SpringBoot源码学习之路（一、Hello World案例探究之依赖版本管理、启动器和主启动类）

深藏阁楼爱情的钟 2022-05-19 04:05 64阅读 0赞

## 一、Spring Boot 简介 ##

> 简化Spring应用开发的一个框架；
> 
> 整个Spring技术栈的一个大整合；
> 
> J2EE开发的一站式解决方案；

## 二、Spring Boot HelloWorld案例 ##

需求：浏览器发送hello请求，服务器接受请求并处理，响应Hello World字符串。  
实现流程如下：

#### 1、创建一个maven工程；（jar打包方式） ####

#### 2、导入spring boot相关的依赖 ####

<parent>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-parent</artifactId>
            <version>1.5.9.RELEASE</version>
        </parent>
        <dependencies>
            <dependency>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-starter-web</artifactId>
            </dependency>
        </dependencies>

#### 3、编写一个主程序；启动Spring Boot应用 ####

/** * @SpringBootApplication 来标注一个主程序类，说明这是一个Spring Boot应用 */
    @SpringBootApplication
    public class HelloWorldMainApplication { 
    
        public static void main(String[] args) {
    
            // Spring应用启动起来
            SpringApplication.run(HelloWorldMainApplication.class,args);
        }
    }

#### 4、编写相关的Controller、Service ####

@Controller
    public class HelloController { 
    
        @ResponseBody
        @RequestMapping("/hello")
        public String hello(){
            return "Hello World!";
        }
    }

#### 5、运行主程序测试 ####

#### 6、简化部署（加入springboot maven插件） ####

<build>
            <plugins>
                <plugin>
                    <groupId>org.springframework.boot</groupId>
                    <artifactId>spring-boot-maven-plugin</artifactId>
                </plugin>
            </plugins>
        </build>

maven的package命令会自动调用该依赖进行打包。  
将这个应用打成jar包，直接使用java -jar的命令进行执行即可。

## 三、Hello World案例探究 ##

#### 1、pom.xml中的标签中的父项目 ####

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.5.9.RELEASE</version>
    </parent>

查看spring-boot-dependencies-1.5.9.RELEASE.pom，发现其还有父项目：

<parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-dependencies</artifactId>
        <version>1.5.9.RELEASE</version>
        <relativePath>../../spring-boot-dependencies</relativePath>
    </parent>

再继续查看spring-boot-dependencies-1.5.9.RELEASE.pom发现内有大量版本配置信息如下：

<properties>
        <activemq.version>5.15.4</activemq.version>
        <antlr2.version>2.7.7</antlr2.version>
        <appengine-sdk.version>1.9.64</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>
        <caffeine.version>2.6.2</caffeine.version>
        <cassandra-driver.version>3.4.0</cassandra-driver.version>
        <classmate.version>1.3.4</classmate.version>
        <commons-codec.version>1.11</commons-codec.version>
        <commons-dbcp2.version>2.2.0</commons-dbcp2.version>
        <commons-lang3.version>3.7</commons-lang3.version>
        <commons-pool.version>1.6</commons-pool.version>
        <commons-pool2.version>2.5.0</commons-pool2.version>
        <couchbase-cache-client.version>2.1.0</couchbase-cache-client.version>
        <couchbase-client.version>2.5.9</couchbase-client.version>
        <derby.version>10.14.1.0</derby.version>
        <dom4j.version>1.6.1</dom4j.version>
        <dropwizard-metrics.version>3.2.6</dropwizard-metrics.version>
        <ehcache.version>2.10.5</ehcache.version>
        <ehcache3.version>3.5.2</ehcache3.version>
        <elasticsearch.version>5.6.10</elasticsearch.version>
        <embedded-mongo.version>2.0.3</embedded-mongo.version>
        <exec-maven-plugin.version>1.5.0</exec-maven-plugin.version>
        <flatten-maven-plugin.version>1.0.0</flatten-maven-plugin.version>
        <flyway.version>5.0.7</flyway.version>
        <freemarker.version>2.3.28</freemarker.version>
        <git-commit-id-plugin.version>2.2.3</git-commit-id-plugin.version>
        <glassfish-el.version>3.0.0</glassfish-el.version>
        <groovy.version>2.4.15</groovy.version>
        <gson.version>2.8.5</gson.version>
        <h2.version>1.4.197</h2.version>
        <hamcrest.version>1.3</hamcrest.version>
        <hazelcast.version>3.9.4</hazelcast.version>
        <hazelcast-hibernate5.version>1.2.3</hazelcast-hibernate5.version>
        <hibernate.version>5.2.17.Final</hibernate.version>
        <hibernate-jpa-2.1-api.version>1.0.2.Final</hibernate-jpa-2.1-api.version>
        <hibernate-validator.version>6.0.10.Final</hibernate-validator.version>
        <hikaricp.version>2.7.9</hikaricp.version>
        <hsqldb.version>2.4.1</hsqldb.version>
        <htmlunit.version>2.29</htmlunit.version>
        <httpasyncclient.version>4.1.3</httpasyncclient.version>
        <httpclient.version>4.5.5</httpclient.version>
        <httpcore.version>4.4.9</httpcore.version>
        <infinispan.version>9.1.7.Final</infinispan.version>
        <influxdb-java.version>2.9</influxdb-java.version>
        <jackson.version>2.9.6</jackson.version>
        <janino.version>3.0.8</janino.version>
        <javax-annotation.version>1.3.2</javax-annotation.version>
        <javax-cache.version>1.1.0</javax-cache.version>
        <javax-jaxb.version>2.3.0</javax-jaxb.version>
        <javax-jms.version>2.0.1</javax-jms.version>
        <javax-json.version>1.1.2</javax-json.version>
        <javax-jsonb.version>1.0</javax-jsonb.version>
        <javax-mail.version>1.6.1</javax-mail.version>
        <javax-money.version>1.0.3</javax-money.version>
        <javax-transaction.version>1.2</javax-transaction.version>
        <javax-validation.version>2.0.1.Final</javax-validation.version>
        <jaxen.version>1.1.6</jaxen.version>
        <jaybird.version>3.0.4</jaybird.version>
        <jboss-logging.version>3.3.2.Final</jboss-logging.version>
        <jboss-transaction-spi.version>7.6.0.Final</jboss-transaction-spi.version>
        <jdom2.version>2.0.6</jdom2.version>
        <jedis.version>2.9.0</jedis.version>
        <jersey.version>2.26</jersey.version>
        <jest.version>5.3.3</jest.version>
        <jetty.version>9.4.11.v20180605</jetty.version>
        <jetty-el.version>8.5.24.2</jetty-el.version>
        <jetty-jsp.version>2.2.0.v201112011158</jetty-jsp.version>
        <jmustache.version>1.14</jmustache.version>
        <jna.version>4.5.1</jna.version>
        <joda-time.version>2.9.9</joda-time.version>
        <johnzon-jsonb.version>1.1.7</johnzon-jsonb.version>
        <jolokia.version>1.5.0</jolokia.version>
        <jooq.version>3.10.7</jooq.version>
        <jsonassert.version>1.5.0</jsonassert.version>
        <json-path.version>2.4.0</json-path.version>
        <jstl.version>1.2</jstl.version>
        <jtds.version>1.3.1</jtds.version>
        <junit.version>4.12</junit.version>
        <junit-jupiter.version>5.1.1</junit-jupiter.version>
        <junit-platform.version>1.1.0</junit-platform.version>
        <kafka.version>1.0.1</kafka.version>
        <kotlin.version>1.2.41</kotlin.version>
        <lettuce.version>5.0.4.RELEASE</lettuce.version>
        <liquibase.version>3.5.5</liquibase.version>
        <log4j2.version>2.10.0</log4j2.version>
        <logback.version>1.2.3</logback.version>
        <lombok.version>1.16.22</lombok.version>
        <mariadb.version>2.2.5</mariadb.version>
        <maven-antrun-plugin.version>1.8</maven-antrun-plugin.version>
        <maven-assembly-plugin.version>3.1.0</maven-assembly-plugin.version>
        <maven-clean-plugin.version>3.0.0</maven-clean-plugin.version>
        <maven-compiler-plugin.version>3.7.0</maven-compiler-plugin.version>
        <maven-dependency-plugin.version>3.0.2</maven-dependency-plugin.version>
        <maven-deploy-plugin.version>2.8.2</maven-deploy-plugin.version>
        <maven-eclipse-plugin.version>2.10</maven-eclipse-plugin.version>
        <maven-enforcer-plugin.version>3.0.0-M1</maven-enforcer-plugin.version>
        <maven-failsafe-plugin.version>2.21.0</maven-failsafe-plugin.version>
        <maven-help-plugin.version>2.2</maven-help-plugin.version>
        <maven-install-plugin.version>2.5.2</maven-install-plugin.version>
        <maven-invoker-plugin.version>3.1.0</maven-invoker-plugin.version>
        <maven-jar-plugin.version>3.0.2</maven-jar-plugin.version>
        <maven-javadoc-plugin.version>3.0.0</maven-javadoc-plugin.version>
        <maven-resources-plugin.version>3.0.1</maven-resources-plugin.version>
        <maven-shade-plugin.version>2.4.3</maven-shade-plugin.version>
        <maven-site-plugin.version>3.6</maven-site-plugin.version>
        <maven-source-plugin.version>3.0.1</maven-source-plugin.version>
        <maven-surefire-plugin.version>2.21.0</maven-surefire-plugin.version>
        <maven-war-plugin.version>3.1.0</maven-war-plugin.version>
        <micrometer.version>1.0.5</micrometer.version>
        <mockito.version>2.15.0</mockito.version>
        <mongodb.version>3.6.4</mongodb.version>
        <mongo-driver-reactivestreams.version>1.7.1</mongo-driver-reactivestreams.version>
        <mssql-jdbc.version>6.2.2.jre8</mssql-jdbc.version>
        <mysql.version>5.1.46</mysql.version>
        <narayana.version>5.8.2.Final</narayana.version>
        <nekohtml.version>1.9.22</nekohtml.version>
        <neo4j-ogm.version>3.1.0</neo4j-ogm.version>
        <netty.version>4.1.25.Final</netty.version>
        <nio-multipart-parser.version>1.1.0</nio-multipart-parser.version>
        <postgresql.version>42.2.2</postgresql.version>
        <quartz.version>2.3.0</quartz.version>
        <querydsl.version>4.1.4</querydsl.version>
        <rabbit-amqp-client.version>5.1.2</rabbit-amqp-client.version>
        <reactive-streams.version>1.0.2</reactive-streams.version>
        <reactor-bom.version>Bismuth-SR10</reactor-bom.version>
        <rest-assured.version>3.0.7</rest-assured.version>
        <rxjava.version>1.3.8</rxjava.version>
        <rxjava2.version>2.1.14</rxjava2.version>
        <rxjava-adapter.version>1.2.1</rxjava-adapter.version>
        <selenium.version>3.9.1</selenium.version>
        <selenium-htmlunit.version>2.29.3</selenium-htmlunit.version>
        <sendgrid.version>4.1.2</sendgrid.version>
        <servlet-api.version>3.1.0</servlet-api.version>
        <simple-json.version>1.1.1</simple-json.version>
        <slf4j.version>1.7.25</slf4j.version>
        <snakeyaml.version>1.19</snakeyaml.version>
        <solr.version>6.6.4</solr.version>
        <spring.version>5.0.7.RELEASE</spring.version>
        <spring-amqp.version>2.0.4.RELEASE</spring-amqp.version>
        <spring-batch.version>4.0.1.RELEASE</spring-batch.version>
        <spring-cloud-connectors.version>2.0.2.RELEASE</spring-cloud-connectors.version>
        <spring-data-releasetrain.version>Kay-SR8</spring-data-releasetrain.version>
        <spring-hateoas.version>0.24.0.RELEASE</spring-hateoas.version>
        <spring-integration.version>5.0.6.RELEASE</spring-integration.version>
        <spring-kafka.version>2.1.7.RELEASE</spring-kafka.version>
        <spring-ldap.version>2.3.2.RELEASE</spring-ldap.version>
        <spring-plugin.version>1.2.0.RELEASE</spring-plugin.version>
        <spring-restdocs.version>2.0.1.RELEASE</spring-restdocs.version>
        <spring-retry.version>1.2.2.RELEASE</spring-retry.version>
        <spring-security.version>5.0.6.RELEASE</spring-security.version>
        <spring-session-bom.version>Apple-SR3</spring-session-bom.version>
        <spring-ws.version>3.0.1.RELEASE</spring-ws.version>
        <sqlite-jdbc.version>3.21.0.1</sqlite-jdbc.version>
        <statsd-client.version>3.1.0</statsd-client.version>
        <sun-mail.version>1.6.1</sun-mail.version>
        <thymeleaf.version>3.0.9.RELEASE</thymeleaf.version>
        <thymeleaf-extras-data-attribute.version>2.0.1</thymeleaf-extras-data-attribute.version>
        <thymeleaf-extras-java8time.version>3.0.1.RELEASE</thymeleaf-extras-java8time.version>
        <thymeleaf-extras-springsecurity4.version>3.0.2.RELEASE</thymeleaf-extras-springsecurity4.version>
        <thymeleaf-layout-dialect.version>2.3.0</thymeleaf-layout-dialect.version>
        <tomcat.version>8.5.31</tomcat.version>
        <unboundid-ldapsdk.version>4.0.6</unboundid-ldapsdk.version>
        <undertow.version>1.4.25.Final</undertow.version>
        <versions-maven-plugin.version>2.3</versions-maven-plugin.version>
        <webjars-hal-browser.version>3325375</webjars-hal-browser.version>
        <webjars-locator-core.version>0.35</webjars-locator-core.version>
        <wsdl4j.version>1.6.3</wsdl4j.version>
        <xml-apis.version>1.4.01</xml-apis.version>
        <xml-maven-plugin.version>1.0.1</xml-maven-plugin.version>
        <xmlunit2.version>2.5.1</xmlunit2.version>
    </properties>

由此可以看出spring-boot利用该项目管理应用中的所有依赖版本。  
spring-boot-dependencies是Spring Boot的版本仲裁中心，所以我们一般导入依赖默认都是不需要写版本的。  
当然如果没有在dependencies里面管理的依赖自然是需要声明版本号的。

#### 2、启动器 ####

Spring Boot将所有的功能场景都抽取出来，做成一个个的starters（启动器）,只需要在项目里面引入这些starter相关场景的所有依赖都会导入进来。要用什么功能就导入什么场景的启动器。

**spring-boot-starter-web** :  
spring-boot的web场景启动器；帮我们导入了web模块正常运行所依赖的组件；

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

#### 2、主程序类 ####

/** * @SpringBootApplication 来标注一个主程序类，说明这是一个Spring Boot应用 */
    @SpringBootApplication
    public class HelloWorldMainApplication { 
    
        public static void main(String[] args) {
            // Spring应用启动起来
            SpringApplication.run(HelloWorldMainApplication.class,args);
        }
    }

**@SpringBootApplication**:  
Spring Boot应用标注在某个类上说明这个类是SpringBoot的主配置类，SpringBoot就应该运行这个类的main方法来启动SpringBoot应用；  
查看**@SpringBootApplication**注解的源码如下：

@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 { 
        ........
    }

接下来探究下其下各个注解的作用：

--------------------

（1）**@SpringBootConfiguration注解**: Spring Boot的配置类，标注在某个类上，表示这是一个Spring Boot的配置类；

查看**@SpringBootConfiguration**源码内容：

@Target({ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Configuration//配置类上会标注这个注解；配置类可以理解为spring中的配置文件
    public @interface SpringBootConfiguration { 
    }

带有**@SpringBootConfiguration**注解的组件，也是容器中的一个组件。(**@SpringBootConfiguration**注解源码有**@Configuration注解**，**@Configuration注解**中源码有**@Component**)；

--------------------

（2）**@EnableAutoConfiguration注解**：开启自动配置功能；

以前我们需要配置的东西，Spring Boot帮我们自动配置；**@EnableAutoConfiguration**告诉SpringBoot开启自动配置功能；这样自动配置才能生效；

查看其源码：

@Target({ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage //自动配置包
    @Import({AutoConfigurationImportSelector.class})//Spring的底层注解@Import，给容器中导入一个组件；
    public @interface EnableAutoConfiguration { 
        String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
        Class<?>[] exclude() default {};
    
        String[] excludeName() default {};
    }

上面源码需要注意的是有引入一个 “导入组件选择器”，就是**@Import(\{AutoConfigurationImportSelector.class\})中的AutoConfigurationImportSelector**（导入组件选择器），作用是选择导入那些组件（实例）进入容器。其会将需要导入容器中的组件（实例）以全类名的方法返回，这些组件就会被添加到容器中。  
**AutoConfigurationImportSelector.class**源码中有这个这样的函数：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
            //将所有需要导入的组件以全类名的方式返回；断点后可以看到导入的组件都有哪些
            List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
        }

会给容器中导入非常多的自动配置类（xxxAutoConfiguration）；就是给容器中导入这个场景需要的所有组件，并配置好这些组件；  
![这里写图片描述][70]  
有了自动配置类，免去了我们手动编写配置注入功能组件等的工作；

Spring Boot在启动的时候从类路径下的META-INF/spring.factories中获取EnableAutoConfiguration指定的值，将这些值作为自动配置类导入到容器中，自动配置类就生效，帮我们进行自动配置工作；

“J2EE的整体整合解决方案”和”自动配置”都体现在spring-boot-autoconfigure-1.5.9.RELEASE.jar；可着重阅读这一块的源码。

--------------------

## 四、使用Spring Initializer快速创建Spring Boot项目 ##

IDE都支持使用Spring的项目创建向导快速创建一个Spring Boot项目；  
选择我们需要的模块；向导会联网创建Spring Boot项目；  
默认生成的Spring Boot项目；

**项目resources文件夹中的目录结构**：  
1. static：保存所有的静态资源； js、css、images；  
2. templates：保存所有的模板页面；（Spring Boot默认jar包使用嵌入式的Tomcat，默认不支持JSP页面）；可以使用模板 引擎（freemarker、thymeleaf）；  
3.application.properties：Spring Boot应用的配置文件；可以修改一些默认设置；

[70]: /images/20220519/f1af56320b844366bffbcb612ba2f503.png