Spring学习--通过注解配置 Bean (一)

在 classpath 中扫描组件：

组件扫描(component scanning)： Spring 能够从 classpath 下自动扫描 , 侦测和实例化具有特定注解的组件。

特定组件包括：

• @Component：基本注解 , 标识了一个受 Spring 管理的组件。
• @Repository：标识持久层组件。
• @Service：标识服务层(业务层)组件。
• @Controller：标识表现层组件。

对于扫描到的组件 , Spring 有默认的命名策略：使用非限定类名 , 第一个字母小写。也可以通过 value 属性值标识组件的名称。

 <?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.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd">

     <!-- 指定 Spring IOC 容器扫描的包-->
     <context:component-scan base-package="com.itdjx.spring.annotation">

     </context:component-scan>

 </beans>

注意：一定要添加 context 命名空间。

 package com.itdjx.spring.annotation;

 import org.springframework.stereotype.Component;

 /**
  * @author Wáng Chéng Dá
  * @create 2017-03-03 8:44
  */
 @Component
 public class TestObject {
 }

 package com.itdjx.spring.annotation.controller;

 import org.springframework.stereotype.Controller;

 /**
  * @author Wáng Chéng Dá
  * @create 2017-03-03 9:28
  */
 @Controller
 public class UserController {

     public void execute() {
         System.out.println("I am UserController's execute method...");
     }
 }

 package com.itdjx.spring.annotation.service;

 import org.springframework.stereotype.Service;

 /**
  * @author Wáng Chéng Dá
  * @create 2017-03-03 9:26
  */
 @Service
 public class UserService {

     public void add() {
         System.out.println("I am UserService's add method...");
     }
 }

 package com.itdjx.spring.annotation.repository;

 import org.springframework.stereotype.Repository;

 /**
  * @author Wáng Chéng Dá
  * @create 2017-03-03 9:24
  */
 @Repository("userRepository")
 public class UserRepositoryImpl implements UserRepository {

     @Override
     public void sava() {
         System.out.println("I am UserRepositoryImpl's sava method...");
     }
 }

 package com.itdjx.spring.annotation.repository;

 /**
  * @author Wáng Chéng Dá
  * @create 2017-03-03 9:23
  */
 public interface UserRepository {

     void sava();

 }

 package com.itdjx.spring.annotation;

 import com.itdjx.spring.annotation.controller.UserController;
 import com.itdjx.spring.annotation.repository.UserRepository;
 import com.itdjx.spring.annotation.service.UserService;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.support.ClassPathXmlApplicationContext;

 /**
  * @author Wáng Chéng Dá
  * @create 2017-03-03 9:34
  */
 public class Main {

     public static void main(String[] args) {
         ApplicationContext cxt = new ClassPathXmlApplicationContext("beans-annotation.xml");
         TestObject testObject = (TestObject) cxt.getBean("testObject");
         System.out.println(testObject);

         UserController userController = (UserController) cxt.getBean("userController");
         System.out.println(userController);

         UserService userService = (UserService) cxt.getBean("userService");
         System.out.println(userService);

         UserRepository userRepository = (UserRepository) cxt.getBean("userRepository");
         System.out.println(userRepository);

     }
 }

控制台输出：

com.itdjx.spring.annotation.TestObject@27ba32
com.itdjx.spring.annotation.controller.UserController@f82753
com.itdjx.spring.annotation.service.UserService@10fe47a
com.itdjx.spring.annotation.repository.UserRepositoryImpl@2b0582

注意：Spring 默认的命名策略：使用非限定类名 , 第一个字母小写。也可以通过 value 属性值标识组件的名称。

若不修改 @Repository("userRepository") 默认的 value 值 , bean 的 id 值是 userRepositoryImpl , UserRepository userRepository = (UserRepository) cxt.getBean("userRepository"); 会抛出找不到 userRepository 这个 bean 节点。

异常：

Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException: No bean named 'userRepository' available
at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeanDefinition(DefaultListableBeanFactory.java:687)
at org.springframework.beans.factory.support.AbstractBeanFactory.getMergedLocalBeanDefinition(AbstractBeanFactory.java:1207)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:284)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1081)
at com.itdjx.spring.annotation.Main.main(Main.java:26)