【Spring】Spring的@Autowire注入Bean的规则测试

背景

在项目中使用Spring的Bean，一般都使用默认的Bean的单例，并且结合@Autowire使用。

实在有同一个类型多个实例的情况，也使用@Qualifier或@Resource实现注入。

所以，对@Autowire的注入规则并不是特别的清楚。

今天突然想起这个疑惑，就用简单的实验确认一下。

实验的基础类

AppleBean，里面有个字符串的属性，和覆盖toString方法，用于在打印日志里更明显地分辨不同的bean：

public class AppleBean {

    private String content;

    public String getContent() {
        return content;
    }

    public void setContent(String content) {
        this.content = content;
    }

    @Override
    public String toString() {
        return "AppleBean{" +
                "content='" + content + '\'' +
                '}';
    }

}

当容器中该类型只有一个Bean实例时，按类型注入，与Bean名字无关

注册一个Bean，叫appleBean：

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

@Configuration
public class BeanConfig {

    @Bean
    public AppleBean appleBean() {
        AppleBean appleBean = new AppleBean();
        appleBean.setContent("1");
        return appleBean;
    }

}

将AppleBean注入到一个Controller中，并在此Controller的方法中打印AppleBean：

import com.example.demo.bean.AppleBean;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HeartBeatController {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    @Autowired
    private AppleBean appleBean;

    @GetMapping("/test")
    public String test() {
        this.logger.info("appleBean -> {}", appleBean);
        return "SUCCESS";
    }

}

运行程序，并访问此测试接口，可见日志，appleBean成功注入：

com.example.demo.HeartBeatController     : appleBean -> AppleBean{content='1'}

修改注入的属性名为appleBean2，与注册的bean名字appleBean是不一致的哦：

import com.example.demo.bean.AppleBean;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HeartBeatController {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    @Autowired
    private AppleBean appleBean2;

    @GetMapping("/test")
    public String test() {
        this.logger.info("appleBean -> {}", appleBean2);
        return "SUCCESS";
    }

}

再运行，日志如下，appleBean也正常注入进来了：

com.example.demo.HeartBeatController     : appleBean -> AppleBean{content='1'}

此节结论：

当容器中该类型只有一个Bean实例时，按类型注入，与Bean名字无关

当容器中该类型只有多个Bean实例时，按类型注入，与Bean名字有关

我们实例化两个AppleBean，名字分别为appleBean、appleBean2：

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

@Configuration
public class BeanConfig {

    @Bean
    public AppleBean appleBean() {
        AppleBean appleBean = new AppleBean();
        appleBean.setContent("1");
        return appleBean;
    }

    @Bean
    public AppleBean appleBean2() {
        AppleBean appleBean = new AppleBean();
        appleBean.setContent("2");
        return appleBean;
    }

}

我们注入appleBean2试试：

import com.example.demo.bean.AppleBean;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HeartBeatController {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    @Autowired
    private AppleBean appleBean2;

    @GetMapping("/test")
    public String test() {
        this.logger.info("appleBean -> {}", appleBean2);
        return "SUCCESS";
    }

}

运行，可见日志，与名字精准匹配的Bean被注入进来：

com.example.demo.HeartBeatController     : appleBean -> AppleBean{content='2'}

我们注入appleBean3试试，appleBean3是一个不存在的Bean名字：

import com.example.demo.bean.AppleBean;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class HeartBeatController {

    private Logger logger = LoggerFactory.getLogger(this.getClass());

    @Autowired
    private AppleBean appleBean3;

    @GetMapping("/test")
    public String test() {
        this.logger.info("appleBean -> {}", appleBean3);
        return "SUCCESS";
    }

}

日志如下，启动时就报异常了：

Field appleBean3 in com.example.demo.HeartBeatController required a single bean, but 2 were found:
	- appleBean: defined by method 'appleBean' in class path resource [com/example/demo/bean/BeanConfig.class]
	- appleBean2: defined by method 'appleBean2' in class path resource [com/example/demo/bean/BeanConfig.class]

我们再实例化另一种类（BoyBean，不是刚才的AppleBean哦），但将它注册的Bean名叫appleBean3：

@Bean
    public BoyBean appleBean3() {
        BoyBean boyBean = new BoyBean();
        return boyBean;
    }

再运行，仍然报错，与上面的异常是一致的：

Field appleBean3 in com.example.demo.HeartBeatController required a single bean, but 2 were found:
	- appleBean: defined by method 'appleBean' in class path resource [com/example/demo/bean/BeanConfig.class]
	- appleBean2: defined by method 'appleBean2' in class path resource [com/example/demo/bean/BeanConfig.class]

此节结论：

当容器中该类型只有多个Bean实例时，按类型注入，与Bean名字有关