从 Spring 2.5 开始就可以使用注解来配置依赖注入。注解连线在默认情况下在 Spring 容器中不打开。因此，在可以使用基于注解的连线之前，我们将需要在我们的 Spring 配置文件中启用它:

<?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/>
   <!-- bean definitions go here -->

</beans>

一些重要的注解：

@Required: 应用于 bean 属性的 setter 方法。
@Autowired: 应用到 bean 属性的 setter 方法，非 setter 方法，构造函数和属性。
JSR-250 Annotations: Spring 支持 JSR-250 的基础的注解，其中包括了 @Resource，@PostConstruct 和 @PreDestroy 注解。

Spring @Required 注释

@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。

看个例子：

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;
   }
}

MainApp:

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() );
   }
}

Beans.xml:

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

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

</beans>

按照上述的beans.xml执行应用，会遇到下列错误：Property ‘age’ is required for bean ‘student’

Jul 25, 2020 11:31:37 AM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh
INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@4ee285c6: startup date [Sat Jul 25 11:31:37 CST 2020]; root of context hierarchy
Jul 25, 2020 11:31:37 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions
INFO: Loading XML bean definitions from class path resource [Beans.xml]
Jul 25, 2020 11:31:38 AM org.springframework.context.support.ClassPathXmlApplicationContext refresh
WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: 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'
Exception in thread "main" org.springframework.beans.factory.BeanCreationException: 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'
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:591)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:502)
	at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:312)
	at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)
	at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:310)
	at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)
	at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:758)
	at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:868)
	at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)
	at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)
	at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)
	at com.sap.MainApp.main(MainApp.java:8)
Caused by: org.springframework.beans.factory.BeanInitializationException: Property 'age' is required for bean 'student'
	at org.springframework.beans.factory.annotation.RequiredAnnotationBeanPostProcessor.postProcessPropertyValues(RequiredAnnotationBeanPostProcessor.java:156)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1344)
	at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582)
	... 11 more

在Beans.xml里重新启用age property的注入，错误消失。

经过实际测试，Student bean里这个setter方法的@Required注解，加或不加，结果都一致。

Spring 基于注解(annotation)的配置之@Required注解

Spring @Required 注释