起步依赖分析

分析spring-boot-starter-parent

按住Ctrl点击pom.xml中的spring-boot-starter-parent，跳转到了spring-boot-starter-parent的pom.xml，xml配置如下（只摘抄了部分重点配置）：

<parent>

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

  <artifactId>spring-boot-dependencies</artifactId>

  <version>2.2.5.RELEASE</version>

  <relativePath>../../spring-boot-dependencies</relativePath>

</parent>

按住Ctrl点击pom.xml中的spring-boot-starter-dependencies，跳转到了spring-boot-starter-dependencies的pom.xml，xml配置如下（只摘抄了部分重点配置）：

<properties>

  <activemq.version>5.15.11</activemq.version>

  <antlr2.version>2.7.7</antlr2.version>

  <appengine-sdk.version>1.9.78</appengine-sdk.version>

  <artemis.version>2.10.1</artemis.version>

  <aspectj.version>1.9.5</aspectj.version>

  <assertj.version>3.13.2</assertj.version>

  <atomikos.version>4.0.6</atomikos.version>

  <awaitility.version>4.0.2</awaitility.version>

  <bitronix.version>2.1.4</bitronix.version>

  <build-helper-maven-plugin.version>3.0.0</build-helper-maven-plugin.version>

  <byte-buddy.version>1.10.8</byte-buddy.version>

  ...  ....  ...

  <dependencyManagement>

  <dependencies>

    <dependency>

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

      <artifactId>spring-boot</artifactId>

      <version>2.2.5.RELEASE</version>

    </dependency>

    <dependency>

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

      <artifactId>spring-boot-test</artifactId>

      <version>2.2.5.RELEASE</version>

    </dependency>

    ... ... ...

    </dependencies>

</dependencyManagement>

<build>

  <pluginManagement>

    <plugins>

      <plugin>

        <groupId>org.apache.johnzon</groupId>

        <artifactId>johnzon-maven-plugin</artifactId>

        <version>${johnzon.version}</version>

      </plugin>

      <plugin>

        <groupId>org.jetbrains.kotlin</groupId>

        <artifactId>kotlin-maven-plugin</artifactId>

        <version>${kotlin.version}</version>

      </plugin>

      <plugin>

        <groupId>org.jooq</groupId>

        <artifactId>jooq-codegen-maven</artifactId>

        <version>${jooq.version}</version>

      </plugin>

      <plugin>

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

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

        <version>2.2.5.RELEASE</version>

      </plugin>

            ... ... ...

        </plugins>

      </pluginManagement>

</build>

从上面的spring-boot-starter-dependencies的pom.xml中我们可以发现，一部分坐标的版本、依赖管理、插件管理已经定义好，所以我们的SpringBoot工程继承spring-boot-starter-parent后已经具备版本锁定等配置了。所以起步依赖的作用就是进行依赖的传递。

分析spring-boot-starter-web

按住Ctrl点击pom.xml中的spring-boot-starter-web，跳转到了spring-boot-starter-web的pom.xml，xml配置如下（只摘抄了部分重点配置）：

<parent>

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

  <artifactId>spring-boot-starters</artifactId>

  <version>2.2.5.RELEASE</version>

</parent>

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

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

<version>2.2.5.RELEASE</version>

<name>Spring Boot Web Starter</name>

 <dependencies>

    <dependency>

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

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

      <version>2.2.5.RELEASE</version>

      <scope>compile</scope>

    </dependency>

    <dependency>

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

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

      <version>2.2.5.RELEASE</version>

      <scope>compile</scope>

    </dependency>

    <dependency>

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

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

      <version>2.2.5.RELEASE</version>

      <scope>compile</scope>

    </dependency>

    <dependency>

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

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

      <version>2.2.5.RELEASE</version>

      <scope>compile</scope>

      <exclusions>

        <exclusion>

          <artifactId>tomcat-embed-el</artifactId>

          <groupId>org.apache.tomcat.embed</groupId>

        </exclusion>

      </exclusions>

    </dependency>

    <dependency>

      <groupId>org.springframework</groupId>

      <artifactId>spring-web</artifactId>

      <version>5.2.4.RELEASE</version>

      <scope>compile</scope>

    </dependency>

    <dependency>

      <groupId>org.springframework</groupId>

      <artifactId>spring-webmvc</artifactId>

      <version>5.2.4.RELEASE</version>

      <scope>compile</scope>

    </dependency>

  </dependencies>

</project>

自动配置原理解析

按住Ctrl点击查看启动类MySpringBootApplication上的注解@SpringBootApplication

package xyz.ytfs;

import org.springframework.boot.SpringApplication;

import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication

public class SpringbootQuick2Application {

    public static void main(String[] args) {

        SpringApplication.run(SpringbootQuick2Application.class, args);

    }

}
/*

 * Copyright 2012-2019 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

 *

 *      https://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;

import java.lang.annotation.Documented;

import java.lang.annotation.ElementType;

import java.lang.annotation.Inherited;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

import org.springframework.boot.SpringBootConfiguration;

import org.springframework.boot.context.TypeExcludeFilter;

import org.springframework.context.annotation.Bean;

import org.springframework.context.annotation.ComponentScan;

import org.springframework.context.annotation.ComponentScan.Filter;

import org.springframework.context.annotation.Configuration;

import org.springframework.context.annotation.FilterType;

import org.springframework.core.annotation.AliasFor;

import org.springframework.data.repository.Repository;

/**

 * Indicates a {@link Configuration configuration} class that declares one or more

 * {@link Bean @Bean} methods and also triggers {@link EnableAutoConfiguration

 * auto-configuration} and {@link ComponentScan component scanning}. This is a convenience

 * annotation that is equivalent to declaring {@code @Configuration},

 * {@code @EnableAutoConfiguration} and {@code @ComponentScan}.

 *

 * @author Phillip Webb

 * @author Stephane Nicoll

 * @author Andy Wilkinson

 * @since 1.2.0

 */

@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 {

   /**

    * Exclude specific auto-configuration classes such that they will never be applied.

    * @return the classes to exclude

    */

   @AliasFor(annotation = EnableAutoConfiguration.class)

   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

    */

   @AliasFor(annotation = EnableAutoConfiguration.class)

   String[] excludeName() default {};

       ... ... ...

}

其中，

@SpringBootConfiguration：等同与@Configuration，既标注该类是Spring的一个配置类

@EnableAutoConfiguration：SpringBoot自动配置功能开启

按住Ctrl点击查看注解@EnableAutoConfiguration

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class)

public @interface EnableAutoConfiguration {

    ... ... ...

}

其中，@Import(AutoConfigurationImportSelector.class) 导入了AutoConfigurationImportSelector类

按住Ctrl点击查看AutoConfigurationImportSelector源码

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());

}

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;

}

其中，SpringFactoriesLoader.loadFactoryNames 方法的作用就是从META-INF/spring.factories文件中读取指定类对应的类名称列表

spring.factories 文件中有关自动配置的配置信息如下：

... ... ...

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,\

... ... ...

上面配置文件存在大量的以Configuration为结尾的类名称，这些类就是存有自动配置信息的类，而SpringApplication在获取这些类名后再加载

我们以ServletWebServerFactoryAutoConfiguration为例来分析源码：

@Configuration(proxyBeanMethods = false)

@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)

@ConditionalOnClass(ServletRequest.class)

@ConditionalOnWebApplication(type = Type.SERVLET)

@EnableConfigurationProperties(ServerProperties.class)

@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,

      ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,

      ServletWebServerFactoryConfiguration.EmbeddedJetty.class,

      ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })

public class ServletWebServerFactoryAutoConfiguration {

    ... ... ...

    }

其中，

@EnableConfigurationProperties(ServerProperties.class) 代表加载ServerProperties服务器配置属性类

进入ServerProperties.class源码如下：

@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)

public class ServerProperties {

   /**

    * Server HTTP port.

    */

   private Integer port;

   /**

    * Network address to which the server should bind.

    */

   private InetAddress address;;

      ... ... ...

}

其中，

prefix = "server" 表示SpringBoot配置文件中的前缀，SpringBoot会将配置文件中以server开始的属性映射到该类的字段中。映射关系如下

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