Java框架（三）--Spring IoC容器与Bean管理（7）--基于注解配置IoC容器

基于注解配置IoC容器

基于注解的优势

摆脱繁琐的XML形式的bean与依赖注入配置。
基于“声明式”的原则，更适合轻量级的现代企业应用。
让代码可读性变得更好，研发人员拥有更好的开发体验。

三类注解

组件类型注解 - 声明当前类的功能与职责。
自动装配注解 - 根据属性特征自动注入对象。
元数据注解 - 更细化的辅助IoC容器管理对象的注解。

四种组件类型注解

在这里插入图片描述

开启组件扫描

在这里插入图片描述

代码演示

打开IDEA创建新的maven工程，在pom.xml引入Spring依赖

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.ql.spring</groupId>
    <artifactId>ioc_annotation</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>
    <repositories>
        <repository>
            <id>aliyun</id>
            <name>aliyun</name>
            <url>https://maven.aliyun.com/repository/public</url>
        </repository>
    </repositories>
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.2.6.RELEASE</version>
        </dependency>
    </dependencies>
</project>
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在resources目录下创建Spring配置文件applicationContext.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
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">
    <!-- 在IoC容器初始化时自动扫描四种组件类型注解并完成实例化
         @Repository
         @Service
         @Controller
         @Component
    -->
    <context:component-scan base-package="com.ql.spring"/>
</beans>
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在com.ql.spring.ioc.dao包下创建UserDao类

package com.ql.spring.ioc.dao;

import org.springframework.stereotype.Repository;
//组件类型注解默认beanId为类名首字母小写
//beanId = userDao
@Repository
public class UserDao {
}

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在com.ql.spring.ioc.service包下创建UserService类

package com.ql.spring.ioc.service;

import org.springframework.stereotype.Service;

@Service
public class UserService {
}

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在com.ql.spring.ioc.controller包下创建UserController类

package com.ql.spring.ioc.controller;

import org.springframework.stereotype.Controller;

@Controller
public class UserController {
}

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在com.ql.spring.ioc.utils包下创建StringUtils类

package com.ql.spring.ioc.utils;

import org.springframework.stereotype.Component;

@Component
public class StringUtils {
}

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然后com.ql.spring.ioc包下创建入口类SpringApplication执行

package com.ql.spring.ioc;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpringApplication {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
        String[] ids = context.getBeanDefinitionNames();
        for (String id: ids) {
            System.out.println(id + ":" + context.getBean(id));
        }
    }
}
/*
userController:com.ql.spring.ioc.controller.UserController@6483f5ae
userDao:com.ql.spring.ioc.dao.UserDao@b9afc07
userService:com.ql.spring.ioc.service.UserService@382db087
stringUtils:com.ql.spring.ioc.utils.StringUtils@73d4cc9e
org.springframework.context.annotation.internalConfigurationAnnotationProcessor:org.springframework.context.annotation.ConfigurationClassPostProcessor@80169cf
org.springframework.context.annotation.internalAutowiredAnnotationProcessor:org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor@5427c60c
org.springframework.context.annotation.internalCommonAnnotationProcessor:org.springframework.context.annotation.CommonAnnotationBeanPostProcessor@15bfd87
org.springframework.context.event.internalEventListenerProcessor:org.springframework.context.event.EventListenerMethodProcessor@543e710e
org.springframework.context.event.internalEventListenerFactory:org.springframework.context.event.DefaultEventListenerFactory@57f23557
*/
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两类自动装配注解

在这里插入图片描述

按类型装配@Autowired

修改com.ql.spring.ioc.dao包下的UserDao

package com.ql.spring.ioc.dao;

import org.springframework.stereotype.Repository;
//组件类型注解默认beanId为类名首字母小写
//beanId = userDao
@Repository
public class UserDao {
    public UserDao() {
        System.out.println("正在创建UserDao:"+this);
    }
}

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修改com.ql.spring.ioc.service包下的UserService

package com.ql.spring.ioc.service;

import com.ql.spring.ioc.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserService {
    private UserService(){
        System.out.println("正在创建UserService:"+this);
    }
    @Autowired
    private UserDao udao;

    public UserDao getUdao() {
        return udao;
    }

    public void setUdao(UserDao udao) {
        System.out.println("setUdao:"+udao);
        this.udao = udao;
    }
}

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然后修改入口类SpringApplication重新执行

package com.ql.spring.ioc;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpringApplication {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
		UserService userService = context.getBean("userService", UserService.class);
        System.out.println(userService.getUdao());
    }
}
/*
正在创建UserDao:com.ql.spring.ioc.dao.UserDao@149494d8
正在创建UserService:com.ql.spring.ioc.service.UserService@710726a3
com.ql.spring.ioc.dao.UserDao@149494d8
*/
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从运行结果得知，装配注解放在属性上，不执行set方法。其原理为：Spring IoC容器会自动通过反射技术将属性private修饰符自动修改改为public，直接进行赋值。所以日常开发时如果用注解注入时不会生成set方法。

如果把@Autowired注解放在set方法上再执行，如下：

package com.ql.spring.ioc.service;

import com.ql.spring.ioc.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserService {
    private UserService(){
        System.out.println("正在创建UserService:"+this);
    }
    //@Autowired
    private UserDao udao;

    public UserDao getUdao() {
        return udao;
    }

    @Autowired
    public void setUdao(UserDao udao) {
        System.out.println("setUdao:"+udao);
        this.udao = udao;
    }
}
/*
正在创建UserDao:com.ql.spring.ioc.dao.UserDao@306279ee
正在创建UserService:com.ql.spring.ioc.service.UserService@77846d2c
setUdao:com.ql.spring.ioc.dao.UserDao@306279ee
com.ql.spring.ioc.dao.UserDao@306279ee
*/
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得知：如果装配注解放在set方法上，则自动按类型/名称对set方法参数进行注入。

下面再分析@Autowired注解：
在com.ql.spring.ioc.dao包下创建IUserDao接口

package com.ql.spring.ioc.dao;

public interface IUserDao {
}

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然后UserDao实现这个接口

package com.ql.spring.ioc.dao;

import org.springframework.stereotype.Repository;

@Repository
public class UserDao implements IUserDao{
    public UserDao() {
        System.out.println("正在创建UserDao:"+this);
    }
}
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在com.ql.spring.ioc.dao包下再创建一个UserOracleDao类也实现IUserDao接口

package com.ql.spring.ioc.dao;

import org.springframework.stereotype.Repository;

@Repository
public class UserOracleDao implements IUserDao{
    public UserOracleDao() {
        System.out.println("正在创建UserOracleDao:"+this);
    }
}

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此时在UserService中属性注解注入形式，然后运行代码就会报错

package com.ql.spring.ioc.service;

import com.ql.spring.ioc.dao.IUserDao;
import com.ql.spring.ioc.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserService {
    private UserService(){
        System.out.println("正在创建UserService:"+this);
    }
    @Autowired
    private IUserDao udao;

    public IUserDao getUdao() {
        return  udao;
    }
}
//NoUniqueBeanDefinitionException: No qualifying bean of type 'com.ql.spring.ioc.dao.IUserDao' available: expected single matching bean but found 2: userDao,userOracleDao
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报错的原因为按类型注入时，有两个对象都实现了IUserDao接口，所以不知道该注入哪个bean。
解决方案有几种：
1、去掉两个实现类其中一个类的注解，如去掉UserDao类的@Repository注解。
2、两个实现类其中一个类再加个注解，如UserOracleDao类再加个注解为@Primary，@Primary代表主要的意思，其含义是如果IoC容器中出现了多个相同类型的对象的话，默认采用@Primary所描述的对象完成注入。
3、注入注解选择@Resource按名称装配。

按名称装配@Resource

1、@Resource设置name属性，则按name在IoC容器中将bean注入。
2、@Resource未设置name属性。
2.1、以属性名作为bean name在IoC容器中匹配bean，如有匹配则注入。
2.2、按属性名未匹配，则按类型进行匹配，同@Autowired，需加入@Primary解决类型冲突。
使用建议：在使用@Resource对象时推荐设置name或保证属性名与bean名称一致。

在com.ql.spring.ioc.service包下创建DepartmentService

package com.ql.spring.ioc.service;

import com.ql.spring.ioc.dao.IUserDao;
import org.springframework.stereotype.Service;

import javax.annotation.Resource;

@Service
public class DepartmentService {
    //@Resource(name="userOracleDao")
    //private IUserDao udao;
    @Resource
    private IUserDao userOracleDao;
        public void joinDepartment(){
        System.out.println(userOracleDao);
    }
}
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然后在入口类SpringApplication改为如下再执行

package com.ql.spring.ioc;

import com.ql.spring.ioc.service.DepartmentService;
import com.ql.spring.ioc.service.UserService;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpringApplication {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
        UserService userService = context.getBean("userService", UserService.class);
        System.out.println(userService.getUdao());

        DepartmentService departmentService = context.getBean("departmentService", DepartmentService.class);
        departmentService.joinDepartment();
        /*        String[] ids = context.getBeanDefinitionNames();
        for (String id: ids) {
            System.out.println(id + ":" + context.getBean(id));
        }*/
    }
}
/*
正在创建UserDao:com.ql.spring.ioc.dao.UserDao@548ad73b
正在创建UserOracleDao:com.ql.spring.ioc.dao.UserOracleDao@4c762604
正在创建UserService:com.ql.spring.ioc.service.UserService@10d59286
com.ql.spring.ioc.dao.UserOracleDao@4c762604
com.ql.spring.ioc.dao.UserOracleDao@4c762604
*/
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如果在DepartmentService中将属性名再改为userDao再运行

package com.ql.spring.ioc.service;

import com.ql.spring.ioc.dao.IUserDao;
import org.springframework.stereotype.Service;

import javax.annotation.Resource;

@Service
public class DepartmentService {
    //@Resource(name="userOracleDao")
    //private IUserDao udao;
    @Resource
    private IUserDao userDao;
    public void joinDepartment(){
        System.out.println(userDao);
    }
}
/*
正在创建UserDao:com.ql.spring.ioc.dao.UserDao@548ad73b
正在创建UserOracleDao:com.ql.spring.ioc.dao.UserOracleDao@4c762604
正在创建UserService:com.ql.spring.ioc.service.UserService@10d59286
com.ql.spring.ioc.dao.UserOracleDao@4c762604
com.ql.spring.ioc.dao.UserDao@548ad73b
*/
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其他元数据注解

在这里插入图片描述
在resources目录添加属性文件config.properties添加内容为

metaData = ql.com
1

在applicationContext.xml中添加配置

<?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
        https://www.springframework.org/schema/beans/spring-beans.xsd
        http://www.springframework.org/schema/context
        https://www.springframework.org/schema/context/spring-context.xsd">
    <!--通过Spring IoC容器初始化时加载属性文件-->
    <context:property-placeholder location="classpath:config.properties"/>
    <!-- 在IoC容器初始化时自动扫描四种组件类型注解并完成实例化
         @Repository
         @Service
         @Controller
         @Component
    -->
    <context:component-scan base-package="com.ql.spring"/>
</beans>
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修改UserService类，添加相关注解

package com.ql.spring.ioc.service;

import com.ql.spring.ioc.dao.IUserDao;
import com.ql.spring.ioc.dao.UserDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Scope;
import org.springframework.stereotype.Service;

import javax.annotation.PostConstruct;

@Service
@Scope("prototype")//设置单例/多例，在XML中bean scope完全相同
public class UserService {
    @Value("${metaData}")
    private String metaData;

    private UserService(){
        System.out.println("正在创建UserService:"+this);
    }

    @PostConstruct//XML中bean init-method完全相同
    public void init(){
        System.out.println("初始化UserService对象,metaData="+metaData);
    }
    @Autowired
    private IUserDao udao;

    public IUserDao getUdao() {
        return  udao;
    }
}

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运行入口类SpringApplication.java

package com.ql.spring.ioc;

import com.ql.spring.ioc.service.DepartmentService;
import com.ql.spring.ioc.service.UserService;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpringApplication {
    public static void main(String[] args) {
        ApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
        UserService userService = context.getBean("userService", UserService.class);
        System.out.println(userService.getUdao());
    }
}
/*
正在创建UserDao:com.ql.spring.ioc.dao.UserDao@1efee8e7
正在创建UserOracleDao:com.ql.spring.ioc.dao.UserOracleDao@1ee807c6
正在创建UserService:com.ql.spring.ioc.service.UserService@3c0a50da
初始化UserService对象,metaData=ql.com
com.ql.spring.ioc.dao.UserOracleDao@1ee807c6
*/
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原文地址：https://blog.csdn.net/qq_32091929/article/details/125571292