spring boot遵循“约定由于配置”的原则，使用annotation对一些常规的配置项做默认配置，减少或不使用xml配置，让你的项目快速运行起来。spring boot的神奇不是借助代码的生成来实现的，而是通过条件注解来实现的。

　　自动配置AutoConfiguration是实现spring boot的重要原理，理解AutoConfiguration的运行原理特别重要，自己写一个AutoConfiguration可以加深我们对spring boot的理解。

1、定义Type-safe Configuration Properties

@ConfigurationProperties("author")//1
public class AuthorPropertis {
    private static final String NAME = "pyj";

    private static final String PWD = "12324";

    private String name = NAME;//2

    private String pwd = PWD;

    private String[] arrayProps;//3

    private Properties properties = new Properties();//4

    private List<Map<String, Object>> listProp1 = new ArrayList();//5

    private List<String> listProp2 = new ArrayList();//6

    private Map<String, Object> mapProps = new HashMap();//7 

    getter and setter...
}

1、@ConfigurationProperties映射application.yml以author为前缀的配置

2、author.name的默认值是pyj

3-7：对于容器对象的属性注入

2、定义一个简单的bean

public class HelloService {

    private String msg;

　　getter and setter...

 }

注意：不用纠结HelloService的内容，它就是一个简单的bean。

3、定义AutoConfiguration

@Configuration//1
@ConditionalOnClass(HelloService.class)//2
@EnableConfigurationProperties(AuthorPropertis.class)//3
@ConditionalOnProperty(prefix = "author",value = "enabled",matchIfMissing = true)//4
public class AuthorAutoConfiguration {

    private final AuthorPropertis authorPropertis;

    public AuthorAutoConfiguration(AuthorPropertis authorPropertis) {//5
        this.authorPropertis = authorPropertis;
    }

    @Bean//6
    @ConditionalOnMissingBean(HelloService.class)//7
    public HelloService helloService() throws JsonProcessingException {
        HelloService helloService = new HelloService();
        helloService.setMsg(authorPropertis.getName() +" :" + authorPropertis.getPwd());

        ObjectMapper objectMapper = new ObjectMapper();
        System.out.println("arrayProps: " + objectMapper.writeValueAsString(authorPropertis.getArrayProps()));
        System.out.println("listProp1: " + objectMapper.writeValueAsString(authorPropertis.getListProp1()));
        System.out.println("listProp2: " + objectMapper.writeValueAsString(authorPropertis.getListProp2()));
        System.out.println("mapProps: " + objectMapper.writeValueAsString(authorPropertis.getMapProps()));
        System.out.println("Props: " + objectMapper.writeValueAsString(authorPropertis.getProperties()));
        return helloService;
    }
}

1、@Configuration声明为一个配置类

2、@ConditionalOnClass表示HelloService在类路径下的时候调用

3、@EnableConfigurationProperties使用AuthorPropertis的配置

4、@ConditionalOnProperty指定的author是否有默认值

5、注入AuthorPropertis

6-7、当不存在HelloService的bean时候，新建一个HelloService的bean

4、在spring.factories注册

org.springframework.boot.autoconfigure.EnableAutoConfiguration=  com.github.gin.springboot.config.AuthorAutoConfiguration

　　在resources新建META-INF目录，再新建spring.factories，注册上AuthorAutoConfiguration，让spring boot知道为你自动配置。

　　如果不明白这个的原理，可以看看@EnableAutoConfiguration通过@Import({EnableAutoConfigurationImportSelector.class})来读取spring.factories

5、配置文件application.yml

debug: true

author:
  name: pyj
  pwd: 1234
  properties:
    reasonable: ‘true‘
    returnPageInfo: ‘true‘
  listProp1:
    - name: abc
      value: sgaw
    - name: efg
      value: sagsag
  listProp2:
    - config2Value1
    - config2Vavlue2
  mapProps:
    reasonable: true
    returnPageInfo: true
  arrayProps: 1,2,3,4,5

6、测试

@RestController
public class TestController {

    @Autowired
    HelloService helloService;

    @RequestMapping("/")
    public String index(){
        return "hello world!";
    }

    @RequestMapping("/hello")
    public String hello(){
        return helloService.getMsg();
    }
}

运行mvn spring-boot:run，如果看到结果，那么证明是没问题的

　　上面的AuthorPropertis 、HelloService 、AuthorAutoConfiguration 可以作为一个独立的starter pom为其他项目提供一个通用的服务。虽然spring boot覆盖了大部分的使用场景，但也不是全部都有，例如mybatis就没有spring boot的自动配置，所以我们可以利用AutoConfiguration起到同样的效果。

　　实例代码地址：有爱自取