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/beanshttp://www.springframework.org/schema/beans/spring-beans-3.0.xsdhttp://www.springframework.org/schema/contexthttp://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;@Requiredpublic void setAge(Integer age) {this.age = age;}public Integer getAge() {return age;}@Requiredpublic 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/beanshttp://www.springframework.org/schema/beans/spring-beans-3.0.xsdhttp://www.springframework.org/schema/contexthttp://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。

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