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OSIDs strictly define the permissible checked an unchecked exceptions which may pass. They are straightforward in the most basic of method implementations. However, some semantic analysis is necessary when creating a chain of methods to help convey what may have gone wrong to your consumer or ultimately your end user.
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In most cases, there is no reason to catch org.osid.OsidRuntimeException of any of its subclasses.
Calling Other OSIDs
Exception
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Misalignment
Sometimes exceptions don’t align and need some attention.
| Code Block | ||
|---|---|---|
| ||
public class Activity
implements org.osid.learning.Activity {
private org.osid.id.Id objectiveId;
private org.osid.learning.ObjectiveLookupSession objectives;
...
@OSID
public org.osid.id.Id getObjectiveId() {
return (this.objectiveId);
}
@OSID
public org.osid.learning.Objective getObjective()
throws org.osid.OperationFailedException {
try {
return (this.objectives.getObjective(getObjectiveId()));
} catch (org.osid.DoesNotExistExceptionNotFoundException dnenfe) {
throw new org.osid.OperationFailedException("for some strange reason, there is no Objective for this Activity. Maybe we're talking to the wrong provider?", dnenfe)
} catch (org.osid.PermissionDeniedException pde) {
throw new org.osid.OperationFailedException("for some inexplicable reason you cannot see the Objective for your Activity. Authorization setup is fakakta. ", pde);
}
}
... |
In the above example, the getObjective() call is implemented using an ObjectiveLookupSession. ObjectiveLookupSession.getObjective defines DoesNotExistException NotFoundtException and PermissionDeniedException not present in Activity.getObjective() (ignoring the unchecked exceptions which imply a programming/integration problem which should not be handled here).
The Activity says that it has an Objective. Therefore, the Objective must exist. To say that Activity.getObjective() should also throw a DoesNotExistException NotFoundException ignores this tenet. If for whatever reason, the provider cannot come up with one should be considered an error due to the result of a breakage in connectivity, data integrity, authorization, configuration, or something which should not occur in normal operations. Semantics like this is what generally causes exception misalignments across method calls.
Do we worry only when exceptions don’t line up?
Exception Alignment
In this case we decided to override the Objective in an orchestration layer.
| Code Block | ||
|---|---|---|
| ||
public class ObjectiveFetchingActivityLookupSession
implements org.osid.learning.ActivityLookupSession {
private org.osid.learning.ActivityLookupSession activityLookupSession;
private org.osid.learning.ObjectiveLookupSession objectiveLookupSession;
...
@OSID
public org.osid.learning.Activity getActivity(org.osid.id.Id activityId)
throws org.osid.NotFoundException,
org.osid.OperationFailedException,
org.osid.PermsissionDeniedException {
org.osid.learning.Activity activity = this.activityLookupSession.getActivity(activityId);
org.osid.learning.Objective objective;
try {
objective = this.objectiveLookupSession.getObjective(activity.getObjectiveId());
} catch (org.osid.NotFoundException nfe) {
throw new org.osid.OperationFailedException("cannot resolve Objective for " + activityId, nfe);
}
return (new ObjectiveOverlayActvity(activity, objective);
}
...
|
The first thing this method does is call getActivity() from some underlying provider. It’s similar to the basic decorator/adapter case in the first section. The semantics align so the exceptions align on this call.
The exceptions from getObjective() also align with getActivity(). But the semantics do not. The OSID Consumer is asking for an Activity and to be told the Activity does not exist when the Id reference of the Objective was not found is a problem. The NotFoundException in getActivity() describes the Activity, not the Objective nor any other call this method implementation may use. The fact that we decided to implement this method in such a way to cause this error condition means our service is broken when getObjective() tosses a NotFoundException at us. It should be caught and not allowed to pass through.
When acting as an OSID Consumer within an OSID Provider, look at the defined exceptions and to what they pertain in order to determine what to catch. Muddling through by adding try/catch blocks only when the compiler complains misses these semantics.
Method Factoring
...
Kicking The Can
Here’s an example of an attempt to replace a GradeSystem for a Course using the kick-the-can method:
| Code Block |
|---|
public class CourseLookupSession
implements org.osid.course.CourseLookupSession {
private org.osid.course.CourseLookupSession courseLookupSession;
private org.osid.grading.GradeSystemLookupSession gradeSystemLookupSession;
private org.osid.relationship.RelationshipLookupSession relationshipLookupSession;
@OSID
public org.osid.course.CourseList getCourses()
throws org.osid.OperationFailedException,
org.osid.PermissionDeniedException {
return (processCourses(this.courseLookupSession.getCourses()));
}
private org.osid.course.CourseList processCourses(org.osid.course.CourseList courses)
throws org.osid.OperationFailedException,
org.osid.PermissionDeniedException {
MutableCourseList ret = new MutableCourseList();
while (courses.hasNext()) {
ret.add(processCourse(courses.getNextCourse()));
}
ret.done();
return (ret);
}
private org.osid.course.Course processCourse(org.osid.course.Course course)
throws org.osid.OperationFailedException,
org.osid.PermissionDeniedException {
if (course.isGraded()) {
return (processGradingOptions(course));
} else {
return (course);
}
}
private org.osid.course.Course processGradingOptions(org.osid.course.Courss course)
throws org.osid.OperationFailedException,
org.osid.PermissionDeniedException {
Collection<org.osid.grading.GradeSystem> mappedGradeSystems = new ArrayList<>();
try (org.osid.grading.GradeSystemList gradeSystems = course.getGradingOptions()) {
while (ids.hasNext()) {
org.osid.grading.GradeSystem gradeSystem = gradeSystems.getNextGradeSystem();
try {
mappedGradeSystems.add(getNewGradingOption(gradeSystem.getId()));
} catch (org.osid.NotFoundException nfe) {
mappedGradeSystems.add(gradeSystem);
}
}
}
return (mapCourseToNewGradeSystems(course, mappedGradeSystems);
}
private org.osid.grading.GradeSystem getNewGradingOption(org.osid.id.Id gradeSystemid)
throws org.osid.NotFoundException,
org.osid.OperationFailedException,
org.osid.PermissionDeniedException {
try (org.osid.relationship.RelationshipList mappings = this.relationLookupSession.getRelationshipsForSource(gradeSystemId)) {
if (mappings.hasNext()) {
return (this.gradeSystemLookupSession.getGradeSystem(mappings.getNextRelationship().getDestinationId()));
} else {
throw new org.osid.NotFoundException("cannot map " + gradeSystemId);
}
}
}
private org.osid.course.Course mapCourseToNewGradeSystems(course, mappedGradeSystems)
throws org.osid.OperationFailedException,
org.osid.PermissionDeniedException {
return (new CourseWrapper(course, mappedGradeSystems));
} |
The above code pushes the logic of each loop into a separate method. The resulting flow describes a procedure. Some of the symptoms include:
a trail of private methods strung together
method names with the word process in them
lack of clarity over which method is responsible for what job
the inability to reuse any part of the trail other than the head
one utility for the list and another for an individual element on the list
needing to declare the same exceptions at each point and the resulting exceptions actually thrown could come from anywhere along the trail.
This factoring approach worked fine in C & Fortran.
See Also
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