3. SpringBoot原理分析

1. 起步依賴原理分析

1.1 分析spring-boot-starter-parent

按住Ctrl點擊pom.xml中的spring-boot-starter-parent，跳轉(zhuǎn)到了spring-boot-starter-parent的pom.xml听绳，xml配置如下（只摘抄了部分重點配置）：

<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點擊pom.xml中的spring-boot-starter-dependencies亚隅，跳轉(zhuǎn)到了spring-boot-starter-dependencies的pom.xml，xml配置如下（只摘抄了部分重點配置）：

<!-- 依賴關(guān)系 -->
<dependencies>
    <!-- 此項目運行使用 junit旨巷，所以此項目依賴 junit -->
    <dependency>
        <!-- junit 的項目名稱 -->
        <groupId>junit</groupId>
        
        <!-- junit 的模塊名稱 -->
        <artifactId>junit</artifactId>
        
        <!-- junit 版本 -->
        <version>4.9</version>
        
        <!-- 依賴范圍：單元測試時使用 junit -->
        <scope>test</scope>
    </dependency>


<build>
    <plugins>
        <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-compiler-plugin</artifactId>
            <configuration>
                <source>1.8</source>
                <target>1.8</target>
                <encoding>UTF-8</encoding>
            </configuration>
        </plugin>
    </plugins>
</build>


<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>

從上面的spring-boot-starter-dependencies的pom.xml中我們可以發(fā)現(xiàn)，一部分坐標(biāo)的版本、依賴管理环葵、插件管理已經(jīng)定義好，所以我們的SpringBoot工程繼承spring-boot-starter-parent后已經(jīng)具備版本鎖定等配置了宝冕。所以起步依賴的作用就是進(jìn)行依賴的傳遞张遭，從而實現(xiàn)版本的控制。

1.2 分析spring-boot-starter-web

按住Ctrl點擊pom.xml中的spring-boot-starter-web地梨，跳轉(zhuǎn)到了spring-boot-starter-web的pom.xml菊卷，xml配置如下（只摘抄了部分重點配置）：

<?xml version="1.0" encoding="UTF-8"?>
<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0
http://maven.apache.org/xsd/maven-4.0.0.xsd"
xmlns="http://maven.apache.org/POM/4.0.0"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starters</artifactId>
        <version>2.0.1.RELEASE</version>
    </parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <version>2.0.1.RELEASE</version>
    <name>Spring Boot Web Starter</name>
    
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
            <version>2.0.1.RELEASE</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-json</artifactId>
            <version>2.0.1.RELEASE</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-tomcat</artifactId>
            <version>2.0.1.RELEASE</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.hibernate.validator</groupId>
            <artifactId>hibernate-validator</artifactId>
            <version>6.0.9.Final</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>5.0.5.RELEASE</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>5.0.5.RELEASE</version>
            <scope>compile</scope>
        </dependency>
    </dependencies>
</project>

從上面的spring-boot-starter-web的pom.xml中我們可以發(fā)現(xiàn)，spring-boot-starter-web就是將web開發(fā)要使用的spring-web宝剖、spring-webmvc等坐標(biāo)進(jìn)行了“打包”洁闰，這樣我們的工程只要引入spring-boot-starter-web起步依賴的坐標(biāo)就可以進(jìn)行web開發(fā)了，同樣體現(xiàn)了依賴傳遞的作用万细。

2. 自動配置原理解析

按住Ctrl點擊查看啟動類MySpringBootApplication上的注解@SpringBootApplication

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

注解@SpringBootApplication的源碼

@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 {};
    ... ... ...
}

其中扑眉，
@SpringBootConfiguration：等同與@Configuration，既標(biāo)注該類是Spring的一個配置類
@EnableAutoConfiguration：SpringBoot自動配置功能開啟

按住Ctrl點擊查看注解@EnableAutoConfiguration

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

其中赖钞，@Import(AutoConfigurationImportSelector.class) 導(dǎo)入了AutoConfigurationImportSelector類

按住Ctrl點擊查看AutoConfigurationImportSelector源碼

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;
}

其中腰素，SpringFactoriesLoader.loadFactoryNames 方法的作用就是從META-INF/spring.factories文件中讀取指定類對應(yīng)的類名稱列表

image.png

spring.factories 文件中有關(guān)自動配置的配置信息如下：

... ... ...
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConf
iguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration
,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfigu
ration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
... ... ...

上面配置文件存在大量的以Configuration為結(jié)尾的類名稱，這些類就是存有自動配置信息的類仁烹，而SpringApplication在獲取這些類名后再加載

我們以ServletWebServerFactoryAutoConfiguration為例來分析源碼：

@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 {
    ... ... ...
}

其中耸弄，
@EnableConfigurationProperties(ServerProperties.class) 代表加載ServerProperties服務(wù)器配置屬性類
進(jìn)入ServerProperties.class源碼如下：

@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配置文件中的前綴卓缰，SpringBoot會將配置文件中以server開始的屬性映射到該類的字段中计呈。映射關(guān)系如下：