SpringBoot-03-原理分析(IEDA)

3.1 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>2.0.1.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>5.15.3</activemq.version>
    <antlr2.version>2.7.7</antlr2.version>
    <appengine-sdk.version>1.9.63</appengine-sdk.version>
    <artemis.version>2.4.0</artemis.version>
    <aspectj.version>1.8.13</aspectj.version>
    <assertj.version>3.9.1</assertj.version>
    <atomikos.version>4.0.6</atomikos.version>
    <bitronix.version>2.1.4</bitronix.version>
    <build-helper-maven-plugin.version>3.0.0</build-helper-maven-plugin.version>
    <byte-buddy.version>1.7.11</byte-buddy.version>
... ... ...
</properties>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot</artifactId>
<version>2.0.1.RELEASE</version>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-test</artifactId>
<version>2.0.1.RELEASE</version>
</dependency>
... ... ...
</dependencies>
</dependencyManagement>
<build>
<pluginManagement>
<plugins>
<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>2.0.1.RELEASE</version>
</plugin>
... ... ...
</plugins>
</pluginManagement>
</build>

3.2 分析spring-boot-starter-web

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。

<?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>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starters</artifactId>
        <version>1.5.9.RELEASE</version>
    </parent>
    <artifactId>spring-boot-starter-web</artifactId>
    <name>Spring Boot Web Starter</name>
    <description>Starter for building web, including RESTful, applications using Spring
        MVC. Uses Tomcat as the default embedded container</description>
    <url>http://projects.spring.io/spring-boot/</url>
    <organization>
        <name>Pivotal Software, Inc.</name>
        <url>http://www.spring.io</url>
    </organization>
    <properties>
        <main.basedir>${basedir}/../..</main.basedir>
    </properties>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
        </dependency>
        <dependency>
            <groupId>org.hibernate</groupId>
            <artifactId>hibernate-validator</artifactId>
        </dependency>
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-databind</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
        </dependency>
    </dependencies>
</project>

从上面的spring-boot-starter-web的pom.xml中我们可以发现，spring-boot-starter-web就是将web开发要使用的 spring-web、spring-webmvc等坐标进行了“打包”，这样我们的工程只要引入spring-boot-starter-web起步依赖的 坐标就可以进行web开发了，同样体现了依赖传递的作用。

3.3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 com.dj.springboot;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringBoot02ConfigApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBoot02ConfigApplication.class, args);
    }

}

注解@SpringBootApplication的源码

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 {
    @AliasFor(
        annotation = EnableAutoConfiguration.class,
        attribute = "exclude"
    )
    Class<?>[] exclude() default {};

    @AliasFor(
        annotation = EnableAutoConfiguration.class,
        attribute = "excludeName"
    )
    String[] excludeName() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackages"
    )
    String[] scanBasePackages() default {};

    @AliasFor(
        annotation = ComponentScan.class,
        attribute = "basePackageClasses"
    )
    Class<?>[] scanBasePackageClasses() 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

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

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

    String[] excludeName() default {};
}

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) {
　　... ... ...
　　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 StringUtils.toStringArray(configurations);
}
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,AnnotationAttributes attributes) {
　　List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
　　　　　getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()
　　);
　　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

 

 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
@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：