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 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。