Spring IOC基于XML和注解管理Bean（二）

爱被打了一巴掌 2023-10-12 14:03 52阅读 0赞

[Spring IOC基于XML和注解管理Bean（一）][Spring IOC_XML_Bean]

#### 文章目录 ####

*   *   *  2.9、实验八：p命名空间
         *  2.10、实验九：引入外部属性文件
         *  2.11、实验十：bean的作用域
         *  2.12、实验十一：bean生命周期
         *  2.13、实验十二：FactoryBean
         *  2.14、实验十三：基于xml自动装配
     *  3、基于注解管理Bean
     *   *  3.1、搭建子模块spring6-ioc-annotation
         *  3.2、开启组件扫描
         *  3.3、使用注解定义 Bean
         *  3.4、实验一：@Autowired注入
         *   *  ①场景一：属性注入
             *  ②场景二：set注入
             *  ③场景三：构造方法注入
             *  ④场景四：形参上注入
             *  ⑤场景五：只有一个构造函数，无注解
             *  ⑥场景六：@Autowired注解和@Qualifier注解联合
         *  3.5、实验二：@Resource注入
         *   *  ①场景一：根据name注入
             *  ②场景二：name未知注入
             *  ③场景三 其他情况
         *  3.6、Spring全注解开发

#### 2.9、实验八：p命名空间 ####

引入p命名空间

<?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:util="http://www.springframework.org/schema/util"
           xmlns:p="http://www.springframework.org/schema/p"
           xsi:schemaLocation="http://www.springframework.org/schema/util
           http://www.springframework.org/schema/util/spring-util.xsd
           http://www.springframework.org/schema/beans
           http://www.springframework.org/schema/beans/spring-beans.xsd">

引入p命名空间后，可以通过以下方式为bean的各个属性赋值

#### 2.10、实验九：引入外部属性文件 ####

**①加入依赖**

<dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.30</version>
    </dependency>
    
    
    <dependency>
        <groupId>com.alibaba</groupId>
        <artifactId>druid</artifactId>
        <version>1.2.15</version>
    </dependency>

**②创建外部属性文件**

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jdbc.user=root
    jdbc.password=123456
    jdbc.url=jdbc:mysql://localhost:3306/ssm?serverTimezone=UTC
    jdbc.driver=com.mysql.cj.jdbc.Driver

**③引入属性文件**

引入context 名称空间

<?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">
    
    </beans>

<context:property-placeholder location="classpath:jdbc.properties"/>

注意：在使用 `<context:property-placeholder>` 元素加载外包配置文件功能前，首先需要在 XML 配置的一级标签 `<beans>` 中添加 context 相关的约束。

**④配置bean**

**⑤测试**

@Test
    public void testDataSource() throws SQLException {
        
        ApplicationContext ac = new ClassPathXmlApplicationContext("spring-datasource.xml");
        DataSource dataSource = ac.getBean(DataSource.class);
        Connection connection = dataSource.getConnection();
        System.out.println(connection);
    }

#### 2.11、实验十：bean的作用域 ####

**①概念**

在Spring中可以通过配置bean标签的`scope`属性来指定bean的作用域范围，各取值含义参加下表：

<table> 
 <thead> 
  <tr> 
   <th>取值</th> 
   <th>含义</th> 
   <th>创建对象的时机</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>singleton（默认）</td> 
   <td>在IOC容器中，这个bean的对象始终为单实例</td> 
   <td>IOC容器初始化时</td> 
  </tr> 
  <tr> 
   <td>prototype</td> 
   <td>这个bean在IOC容器中有多个实例</td> 
   <td>获取bean时</td> 
  </tr> 
 </tbody> 
</table>

如果是在WebApplicationContext环境下还会有另外几个作用域（但不常用）：

<table> 
 <thead> 
  <tr> 
   <th>取值</th> 
   <th>含义</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>request</td> 
   <td>在一个请求范围内有效</td> 
  </tr> 
  <tr> 
   <td>session</td> 
   <td>在一个会话范围内有效</td> 
  </tr> 
 </tbody> 
</table>

**②创建类User**

package org.example.bean;
    public class User {
        
    
        private Integer id;
    
        private String username;
    
        private String password;
    
        private Integer age;
    
        public User() {
        
        }
    
        public User(Integer id, String username, String password, Integer age) {
        
            this.id = id;
            this.username = username;
            this.password = password;
            this.age = age;
        }
    
        public Integer getId() {
        
            return id;
        }
    
        public void setId(Integer id) {
        
            this.id = id;
        }
    
        public String getUsername() {
        
            return username;
        }
    
        public void setUsername(String username) {
        
            this.username = username;
        }
    
        public String getPassword() {
        
            return password;
        }
    
        public void setPassword(String password) {
        
            this.password = password;
        }
    
        public Integer getAge() {
        
            return age;
        }
    
        public void setAge(Integer age) {
        
            this.age = age;
        }
    
        @Override
        public String toString() {
        
            return "User{" +
                    "id=" + id +
                    ", username='" + username + '\'' +
                    ", password='" + password + '\'' +
                    ", age=" + age +
                    '}';
        }
    }

**③配置bean**

<bean class="org.example.bean.User" scope="prototype"></bean>

**④测试**

@Test
    public void testBeanScope(){
        
        ApplicationContext ac = new ClassPathXmlApplicationContext("spring-scope.xml");
        User user1 = ac.getBean(User.class);
        User user2 = ac.getBean(User.class);
        System.out.println(user1==user2);
    }

#### 2.12、实验十一：bean生命周期 ####

**①具体的生命周期过程**

*  1、bean对象创建（调用无参构造器）
 *  2、给bean对象设置属性
 *  3、bean的后置处理器（初始化之前）
 *  4、bean对象初始化（需在配置bean时指定初始化方法）
 *  5、bean的后置处理器（初始化之后）
 *  6、bean对象就绪可以使用
 *  7、bean对象销毁（需在配置bean时指定销毁方法）
 *  8、IOC容器关闭

**②修改类User**

public class User {
        
    
        private Integer id;
    
        private String username;
    
        private String password;
    
        private Integer age;
    
        public User() {
        
            System.out.println("生命周期：1、创建对象");
        }
    
        public User(Integer id, String username, String password, Integer age) {
        
            this.id = id;
            this.username = username;
            this.password = password;
            this.age = age;
        }
    
        public Integer getId() {
        
            return id;
        }
    
        public void setId(Integer id) {
        
            System.out.println("生命周期：2、依赖注入");
            this.id = id;
        }
    
        public String getUsername() {
        
            return username;
        }
    
        public void setUsername(String username) {
        
            this.username = username;
        }
    
        public String getPassword() {
        
            return password;
        }
    
        public void setPassword(String password) {
        
            this.password = password;
        }
    
        public Integer getAge() {
        
            return age;
        }
    
        public void setAge(Integer age) {
        
            this.age = age;
        }
    
        public void initMethod(){
        
            System.out.println("生命周期：3、初始化");
        }
    
        public void destroyMethod(){
        
            System.out.println("生命周期：5、销毁");
        }
    
        @Override
        public String toString() {
        
            return "User{" +
                    "id=" + id +
                    ", username='" + username + '\'' +
                    ", password='" + password + '\'' +
                    ", age=" + age +
                    '}';
        }
    }

> 注意其中的`initMethod()`和`destroyMethod()`，可以通过配置bean指定为初始化和销毁的方法

**③配置bean**

<bean class="org.example.bean.User" scope="prototype" init-method="initMethod" destroy-method="destroyMethod">
        <property name="id" value="1001"></property>
        <property name="username" value="admin"></property>
        <property name="password" value="123456"></property>
        <property name="age" value="23"></property>
    </bean>

**④测试**

@Test
    public void testLife(){
        
        ClassPathXmlApplicationContext ac = new ClassPathXmlApplicationContext("spring-lifecycle.xml");
        User bean = ac.getBean(User.class);
        System.out.println("生命周期：4、通过IOC容器获取bean并使用");
        ac.close();
    }

**⑤bean的后置处理器**

bean的后置处理器会在生命周期的初始化前后添加额外的操作，需要实现`BeanPostProcessor`接口，且配置到IOC容器中，需要注意的是，bean后置处理器不是单独针对某一个bean生效，而是针对IOC容器中所有bean都会执行

创建bean的后置处理器：

package org.example.process;
        
    import org.springframework.beans.BeansException;
    import org.springframework.beans.factory.config.BeanPostProcessor;
    
    public class MyBeanProcessor implements BeanPostProcessor {
        
        
        @Override
        public Object postProcessBeforeInitialization(Object bean, String beanName) throws BeansException {
        
            System.out.println("☆☆☆" + beanName + " = " + bean);
            return bean;
        }
        
        @Override
        public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
        
            System.out.println("★★★" + beanName + " = " + bean);
            return bean;
        }
    }

在IOC容器中配置后置处理器：

<bean id="myBeanProcessor" class="org.example.process.MyBeanProcessor"/>

#### 2.13、实验十二：FactoryBean ####

**①简介**

`FactoryBean`是Spring提供的一种整合第三方框架的常用机制。和普通的bean不同，配置一个FactoryBean类型的bean，在获取bean的时候得到的并不是class属性中配置的这个类的对象，而是`getObject()`方法的返回值。通过这种机制，Spring可以帮我们把复杂组件创建的详细过程和繁琐细节都屏蔽起来，只把最简洁的使用界面展示给我们。

整合Mybatis时，Spring就是通过FactoryBean机制来帮我们创建`SqlSessionFactory`对象的。

/*
     * Copyright 2002-2020 the original author or authors.
     *
     * Licensed under the Apache License, Version 2.0 (the "License");
     * you may not use this file except in compliance with the License.
     * You may obtain a copy of the License at
     *
     *      https://www.apache.org/licenses/LICENSE-2.0
     *
     * Unless required by applicable law or agreed to in writing, software
     * distributed under the License is distributed on an "AS IS" BASIS,
     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     * See the License for the specific language governing permissions and
     * limitations under the License.
     */
    package org.springframework.beans.factory;
    
    import org.springframework.lang.Nullable;
    
    /**
     * Interface to be implemented by objects used within a {@link BeanFactory} which
     * are themselves factories for individual objects. If a bean implements this
     * interface, it is used as a factory for an object to expose, not directly as a
     * bean instance that will be exposed itself.
     *
     * <p><b>NB: A bean that implements this interface cannot be used as a normal bean.</b>
     * A FactoryBean is defined in a bean style, but the object exposed for bean
     * references ({@link #getObject()}) is always the object that it creates.
     *
     * <p>FactoryBeans can support singletons and prototypes, and can either create
     * objects lazily on demand or eagerly on startup. The {@link SmartFactoryBean}
     * interface allows for exposing more fine-grained behavioral metadata.
     *
     * <p>This interface is heavily used within the framework itself, for example for
     * the AOP {@link org.springframework.aop.framework.ProxyFactoryBean} or the
     * {@link org.springframework.jndi.JndiObjectFactoryBean}. It can be used for
     * custom components as well; however, this is only common for infrastructure code.
     *
     * <p><b>{@code FactoryBean} is a programmatic contract. Implementations are not
     * supposed to rely on annotation-driven injection or other reflective facilities.</b>
     * {@link #getObjectType()} {@link #getObject()} invocations may arrive early in the
     * bootstrap process, even ahead of any post-processor setup. If you need access to
     * other beans, implement {@link BeanFactoryAware} and obtain them programmatically.
     *
     * <p><b>The container is only responsible for managing the lifecycle of the FactoryBean
     * instance, not the lifecycle of the objects created by the FactoryBean.</b> Therefore,
     * a destroy method on an exposed bean object (such as {@link java.io.Closeable#close()}
     * will <i>not</i> be called automatically. Instead, a FactoryBean should implement
     * {@link DisposableBean} and delegate any such close call to the underlying object.
     *
     * <p>Finally, FactoryBean objects participate in the containing BeanFactory's
     * synchronization of bean creation. There is usually no need for internal
     * synchronization other than for purposes of lazy initialization within the
     * FactoryBean itself (or the like).
     *
     * @author Rod Johnson
     * @author Juergen Hoeller
     * @since 08.03.2003
     * @param <T> the bean type
     * @see org.springframework.beans.factory.BeanFactory
     * @see org.springframework.aop.framework.ProxyFactoryBean
     * @see org.springframework.jndi.JndiObjectFactoryBean
     */
    public interface FactoryBean<T> {
        
    
        /**
         * The name of an attribute that can be
         * {@link org.springframework.core.AttributeAccessor#setAttribute set} on a
         * {@link org.springframework.beans.factory.config.BeanDefinition} so that
         * factory beans can signal their object type when it can't be deduced from
         * the factory bean class.
         * @since 5.2
         */
        String OBJECT_TYPE_ATTRIBUTE = "factoryBeanObjectType";
    
        /**
         * Return an instance (possibly shared or independent) of the object
         * managed by this factory.
         * <p>As with a {@link BeanFactory}, this allows support for both the
         * Singleton and Prototype design pattern.
         * <p>If this FactoryBean is not fully initialized yet at the time of
         * the call (for example because it is involved in a circular reference),
         * throw a corresponding {@link FactoryBeanNotInitializedException}.
         * <p>As of Spring 2.0, FactoryBeans are allowed to return {@code null}
         * objects. The factory will consider this as normal value to be used; it
         * will not throw a FactoryBeanNotInitializedException in this case anymore.
         * FactoryBean implementations are encouraged to throw
         * FactoryBeanNotInitializedException themselves now, as appropriate.
         * @return an instance of the bean (can be {@code null})
         * @throws Exception in case of creation errors
         * @see FactoryBeanNotInitializedException
         */
        @Nullable
        T getObject() throws Exception;
    
        /**
         * Return the type of object that this FactoryBean creates,
         * or {@code null} if not known in advance.
         * <p>This allows one to check for specific types of beans without
         * instantiating objects, for example on autowiring.
         * <p>In the case of implementations that are creating a singleton object,
         * this method should try to avoid singleton creation as far as possible;
         * it should rather estimate the type in advance.
         * For prototypes, returning a meaningful type here is advisable too.
         * <p>This method can be called <i>before</i> this FactoryBean has
         * been fully initialized. It must not rely on state created during
         * initialization; of course, it can still use such state if available.
         * <p><b>NOTE:</b> Autowiring will simply ignore FactoryBeans that return
         * {@code null} here. Therefore it is highly recommended to implement
         * this method properly, using the current state of the FactoryBean.
         * @return the type of object that this FactoryBean creates,
         * or {@code null} if not known at the time of the call
         * @see ListableBeanFactory#getBeansOfType
         */
        @Nullable
        Class<?> getObjectType();
    
        /**
         * Is the object managed by this factory a singleton? That is,
         * will {@link #getObject()} always return the same object
         * (a reference that can be cached)?
         * <p><b>NOTE:</b> If a FactoryBean indicates to hold a singleton object,
         * the object returned from {@code getObject()} might get cached
         * by the owning BeanFactory. Hence, do not return {@code true}
         * unless the FactoryBean always exposes the same reference.
         * <p>The singleton status of the FactoryBean itself will generally
         * be provided by the owning BeanFactory; usually, it has to be
         * defined as singleton there.
         * <p><b>NOTE:</b> This method returning {@code false} does not
         * necessarily indicate that returned objects are independent instances.
         * An implementation of the extended {@link SmartFactoryBean} interface
         * may explicitly indicate independent instances through its
         * {@link SmartFactoryBean#isPrototype()} method. Plain {@link FactoryBean}
         * implementations which do not implement this extended interface are
         * simply assumed to always return independent instances if the
         * {@code isSingleton()} implementation returns {@code false}.
         * <p>The default implementation returns {@code true}, since a
         * {@code FactoryBean} typically manages a singleton instance.
         * @return whether the exposed object is a singleton
         * @see #getObject()
         * @see SmartFactoryBean#isPrototype()
         */
        default boolean isSingleton() {
        
            return true;
        }
    }

**②创建类UserFactoryBean**

package org.example.bean;
    public class UserFactoryBean implements FactoryBean<User> {
        
        @Override
        public User getObject() throws Exception {
        
            return new User();
        }
    
        @Override
        public Class<?> getObjectType() {
        
            return User.class;
        }
    }

**③配置bean**

**④测试**

@Test
    public void testUserFactoryBean(){
        
        //获取IOC容器
        ApplicationContext ac = new ClassPathXmlApplicationContext("spring-factorybean.xml");
        User user = (User) ac.getBean("user");
        System.out.println(user);
    }

#### 2.14、实验十三：基于xml自动装配 ####

> 自动装配：
> 
> 根据指定的策略，在IOC容器中匹配某一个bean，自动为指定的bean中所依赖的类类型或接口类型属性赋值

**①场景模拟**

创建类UserController

package org.example.autowire.controller;
    public class UserController {
        
    
        private UserService userService;
    
        public void setUserService(UserService userService) {
        
            this.userService = userService;
        }
    
        public void saveUser(){
        
            userService.saveUser();
        }
    
    }

创建接口UserService

package org.example.autowire.service;
    public interface UserService {
        
    
        void saveUser();
    
    }

创建类UserServiceImpl实现接口UserService

package org.example.autowire.service.impl;
    public class UserServiceImpl implements UserService {
        
    
        private UserDao userDao;
    
        public void setUserDao(UserDao userDao) {
        
            this.userDao = userDao;
        }
    
        @Override
        public void saveUser() {
        
            userDao.saveUser();
        }
    
    }

创建接口UserDao

package org.example.autowire.dao;
    public interface UserDao {
        
    
        void saveUser();
    
    }

创建类UserDaoImpl实现接口UserDao

package org.example.autowire.dao.impl;
    public class UserDaoImpl implements UserDao {
        
    
        @Override
        public void saveUser() {
        
            System.out.println("保存成功");
        }
    
    }

**②配置bean**

> 使用bean标签的`autowire`属性设置自动装配效果
> 
> 自动装配方式：`byType`
> 
> `byType`：根据类型匹配IOC容器中的某个兼容**类型**的bean，为属性自动赋值
> 
> 若在IOC中，没有任何一个兼容类型的bean能够为属性赋值，则该属性不装配，即值为默认值null
> 
> 若在IOC中，有多个兼容类型的bean能够为属性赋值，则抛出异常NoUniqueBeanDefinitionException

> 自动装配方式：`byName`
> 
> `byName`：将自动装配的属性的属性名，作为bean的**id**在IOC容器中匹配相对应的bean进行赋值

**③测试**

@Test
    public void testAutoWireByXML(){
        
        ApplicationContext ac = new ClassPathXmlApplicationContext("autowire-xml.xml");
        UserController userController = ac.getBean(UserController.class);
        userController.saveUser();
    }

### 3、基于注解管理Bean ###

从 Java 5 开始，Java 增加了对注解（Annotation）的支持，它是代码中的一种特殊标记，可以在编译、类加载和运行时被读取，执行相应的处理。开发人员可以通过注解在不改变原有代码和逻辑的情况下，在源代码中嵌入补充信息。

Spring 从 2.5 版本开始提供了对注解技术的全面支持，我们可以使用注解来实现自动装配，简化 Spring 的 XML 配置。

Spring 通过注解实现自动装配的步骤如下：

1.  引入依赖
2.  开启组件扫描
3.  使用注解定义 Bean
4.  依赖注入

#### 3.1、搭建子模块spring6-ioc-annotation ####

**①搭建模块**

搭建方式如：spring6-ioc-xml

**②引入配置文件**

引入spring-ioc-xml模块日志log4j2.xml

**③添加依赖**

<dependencies>
        
        
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>6.0.3</version>
        </dependency>
    
        
        <dependency>
            <groupId>org.junit.jupiter</groupId>
            <artifactId>junit-jupiter-api</artifactId>
        </dependency>
    
        
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
            <version>2.19.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-slf4j2-impl</artifactId>
            <version>2.19.0</version>
        </dependency>
    </dependencies>

#### 3.2、开启组件扫描 ####

Spring 默认不使用注解装配 Bean，因此我们需要在 Spring 的 XML 配置中，通过 `<context:component-scan>` 元素开启 Spring Beans的自动扫描功能。开启此功能后，Spring 会自动从扫描指定的包（`base-package` 属性设置）及其子包下的所有类，如果类上使用了 `@Component` 注解，就将该类装配到容器中。

<?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.xsd">
        
        <context:component-scan base-package="org.example"></context:component-scan>
    </beans>

注意：在使用 `<context:component-scan>` 元素开启自动扫描功能前，首先需要在 XML 配置的一级标签 `<beans>` 中添加 context 相关的约束。

**情况一：最基本的扫描方式**

<context:component-scan base-package="org.example">
    </context:component-scan>

**情况二：指定要排除的组件**

<context:component-scan base-package="org.example">
        
        
        <context:exclude-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
            
    </context:component-scan>

**情况三：仅扫描指定组件**

<context:component-scan base-package="org.example" use-default-filters="false">
        
        
        
        
        <context:include-filter type="annotation" expression="org.springframework.stereotype.Controller"/>
    	
    </context:component-scan>

#### 3.3、使用注解定义 Bean ####

Spring 提供了以下多个注解，这些注解可以直接标注在 Java 类上，将它们定义成 Spring Bean。

<table> 
 <thead> 
  <tr> 
   <th>注解</th> 
   <th>说明</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>@Component</td> 
   <td>该注解用于描述 Spring 中的 Bean，它是一个泛化的概念，仅仅表示容器中的一个组件（Bean），并且可以作用在应用的任何层次，例如 Service 层、Dao 层等。 使用时只需将该注解标注在相应类上即可。</td> 
  </tr> 
  <tr> 
   <td>@Repository</td> 
   <td>该注解用于将数据访问层（Dao 层）的类标识为 Spring 中的 Bean，其功能与 @Component 相同。</td> 
  </tr> 
  <tr> 
   <td>@Service</td> 
   <td>该注解通常作用在业务层（Service 层），用于将业务层的类标识为 Spring 中的 Bean，其功能与 @Component 相同。</td> 
  </tr> 
  <tr> 
   <td>@Controller</td> 
   <td>该注解通常作用在控制层（如SpringMVC 的 Controller），用于将控制层的类标识为 Spring 中的 Bean，其功能与 @Component 相同。</td> 
  </tr> 
 </tbody> 
</table>

#### 3.4、实验一：@Autowired注入 ####

单独使用`@Autowired`注解，**默认根据类型装配**。【默认是**byType**】

查看源码：

package org.springframework.beans.factory.annotation;
    
    import java.lang.annotation.Documented;
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target({
        ElementType.CONSTRUCTOR, ElementType.METHOD, ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Autowired {
        
        boolean required() default true;
    }

源码中有两处需要注意：

*  第一处：该注解可以标注在哪里？
 *   *  构造方法上
     *  方法上
     *  形参上
     *  属性上
     *  注解上
 *  第二处：该注解有一个`required`属性，默认值是true，表示在注入的时候要求被注入的Bean必须是存在的，如果不存在则报错。如果required属性设置为false，表示注入的Bean存在或者不存在都没关系，存在的话就注入，不存在的话，也不报错。

##### ①场景一：属性注入 #####

创建UserDao接口

package org.example.dao;
    
    public interface UserDao {
        
    
        public void print();
    }

创建UserDaoImpl实现

package org.example.dao.impl;
    
    import org.example.dao.UserDao;
    import org.springframework.stereotype.Repository;
    
    @Repository
    public class UserDaoImpl implements UserDao {
        
    
        @Override
        public void print() {
        
            System.out.println("Dao层执行结束");
        }
    }

创建UserService接口

package org.example.spring6.service;
    
    public interface UserService {
        
    
        public void out();
    }

创建UserServiceImpl实现类

package org.example.spring6.service.impl;
    
    import org.example.spring6.dao.UserDao;
    import org.example.spring6.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        @Autowired
        private UserDao userDao;
    
        @Override
        public void out() {
        
            userDao.print();
            System.out.println("Service层执行结束");
        }
    }

创建UserController类

package org.exampleu.spring6.controller;
    
    import org.example.spring6.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Controller;
    
    @Controller
    public class UserController {
        
    
        @Autowired
        private UserService userService;
    
        public void out() {
        
            userService.out();
            System.out.println("Controller层执行结束。");
        }
    
    }

**测试一**

package org.example.spring6.bean;
    
    import org.example.spring6.controller.UserController;
    import org.junit.jupiter.api.Test;
    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    public class UserTest {
        
    
        private Logger logger = LoggerFactory.getLogger(UserTest.class);
    
        @Test
        public void testAnnotation(){
        
            ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");
            UserController userController = context.getBean("userController", UserController.class);
            userController.out();
            logger.info("执行成功");
        }
    
    
    }

以上构造方法和setter方法都没有提供，经过测试，仍然可以注入成功。

##### ②场景二：set注入 #####

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        private UserDao userDao;
    
        @Autowired
        public void setUserDao(UserDao userDao) {
        
            this.userDao = userDao;
        }
    
        @Override
        public void out() {
        
            userDao.print();
            System.out.println("Service层执行结束");
        }
    }

修改UserController类

package org.example.controller;
    
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Controller;
    
    @Controller
    public class UserController {
        
    
        private UserService userService;
    
        @Autowired
        public void setUserService(UserService userService) {
        
            this.userService = userService;
        }
    
        public void out() {
        
            userService.out();
            System.out.println("Controller层执行结束。");
        }
    
    }

测试：成功调用

##### ③场景三：构造方法注入 #####

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        private UserDao userDao;
    
        @Autowired
        public UserServiceImpl(UserDao userDao) {
        
            this.userDao = userDao;
        }
    
        @Override
        public void out() {
        
            userDao.print();
            System.out.println("Service层执行结束");
        }
    }

修改UserController类

package org.example.controller;
    
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Controller;
    
    @Controller
    public class UserController {
        
    
        private UserService userService;
    
        @Autowired
        public UserController(UserService userService) {
        
            this.userService = userService;
        }
    
        public void out() {
        
            userService.out();
            System.out.println("Controller层执行结束。");
        }
    
    }

测试：成功调用

> 注：在 Spring4.x 中增加了新的特性：如果类只提供了一个带参数的构造方法，则不需要对对其内部的属性写 @Autowired 注解，Spring 会自动为你注入属性。（请看场景五）

##### ④场景四：形参上注入 #####

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.spring6.dao.UserDao;
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        private UserDao userDao;
    
        public UserServiceImpl(@Autowired UserDao userDao) {
        
            this.userDao = userDao;
        }
    
        @Override
        public void out() {
        
            userDao.print();
            System.out.println("Service层执行结束");
        }
    }

修改UserController类

package org.example.controller;
    
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Controller;
    
    @Controller
    public class UserController {
        
    
        private UserService userService;
    
        public UserController(@Autowired UserService userService) {
        
            this.userService = userService;
        }
    
        public void out() {
        
            userService.out();
            System.out.println("Controller层执行结束。");
        }
    
    }

测试：成功调用

##### ⑤场景五：只有一个构造函数，无注解 #####

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Qualifier;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        @Autowired
        private UserDao userDao;
    
        public UserServiceImpl(UserDao userDao) {
        
            this.userDao = userDao;
        }
    
        @Override
        public void out() {
        
            userDao.print();
            System.out.println("Service层执行结束");
        }
    }

测试通过

**当有参数的构造方法只有一个时，@Autowired注解可以省略。**

说明：有多个构造方法时呢？大家可以测试（再添加一个无参构造函数），测试报错

##### ⑥场景六：@Autowired注解和@Qualifier注解联合 #####

添加dao层实现

package org.example.dao.impl;
    
    import org.example.dao.UserDao;
    import org.springframework.stereotype.Repository;
    
    @Repository
    public class UserDaoRedisImpl implements UserDao {
        
    
        @Override
        public void print() {
        
            System.out.println("Redis Dao层执行结束");
        }
    }

测试：测试异常

错误信息中说：不能装配，UserDao这个Bean的数量等于2

怎么解决这个问题呢？**当然要byName，根据名称进行装配了。**

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        @Autowired
        @Qualifier("userDaoImpl") // 指定bean的名字
        private UserDao userDao;
    
        @Override
        public void out() {
        
            userDao.print();
            System.out.println("Service层执行结束");
        }
    }

**总结**

*  `@Autowired`注解可以出现在：属性上、构造方法上、构造方法的参数上、setter方法上。
 *  当带参数的构造方法只有一个，@Autowired注解可以省略。（）
 *  @Autowired注解默认根据类型注入。如果要根据名称注入的话，需要配合`@Qualifier`注解一起使用。

#### 3.5、实验二：@Resource注入 ####

`@Resource`注解也可以完成属性注入。那它和@Autowired注解有什么区别？

*  `@Resource`注解是JDK扩展包中的，也就是说属于JDK的一部分。所以该注解是标准注解，更加具有通用性。(JSR-250标准中制定的注解类型。JSR是Java规范提案。)
 *  @Autowired注解是Spring框架自己的。
 *  **@Resource注解默认根据名称装配byName，未指定name时，使用属性名作为name。通过name找不到的话会自动启动通过类型byType装配。**
 *  **@Autowired注解默认根据类型装配byType，如果想根据名称装配，需要配合@Qualifier注解一起用。**
 *  @Resource注解用在属性上、setter方法上。
 *  @Autowired注解用在属性上、setter方法上、构造方法上、构造方法参数上。

@Resource注解属于JDK扩展包，所以不在JDK当中，需要额外引入以下依赖：【**如果是JDK8的话不需要额外引入依赖。高于JDK11或低于JDK8需要引入以下依赖。**】

<dependency>
        <groupId>jakarta.annotation</groupId>
        <artifactId>jakarta.annotation-api</artifactId>
        <version>2.1.1</version>
    </dependency>

源码：

package jakarta.annotation;
    
    import java.lang.annotation.ElementType;
    import java.lang.annotation.Repeatable;
    import java.lang.annotation.Retention;
    import java.lang.annotation.RetentionPolicy;
    import java.lang.annotation.Target;
    
    @Target({
        ElementType.TYPE, ElementType.FIELD, ElementType.METHOD})
    @Retention(RetentionPolicy.RUNTIME)
    @Repeatable(Resources.class)
    public @interface Resource {
        
        String name() default "";
    
        String lookup() default "";
    
        Class<?> type() default Object.class;
    
        Resource.AuthenticationType authenticationType() default Resource.AuthenticationType.CONTAINER;
    
        boolean shareable() default true;
    
        String mappedName() default "";
    
        String description() default "";
    
        public static enum AuthenticationType {
        
            CONTAINER,
            APPLICATION;
    
            private AuthenticationType() {
        
            }
        }
    }

##### ①场景一：根据name注入 #####

修改UserDaoImpl类

package org.example.dao.impl;
    
    import org.example.dao.UserDao;
    import org.springframework.stereotype.Repository;
    
    @Repository("myUserDao")
    public class UserDaoImpl implements UserDao {
        
    
        @Override
        public void print() {
        
            System.out.println("Dao层执行结束");
        }
    }

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import jakarta.annotation.Resource;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Qualifier;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        @Resource(name = "myUserDao")
        private UserDao myUserDao;
    
        @Override
        public void out() {
        
            myUserDao.print();
            System.out.println("Service层执行结束");
        }
    }

测试通过

##### ②场景二：name未知注入 #####

修改UserDaoImpl类

修改UserServiceImpl类

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import jakarta.annotation.Resource;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Qualifier;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        @Resource
        private UserDao myUserDao;
    
        @Override
        public void out() {
        
            myUserDao.print();
            System.out.println("Service层执行结束");
        }
    }

测试通过

当`@Resource`注解使用时没有指定name的时候，还是根据name进行查找，这个name是 属性名 。

##### ③场景三 其他情况 #####

修改UserServiceImpl类，userDao1属性名不存在

package org.example.service.impl;
    
    import org.example.dao.UserDao;
    import org.example.service.UserService;
    import jakarta.annotation.Resource;
    import org.springframework.beans.factory.annotation.Autowired;
    import org.springframework.beans.factory.annotation.Qualifier;
    import org.springframework.stereotype.Service;
    
    @Service
    public class UserServiceImpl implements UserService {
        
    
        @Resource
        private UserDao userDao1;
    
        @Override
        public void out() {
        
            userDao1.print();
            System.out.println("Service层执行结束");
        }
    }

测试异常

根据异常信息得知：显然当通过name找不到的时候，自然会启动byType进行注入，以上的错误是因为UserDao接口下有两个实现类导致的。所以根据类型注入就会报错。

@Resource的set注入可以自行测试

**总结：**

@Resource注解：**默认byName注入**，没有指定name时把属性名当做name，**根据name找不到时，才会byType注入**。byType注入时，某种类型的Bean只能有一个

#### 3.6、Spring全注解开发 ####

全注解开发就是不再使用spring配置文件了，写一个配置类来代替配置文件。

package org.example.config;
    
    import org.springframework.context.annotation.ComponentScan;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    //@ComponentScan({"org.example.controller", "org.example.service","org.example.dao"})
    @ComponentScan("org.example")
    public class Spring6Config {
        
    }

测试类

@Test
    public void testAllAnnotation(){
        
        ApplicationContext context = new AnnotationConfigApplicationContext(Spring6Config.class);
        UserController userController = context.getBean("userController", UserController.class);
        userController.out();
        logger.info("执行成功");
    }

[Spring IOC_XML_Bean]: https://blog.csdn.net/weixin_43847283/article/details/131153799