注解开发定义Bean对象

【第一步】在applicationContext.xml中开启Spring注解包扫描

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:context="http://www.springframework.org/schema/context"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">
    
	<!--扫描org.example包及其子包下的类中注解-->
    <context:component-scan base-package="org.example"/>
</beans>

 【第二步】在类上使用@Component注解定义Bean。

@Component("bookDao")
public class BookDaoImpl implements BookDao {
    public void save() {
        System.out.println("book dao save ...");
    }
}

@Component
public class BookServiceImpl implements BookService {
    private BookDao bookDao;

    public void setBookDao(BookDao bookDao) {
        this.bookDao = bookDao;
    }

    public void save() {
        System.out.println("book service save ...");
        bookDao.save();
    }
}

补充说明：如果@Component注解没有使用参数指定Bean的名称，那么类名首字母小写就是Bean在IOC容器中的默认名称。例如：BookServiceImpl对象在IOC容器中的名称是bookServiceImpl。

【第三步】在测试类中获取Bean对象

public class AppForAnnotation {
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("applicationContext.xml");
        BookDao bookDao = (BookDao) ctx.getBean("bookDao");
        System.out.println(bookDao);
        //按类型获取bean
        BookService bookService = ctx.getBean(BookService.class);
        System.out.println(bookService);
    }
}

注意：在测试类中不要调用bookService的save方法，因为还没有给BookServiceImpl中的bookDao赋值，调用bookService的save方法会出现空指针异常。

@Component三个衍生注解

说明：加粗的注解为常用注解

• Spring提供

  @Component注解的三个衍生注解

  • @Controller：用于表现层bean定义

  • @Service：用于业务层bean定义

  • @Repository：用于数据层bean定义

    @Repository("bookDao")
    public class BookDaoImpl implements BookDao {
    }
    
    @Service
    public class BookServiceImpl implements BookService {
    }