如果drools决策表中没有条件适用，如何设置默认值？

Question

I am working with drools 7.69.0,

My purpose is to convert the below condition into a decision table at firstly,

Stream.of(usecase1, usecase2, usecase3, usecase4).collect(Collectors.toSet()).contains(statusRequest.getUseCase())

|| (usecase6 == statusRequest.getUseCase() && aspect == statusRequest.getAspect())

|| (usecase5 != statusRequest.getUseCase() && statusRequest.isCorrection())

So how to apply default action if none of the conditions are applied?

Answer 1

There are two ways to do a "default" value:

1. Set the default in the highest priority rule, then rely on subsequent rules to overwrite it. If no subsequent rules are valid, then the default remains.
2. Set the default in the lowest priority rule, and have higher priority rules exit fast or set some condition such that the lowest priority rule is no longer valid.

In a decision table, #1 is easiest if you have Sequential set to TRUE in the Rule Set area. Default your "default case" rule in the first row, so it fires first. Give it no conditions, so it always triggers.

It would look something like this:

The second is also possible, but for it to work you need to be able key off of something that all of the other "real" rules will cause to no longer be valid.

In your case, all of your "real" rules are setting a value on the 'status' field of the model. Therefore, it stands to reason that if there is no value set on that field, we can set our default value.

This will require two things:

• changing your ACTION to use a modify so that the results are visible to the conditional checks
• changing your data type for the 'status' field to allow a null value; by seeing that the value is originally 'null', we'll know that no other rules have fired and that we can thus set the default value.

The cool thing about this version is that it doesn't rely on rule order, so you don't need to set the Sequential property, and it also simply doesn't matter when the rule fires. If the rule fires first, cool: the "real" rules will simply overwrite the value. If it fires last, also cool; that means that none of the other rules fired. Somewhere in the middle? Still fine! Since none of the other rules care about the value of status, it doesn't matter when the rule fires as long as it fires only when the status is null.