Spring中Interface与对应Impl Bean扫描冲突问题

最近在做一个SpringBoot的脚手架，熟悉Spring的各种特性与一些第三方工具的使用。在做到Interface与Impl注入到Controller调用的时候报Bean冲突问题。

问题重现

定义一个Interface文件 – GreetingService

package com.zackku.service.hello.service;

import ...

public interface GreetingService {
    List<Greeting> findGreats(String content, Integer offset, Integer rows);
}

对应的Impl实现 – GreetingServiceImpl

package com.zackku.service.hello.service.impl;

import ...

@Service
public class GreetingServiceImpl implements GreetingService {
    private static final String TEMPLATE = "this is %s!";

    private GreetingMapper greetingMapper;

    @Autowired
    public GreetingServiceImpl(GreetingMapper greetingMapper) {
        this.greetingMapper = greetingMapper;
    }

    @Override
    public List<Greeting> findGreats(String content, Integer offset, Integer rows) {
        List<Greeting> greets = greetingMapper.find(content, offset, rows);
        return greets;
    }
}

注入调用方 – GreetingController

package com.zackku.api.hello.controller;

import ...
@RestController
@RequestMapping("/greeting")
public class GreetingController {
    private GreetingService greetingService;

    @Autowired
    public GreetingController(GreetingService greetService) {
        this.greetingService = greetService;
    }

    @RequestMapping("/findList")
    public List<Greeting> find(@RequestParam(value = "content") String content,
                               @RequestParam(value = "offset", defaultValue = "0") Integer offset,
                               @RequestParam(value = "rows", defaultValue = "2") Integer rows) {
        List<Greeting> greets = greetingService.findGreats(content, offset, rows);
        return greets;
    }
}

以上代码在项目启动时候会报错

Parameter 0 of constructor in com.zackku.api.hello.controller.GreetingController required a single bean, but 2 were found:
- greetingServiceImpl: defined in file […/com/zackku/service/hello/service/impl/GreetingServiceImpl.class]
- greetingService: defined in file […/com/zackku/service/hello/service/GreetingService.class]

原因分析

居然Service的Interface和Impl会冲突？百思不得其解，况且我的Interface也没写注入的注解，即使写了也不应该会冲突。因为Spring的接口注入规则就是只会注入对应接口实现，除非实现有多个。
问题找了好久，最后Debug源码发现，在GreetingController 中 greetingService.findGreats的调用居然被MapperProxy代理。这样的话大概猜想到是GreetingService被多次扫描注册了。
回看组件扫描的配置

package com.zackku.api;
import ...
@ComponentScan(basePackages = {"com.zackku"})
@MapperScan(basePackages = {"com.zackku.service"})
@EnableAutoConfiguration
@EnableCaching
public class ApiApplication extends SpringBootServletInitializer {
    public static void main(String args[]) {
        SpringApplication.run(ApiApplication.class, args);
    }
}

再看看MapperScan的扫描原理：MapperScan在指定的basePackages下，扫描所有的interface并注册。

原来MapperScan并没有想象中那么智能去扫指定的Mapper。当ComponentScan与MapperScan扫的路径有交集的时候，里面的interface会被重复的注册使用。最终导致报Bean冲突错误。

解决方法

MapperScan注解有一个markerInterface的参数，

The scanner will register all interfaces in the base package that also have
the specified interface class as a parent.

也就是说，MapperScan可以只扫描继承该类的接口
具体如下：

1. 新增一个MapperInterface接口,空的就行

   public interface MapperInterface {}

2. GreetingMapper继承该接口

   public interface GreetingMapper extends MapperInterface {}

以后新增的Mapper也要继承该接口

1. 修改扫描配置

   @MapperScan(basePackages = {"com.zackku.service"}, markerInterface = MapperInterface.class)

这样就完美解决问题了。

源码参考

Java web 项目脚手架： https://github.com/Zack-Ku/java-web-scaffold