使用Spring容器最简单的代码姿势

如果仅仅是为了测试简单使用一下Spring的IOC容器，或者研究一下Spring的容器的源码实现，那么在搭建Spring工程的时候，不需要复杂的xml配置。Spring3.0之后提供了Java注解配置的方式来启动Spring容器。下面是最简单的启动Spring容器的方式：

1.依赖包只需要spring-context即可：

<?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.springexample</groupId>
    <artifactId>spring-ioc</artifactId>
    <version>1.0-SNAPSHOT</version>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.1.8.RELEASE</version>
        </dependency>
    </dependencies>

</project>

2.一个启动测试类：

import com.spring.ioc.SpringConfig;
import com.spring.ioc.UserService;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

public class Application {

    public static void main(String[] args) {
        AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(SpringConfig.class);
        UserService userService = context.getBean(UserService.class);
        System.out.println(userService.getSysUser());
    }
}

3.一个配置类（@Configuration注解的类相当于一个xml文件），用于扫描和注册Bean，@ComponentScan注解用于要扫描哪个包下的Bean（即扫描@Service、@Repository等注解的Bean）：

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@ComponentScan("com.spring")
@Configuration
public class SpringConfig {

}

4.模拟一个业务类，类上加上@Service注解：

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;

@Service
public class UserService {

    @Autowired
    private UserDao userDao;

    public SysUser getSysUser() {
        return userDao.getSysUser();
    }
}

5.模拟Dao服务，类上加上@Repository注解，提供数据访问服务：

import org.springframework.stereotype.Repository;

@Repository
public class UserDao {
    public SysUser getSysUser() {
        SysUser sysUser = new SysUser();
        sysUser.setUserId(2);
        sysUser.setUserName("Tom");
        return sysUser;
    }
}

6.一个实体类，用于封装业务数据：

public class SysUser {

    private Integer userId;
    private String userName;

    public Integer getUserId() {
        return userId;
    }

    public void setUserId(Integer userId) {
        this.userId = userId;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    @Override
    public String toString() {
        return "SysUser{" +
                "userId=" + userId +
                ", userName='" + userName + '\'' +
                '}';
    }
}

6.执行测试类Application中的main方法，查看输出结果：

SysUser{userId=2, userName='Tom'}