with a return nil. But this would lead to very bad programming.
Apple says:
Important: In many environments, use of exceptions is fairly commonplace. For example, you might throw an exception to signal that a routine could not execute normally—such as when a file is missing or data could not be parsed correctly. Exceptions are resource-intensive in Objective-C. You should not use exceptions for general flow-control, or simply to signify errors. Instead you should use the return value of a method or function to indicate that an error has occurred, and provide information about the problem in an error object.
I think you are absolutely right with your assumption for Errors and for it framework provide a set of methods (UIWebView error handling ), But your assumption for Exception partially right because the exception only occurred if we do something wrong which is not allowed by the framework and can be fixed. (for example accessing a member from an array beyond its limit).
and will result in application crash.