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"Drools用户指南7.46.0.Final：适用于初学者的声明式编程教程"

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The "Drools 使用手册User Guide 7.46.0.Final" is a comprehensive resource designed for newcomers to Drools, a powerful business rules management system. This guide is essential for those looking to transition from classical procedural programming to declarative programming with Drools. Intended for beginners, this book provides a step-by-step tutorial to help individuals seamlessly implement algorithms and business rules with Drools. The shift from procedural to declarative programming may initially appear daunting, but with the assistance of the tutorials provided in this guide, users can quickly adapt to the new paradigm. Authored by a seasoned Drools trainer, this manual offers practical guidance and insights to assist readers in effectively utilizing Drools for their programming needs. By following the instructions outlined in this guide, users can confidently navigate the complexities of Drools and leverage its capabilities to streamline their development processes. From understanding the basics of Drools to mastering advanced functionalities, the "Drools 使用手册User Guide 7.46.0.Final" equips readers with the knowledge and skills needed to maximize the potential of Drools in their projects. Whether you are an experienced programmer seeking to enhance your skillset or a novice looking to learn a new programming language, this guide is an invaluable companion on your journey to mastering Drools.

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Then you should add the following unit test to the TestLesson1 java class



And in the console you should see this :

The first line is generated by the first test. To be able to see the difference when a new test is

started, we can add the following code to our JUnit test case :



and now the console should look like this :



The first test is passed and we see the output generated when the rule is fired. The second test

first inserts an object, then updates it the retract it. To update an object, you first have to

memorize the fact handle :



and then you have to use that fact handle to tell drools the object was updated :



and the same applies when you want to retract (now called delete in drools 6) :



as we call the "fireAllRules()" method after we retract the only fact that was in the working

memory, the rule "Your First Rule revisited" is not fired.



When and how is a rule fired?



Let us take an example :

FactHandle handlea = sessionStatefull.insert(a);

 a.setBalance(12.0);

 sessionStatefull.update(handlea, a);

sessionStatefull.delete(handlea);

We insert an Account, call FireAllRules, and then call a second-time fireAllRules on the same

session. What shall happen? How many times the rule "Your First Rule revisited" shall be fired?

Here is the result :



The rule is fired only once. Maybe it is because we did not modify the Account object? Let us do

another example that modifies the account object we pass to the session.



Here is the result :



The rule is still not fired a second time. Just modifying the object does not do the job.

Let us modify the example like this by telling the rule engine we modified the fact :



Before calling the second fireAllRules, we tell drools that the fact we inserted before was updated.

In our case, we did not modify its data. Here is the result :



The rule was executed a second time.

Here is what is happening when the FireAllRules method is called on a stateful session :

drools will look at all rules that can apply and put it in its agenda.

drools will execute the rule that is on top of its agenda

Once fired, the rule will be deactivated

We have to tell drools of a state change in one of the facts in the when part (lhs) to make him

reconsider the rule.

A state change can be an insert, update, or delete (retract).

In the last example, we tell drools that a fact has been updated



So, therefore, considering the previously inserted fact has been updated, drools reconsider the

rule. As in the rule "Your First Rule revisited" there is no condition on the attributes, the rule is

then fired.

But we can also do that in the then (RHS) part of a rule: insert, update, retract.

Here is the test case :



sessionStatefull.update(handle, a);

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