SpringBoot開發筆記-(2) springboot原理初探-解析helloworld

上一節:SpringBoot開發筆記-(1) springboot簡介和helloworld

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2.1 父依賴: starter-parent

helloworld查看依賴: 項目依賴了 spring-boot-stater-parent:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
</dependency>

spring-boot-stater-parent點進去, 裏面還依賴了個parent:

<parent>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-dependencies</artifactId>
  <version>2.3.2.RELEASE</version>
</parent>

spring-boot-dependencies再點進去:

一大堆 properties

一大堆 dependencies

可見:

spring-boot-dependencies 纔是springboot 全部依賴真正管理者;

2.2 導入的依賴: starter-web

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId> 
</dependency>

點進去: spring-boot-starter-web幫咱們導入了web項目所依賴的依賴項:

<dependencies>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <version>2.3.2.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-json</artifactId>
    <version>2.3.2.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-tomcat</artifactId>
    <version>2.3.2.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>5.2.8.RELEASE</version>
    <scope>compile</scope>
  </dependency>
  <dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-webmvc</artifactId>
    <version>5.2.8.RELEASE</version>
    <scope>compile</scope>
  </dependency>
</dependencies>

1. spring-boot-starter
2. spring-boot-starter-json
3. spring-boot-starter-tomcat
4. spring-web
5. spring-webmvc

starter: spring boot幫助管理依賴資源;

spring提供了一系列starter: mail/web/jpa/mq...

2.3 @SpringBootApplication註解

註解類:

//....SpringBootApplication...
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration // 1
@EnableAutoConfiguration // 2
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

.....
    
//....SpringBootConfiguration...
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration    // 3
public @interface SpringBootConfiguration {
    
.....
    
//....Configuration...
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component // 4
public @interface Configuration {

主要註解類:

2.4 SpringBootApplication之@SpringBootConfiguration

SpringBootConfiguration能夠看到, 有 Configuration因此:

2.4.1 聲明Application類爲一個配置類

SpringBootApplication -> SpringBootConfiguration -> Configuration -> Component

註解了 @SpringBootApplication 就有了 @SpringBootConfiguration;

就有了 @Configuration 的功能; 即:此類是配置類!

就有了 @Component 的功能;

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration // 繼承了Component
public @interface SpringBootConfiguration {

上面說明: HelloApplication.java註解了@SpringBootApplication 它就是一個配置類, 能夠被 AnnotationConfigApplicationContext加載掃描起來!

2.5 SpringBootApplication之@EnableAutoConfiguration

按需自動注入spring.factories組件

2.5.1 EnableAutoConfiguration註解原理

1. 路徑: org.springframework.boot.autoconfigure.EnableAutoConfiguration

2. 此類所屬jar包 spring-boot-autoconfigure.jar 目錄下有 META-INF/spring.factories文件, 內容以下: 能夠這麼理解: 下面全部類都是配置類, 做用都相似於寫一個 @Configuration的類, 在其中初始化相應所須要的的 @Bean;

   # Initializers
   org.springframework.context.ApplicationContextInitializer=\
   org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
   org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener
   
   # Application Listeners
   org.springframework.context.ApplicationListener=\
   org.springframework.boot.autoconfigure.BackgroundPreinitializer
   
   # Auto Configuration Import Listeners
   org.springframework.boot.autoconfigure.AutoConfigurationImportListener=\
   org.springframework.boot.autoconfigure.condition.ConditionEvaluationReportAutoConfigurationImportListener
   
   # Auto Configuration Import Filters
   org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
   org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
   org.springframework.boot.autoconfigure.condition.OnClassCondition,\
   org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition
   
   # Auto Configure
   org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
   org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
   org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
   org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
   ............省略100多個....
   
   # Failure analyzers
   org.springframework.boot.diagnostics.FailureAnalyzer=\
   org.springframework.boot.autoconfigure.data.redis.RedisUrlSyntaxFailureAnalyzer,\
   org.springframework.boot.autoconfigure.diagnostics.analyzer.NoSuchBeanDefinitionFailureAnalyzer,\
   org.springframework.boot.autoconfigure.flyway.FlywayMigrationScriptMissingFailureAnalyzer,\
   org.springframework.boot.autoconfigure.jdbc.DataSourceBeanCreationFailureAnalyzer,\
   org.springframework.boot.autoconfigure.jdbc.HikariDriverConfigurationFailureAnalyzer,\
   org.springframework.boot.autoconfigure.r2dbc.ConnectionFactoryBeanCreationFailureAnalyzer,\
   org.springframework.boot.autoconfigure.session.NonUniqueSessionRepositoryFailureAnalyzer
   
   # Template availability providers
   org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider=\
   org.springframework.boot.autoconfigure.freemarker.FreeMarkerTemplateAvailabilityProvider,\
   org.springframework.boot.autoconfigure.mustache.MustacheTemplateAvailabilityProvider,\
   org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAvailabilityProvider,\
   org.springframework.boot.autoconfigure.thymeleaf.ThymeleafTemplateAvailabilityProvider,\
   org.springframework.boot.autoconfigure.web.servlet.JspTemplateAvailabilityProvider

2.5.2 EnableAutoConfiguration的繼承關係

//....EnableAutoConfiguration...
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage // 5
@Import(AutoConfigurationImportSelector.class) // 6
public @interface EnableAutoConfiguration {}

//....AutoConfigurationPackage...
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)     // 7
public @interface AutoConfigurationPackage {}


// ...Registrar...
// ...ImportBeanDefinitionRegistrar...
/**
 * {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
 * configuration.
 */
static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

    @Override
    public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
        register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
    }

    @Override
    public Set<Object> determineImports(AnnotationMetadata metadata) {
        return Collections.singleton(new PackageImports(metadata));
    }

}
// ImportBeanDefinitionRegistrar 能夠在spring容器bean初始化的時候注入組件到容器中

// 6 處是一個 ImportSelector

// 7 處是一個ImportBeanDefinitionRegistrar

這兩處都是往容器中註冊bean, 具體參見筆記:

spring註解驅動開發-(5) 向Spring容器中註冊組件的方法

ImportBeanDefinitionRegistrar 能夠在spring容器bean初始化的時候注入組件到容器中: 在這裏打斷點能夠看到: 會把 com.niewj.springboot 包目錄註冊進去; 這樣, 此包下全部子包中的spring註解的類就都會被掃描, 並初始化註冊到spring容器的上下文;

這樣的話, spring容器在啓動容器的時候, 就會按需加載 spring-boot-autoconfigure.jar包下的META-INF/spring.factories中配置的加載類; 達到自動配置注入bean到容器中的效果(目的就是方便開發, 不須要可憐的程序員一個一個寫@Configuration的類, 而後本身寫@Bean的方法註冊組件) 這種機制就是 SPI 深刻理解SPI機制