Spring中存储Bean的相关注解及用法

 把在Spring的IOC容器中存放的对象,叫做bean.本文着重介绍bean的存储相关的注解及其用法.

1. 三种获取bean的方法

这里只介绍三种获取bean的方法:

Object getBean(String var1)
Object getBean(Class<T> var1)
Object getBean(String var1,Class<T> var2)

分别是根据bean的名称获取,根据bean的类型获取,根据bean的名称和类型获取.我们获取到的bean对象都是同一个,属于单例.

bean的命名:

bean名称以小写字母开头，然后使用驼峰式大小写.

比如: UserController这个类的bean的名称是userController. 

也有特殊情况:当有多个字符并且第一个和第⼆个字符都是大写时,将保留原始的大小写.

比如: UController这个类的bean名称是UController.

2. 类注解

2.1 @Controller(控制器存储)

@Controller
public class UserController {
    public void sayHi() {
        System.out.println("hi,controller");
    }
}

获取到UserController对象,调用sayHi方法:

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		UserController userController = context.getBean(UserController.class);
		userController.sayHi();
	}

这就是通过类名的方式来获取bean.

2.2 @Service(服务存储)

@Service
public class UserService {
    public void sayHi() {
        System.out.println("hi,service");
    }
}

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		UserService userService = context.getBean(UserService.class);
		userService.sayHi();
	}

2.3 @Repository(仓库存储)

@Repository
public class UserRepository {
    public void sayHi() {
        System.out.println("hi,repository");
    }
}

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		UserRepository userRepository = context.getBean(UserRepository.class);
		userRepository .sayHi();
	}

2.4 @Component(组件存储)

@Component
public class UserComponent {
    public void sayHi() {
        System.out.println("hi,component");
    }
}

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		UserComponent userComponent = context.getBean(UserComponent.class);
		userComponent.sayHi();
	}

2.5 @Configuration(配置存储)

@Configuration
public class UserConfiguration {
    public void sayHi() {
        System.out.println("hi,configuration");
    }
}

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		UserConfiguration userConfiguration = context.getBean(UserConfiguration.class);
		userConfiguration.sayHi();
	}

3. 方法注解@Bean

只使用类注解,有些问题就无法解决:外部包里面的类无法添加类注解,一个类可能有多个数据源需要多个对象.这时,我们仅仅使用类注解无法解决问题,就引入了方法注解@Bean.

使用@Bean注解时,需要搭配上述的类注解一同使用.

3.1 获取单个bean

@Component
public class BeanConfig {
    @Bean
    public User user(){
        User user = new User();
        user.setName("zhangsan");
        user.setAge(22);
        return user;
    }
}

@Data
public class User {
    public String name;
    public int age;
}

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		User user = context.getBean(User.class);
		System.out.println(user);
	}

3.2 获取多个bean

这时必须使用名称来获取bean:

@Component
public class BeanConfig {
    @Bean
    public User user1(){
        User user = new User();
        user.setName("zhangsan");
        user.setAge(22);
        return user;
    }
    @Bean
    public User user2(){
        User user = new User();
        user.setName("lisi");
        user.setAge(19);
        return user;
    }
}

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		User user1 = (User)context.getBean("user1");
		User user2 = (User)context.getBean("user2");
		System.out.println(user1);
		System.out.println(user2);
	}

3.3 bean的重命名

@Component
public class BeanConfig {
    @Bean(name = {"u1","u2"})
    public User user1(){
        User user = new User();
        user.setName("zhangsan");
        user.setAge(22);
        return user;
    }
}

"name="这部分可以省略,如果只有一个重新命名的名字,{}也可以省略.

public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		User user1 = (User)context.getBean("u1");
		System.out.println(user1);
	}

3.4 扫描路径

我们之前加上了@Bean注解不一定会生效,如果想要生效,需要Spring扫描到.而Spring扫描的默认路径是Application这个类所在的包中.我如果将它放入controller包中,Spring就扫描不到component这个包下面的bean了.我们可以通过@ComponentScan来指定扫描路径.

@ComponentScan("{com.lzq.demo}")
@SpringBootApplication
public class SpringIocApplication {
	public static void main(String[] args) {
		ApplicationContext context = SpringApplication.run(SpringIocApplication.class, args);
		User user1 = (User)context.getBean("u1");
		System.out.println(user1);
	}
}