@Autowired自动装配,@Bean注入@Primary,@Qualifier优先级讲解_Java教程

Autowired自动装配

spring利用依赖注入(DI)，完成对IOC容器中的各个组件的依赖关系赋值

对同一个Dao类，既有 @Bean 注解声明，又有Autowired 自动装配，分析一下几种情况：

第一种情况

1、如果Dao类中声明了@Repository，且@ComponentScan 添加了dao扫描，则默认会创建一个testDao在IOC容器中。

2、如果在config中指定了Bean注解，此时：

a、如果Bean注解的方法名也是testDao ，则会覆盖默认对象

1.新建maven工程

<groupId>com.MySpring</groupId> <artifactId>demo</artifactId>

新建TestController.java TestService.java TestDao; 分别建在指定的包内

@Autowired自动装配,@Bean注入@Primary,@Qualifier优先级讲解

pom.xml 文件如下：

<?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.MySpring</groupId>
  <artifactId>demo</artifactId>
  <version>1.0-SNAPSHOT</version>
  <dependencies>
      <dependency>
          <groupId>org.springframework</groupId>
          <artifactId>spring-context</artifactId>
          <version>5.0.6.RELEASE</version>
      </dependency>
      <dependency>
          <groupId>junit</groupId>
          <artifactId>junit</artifactId>
          <version>4.12</version>
          <scope>test</scope>
      </dependency>
  </dependencies>
</project>

1.TestController 类：

package com.MySpring.demo.controller;
import com.MySpring.demo.service.TestService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
@Controller
public class TestController {
  @Autowired
  private TestService testService;
}

2.TestDao 类：

package com.MySpring.demo.dao;
import org.springframework.stereotype.Repository;
@Repository
public class TestDao {
  public String getMessage( ) {
      return message;
  }
  public void setMessage( String message ) {
      this.message = message;
  }
  private  String message ="1";
  @Override
  public String toString( ) {
      return "TestDao{" +
              "message='" + message + '\'' +
              '}';
  }
}

3.TestService

package com.MySpring.demo.service;
import com.MySpring.demo.dao.TestDao;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class TestService {
  @Autowired
  private TestDao testDao;
  public void print(){
      System.out.printf( "[%s] from testService.\n" ,testDao);
  }
}

4.AppConfig

package com.MySpring.demo;
import com.MySpring.demo.dao.TestDao;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
@Configuration
@ComponentScan({"com.MySpring.demo.controller","com.MySpring.demo.service","com.MySpring.demo.dao",})
public class AppConfig {
  @Bean
  public TestDao testDao(){
      TestDao dao = new TestDao();
      dao.setMessage( "2" );
      return dao;
  }
}

5.新建测试类：

package com.MySpring.demo;
import com.MySpring.demo.dao.TestDao;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
@Configuration
@ComponentScan({"com.MySpring.demo.controller","com.MySpring.demo.service","com.MySpring.demo.dao",})
public class AppConfig {
  @Bean
  public TestDao testDao(){
      TestDao dao = new TestDao();
      dao.setMessage( "2" );
      return dao;
  }
}

输出结果：

@Autowired自动装配,@Bean注入@Primary,@Qualifier优先级讲解

第二种情况

如果IOC容器中有两个Dao实例，则@Autowired 自动装配会根据变量名称去IOC容器中寻找，对应的实例装载，若找不到则报错，默认是必须存在，Autowired 源码如下：

当然，也可以设置required为false

public @interface Autowired {
	/**
	 * Declares whether the annotated dependency is required.
	 * <p>Defaults to {@code true}.
	 */
	boolean required() default true;
}

1. 更改AppConfig类中 Bean注解的方法名为testDao2

@Autowired自动装配,@Bean注入@Primary,@Qualifier优先级讲解