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
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
