异常执行数据流，为什么抛出异常时我的对象为null？

Question

I'm not sure of why this is happening, but I have a simple setup where a caller's return value is null.

I call a function which might throw registered exceptions. When it does, myXDSConsumerRequestHandler stays null. The problem is that I'm able to recover from the registered events (checked the object on the callee). So how do I call the retrieveDocuments and get my object back?

I understand the flow is broken when the exception is thrown, so should I be catching the exceptions at a higher level?

This is the caller:

try {
    myXDSConsumerRequestHandler = 
        RetrieveDocSetUtil.retrieveDocuments(NIST, multipleRetrieveMap);
} catch (VerboseIllegalArgumentException e) {

} catch (XDSException e) {

}

This is the callee:

public static RetrieveDocumentSetImpl retrieveDocuments(
        String repoURL, Map<String, String> docRepoMap) 
    throws VerboseIllegalArgumentException, XDSException {

    RetrieveDocumentSetImpl myXDSConsumerRequestHandler = 
            new RetrieveDocumentSetImpl(repoURL);
    myXDSConsumerRequestHandler.retrieveDocumentSet(docRepoMap);
    return myXDSConsumerRequestHandler;
}

Thank you!

Answer 1

Your code is executed in the following order:

1. Call retrieveDocuments
2. Assign the result to myXDSConsumerRequestHandler

If an exception is thrown during step 1, step 2 will never happen.

In general, you cannot both throw an exception and return a value.

Answer 2

If retrieveDocuments() throws an exception then it never gets a chance to return something. The statement myXDSConsumerRequestHandler = RetrieveDocSetUtil.retrieveDocuments() does not finish normally.

It is not possible to both catch an exception from a method and receive the return value.

If retrieveDocuments() has something meaningful it can return to callers even when these exceptions are encountered, then this method should be catching exceptions internally and returning an appropriate return value rather than allowing the exception to propagate up to the calling method.

Answer 3

You problem comes from having a methid doing too much. It both obtains a RetrieveDocumentSetImpl and attempts to use it.

RetrieveDocumentSetImpl myXDSConsumerRequestHandler = 
        new RetrieveDocumentSetImpl(repoURL);
myXDSConsumerRequestHandler.retrieveDocumentSet(docRepoMap);
return myXDSConsumerRequestHandler;

Separate this into two methods:

RetrieveDocumentSetImpl myXDSConsumerRequestHandler = 
        new RetrieveDocumentSetImpl(repoURL);
return myXDSConsumerRequestHandler;

which either return a handler or fails, throwing an Exception, and

 myXDSConsumerRequestHandler.retrieveDocumentSet(docRepoMap);

Then your caller can call the first method, grabbing the return code, and then if they so choose try various things such as cal the second, catching the indidual exceptions they throw.

Answer 4

Here are my suggestions

1. Add proper handling on public retrieveDocuments(NIST, multipleRetrieveMap) method to ensure that proper parameters are passed to your method. This will address exceptions specially related to VerboseIllegalArgumentException the parameter validation/handling should be done before you pass the paramters to the retrieveDocuments method. Im afraid that if you encounter an exception you can no longer retreive your object for the reason stated by the previous posters It is not possible to both catch an exception from a method and receive the return value. thats why proper validation and handling is very important.
2. After determining the cause of your exception, define/create your own exceptions. This will save you a lot of time and headache in the future.