Spring Boot原理分析
阿新 • • 發佈：2020-07-05
一、起步依賴原理分析
1. spring-boot-starter-parent（版本控制）
按住Ctrl點選pom.xml中的spring-boot-starter-parent，跳轉到了spring-boot-starter-parent的pom.xml，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點選spring-boot-starter-dependencies，跳轉到了spring-boot-starter-dependencies的pom.xml，xml配置如下（只摘抄了部分重點配置）：
<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>
從上面的spring-boot-starter-dependencies的pom.xml中我們可以發現，一部分版本的座標、依賴管理、外掛管理已經定義好，所以繼承spring-boot-starter-parent使得Spring Boot工程具有了版本控制的功能。
起步依賴的作用就是進行依賴的傳遞。
2. spring-boot-starter-web（web開發）
按住Ctrl點選pom.xml中的spring-boot-starter-web，跳轉到了spring-boot-starter-web的pom.xml，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中我們可以發現，spring-boot-starter-web就是將web開發要使用的spring-web、spring-webmvc等座標進行了“打包”，這樣我們的工程只要引入spring-boot-starter-web起步依賴的座標就可以進行web開發了，同樣體現了依賴傳遞的作用。
二、自動配置原理分析
按住Ctrl點選檢視啟動類MySpringBootApplication上的註解@SpringBootApplication
@SpringBootApplication
public class MySpringBootApplication {
public static void main(String[] args) {
SpringApplication.run(MySpringBootApplication.class);
}
}
註解@SpringBootApplication的原始碼
@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，
@SpringBootConfiguration：等同與@Configuration，即標註該類是Spring的一個配置類
@EnableAutoConfiguration：SpringBoot自動配置功能開啟
按住Ctrl點選檢視註解@EnableAutoConfiguration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
... ... ...
}
其中，@Import(AutoConfigurationImportSelector.class) 匯入了AutoConfigurationImportSelector類
按住Ctrl點選檢視AutoConfigurationImportSelector原始碼
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，SpringFactoriesLoader.loadFactoryNames 方法的作用就是從META-INF/spring.factories檔案中讀取指定類對應的類名稱列表
spring.factories 檔案中有關自動配置的配置資訊如下：
... ... ...
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,\
... ... ...
上面配置檔案存在大量的以Configuration為結尾的類名稱，這些類就是存有自動配置資訊的類，而SpringApplication在獲取這些類名後再載入。
我們以ServletWebServerFactoryAutoConfiguration為例來分析原始碼：
@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，
@EnableConfigurationProperties(ServerProperties.class) 代表載入ServerProperties伺服器配置屬性類
進入ServerProperties.class原始碼如下：
@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配置檔案中的字首，SpringBoot會將配置檔案中以server開始的屬性對映到該類的欄位中。對映關係如下：