Grails陷阱之一

最近在摆弄Grails，感觉Grails还是很好用的，不过还是遇到一些小问题。因为是基于groovy这样的动态语言，因此许多问题是要到运行时才抛出异常，而有些时候所抛出的异常并不能反应问题的实质，结果往往令人摸不到头脑，从而构成了陷阱（gotchas）。我也经过了多次这样的抓狂经历。

Grails陷阱之一：Domain Class 中的关联不能指向抽象类

Grails的domain class可以继承抽象类。比如：

abstract class Pet {
	String name
	boolean eat(Food food) {
		println("$name is eating ${food.toString()}...")
	}
}
class Cat extends Pet {
}
class Dog extends Pet {
}

abstract class Food {
}
class Fish extends Food {
	String toString() { 'a fish' }
}
class Bone extends Food {
	String toString() { 'a bone' }
}

class Person {
	
	String name
	
	boolean feed(Pet pet, Food food) {
		pet.eat(food)
	}
}

/**********************************************************/

import grails.test.*

class FeedPetTests extends GrailsUnitTestCase {
    protected void setUp() {
        super.setUp()
    }

    protected void tearDown() {
        super.tearDown()
    }

    void testFeed() {
		def hax = new Person(name:'Hax')
		def tom = new Cat(name:'Tom')
		def snoopy = new Dog(name:'Snoopy')
		def fish = new Fish()
		def bone = new Bone()
		hax.feed(tom, fish)
		hax.feed(tom, bone)
		hax.feed(snoopy, fish)
		hax.feed(snoopy, bone)
    }
}

上述代码可以编译和运行成功。

--Output from testFeed--
Tom is eating a fish...
Tom is eating a bone...
Snoopy is eating a fish...
Snoopy is eating a bone...

如果我们给Person加上一个pets关联的话：

class Person {
	
    static hasMany = [pets:Pet] // 关联到抽象类Pet

	String name
	
	boolean feed(Pet pet, Food food) {
		pet.eat(food)
	}
}

结果可以编译，但是运行时出现异常：

gant.TargetExecutionException: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'messageSource': Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'transactionManager': Cannot resolve reference to bean 'sessionFactory' while setting bean property 'sessionFactory'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory': Invocation of init method failed; nested exception is java.lang.NullPointerException

        at gant.Gant$_dispatch_closure4.doCall(Gant.groovy:331)
        at gant.Gant$_dispatch_closure6.doCall(Gant.groovy:334)
        at gant.Gant$_dispatch_closure6.doCall(Gant.groovy)
        at gant.Gant.withBuildListeners(Gant.groovy:344)
        at gant.Gant.this$2$withBuildListeners(Gant.groovy)
        at gant.Gant$this$2$withBuildListeners.callCurrent(Unknown Source)
        at gant.Gant.dispatch(Gant.groovy:334)
        at gant.Gant.this$2$dispatch(Gant.groovy)
        at gant.Gant.invokeMethod(Gant.groovy)
        at gant.Gant.processTargets(Gant.groovy:495)
        at gant.Gant.processTargets(Gant.groovy:480)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'messageSource': Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'transactionManager': Cannot resolve reference to bean 'sessionFactory' while setting bean property 'sessionFactory'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory': Invocation of init method failed; nested exception is java.lang.NullPointerException
        at java.security.AccessController.doPrivileged(Native Method)
        at _GrailsBootstrap_groovy$_run_closure2_closure13.doCall(_GrailsBootstrap_groovy:86)
        at _GrailsBootstrap_groovy$_run_closure2_closure13.doCall(_GrailsBootstrap_groovy)
        at _GrailsSettings_groovy$_run_closure10.doCall(_GrailsSettings_groovy:269)
        at _GrailsBootstrap_groovy$_run_closure2.doCall(_GrailsBootstrap_groovy:84)
        at _GrailsBootstrap_groovy$_run_closure7.doCall(_GrailsBootstrap_groovy:142)
        at _GrailsTest_groovy$_run_closure7.doCall(_GrailsTest_groovy:238)
        at _GrailsTest_groovy$_run_closure7.doCall(_GrailsTest_groovy)
        at _GrailsTest_groovy$_run_closure1_closure19.doCall(_GrailsTest_groovy:106)
        at _GrailsTest_groovy$_run_closure1.doCall(_GrailsTest_groovy:92)
        at TestApp$_run_closure1.doCall(TestApp.groovy:66)
        at gant.Gant$_dispatch_closure4.doCall(Gant.groovy:324)
        ... 10 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'transactionManager': Cannot resolve reference to bean 'sessionFactory' while setting bean property 'sessionFactory'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory': Invocation of init method failed; nested exception is java.lang.NullPointerException
        at java.security.AccessController.doPrivileged(Native Method)
        ... 22 more
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory': Invocation of init method failed; nested exception is java.lang.NullPointerException

        at java.security.AccessController.doPrivileged(Native Method)
        ... 23 more
Caused by: java.lang.NullPointerException
        ... 24 more

光看这些异常你肯定丈二和尚摸不着头脑。我就在这个上栽了跟头，浪费了好多时间去定位问题根源。

问题本身其实不难理解，涉及到数据库关联的，不管是一对一、一对多或多对多，都要指向一个实际的数据库表，而grails是不会为抽象类建立对应的数据库表的，也就是说，在grails中抽象类本身并不是domain class，而只是提供了一种代码复用的手段（也就是节约了一点重复性代码，减少了一些C&P）。

解决方法很简单，就是把Pet抽象类改为具体类，或者修改你的关联，不要关联到抽象类，而是直接关联到具体类。

类似的，如果你关联了一个interface，也会抛出差不多的异常，不过在/grails-app/domain下放置interface会编译失败，你必须放到/src/groovy下，而所以一般来说你不太会遇到interface的问题。