《Drools7.0.0.Final规则引擎教程》第4章 4.6 结果条件

结果条件

在Java中，如果有重复的代码我们会考虑进行重构，抽取公共方法或继承父类，以减少相同的代码在多处出现，达到代码的最优管理和不必要的麻烦。Drools同样提供了类似的功能。下面我们以实例来逐步说明。

像下面最原始的两条规则，有相同的业务判断，也有不同的地方：

package com.rules.conditional
import com.secbro.drools.model.Customer;
import com.secbro.drools.model.Car;

rule "conditional1:Give 10% discount to customers older than 60"
    agenda-group "conditional1"
when
    $customer : Customer( age > 60 )
then
    modify($customer) { setDiscount( 0.1 ) };
    System.out.println("Give 10% discount to customers older than 60");
end

rule "conditional1:Give free parking to customers older than 60"
agenda-group "conditional1"
when
    $customer : Customer( age > 60 )
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
    System.out.println("Give free parking to customers older than 60");
end

现在Drools提供了extends特性，也就是一个规则可以继承另外一个规则，并获得其约束条件。改写之后执行效果相同，代码如下：

package com.rules.conditional
import com.secbro.drools.model.Customer;
import com.secbro.drools.model.Car;

rule "conditional2:Give 10% discount to customers older than 60"
    agenda-group "conditional2"
when
    $customer : Customer( age > 60 )
then
    modify($customer) { setDiscount( 0.1 ) };
    System.out.println("conditional2:Give 10% discount to customers older than 60");
end

rule "conditional2:Give free parking to customers older than 60"
    extends "conditional2:Give 10% discount to customers older than 60"
agenda-group "conditional2"
when
    $car : Car ( owner == $customer )
then
    modify($car) { setFreeParking( true ) };
    System.out.println("conditional2:Give free parking to customers older than 60");
end

我们可以看到上面使用了extends，后面紧跟的是另外一条规则的名称。这样，第二条规则同时拥有了第一条规则的约束条件。只需要单独写此条规则自身额外需要的约束条件即可。那么，现在是否是最优的写法吗？当然不是，还可以将两条规则合并成一条来规则。这就用到了do和标记。

package com.rules.conditional
import com.secbro.drools.model.Customer;
import com.secbro.drools.model.Car;

rule "conditional3:Give 10% discount to customers older than 60"
    agenda-group "conditional3"
when
    $customer : Customer( age > 60 )
    do[giveDiscount]
    $car : Car(owner == $customer)
then
    modify($car) { setFreeParking(true) };
        System.out.println("conditional3:Give free parking to customers older than 60");
then[giveDiscount]
    modify($customer){
        setDiscount(0.1)
    };
    System.out.println("conditional3:Give 10% discount to customers older than 60");
end

在then中标记了giveDiscount处理操作，在when中用do来调用标记的操作。这样也当第一个约束条件判断完成之后，就执行标记giveDiscount中的操作，然后继续执行Car的约束判断，通过之后执行默认的操作。

在then中还可以添加一些判断来执行标记的操作，这样就不必每次都执行do操作，而是每当满足if条件之后才执行：

package com.rules.conditional
import com.secbro.drools.model.Customer;
import com.secbro.drools.model.Car;

rule "conditional4:Give 10% discount to customers older than 60"
    agenda-group "conditional4"
when
    $customer : Customer( age > 60 )
    if(type == "Golden") do[giveDiscount]
    $car : Car(owner == $customer)
then
    modify($car) { setFreeParking(true) };
        System.out.println("conditional4:Give free parking to customers older than 60");
then[giveDiscount]
    modify($customer){
        setDiscount(0.1)
    };
    System.out.println("conditional4:Give 10% discount to customers older than 60");
end

同时，还可以通过break来中断后续的判断。

package com.rules.conditional
import com.secbro.drools.model.Customer;
import com.secbro.drools.model.Car;

rule "conditional5:Give 10% discount to customers older than 60"
    agenda-group "conditional5"
when
    $customer : Customer( age > 60 )
    if(type == "Golden") do[giveDiscount10]
    else if (type == "Silver") break[giveDiscount5]
    $car : Car(owner == $customer)
then
    modify($car) { setFreeParking(true) };
        System.out.println("conditional5:Give free parking to customers older than 60");
then[giveDiscount10]
    modify($customer){
        setDiscount(0.1)
    };
    System.out.println("giveDiscount10:Give 10% discount to customers older than 60");
then[giveDiscount5]
    modify($customer){
        setDiscount(0.05)
    };
    System.out.println("giveDiscount5:Give 10% discount to customers older than 60");
end

以上规则的执行测试代码如下，执行结果可自行尝试，源代码已经存放在GitHub：https://github.com/secbr/drools。

后语

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