Spring @Required 注解

@Required注解

@Required注解适用于bean属性setter方法，它指示在配置时必须在XML配置文件中填充受影响的bean属性。否则，容器将引发BeanInitializationException异常。以下是显示@Required注解的用法的示例。

实例

假设我们拥有一个运行良好的Eclipse IDE，并采取以下步骤来创建一个Spring应用程序：

创建一个名称为SpringExample的项目，并在创建的项目的src文件夹下创建一个包com.jc2182
使用“添加外部JAR”选项添加所需的Spring库，如“Spring Hello World示例”一章中所述。
在com.jc2182包下创建Java类Student 和 MainApp。
在src文件夹下创建Beans配置文件Beans.xml。
最后一步是创建所有Java文件和Bean配置文件的内容，然后按以下说明运行应用程序。

以下是Student.java内容。

package com.jc2182;

public class TextEditor {
   private SpellChecker spellChecker;
   
   // a setter method to inject the dependency.
   public void setSpellChecker(SpellChecker spellChecker) {
      System.out.println("Inside setSpellChecker." );
      this.spellChecker = spellChecker;
   }
   
   // a getter method to return spellChecker
   public SpellChecker getSpellChecker() {
      return spellChecker;
   }
   public void spellCheck() {
      spellChecker.checkSpelling();
   }
}

以下是MainApp.java文件的内容。

package com.jc2182;

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");
      
      HelloWorld objA = (HelloWorld) context.getBean("helloWorld");
      objA.getMessage1();
      objA.getMessage2();

      HelloChina objB = (HelloChina) context.getBean("helloChina");
      objB.getMessage1();
      objB.getMessage2();
      objB.getMessage3();
   }
}

以下是配置文件的beans.xml它可以对基于setter方法注入配置

<?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: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.jc2182.Student">
      <property name = "name" value = "Zara" />

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

</beans>

完成创建源和Bean配置文件后，尝试运行，它将包含以下错误消息：

Error creating bean with name 'student' defined in class path resource [Beans.xml]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanInitializationException: Property 'age' is required for bean 'student'

接下来，您可以按照以下方式从“age”属性中删除注释，然后尝试上述示例-

<?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.jc2182.Student">
      <property name = "name" value = "Zara" />

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

</beans>

上面的示例将产生以下结果:

Name : Zara
Age : 11

Spring @Required 注解

@Required注解

实例