Spring 的@Required注釋

本例子源於：W3Cschool，在此做一個記錄

@Required注釋為為了保證所對應的屬性必須被設置，@Required 注釋應用於 bean 屬性的 setter 方法，它表明受影響的 bean 屬性在配置時必須放在 XML 配置文件中，否則容器就會拋出一個 BeanInitializationException 異常。下面顯示的是一個使用 @Required 注釋的示例。直接的理解就是，如果你在某個java類的某個set方法上使用了該注釋，那么該set方法對應的屬性在xml配置文件中必須被設置，否則就會報錯！！！

例子如下：

Studnent.java

package com.how2java.w3cschool.required; import org.springframework.beans.factory.annotation.Required; public class Student { private Integer age; private String name; public Integer getAge() { return age; } @Required //該注釋放在的是set方法中，如果沒有在xml配置文件中配置相關的屬性，就會報錯
    public void setAge(Integer age) { this.age = age; } public String getName() { return name; } @Required public void setName(String name) { this.name = name; } }

 

MainApp.java

package com.how2java.w3cschool.required; import org.springframework.context.ApplicationContext; import org.springframework.context.support.ClassPathXmlApplicationContext; public class MainApp { public static void main(String[] args) { ApplicationContext context = new ClassPathXmlApplicationContext("beanrequired.xml"); Student student = (Student) context.getBean("student"); System.out.println("Name:" + student.getName()); System.out.println("age:" + student.getAge()); } }

第一次的xml如下：

<?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:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:annotation-config/>
    <!-- student bean的定義 -->
    <bean id = "student" class="com.how2java.w3cschool.required.Student">
    <property name = "name" value="派大星"/>
    
    </bean>
    
</beans>

應該和注意到Student.java中的@Required注釋應用在了兩個set方法上，但是此時在xml的bean中只設置了一個屬性，因此從報錯，“ Property 'age' is required for bean 'student' ”

 

將對應的屬性設置加上后的xml如下

<?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:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-3.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd">

    <context:annotation-config/>
    <!-- student bean的定義 -->
    <bean id = "student" class="com.how2java.w3cschool.required.Student">
    <property name = "name" value="派大星"/>
    <property name = "age" value="5"/>
    </bean>
    
</beans>

最后的結果輸出為：