required a bean of type XXX that could not be found.

启动springboot时提示找不到自动注入的对象，Field articlesService in com.bowen.web.controller.ApiArticleController required a bean of type ‘com.bowen.service.service.ArticlesService’ that could not be found. 具体如下：

Error starting ApplicationContext. To display the conditions report re-run your application with 'debug' enabled.
2019-10-21 21:53:42.871 ERROR 10704 --- [           main] o.s.b.d.LoggingFailureAnalysisReporter   : 
***************************
APPLICATION FAILED TO START
***************************
Description:
Field articlesService in com.bowen.web.controller.ApiArticleController required a bean of type 'com.bowen.service.service.ArticlesService' that could not be found.
The injection point has the following annotations:
    - @org.springframework.beans.factory.annotation.Autowired(required=true)
Action:
Consider defining a bean of type 'com.bowen.service.service.ArticlesService' in your configuration.
Process finished with exit code 1

分析

检查该接口的实现

发现已经添加了@Service的声明，那么不可能是这个

package com.bowen.service.service.Impl;
import com.bowen.dao.ArticlesDao;
import com.bowen.dao.model.Articles;
import com.bowen.dao.mapper.ArticlesMapper;
import com.bowen.service.service.ArticlesService;
import com.baomidou.mybatisplus.extension.service.impl.ServiceImpl;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import java.util.List;
/**
 * <p>
 * 文章表 服务实现类
 * </p>
 *
 * @author wangbowen
 * @since 2019-10-19
 */
@Service
public class ArticlesServiceImpl implements ArticlesService {
    @Autowired
    private ArticlesDao articlesDao;
    @Override
    public Articles getArticleInfo(Integer id) {
        if(id <= 0){
            return null;
        }
        return articlesDao.getArticleInfo(id);
    }
    @Override
    public int addArticleViewCount(Integer id) {
        if(id <= 0){
            return 0;
        }
        return articlesDao.addArticleViewCount(id);
    }
    @Override
    public int addPraiseCount(Integer id) {
        return 0;
    }
    @Override
    public List<Articles> queryApiArticlesList(Integer type, String title, Integer page, Integer pageSize) {
        return null;
    }
    @Override
    public List<Articles> hotArticlesList() {
        return articlesDao.hotArticlesList();
    }
}

检查springboot启动文件BoboServiceApplication

发现没有包含ComponentScan，由于我这个项目是分模块的项目，启动文件和调用的service不在同一个包，那么可能扫描不到包，导致无法找到自动注入的类。
package com.bowen.service;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;
@SpringBootApplication
public class BoboServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(BoboServiceApplication.class, args);
    }
}

解决方案

BoboServiceApplication 加入如下代码

package com.bowen.service;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;
/**
 * 加入需要扫描的包地址，防止出现找不到注入对象的问题
 */
@ComponentScan(basePackages = {"com.bowen.*"})
@SpringBootApplication
public class BoboServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(BoboServiceApplication.class, args);
    }
}

总结

• ComponentScan做的事情就是告诉Spring从哪里找到bean
• 如果你的其他包都在使用了@SpringBootApplication注解的main app所在的包及其下级包，则你什么都不用做，SpringBoot会自动帮你把其他包都扫描了
• 如果你有一些bean所在的包，不在main app的包及其下级包，那么你需要手动加上@ComponentScan注解并指定那个bean所在的包