SpringBoot之@EnableAutoConfiguration

@EnableAutoConfiguration

首先Spring Boot项目中都会如下启动类：

@SpringBootApplication  
public class Application {  
    public static void main(String[] args) {  
        SpringApplication.run(Application.class, args);  
    }  
} 

从上面代码可以看出，注解@SpringBootApplication和SpringApplication.run()方法是最为重要的部分。这里主要来看看@SpringBootApplication注解部分。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
    ...
}

虽然定义使用了多个Annotation进行了原信息标注，但实际上重要的只有三个Annotation：

1. @Configuration（@SpringBootConfiguration点开查看发现里面还是应用了@Configuration。根据Spring文档， @ SpringBootConfiguration只是Spring标准@Configuration批注的替代方法。 两者之间的唯一区别是@SpringBootConfiguration允许自动找到配置）
2. @EnableAutoConfiguration（启用 SpringBoot 的自动配置机制）
3. @ComponentScan（扫描被@Component (@Service,@Controller)注解的 bean，注解默认会扫描该类所在的包下所有的类）

如果在启动类使用这个三个注解，整个SpringBoot应用依然可以与之前的启动类功能一样。但每次写这3个比较啰嗦，所以写一个@SpringBootApplication方便点。

这三个注解中@Configuration和@ComponentScan对我们来说并不陌生，今天我们的主角是@EnableAutoConfiguration。

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class<?>[] exclude() default {};

    String[] excludeName() default {};
}

　　其中最关键的要属@Import(AutoConfigurationImportSelector.class)，借助AutoConfigurationImportSelector，@EnableAutoConfiguration可以帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot创建并使用的IoC容器。

　　@EnableAutoConfiguration默认会扫描与项目启动类同级或者更低级目录下的所有使用@Configuration注解的类，如果我们想要避开某些配置类的扫描（包括避开一些第三方jar包下面的配置），
我们可以进行如下操作：

@EnableAutoConfiguration(exclude = {
        org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration.class
})

　　借助于Spring框架原有的一个工具类：SpringFactoriesLoader的支持，@EnableAutoConfiguration可以智能的自动配置功效才得以大功告成！

　　在AutoConfigurationImportSelector类中可以看到通过 SpringFactoriesLoader.loadFactoryNames()
　　把 spring-boot-autoconfigure.jar/META-INF/spring.factories中每一个xxxAutoConfiguration文件都加载到容器中，spring.factories文件里每一个xxxAutoConfiguration文件一般都会有下面的条件注解:

• @ConditionalOnClass ： classpath中存在该类时起效
• @ConditionalOnMissingClass ： classpath中不存在该类时起效
• @ConditionalOnBean ： DI容器中存在该类型Bean时起效
• @ConditionalOnMissingBean ： DI容器中不存在该类型Bean时起效
• @ConditionalOnSingleCandidate ： DI容器中该类型Bean只有一个或@Primary的只有一个时起效
• @ConditionalOnExpression ： SpEL表达式结果为true时
• @ConditionalOnProperty ： 参数设置或者值一致时起效
• @ConditionalOnResource ： 指定的文件存在时起效
• @ConditionalOnJndi ： 指定的JNDI存在时起效
• @ConditionalOnJava ： 指定的Java版本存在时起效
• @ConditionalOnWebApplication ： Web应用环境下起效
• @ConditionalOnNotWebApplication ： 非Web应用环境下起效

...
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
...

springboot默认加载了很多开源技术

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.autoconfigure.BackgroundPreinitializer

# Auto Configuration Import Listeners
org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener

# Auto Configuration Import Filters
org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
org.springframework.boot.autoconfigure.condition.OnClassCondition,\
org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityRequestMatcherProviderAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

# Failure analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer

# Template availability providers
org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

SpringFactoriesLoader

　　SpringFactoriesLoader属于Spring框架私有的一种扩展方案(类似于Java的SPI方案java.util.ServiceLoader)，其主要功能就是从指定的配置文件META-INF/spring-factories加载配置，spring-factories是一个典型的java properties文件，只不过Key和Value都是Java类型的完整类名。

　　对于@EnableAutoConfiguration来说，SpringFactoriesLoader的用途稍微不同一些，其本意是为了提供SPI扩展的场景，而在@EnableAutoConfiguration场景中，它更多提供了一种配置查找的功能支持，即根据@EnableAutoConfiguration的完整类名org.springframework.boot.autoconfig.EnableAutoConfiguration作为查找的Key，获得对应的一组@Configuration类。

　　SpringFactoriesLoader是一个抽象类，类中定义的静态属性定义了其加载资源的路径public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"，此外还有三个静态方法：

• loadFactories：加载指定的factoryClass并进行实例化。
• loadFactoryNames：加载指定的factoryClass的名称集合。
• instantiateFactory：对指定的factoryClass进行实例化。

在loadFactories方法中调用了loadFactoryNames以及instantiateFactory方法。

public static <T> List<T> loadFactories(Class<T> factoryClass, ClassLoader classLoader) {
   Assert.notNull(factoryClass, "'factoryClass' must not be null");
   ClassLoader classLoaderToUse = classLoader;
   if (classLoaderToUse == null) {
      classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
   }
   List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
   if (logger.isTraceEnabled()) {
      logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
   }
   List<T> result = new ArrayList<T>(factoryNames.size());
   for (String factoryName : factoryNames) {
      result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
   }
   AnnotationAwareOrderComparator.sort(result);
   return result;
}

loadFactories方法首先获取类加载器，然后调用loadFactoryNames方法获取所有的指定资源的名称集合、接着调用instantiateFactory方法实例化这些资源类并将其添加到result集合中。最后调用AnnotationAwareOrderComparator.sort方法进行集合的排序。

一个例子

上面介绍了很多原理的知识，下面结合一个例子来加深理解，例子展示的是当项目启动时如果某个类存在就自动配置这个Bean，并且这个属性可以在application.properties中配置

新建一个Maven项目,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.chm.test</groupId>
    <artifactId>spring-boot-starter-hello</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>jar</packaging>

    <name>spring-boot-starter-hello</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>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
            <version>2.0.4.RELEASE</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <version>2.0.4.RELEASE</version>
            <optional>true</optional>
        </dependency>
    </dependencies>
</project>

项目目录结构如下：

Hello.java

public class Hello {
    private String msg;

    public String sayHello() {
        return "hello " + msg;
    }
    public String getMsg() {
        return msg;
    }
    public void setMsg(String msg) {
        this.msg = msg;
    }
}

HelloProperties.java

@ConfigurationProperties(prefix = "hello") //获取属性值
public class HelloProperties {
    private static final String MSG = "world";
    private String msg = MSG ;
    public String getMsg() {
        return msg;
    }
    public void setMsg(String msg) {
        this.msg = msg;
    }
}

HelloAutoConfiguration.java

@Configuration
//为带有@ConfigurationProperties注解的Bean提供有效的支持。
// 这个注解可以提供一种方便的方式来将带有@ConfigurationProperties注解的类注入为Spring容器的Bean。
@EnableConfigurationProperties(HelloProperties.class)//开启属性注入,通过@autowired注入
@ConditionalOnClass(Hello.class)//判断这个类是否在classpath中存在，如果存在，才会实例化一个Bean
// The Hello bean will be created if the hello.enable property exists and has a value other than false
// or the property doesn't exist at all.
@ConditionalOnProperty(prefix="hello", value="enabled", matchIfMissing = true)
public class HelloAutoConfiguration {

    @Autowired
    private HelloProperties helloProperties;

    @Bean
    @ConditionalOnMissingBean(Hello.class)//容器中如果没有Hello这个类,那么自动配置这个Hello
    public Hello hello() {
        Hello hello = new Hello();
        hello.setMsg(helloProperties.getMsg());
        return hello;
    }

}

spring.factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
com.chm.test.HelloAutoConfiguration

最后使用mvn package将上面项目打包，使用mvn install:install-file命令将打包文件上传到本地Maven仓库进行测试。下面再新建一个Maven项目用于测试。

pom.xml文件

<?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.chm.test</groupId>
  <artifactId>test-starter</artifactId>
  <version>1.0-SNAPSHOT</version>

  <name>test-starter</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>
    <boot.version>2.0.4.RELEASE</boot.version>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>com.chm.test</groupId>
      <artifactId>spring-boot-starter-hello</artifactId>
      <version>0.0.1-SNAPSHOT</version>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
      <version>${boot.version}</version>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-test</artifactId>
      <version>${boot.version}</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

App.java

@SpringBootApplication
@RestController
public class App {

    @Autowired
    private Hello hello;

    @RequestMapping("/")
    public String index() {
        return hello.sayHello();
    }

    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
}

application.properties

#可以不配置
hello.enabled=true
hello.msg=charmingfst

#以debug模式运行
debug=true

以debug模式运行，可以看到我们的配置：

启动项目，打开浏览器，运行结果如下：

 

 

https://blog.csdn.net/zxc123e/article/details/80222967