Spring @Required注释

@Required注解适用于bean属性的setter方法并且它指示，受影响的bean属性必须在配置时被填充在XML配置文件中，否则容器将抛出BeanInitializationException例外。下面是一个例子，说明使用@ Required注解。

例子:

我们使用Eclipse IDE，然后按照下面的步骤来创建一个Spring应用程序：

步骤	描述
1	Create a project with a name SpringExample and create a package com.manongjc under the src folder in the created project.
2	Add required Spring libraries using Add External JARs option as explained in the Spring Hello World Example chapter.
3	Create Java classes Student and MainApp under the com.manongjc package.
4	Create Beans configuration file Beans.xml under the src folder.
5	The final step is to create the content of all the Java files and Bean Configuration file and run the application as explained below.

这里是Student.java文件的内容：

package com.manongjc;

import org.springframework.beans.factory.annotation.Required;

public class Student {
   private Integer age;
   private String name;

   @Required
   public void setAge(Integer age) {
      this.age = age;
   }
   public Integer getAge() {
      return age;
   }

   @Required
   public void setName(String name) {
      this.name = name;
   }
   public String getName() {
      return name;
   }
}

以下是MainApp.java文件的内容：

package com.manongjc;

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

public class MainApp {
   public static void main(String[] args) {
      ApplicationContext context = 
             new ClassPathXmlApplicationContext("Beans.xml");

      Student student = (Student) context.getBean("student");

      System.out.println("Name : " + student.getName() );
      System.out.println("Age : " + student.getAge() );
   }
}

以下是配置文件beans.xml中的内容：

<?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: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/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">

   <context:annotation-config/>

   <!-- Definition for student bean -->
   <bean id="student" class="com.manongjc.Student">
      <property name="name"  value="Zara" />

      <!-- try without passing age and check the result -->
      <!-- property name="age"  value="11"-->
   </bean>

</beans>

一旦创建源代码和bean配置文件完成后，让我们运行应用程序。如果一切顺利，这将抛出BeanInitializationException异常，并与其他日志信息打印以下错误：

Property 'age' is required for bean 'student'

接下来，可以从“age”属性中删除注释如下尝试后，上面的例子：

<?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: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/context
    http://www.springframework.org/schema/context/spring-context-3.0.xsd">

   <context:annotation-config/>

   <!-- Definition for student bean -->
   <bean id="student" class="com.manongjc.Student">
      <property name="name"  value="Zara" />
      <property name="age"  value="11"/>
   </bean>

</beans>

现在，上面的例子会产生以下结果：

Name : Zara
Age : 11