Spring 的@Required注释

本例子源于：W3Cschool，在此做一个记录

@Required注释为为了保证所对应的属性必须被设置，@Required 注释应用于 bean 属性的 setter 方法，它表明受影响的 bean 属性在配置时必须放在 XML 配置文件中，否则容器就会抛出一个 BeanInitializationException 异常。下面显示的是一个使用 @Required 注释的示例。直接的理解就是，如果你在某个java类的某个set方法上使用了该注释，那么该set方法对应的属性在xml配置文件中必须被设置，否则就会报错！！！

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     //该注释放在的是set方法中，如果没有在xml配置文件中配置相关的属性，就会报错
    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的定义 -->
    <bean id = "student" class="com.how2java.w3cschool.required.Student">
    <property name = "name" value="派大星"/>
    
    </bean>
    
</beans>

应该和注意到Student.java中的@Required注释应用在了两个set方法上，但是此时在xml的bean中只设置了一个属性，因此从报错，“ Property 'age' is required for bean 'student' ”

将对应的属性设置加上后的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的定义 -->
    <bean id = "student" class="com.how2java.w3cschool.required.Student">
    <property name = "name" value="派大星"/>
    <property name = "age" value="5"/>
    </bean>
    
</beans>

最后的结果输出为：