Bean之間的關系：繼承和依賴。

繼承Bean配置

Spring允許繼承bean的配置，被繼承的bean稱為父bean，繼承這個父bean的Bean稱為子Bean。

子Bean從父Bean中繼承配置，包括Bean的屬性配置。

子Bean也可以覆蓋從父Bean繼承過來的配置。

父Bean可以作為配置模板，也可以作為Bean實例，若只想把父Bean作為模板，可以設置<bean>的abstract屬性為true，這樣Spring將不會實例化這個bean。（抽象Bean，抽象Bean不能被實例化，只能被繼承）

並不是<bean>元素裏的所有屬性都會被繼承，比如：autowire，abstract等。

也可以忽略父Bean的class屬性，讓子Bean指定自己的類，而共享相同的屬性配置。但此時abstract必須為true。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd"
 
>
    
    <bean id="address" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="WuDaoKou"></bean>
    
    <bean id="address2" class="logan.spring.study.autowire.Address"
    p:city="Beijing" p:street="DaZhongSi"></bean>


</beans>

package logan.spring.study.relation;

 
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import logan.spring.study.autowire.Address;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-relation.xml");
        Address address = (Address) ctx.getBean("address");
        System.out.println(address);
        
        address = (Address) ctx.getBean("address2");
        System.out.println(address);

    }

}

下面是輸出結果：

五月 20, 2017 4:03:58 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContex[email protected]: startup date [Sat May 20 16:03:58 CST 2017]; root of context hierarchy
五月 20, 2017 4:03:58 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-relation.xml]
Address [city=Beijing, street=WuDaoKou]
Address [city=Beijing, street=DaZhongSi]

可以看到兩個address的內容大部分都是一樣的，所以，我們可以如下使用關系，讓第二個address繼承第一個address

如下配置

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address" class="logan.spring.study.autowire.Address"
    p:city="Beijing1" p:street="WuDaoKou"></bean>
    
    <bean id="address2" p:street="DaZhongSi" parent="address"></bean>


</beans>

下面是輸出結果

五月 20, 2017 4:08:07 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContex[email protected]: startup date [Sat May 20 16:08:07 CST 2017]; root of context hierarchy
五月 20, 2017 4:08:07 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-relation.xml]
Address [city=Beijing1, street=WuDaoKou]
Address [city=Beijing1, street=DaZhongSi]

下面看Bean之間的依賴關系

Spring允許用戶通過depends-on屬性設定Bean前置依賴的Bean，前置依賴的Bean會在本Bean實例化之前創建好。

如果前置依賴於多個Bean，則可以通過都好，空格的方式配置Bean的名稱。

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address" class="logan.spring.study.autowire.Address"
    p:city="Beijing1" p:street="WuDaoKou"></bean>
    
    <bean id="address2" p:street="DaZhongSi" parent="address"></bean>
    
    <bean id="person" class="logan.spring.study.autowire.Person"
    p:name="Tom" p:address-ref="address"></bean>


</beans>

package logan.spring.study.relation;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import logan.spring.study.autowire.Address;
import logan.spring.study.autowire.Person;

public class Main {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans-relation.xml");
        Address address = (Address) ctx.getBean("address");
        System.out.println(address);
        
        address = (Address) ctx.getBean("address2");
        System.out.println(address);
        
        Person person = (Person) ctx.getBean("person");
        System.out.println(person);

    }

}

下面是輸出結果

五月 20, 2017 4:23:24 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContex[email protected]: startup date [Sat May 20 16:23:24 CST 2017]; root of context hierarchy
五月 20, 2017 4:23:25 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-relation.xml]
Address [city=Beijing1, street=WuDaoKou]
Address [city=Beijing1, street=DaZhongSi]
Person [name=Tom, address=Address [city=Beijing1, street=WuDaoKou], car=null]

可以看到car是null，如果我必須要car不是null，也就是說person依賴於null，那麽如何做？

如下做法：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address" class="logan.spring.study.autowire.Address"
    p:city="Beijing1" p:street="WuDaoKou"></bean>
    
    <bean id="address2" p:street="DaZhongSi" parent="address"></bean>
    
    <bean id="person" class="logan.spring.study.autowire.Person"
    p:name="Tom" p:address-ref="address" depends-on="car"></bean>


</beans>

輸出錯誤結果

五月 20, 2017 4:27:50 下午 org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
信息: Refreshing org.springframework.context.support.ClassPathXmlApplicationContex[email protected]: startup date [Sat May 20 16:27:50 CST 2017]; root of context hierarchy
五月 20, 2017 4:27:50 下午 org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
信息: Loading XML bean definitions from class path resource [beans-relation.xml]
五月 20, 2017 4:27:50 下午 org.springframework.context.support.ClassPathXmlApplicationContext refresh
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named ‘car‘ available
Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named ‘car‘ available
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanDefinition(DefaultListableBeanFactory.java:687)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getMergedLocalBeanDefinition(AbstractBeanFactory.java:1207)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:284)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:296)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
    at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:761)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:866)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:542)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
    at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
    at logan.spring.study.relation.Main.main(Main.java:14)

如下配置就不會報錯

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:p="http://www.springframework.org/schema/p"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    
    <bean id="address" class="logan.spring.study.autowire.Address"
    p:city="Beijing1" p:street="WuDaoKou"></bean>
    
    <bean id="address2" p:street="DaZhongSi" parent="address"></bean>
    
    <bean id="car" class="logan.spring.study.autowire.Car"
    p:brand="Audi" p:price="300000"></bean>
    <bean id="person" class="logan.spring.study.autowire.Person"
    p:name="Tom" p:address-ref="address" depends-on="car"></bean>


</beans>

Spring入門第七課