starter-* POM依赖

使用SpringBoot开发时，在pom.xml文件中引入的依赖一般都是形如spring-boot-starter-*。starter依赖是居于某个场景或者功能的，我们引入一个starter依赖之后，它会间接引入实现这个场景或功能所需的其他依赖。我们可以把这些starters称为场景启动器，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。这里以spring-boot-starter-web为例分析。

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

spring-boot-starter-web的间接依赖
我们来看下spring-boot-starter-web的pom文件，它定义了一个父类spring-boot-starters

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starters</artifactId>
        <version>1.5.4.RELEASE</version>
    </parent>
    <artifactId>spring-boot-starter-web</artifactId>
    <name>Spring Boot Web Starter</name>
    <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>

spring-boot-starters的打包类型为pom，它定义好了SpringBoot中所有的starter，同时它的父类为spring-boot-parent

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-parent</artifactId>
        <version>1.5.4.RELEASE</version>
        <relativePath>../spring-boot-parent</relativePath>
    </parent>
    <artifactId>spring-boot-starters</artifactId>
    <packaging>pom</packaging>
    <name>Spring Boot Starters</name>
    <description>Spring Boot Starters</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>
    <modules>
        <module>spring-boot-starter</module>
        <module>spring-boot-starter-activemq</module>
        <module>spring-boot-starter-amqp</module>
        <module>spring-boot-starter-aop</module>
        <module>spring-boot-starter-artemis</module>
        <module>spring-boot-starter-batch</module>
        <module>spring-boot-starter-cache</module>
        <module>spring-boot-starter-cloud-connectors</module>
        <module>spring-boot-starter-data-cassandra</module>
        <module>spring-boot-starter-data-couchbase</module>
        <module>spring-boot-starter-data-elasticsearch</module>
        <module>spring-boot-starter-data-gemfire</module>
        <module>spring-boot-starter-data-jpa</module>
        <module>spring-boot-starter-data-ldap</module>
        <module>spring-boot-starter-data-mongodb</module>
        <module>spring-boot-starter-data-neo4j</module>
        <module>spring-boot-starter-data-redis</module>
        <module>spring-boot-starter-data-rest</module>
        <module>spring-boot-starter-data-solr</module>
        <module>spring-boot-starter-freemarker</module>
        <module>spring-boot-starter-groovy-templates</module>
        <module>spring-boot-starter-hateoas</module>
        <module>spring-boot-starter-integration</module>
        <module>spring-boot-starter-jdbc</module>
        <module>spring-boot-starter-jersey</module>
        <module>spring-boot-starter-jetty</module>
        <module>spring-boot-starter-jooq</module>
        <module>spring-boot-starter-jta-atomikos</module>
        <module>spring-boot-starter-jta-bitronix</module>
        <module>spring-boot-starter-jta-narayana</module>
        <module>spring-boot-starter-logging</module>
        <module>spring-boot-starter-log4j2</module>
        <module>spring-boot-starter-mail</module>
        <module>spring-boot-starter-mobile</module>
        <module>spring-boot-starter-mustache</module>
        <module>spring-boot-starter-actuator</module>
        <module>spring-boot-starter-parent</module>
        <module>spring-boot-starter-security</module>
        <module>spring-boot-starter-social-facebook</module>
        <module>spring-boot-starter-social-twitter</module>
        <module>spring-boot-starter-social-linkedin</module>
        <module>spring-boot-starter-remote-shell</module>
        <module>spring-boot-starter-test</module>
        <module>spring-boot-starter-thymeleaf</module>
        <module>spring-boot-starter-tomcat</module>
        <module>spring-boot-starter-undertow</module>
        <module>spring-boot-starter-validation</module>
        <module>spring-boot-starter-web</module>
        <module>spring-boot-starter-websocket</module>
        <module>spring-boot-starter-web-services</module>
    </modules>

spring-boot-parent的父pom为spring-boot-dependencies，该pom文件中定义了我们所需要具体jar包的version。

<properties>
        <!-- Dependency versions -->
        <activemq.version>5.14.5</activemq.version>
        <antlr2.version>2.7.7</antlr2.version>
        <appengine-sdk.version>1.9.53</appengine-sdk.version>
        <artemis.version>1.5.5</artemis.version>
        <aspectj.version>1.8.10</aspectj.version>
        <assertj.version>2.6.0</assertj.version>
        <atomikos.version>3.9.3</atomikos.version>
        <bitronix.version>2.1.4</bitronix.version>
        <caffeine.version>2.3.5</caffeine.version>
        <cassandra-driver.version>3.1.4</cassandra-driver.version>
        <classmate.version>1.3.3</classmate.version>
        <commons-beanutils.version>1.9.3</commons-beanutils.version>
        <commons-collections.version>3.2.2</commons-collections.version>
        <commons-codec.version>1.10</commons-codec.version>
        <commons-dbcp.version>1.4</commons-dbcp.version>
        <commons-dbcp2.version>2.1.1</commons-dbcp2.version>
        <commons-digester.version>2.1</commons-digester.version>
        <commons-pool.version>1.6</commons-pool.version>
        <commons-pool2.version>2.4.2</commons-pool2.version>
        <couchbase-client.version>2.3.7</couchbase-client.version>
        <couchbase-cache-client.version>2.1.0</couchbase-cache-client.version>
        <crashub.version>1.3.2</crashub.version>
        <derby.version>10.13.1.1</derby.version>
        <dom4j.version>1.6.1</dom4j.version>
        <dropwizard-metrics.version>3.1.4</dropwizard-metrics.version>
        <ehcache.version>2.10.4</ehcache.version>
        <ehcache3.version>3.2.2</ehcache3.version>
        <embedded-mongo.version>1.50.5</embedded-mongo.version>
        <flyway.version>3.2.1</flyway.version>
        <freemarker.version>2.3.26-incubating</freemarker.version>
        <elasticsearch.version>2.4.5</elasticsearch.version>
        <gemfire.version>8.2.4</gemfire.version>
        <glassfish-el.version>3.0.0</glassfish-el.version>
        <gradle.version>2.9</gradle.version>
        <groovy.version>2.4.11</groovy.version>
        <gson.version>2.8.0</gson.version>
        <h2.version>1.4.195</h2.version>
        <hamcrest.version>1.3</hamcrest.version>
        <hazelcast.version>3.7.7</hazelcast.version>
        <hazelcast-hibernate4.version>3.7.1</hazelcast-hibernate4.version>
        <hazelcast-hibernate5.version>1.1.3</hazelcast-hibernate5.version>
        <hibernate.version>5.0.12.Final</hibernate.version>
        <hibernate-validator.version>5.3.5.Final</hibernate-validator.version>
        <hikaricp.version>2.5.1</hikaricp.version>
        <hikaricp-java6.version>2.3.13</hikaricp-java6.version>
        <hikaricp-java7.version>2.4.11</hikaricp-java7.version>
        <hsqldb.version>2.3.5</hsqldb.version>
        <htmlunit.version>2.21</htmlunit.version>
        <httpasyncclient.version>4.1.3</httpasyncclient.version>
        <httpclient.version>4.5.3</httpclient.version>
        <httpcore.version>4.4.6</httpcore.version>
        <infinispan.version>8.2.6.Final</infinispan.version>
        <jackson.version>2.8.8</jackson.version>
        <janino.version>2.7.8</janino.version>
        <javassist.version>3.21.0-GA</javassist.version> <!-- Same as Hibernate -->
        <javax-cache.version>1.0.0</javax-cache.version>
        <javax-mail.version>1.5.6</javax-mail.version>
        <javax-transaction.version>1.2</javax-transaction.version>
        <javax-validation.version>1.1.0.Final</javax-validation.version>
        <jaxen.version>1.1.6</jaxen.version>
        <jaybird.version>2.2.13</jaybird.version>
        <jboss-logging.version>3.3.1.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>2.25.1</jersey.version>
        <jest.version>2.0.4</jest.version>
        <jetty.version>9.4.5.v20170502</jetty.version>
        <jetty-jsp.version>2.2.0.v201112011158</jetty-jsp.version>
        <jetty-el.version>8.0.33</jetty-el.version>
        <jms-api.version>1.1-rev-1</jms-api.version>
        <jmustache.version>1.13</jmustache.version>
        <jna.version>4.2.2</jna.version>
        <joda-time.version>2.9.9</joda-time.version>
        <jolokia.version>1.3.6</jolokia.version>
        <jooq.version>3.9.2</jooq.version>
        <json.version>20140107</json.version>
        <jsonassert.version>1.4.0</jsonassert.version>
        <json-path.version>2.2.0</json-path.version>
        <jstl.version>1.2</jstl.version>
        <jtds.version>1.3.1</jtds.version>
        <junit.version>4.12</junit.version>
        <liquibase.version>3.5.3</liquibase.version>
        <log4j2.version>2.7</log4j2.version>
        <logback.version>1.1.11</logback.version>
        <lombok.version>1.16.16</lombok.version>
        <mariadb.version>1.5.9</mariadb.version>
        <mssql-jdbc.version>6.1.0.jre7</mssql-jdbc.version>
        <mockito.version>1.10.19</mockito.version>
        <mongodb.version>3.4.2</mongodb.version>
        <mysql.version>5.1.42</mysql.version>
        <narayana.version>5.5.24.Final</narayana.version>
        <nekohtml.version>1.9.22</nekohtml.version>
        <neo4j-ogm.version>2.1.3</neo4j-ogm.version>
        <postgresql.version>9.4.1212.jre7</postgresql.version>
        <querydsl.version>4.1.4</querydsl.version>
        <reactor.version>2.0.8.RELEASE</reactor.version>
        <reactor-spring.version>2.0.7.RELEASE</reactor-spring.version>
        <selenium.version>2.53.1</selenium.version>
        <selenium-htmlunit.version>2.21</selenium-htmlunit.version>
        <sendgrid.version>2.2.2</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>1.17</snakeyaml.version>
        <solr.version>5.5.4</solr.version>
        <spock.version>1.0-groovy-2.4</spock.version>
        <spring.version>4.3.9.RELEASE</spring.version>
        <spring-amqp.version>1.7.3.RELEASE</spring-amqp.version>
        <spring-cloud-connectors.version>1.2.4.RELEASE</spring-cloud-connectors.version>
        <spring-batch.version>3.0.7.RELEASE</spring-batch.version>
        <spring-data-releasetrain.version>Ingalls-SR4</spring-data-releasetrain.version>
        <spring-hateoas.version>0.23.0.RELEASE</spring-hateoas.version>
        <spring-integration.version>4.3.10.RELEASE</spring-integration.version>
        <spring-integration-java-dsl.version>1.2.2.RELEASE</spring-integration-java-dsl.version>
        <spring-kafka.version>1.1.6.RELEASE</spring-kafka.version>
        <spring-ldap.version>2.3.1.RELEASE</spring-ldap.version>
        <spring-loaded.version>1.2.7.RELEASE</spring-loaded.version>
        <spring-mobile.version>1.1.5.RELEASE</spring-mobile.version>
        <spring-plugin.version>1.2.0.RELEASE</spring-plugin.version>
        <spring-restdocs.version>1.1.3.RELEASE</spring-restdocs.version>
        <spring-retry.version>1.2.0.RELEASE</spring-retry.version>
        <spring-security.version>4.2.3.RELEASE</spring-security.version>
        <spring-security-jwt.version>1.0.8.RELEASE</spring-security-jwt.version>
        <spring-security-oauth.version>2.0.14.RELEASE</spring-security-oauth.version>
        <spring-session.version>1.3.1.RELEASE</spring-session.version>
        <spring-social.version>1.1.4.RELEASE</spring-social.version>
        <spring-social-facebook.version>2.0.3.RELEASE</spring-social-facebook.version>
        <spring-social-linkedin.version>1.0.2.RELEASE</spring-social-linkedin.version>
        <spring-social-twitter.version>1.1.2.RELEASE</spring-social-twitter.version>
        <spring-ws.version>2.4.0.RELEASE</spring-ws.version>
        <sqlite-jdbc.version>3.15.1</sqlite-jdbc.version>
        <statsd-client.version>3.1.0</statsd-client.version>
        <sun-mail.version>${javax-mail.version}</sun-mail.version>
        <thymeleaf.version>2.1.5.RELEASE</thymeleaf.version>
        <thymeleaf-extras-springsecurity4.version>2.1.3.RELEASE</thymeleaf-extras-springsecurity4.version>
        <thymeleaf-extras-conditionalcomments.version>2.1.2.RELEASE</thymeleaf-extras-conditionalcomments.version>
        <thymeleaf-layout-dialect.version>1.4.0</thymeleaf-layout-dialect.version>
        <thymeleaf-extras-data-attribute.version>1.3</thymeleaf-extras-data-attribute.version>
        <thymeleaf-extras-java8time.version>2.1.0.RELEASE</thymeleaf-extras-java8time.version>
        <tomcat.version>8.5.15</tomcat.version>
        <undertow.version>1.4.15.Final</undertow.version>
        <unboundid-ldapsdk.version>3.2.1</unboundid-ldapsdk.version>
        <webjars-hal-browser.version>9f96c74</webjars-hal-browser.version>
        <webjars-locator.version>0.32-1</webjars-locator.version>
        <wsdl4j.version>1.6.3</wsdl4j.version>
        <xml-apis.version>1.4.01</xml-apis.version>
        <!-- Plugin versions -->
        <build-helper-maven-plugin.version>1.10</build-helper-maven-plugin.version>
        <exec-maven-plugin.version>1.5.0</exec-maven-plugin.version>
        <git-commit-id-plugin.version>2.2.2</git-commit-id-plugin.version>
        <maven-antrun-plugin.version>1.8</maven-antrun-plugin.version>
        <maven-assembly-plugin.version>2.6</maven-assembly-plugin.version>
        <maven-clean-plugin.version>2.6.1</maven-clean-plugin.version>
        <maven-compiler-plugin.version>3.1</maven-compiler-plugin.version>
        <maven-dependency-plugin.version>2.10</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>1.4</maven-enforcer-plugin.version>
        <maven-failsafe-plugin.version>2.18.1</maven-failsafe-plugin.version>
        <maven-install-plugin.version>2.5.2</maven-install-plugin.version>
        <maven-invoker-plugin.version>1.10</maven-invoker-plugin.version>
        <maven-help-plugin.version>2.2</maven-help-plugin.version>
        <maven-jar-plugin.version>2.6</maven-jar-plugin.version>
        <maven-javadoc-plugin.version>2.10.4</maven-javadoc-plugin.version>
        <maven-resources-plugin.version>2.7</maven-resources-plugin.version>
        <maven-shade-plugin.version>2.4.3</maven-shade-plugin.version>
        <maven-site-plugin.version>3.5.1</maven-site-plugin.version>
        <maven-source-plugin.version>2.4</maven-source-plugin.version>
        <maven-surefire-plugin.version>2.18.1</maven-surefire-plugin.version>
        <maven-war-plugin.version>2.6</maven-war-plugin.version>
        <versions-maven-plugin.version>2.2</versions-maven-plugin.version>
    </properties>

这就解释了springboot为我们定义好了依赖包的版本，在开发过程中jar包冲突是个常见且让人头痛的问题，而使用springboot的话由于其帮我们选择好了依赖包的版本，所以很好的解决了包冲突等繁琐问题。如果spring-boot-parent没有定义的jar包，而此时项目中需要使用的话，需要我们自己定义好版本。

程序入口

@SpringBootApplication
public class StartSpringBootMain {

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

我们在主程序类中加入@SpringBootApplication注解就能启用SpringBoot，下面就来看看这个注解类

@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 {
  ......
}

其中@ComponentScan是我们熟悉的组件扫描，其作用是使@Controller、@Service、@Componet等组件生效。这里它排除了两个Filter类TypeExcludeFilter和AutoConfigurationExcludeFilter。

@SpringBootConfiguration注解相当于@Configuration，表示应用该注解的类为配置类，相当于applicationContext.xml文件。

@Configuration
public @interface SpringBootConfiguration {
}

@EnableAutoConfiguration注解表示开启自动配置的功能，之前需要开发人员手动进行的配置，SpringBoot帮我们自动配置。它的源码如下

@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
  ......
}

@AutoConfigurationPackage自动配置包

@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

AutoConfigurationPackages.Registrar的源码如下

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata,
                BeanDefinitionRegistry registry) {
            register(registry, new PackageImport(metadata).getPackageName());
        }

new PackageImport(metadata).getPackageName()会获取主配置类（@SpringBootApplication标注的类）所在的包名，然后会将该包及下面所有子包里面的所有组件扫描到Spring容器中。
@Import注解的含义为给Spring容器中注入一个对象，@Import(EnableAutoConfigurationImportSelector.class)表示给Spring容器中注入EnableAutoConfigurationImportSelector对象，该对象的字面意思为“开启自动配置导入选择器”。同时它还继承自AutoConfigurationImportSelector类

@Deprecated
public class EnableAutoConfigurationImportSelector
        extends AutoConfigurationImportSelector {

AutoConfigurationImportSelector类的作用是将所有需要导入的组件以全类名的方式添加到容器中；其通过selectImports方法筛选出选哟导入的组件的全类名。

    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        try {
            AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                    .loadMetadata(this.beanClassLoader);
            AnnotationAttributes attributes = getAttributes(annotationMetadata);
  //这句获取自动配置类
            List<String> configurations = getCandidateConfigurations(annotationMetadata,
                    attributes);
            configurations = removeDuplicates(configurations);
            configurations = sort(configurations, autoConfigurationMetadata);
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);
            checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = filter(configurations, autoConfigurationMetadata);
            fireAutoConfigurationImportEvents(configurations, exclusions);
            return configurations.toArray(new String[configurations.size()]);
        }
        catch (IOException ex) {
            throw new IllegalStateException(ex);
        }
    }

getCandidateConfigurations方法用来获取自动配置类，该方法内部的实现最终会委托给SpringFactoriesLoader.loadFactoryNames(EnableAutoConfiguration.class,classLoader)；方法，loadFactoryNames方法的源码如下：

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
    String factoryClassName = factoryClass.getName();
    try {
        Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
        ArrayList result = new ArrayList();
        while(urls.hasMoreElements()) {
            URL url = (URL)urls.nextElement();
            Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
            String factoryClassNames = properties.getProperty(factoryClassName);
            result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
        }
        return result;
    } catch (IOException var8) {
        throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
    }
}

可以看到其在Spring Boot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；
我们调式源码的时候可以看到加载的自动配置类96个之多

而这些类是定义在spring.factories文件中有定义的。

今天先分析到这里，还存在没有理清楚的地方，后续有时间的话再更新吧。