4、Spring Boot 2.x 自动配置原理

时间：2021-07-07 06:23:21

1.4 Spring Boot 自动配置原理

简介

spring boot自动配置功能可以根据不同情况来决定spring配置应该用哪个，不应该用哪个，举个例子：

  • Spring的JdbcTemplate是不是在Classpath里面？如果是，并且DataSource也存在，就自动配置一个JdbcTemplate的Bean
  • Thymeleaf是不是在Classpath里面？如果是，则自动配置Thymeleaf的模板解析器、视图解析器、模板引擎

那个这个是怎么实现的呢？原因就在于它利用了Spring的条件化配置，条件化配置允许配置存在于应用中，但是在满足某些特定条件前会忽略这些配置。

要实现条件化配置我们要用到@Conditional条件化注解。

本篇从如下几个方面进行展开：

  1. @Conditional注解
  2. spring boot 的条件化配置详解
  3. spring boot 自动配置源码分析
  4. 自己动手实现spring boot starter pom

1.4.1 @Conditional 注解

@Conditional是由Spring 4提供的一个新特性，用于根据特定条件来控制Bean的创建行为。

1.4.2 Spring Boot 的条件化配置详解

在spring boot项目中会存在一个名为spring-boot-autoconfigure的jar包，条件化配置就是在这个jar里面实现的，它用到了如下的条件化注解，这些注解都是以@ConditionalOn开头的，他们都是应用了@Conditional的组合注解：

  • @ConditionalOnBean ------------------------ 配置了某个特定的Bean
  • @ConditionalMissingBean ------------------- 没有配置特定的Bean
  • @ConditionalOnCalss ----------------------- Classpath里有指定的类
  • @ConditionalOnMissingCalss ---------------- Classpath缺少指定的类
  • @ConditionalOnExpression ------------------ 给定的Spring Expression Language(SpEL)表达式计算
  • @ConditionalOnJava ------------------------ Java的版本匹配特定值或者一个范围值
  • @ConditionalOnProperty -------------------- 指定的配置属性要有一个明确的值
  • @ConditionalOnResource -------------------- ClassPath里有指定的资源
  • @ConditionalOnWebApplication -------------- 这是一个Web应用程序
  • @ConditionalOnNotWebApplication ----------- 这不是一个Web应用程序

以JdbcTemplateAutoConfiguration为例，它里面有这段代码：

@Bean

@Primary

@ConditionalOnMissingBean(JdbcOperations.class)

public JdbcTemplate jdbcTemplate() {

    return new JdbcTemplate(this.dataSource);

}

只有在不存在JdbcOperations（如果查看JdbcTemplate的源码，你会发现JdbcTemplate类实现了JdbcOperations接口）实例的时候，才会初始化一个JdbcTemplate 的Bean。

1.4.3 Spring Boot 自动配置源码分析

spring boot项目的启动类用的注解--@SpringBootApplication是一个组合注解，其中@EnableAutoConfiguration是自动配置相关的。

查看@EnableAutoConfiguration注解源码（本文基于spring-boot-starter-parent 2.1.0.RELEASE）：

@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 {};

}

而这个@EnableAutoConfiguration注解里面有个@Import注解导入了EnableAutoConfigurationImportSelector用来实现具体的功能。

进入AutoConfigurationImportSelector.java里面有个方法selectImports（）调用了方法getAutoConfigurationEntry()，然后getAutoConfigurationEntry()方法又调用了getCandidateConfigurations（），进而调用了SpringFactoriesLoader.loadFactoryNames（）方法

@Override

	public String[] selectImports(AnnotationMetadata annotationMetadata) {

		if (!isEnabled(annotationMetadata)) {

			return NO_IMPORTS;

		}

		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader

				.loadMetadata(this.beanClassLoader);

		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(

				autoConfigurationMetadata, annotationMetadata);

		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());

	}

	/**

	 * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}

	 * of the importing {@link Configuration @Configuration} class.

	 * @param autoConfigurationMetadata the auto-configuration metadata

	 * @param annotationMetadata the annotation metadata of the configuration class

	 * @return the auto-configurations that should be imported

	 */

	protected AutoConfigurationEntry getAutoConfigurationEntry(

			AutoConfigurationMetadata autoConfigurationMetadata,

			AnnotationMetadata annotationMetadata) {

		if (!isEnabled(annotationMetadata)) {

			return EMPTY_ENTRY;

		}

		AnnotationAttributes attributes = getAttributes(annotationMetadata);

		List<String> configurations = getCandidateConfigurations(annotationMetadata,

				attributes);

		configurations = removeDuplicates(configurations);

		Set<String> exclusions = getExclusions(annotationMetadata, attributes);

		checkExcludedClasses(configurations, exclusions);

		configurations.removeAll(exclusions);

		configurations = filter(configurations, autoConfigurationMetadata);

		fireAutoConfigurationImportEvents(configurations, exclusions);

		return new AutoConfigurationEntry(configurations, exclusions);

	}


protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,

			AnnotationAttributes attributes) {

		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(

				getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());

		Assert.notEmpty(configurations,

				"No auto configuration classes found in META-INF/spring.factories. If you "

						+ "are using a custom packaging, make sure that file is correct.");

		return configurations;

	}

在SpringFactoriesLoader.loadFactoryNames（）方法里面，会调用loadSpringFactories（），然后查询META-INF/spring.factories这个配置文件

    // 所要查询的资源文件

	public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

	public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {

		String factoryClassName = factoryClass.getName();

		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());

	}

	private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {

		MultiValueMap<String, String> result = cache.get(classLoader);

		if (result != null) {

			return result;

		}

		try {

			Enumeration<URL> urls = (classLoader != null ?

					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :

					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));

			result = new LinkedMultiValueMap<>();

			while (urls.hasMoreElements()) {

				URL url = urls.nextElement();

				UrlResource resource = new UrlResource(url);

				Properties properties = PropertiesLoaderUtils.loadProperties(resource);

				for (Map.Entry<?, ?> entry : properties.entrySet()) {

					String factoryClassName = ((String) entry.getKey()).trim();

					for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {

						result.add(factoryClassName, factoryName.trim());

					}

				}

			}

			cache.put(classLoader, result);

			return result;

		}

		catch (IOException ex) {

			throw new IllegalArgumentException("Unable to load factories from location [" +

					FACTORIES_RESOURCE_LOCATION + "]", ex);

		}

	}

SpringFactoriesLoader属于Spring框架私有的一种扩展方案，其主要功能就是从指定的配置文件META-INF/spring.factories加载配置。

SpringFactoriesLoader.loadFactoryNames方法会扫描具有META-INF/spring.factories文件的jar包，而我们的spring-boot-autoconfigure.jar里面就有一个这样的文件，此文件中声明了具体有哪些自动配置：

# 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

我们上面提到的JdbcTemplateAutoConfiguration自动配置类就在里面。

1.4.4 自己动手实现spring boot starter pom

接下来，我们就来写一个简单的spring boot starter pom。

  1. 新建starter maven项目 spring-boot-starter-autoconfigure
  2. 修改pom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.hw</groupId>

    <artifactId>spring-boot-starter-autoconfigure</artifactId>

    <version>0.0.1-SNAPSHOT</version>

    <packaging>jar</packaging>

    <name>spring-boot-starter-autoconfigure</name>

    <description>Demo project for Spring Boot Autoconfigure</description>

    <properties>

        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>

        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>

        <java.version>1.8</java.version>

    </properties>

    <dependencies>

        <!-- 这里需要引入spring boot的自动配置作为依赖 -->

        <dependency>

            <groupId>org.springframework.boot</groupId>

            <artifactId>spring-boot-autoconfigure</artifactId>

            <version>2.1.0.RELEASE</version>

        </dependency>

    </dependencies>

    <build>

        <plugins>

            <plugin>

                <groupId>org.springframework.boot</groupId>

                <artifactId>spring-boot-maven-plugin</artifactId>

            </plugin>

        </plugins>

    </build>

</project>
  1. 属性配置 HelloServiceProperties.java


package com.hw.springbootstarterautoconfigure.conifg;

import org.springframework.boot.context.properties.ConfigurationProperties;

/**

 * @Description TODO

 * @Author hw

 * @Date 2018/11/18 22:57

 * @Version 1.0

 */

/**

 * @ConfigurationProperties 自动匹配application.properties文件中hello.msg的值，然后赋值给类属性msg，这里的msg默认值为“spring boot”

 */

@ConfigurationProperties(prefix = "hello")

public class HelloServiceProperties {

    private static final String MSG = "spring boot";

    private String msg = MSG;

    public String getMsg() {

        return msg;

    }

    public void setMsg(String msg) {

        this.msg = msg;

    }

}
  1. 判定依据类HelloService.java，后面的代码会依据此类是否存在，来决定是否生产对应的Bean
package com.hw.springbootstarterautoconfigure.conifg;

/**

 * @Description TODO

 * @Author hw

 * @Date 2018/11/18 23:00

 * @Version 1.0

 */

public class HelloService {

    private String msg;

    public String getMsg() {

        return msg;

    }

    public void setMsg(String msg) {

        this.msg = msg;

    }

    public String sayHello() {

        return "hello " + msg;

    }

}
  1. 自动配置类 HelloServiceAutoConfiguration.java，根据HelloServiceProperties提供的参数，并通过@ConditionalOnClass(HelloService.class)判定HelloService这个类在Classpath中是否存在，存在并且还没有对应的Bean，就生成对应的helloService Bean
package com.hw.springbootstarterautoconfigure.conifg;

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

import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;

import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;

import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;

import org.springframework.boot.context.properties.EnableConfigurationProperties;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.Configuration;

/**

 * @Description TODO

 * @Author hw

 * @Date 2018/11/18 23:01

 * @Version 1.0

 */

@Configuration

@EnableConfigurationProperties(HelloServiceProperties.class)

@ConditionalOnClass(HelloService.class)

@ConditionalOnProperty(prefix = "hello", matchIfMissing = true, value = "enabled")

public class HelloServiceAutoConfiguration {

    @Autowired

    HelloServiceProperties helloServiceProperties;

    @Bean

    @ConditionalOnMissingBean(HelloService.class)

    public HelloService helloService() {

        HelloService service = new HelloService();

        service.setMsg(helloServiceProperties.getMsg());

        return service;

    }

}
  1. 注册配置，需要到META-INF/spring.factories文件中注册改自动配置类：在src/main/source目录下新建该文件，然后进行配置。
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\

com.hw.springbootstarterautoconfigure.conifg.HelloServiceAutoConfiguration
  1. 实现controller HelloController.java
package com.hw.springbootstarterautoconfigure.web;

import com.hw.springbootstarterautoconfigure.conifg.HelloService;

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

import org.springframework.web.bind.annotation.RequestMapping;

import org.springframework.web.bind.annotation.RestController;

/**

 * @Description TODO

 * @Author hw

 * @Date 2018/11/18 23:07

 * @Version 1.0

 */

@RestController

public class HelloController {

    /**

     * 代码中没有配置这个helloService Bean，但是自动配置能够帮忙实例化，因此可以直接注入

     */

    @Autowired

    HelloService helloService;

    @RequestMapping(value = "/helloService")

    public String sayHello() {

        return helloService.sayHello();

    }

}
  1. 启动项目 访问/helloService 接口。 得到 hello spring boot
  2. 在application.properties里面配置hello.msg=hw，然后再次访问/helloService接口。得到 hello hw
  3. 本文完整代码：Spring Boot自动配置

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