Spring系列-Spring boot启动原理

我们上一篇文章新建了一个Spring boot的项目：spring-boot-test，稍微做了一些配置就运行起来了。和以前运行普通的项目相比，Spring boot最大的特点就是没有那一堆的配置文件，而且不用配置Tomcat，直接就可以运行了。那Spring boot做了什么，可以让我们的开发变得这么方便呢？

还是要回到我们的项目中，我们的pom中引入了spring-boot-starter-web的依赖：

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

这个依赖就会引入内置的Tomcat，还有spring-webmvc。

接下来再看启动类：

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

上一篇文章说过，@SpringBootApplication是个组合注解，最主要的就是组合了@Configuration、@EnableAutoConfiguration、@ComponentScan这三个注解。

@Configuration声明当前类是一个配置类，相当于xml文件。@ComponentScan的功能是自动扫描需要装配的类，并自动装配到Spring的Bean容器中。
最重要的就是@EnableAutoConfiguration这个注解了，这个注解是进行自动配置的。我们启动Spring Boot项目的时候，这个注解就要起作用，根据我们引入的jar包，来自动的进行一些配置，省去了我们手动配置的麻烦。比如我们引入spring-boot-starter-web的依赖，就会自动的配置web项目中所需要的默认配置。

下面看下@EnableAutoConfiguration这个注解：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 * @return the classes to exclude
	 */
	Class<?>[] exclude() default {};

	/**
	 * Exclude specific auto-configuration class names such that they will never be
	 * applied.
	 * @return the class names to exclude
	 * @since 1.3.0
	 */
	String[] excludeName() default {};

}

在EnableAutoConfiguration中还定义了两个属性，exclude和excludeName，用来排除一些自动配置的类。

这里最关键的就是@Import(EnableAutoConfigurationImportSelector.class)。
@Import的作用是导入一个或多个配置类。
这里导入了EnableAutoConfigurationImportSelector，这是EnableAutoConfigurationImportSelector的图：

EnableAutoConfigurationImportSelector实现了ImportSelector接口，

public interface ImportSelector {

	/**
	 * Select and return the names of which class(es) should be imported based on
	 * the {@link AnnotationMetadata} of the importing @{@link Configuration} class.
	 */
	String[] selectImports(AnnotationMetadata importingClassMetadata);

}

ImportSelector接口中定义了selectImports方法，从这个方法的注释也可以看到，这个方法是用来扫描并返回需要导入的类的。
EnableAutoConfigurationImportSelector实现了selectImports方法：

@Override
public String[] selectImports(AnnotationMetadata metadata) {
	try {
		AnnotationAttributes attributes = getAttributes(metadata);
		List<String> configurations = getCandidateConfigurations(metadata,
				attributes);
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(metadata, attributes);
		configurations.removeAll(exclusions);
		configurations = sort(configurations);
		recordWithConditionEvaluationReport(configurations, exclusions);
		return configurations.toArray(new String[configurations.size()]);
	}
	catch (IOException ex) {
		throw new IllegalStateException(ex);
	}
}

selectImports中，就会去获取自动配置类，这里最重要的一行就是getCandidateConfigurations方法：

/**
 * Return the auto-configuration class names that should be considered. By default
 * this method will load candidates using {@link SpringFactoriesLoader} with
 * {@link #getSpringFactoriesLoaderFactoryClass()}.
 * @param metadata the source metadata
 * @param attributes the {@link #getAttributes(AnnotationMetadata) annotation
 * attributes}
 * @return a list of candidate configurations
 */
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
		AnnotationAttributes attributes) {
	return SpringFactoriesLoader.loadFactoryNames(
			getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
}

这个getCandidateConfigurations方法，就会返回auto-configuration类。是通过SpringFactoriesLoader.loadFactoryNames来实现的：

/**
 * Load the fully qualified class names of factory implementations of the
 * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
 * class loader.
 * @param factoryClass the interface or abstract class representing the factory
 * @param classLoader the ClassLoader to use for loading resources; can be
 * {@code null} to use the default
 * @see #loadFactories
 * @throws IllegalArgumentException if an error occurs while loading factory names
 */
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
	String factoryClassName = factoryClass.getName();
	try {
		Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
				ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
		List<String> result = new ArrayList<String>();
		while (urls.hasMoreElements()) {
			URL url = urls.nextElement();
			Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
			String factoryClassNames = properties.getProperty(factoryClassName);
			result.addAll(Arrays.asList(StringUtilsmaDelimitedListToStringArray(factoryClassNames)));
		}
		return result;
	}
	catch (IOException ex) {
		throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
				"] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
	}
}

这个方法里就会读取classpath下所有的META-INF/spring.factories文件中的配置，然后返回一个字符串类型的数组。
我们在spring-boot-autoconfigure-1.3.6.RELEASE.jar包的META-INF路径下，就可以找到spring.factories这个文件：

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.logging.AutoConfigurationReportLoggingInitializer

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

# 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.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
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.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
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.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
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.jms.hornetq.HornetQAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.velocity.VelocityAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration

# 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.velocity.VelocityTemplateAvailabilityProvider,\
org.springframework.boot.autoconfigure.web.JspTemplateAvailabilityProvider

这里面就定义了很多自动配置的类，把这些信息读取到并返回之后，就会把这些xxxAutoConfiguration注入Spring 容器中。
这写xxxAutoConfiguration就是自动配置的类，当满足条件的时候，就会自动配置我们的项目。

所以，现在就大体了解了Spring Boot自动配置的原理了，在我们项目启动的时候，因为有了@EnableAutoConfiguration注解，所以会从classpath下搜索所有的META-INF/spring.factories文件，加载里面的xxxAutoConfiguration，然后根据我们引入的jar包触发的条件，来对项目进行自动配置。