Spring Boot是使用SpringApplication引導專案啟動，SpringApplication類為我們引導專案提供了一種便利的方式——通過main()方法直接啟動。大多數情況下，我們可以把專案啟動這個任務直接委託給SpringApplication.run方法。

Spring boot關於自動配置的原始碼在spring-boot-autoconfigure-x.x.x.x.jar中，主要包含了如下圖所示的配置（並未截全）：

執行原理

在第一次使用spring boot的時候，大家都會驚訝於@SpringBootApplication這個註解，有了它馬上就能夠讓整個應用跑起來。實際上它只是一個組合註解，包含@Configuration、@EnableAutoConfiguration、@ComponentScan這三個註解。

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Configuration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication {
    Class<?>[] exclude() default {};
    String[] excludeName() default {};
    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackages"
 

    )
    String[] scanBasePackages() default {};
    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackageClasses"
    )
    Class<?>[] scanBasePackageClasses() default {};
}

它的核心功能是由@EnableAutoConfiguration這個註解提供的，我們來看看@EnableAutoConfiguration的原始碼：

@Target({ElementType.TYPE})
@Retention
 
(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({EnableAutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
    Class<?>[] exclude() default {};
    String[] excludeName() default {};
}

這裡的關鍵功能是@Import註解匯入的配置功能 

EnableAutoConfigurationImportSelector使用SpringFactoriesLoader.loadFactoryNames方法來掃描具有META-INF/spring.factories檔案的jar包，spring-boot-autoconfigure-x.x.x.x.jar裡就有一個spring.factories檔案，這個檔案中聲明瞭有哪些要自動配置。

以MongoDB為例

/*
 * Copyright 2012-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.boot.autoconfigure.mongo;

import java.net.UnknownHostException;

import javax.annotation.PreDestroy;

import com.mongodb.MongoClient;
import com.mongodb.MongoClientOptions;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;

/**
 * {@link EnableAutoConfiguration Auto-configuration} for Mongo.
 *
 * @author Dave Syer
 * @author Oliver Gierke
 * @author Phillip Webb
 */
@Configuration
@ConditionalOnClass(MongoClient.class)
@EnableConfigurationProperties(MongoProperties.class)
@ConditionalOnMissingBean(type = "org.springframework.data.mongodb.MongoDbFactory")
public class MongoAutoConfiguration {

	private final MongoProperties properties;

	private final MongoClientOptions options;

	private final Environment environment;

	private MongoClient mongo;

	public MongoAutoConfiguration(MongoProperties properties,
			ObjectProvider<MongoClientOptions> options, Environment environment) {
		this.properties = properties;
		this.options = options.getIfAvailable();
		this.environment = environment;
	}

	@PreDestroy
	public void close() {
		if (this.mongo != null) {
			this.mongo.close();
		}
	}

	@Bean
	@ConditionalOnMissingBean
	public MongoClient mongo() throws UnknownHostException {
		this.mongo = this.properties.createMongoClient(this.options, this.environment);
		return this.mongo;
	}

}