Spring @Required 註解

@Required 註解應用於 Bean 屬性的 setter 方法，它指示必須在配置時在 XML 配置檔案中填充受影響的 Bean 屬性。否則，容器將丟擲 BeanInitializationException 異常。以下是一個示例，展示了 @Required 註解的使用。

示例

讓我們準備一個可用的 Eclipse IDE，並按照以下步驟建立一個 Spring 應用程式：

以下是 Student.java 檔案的內容：

package com.tutorialspoint;

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 檔案的內容：

package com.tutorialspoint;

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() );
   }
}

以下是配置檔案 Beans.xml 的內容：

<?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.tutorialspoint.Student">
      <property name = "name" value = "Zara" />

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

</beans>

建立原始檔和 Bean 配置檔案後，讓我們執行應用程式。如果應用程式一切正常，它將引發 BeanInitializationException 異常，並列印以下錯誤以及其他日誌訊息：

Property 'age' is required for bean 'student'

接下來，您可以嘗試以上示例，方法是刪除“age”屬性的註釋，如下所示：

<?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.tutorialspoint.Student">
      <property name = "name" value = "Zara" />
      <property name = "age"  value = "11"/>
   </bean>

</beans>

以上示例將產生以下結果：

Name : Zara
Age : 11