（004）spring容器报错expected single matching bean but found的原因及解放方法

　　如果spring容器中存在某个类型的bean有多个，在根据名字获取bean的时候就会报expected single matching bean but found（期望匹配一个bean，但是发现多个）的错误，请看下面的实例。

　　示例1：

　　pom.xml

<?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.edu.spring</groupId>
    <artifactId>spring</artifactId>
    <version>1.0.0</version>

    <name>spring</name>
    <!-- FIXME change it to the project's website -->
    <url>http://www.example.com</url>

    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>4.3.2.RELEASE</version>
        </dependency>
    </dependencies>

</project>

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　　User.java

package com.edu.spring;

import org.springframework.stereotype.Component;

@Component("myUser")
public class User {

}

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　　MyConfig.java

package com.edu.spring;

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

@Configuration
public class MyConfig {
    
    @Bean
    public User createUser(){
        return new User();
    }
}

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　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,MyConfig.class);
        System.out.println(context.getBean(User.class));
        context.close();
    }
}

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　　运行结果如下：

　　报错信息为：Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException: No qualifying bean of type [com.edu.spring.User] is defined: expected single matching bean but found 2: myUser,createUser。

　　现在用名字获取一下，发现这是两个bean，如下：

　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,MyConfig.class);
        System.out.println(context.getBean("myUser"));
        System.out.println(context.getBean("createUser"));
        context.close();
    }
}

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　　运行结果如下：

　　使用getBeansOfType可以获取某个类型的所有bean对象，如下：

　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,MyConfig.class);
        System.out.println(context.getBean("myUser"));
        System.out.println(context.getBean("createUser"));
        System.out.println(context.getBeansOfType(User.class));
        context.close();
    }
}

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　　运行结果如下：

　　示例2：

　　User.java

package com.edu.spring;

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

@Component("myUser")
public class User {
    
    @Autowired
    private UserService userService;

    public void show(){
        userService.show();
    }

}

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　　UserDao.java

package com.edu.spring;

import org.springframework.stereotype.Repository;

@Repository
public class UserDao {

    public void show(){
        System.out.println("--------------show()------------------");
    }
}

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　　UserService.java

package com.edu.spring;

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

@Service
public class UserService {
    
    @Autowired
    private UserDao userDao;

    public void show(){
        userDao.show();
    }
    
}

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　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,UserDao.class,UserService.class);
        System.out.println(context.getBean(UserService.class));
        System.out.println(context.getBean(UserDao.class));
        User user=context.getBean("myUser",User.class);
        user.show();
        context.close();
    }
}

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　　运行结果如下：

　　可以看到，正常运行，不会报错，下面再创建一个UserDao的bean，并且纳入到spring容器中，程序会报错expected single matching bean but found，如下：

　　MyConfig.java

package com.edu.spring;

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

@Configuration
public class MyConfig {
    
    @Bean
    public UserDao createUserDao(){
        return new UserDao();
    }
}

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　　App.java 第一行新添加了 MyConfig.class。

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,UserDao.class,UserService.class,MyConfig.class);
        System.out.println(context.getBean(UserService.class));
        System.out.println(context.getBean(UserDao.class));
        User user=context.getBean("myUser",User.class);
        user.show();
        context.close();
    }
}

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　　运行结果如下：

　　报错原因是以下代码引起

System.out.println(context.getBean(UserDao.class));

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　　注意，UserService中@Autowired注入userDao不会报错，这里的对象userDao即为bean的名称，请看以下代码：

　　App.java 去掉System.out.println(context.getBean(UserDao.class));程序不会报错

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,UserDao.class,UserService.class,MyConfig.class);
        System.out.println(context.getBean(UserService.class));
        //System.out.println(context.getBean(UserDao.class));
        User user=context.getBean("myUser",User.class);
        user.show();
        context.close();
    }
}

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　　运行结果如下：

　　原因出在Autowired注解，Autowired是把spring容器中已经存在的bean注入（装配）到对象中，Autowired优先根据bean类型注入，如果该类型有一个bean，注入成功，如果有多个bean，则根据名字注入，注入bean的名字与属性的名字形同的bean，没有名字相同的会报expected single matching bean but found。

　　UserService.java中属性为createUserDao或者userDao都不会报错。如下：

package com.edu.spring;

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

@Service
public class UserService {
    
    @Autowired
    private UserDao createUserDao;

    public void show(){
        createUserDao.show();
    }
    
}

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　　但是换成别的属性会报expected single matching bean but found，如下：

　　UserService.java

package com.edu.spring;

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

@Service
public class UserService {
    
    @Autowired
    private UserDao ssss;

    public void show(){
        ssss.show();
    }
    
}

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　　App.java

package com.edu.spring;

import org.springframework.context.annotation.AnnotationConfigApplicationContext;


public class App 
{
    public static void main( String[] args )
    {
        AnnotationConfigApplicationContext context=new AnnotationConfigApplicationContext(User.class,UserDao.class,UserService.class,MyConfig.class);
        System.out.println(context.getBean(UserService.class));
        //System.out.println(context.getBean(UserDao.class));
        User user=context.getBean("myUser",User.class);
        user.show();
        context.close();
    }
}

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　　运行结果如下：

　　解决办法，Qualifier注解指定其中的一个bean，如下：

　　UserService.java

package com.edu.spring;

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

@Service
public class UserService {
    
    @Autowired
    @Qualifier("createUserDao")

    private UserDao ssss;

    public void show(){
        ssss.show();
    }
    
}

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　　运行结果如下：

　　还有一种解决方法，在创建bean时用Primary注解，如下：

　　在UserDao.java中添加@Primary

package com.edu.spring;

import org.springframework.context.annotation.Primary;
import org.springframework.stereotype.Repository;

@Repository
@Primary
public class UserDao {

    public void show(){
        System.out.println("--------------show()------------------");
    }
}

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　　或者在MyConfig.java中添加

package com.edu.spring;

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

@Configuration
public class MyConfig {
    
    @Bean
    @Primary
    public UserDao createUserDao(){
        return new UserDao();
    }
}

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来源：https://www.cnblogs.com/javasl/p/11783482.html

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