Springboot 随笔（1） -- 自动引入配置与启动机制

alexqdjay

发布于 2018-05-11 13:58:52

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发布于 2018-05-11 13:58:52

文章被收录于专栏：alexqdjay

为什么用SpringBoot?

同上题记。总结：快速开始，方便搭建，开发web时并不需要Tomcat或者Jetty，甚至连插件都不用（因为自带Tomcat或自配置成Jetty）。

肯定有缺点吧？

一个框架除了知道他的优点，肯定要知道他的缺点。

SpringBoot 缺点如下（暂时发现）：

配置逻辑隐藏太深，所以如果有很多自定义的需要翻源码看，如配置多个Servlet
配置Bean化，替代XML。Bean和XML谁更优？一半一半，所以建议Bean和XML混用（SpringBoot提供这种方式），有时XML定义更加清晰。
文档略少，有时需要翻源码才知道用法。
默认加载的AutoConfig有点多，所以影响启动速度。网上有优化方式，基本思想就是去除@SpringBootApplication，使用自己编写@Import，但是这样SpringBoot的便捷性就没有了。

SpringBoot 的运行机制

1. 引入配置

使用 @Import 引入配置Bean，有三种方式：

1) 直接引入 configuration.class

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Import({DelegatingWebMvcConfiguration.class})
public @interface EnableWebMvc {
}

DelegatingWebMvcConfiguration 就是配置的Configuration类

@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
    //...
}

2) 引入 ImportSelector 接口的实现

public interface ImportSelector {
    String[] selectImports(AnnotationMetadata var1);
}

只要实现该接口，返回需要加载的 Configuration 类名字符串即可，见例子：

static class CacheConfigurationImportSelector implements ImportSelector {
    CacheConfigurationImportSelector() {
    }

    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        CacheType[] types = CacheType.values();
        String[] imports = new String[types.length];

        for(int i = 0; i < types.length; ++i) {
            imports[i] = CacheConfigurations.getConfigurationClass(types[i]);
        }

        return imports;
    }
}

3) 引入 ImportBeanDefinitionRegistrar 接口的实现类

public interface ImportBeanDefinitionRegistrar {
    void registerBeanDefinitions(AnnotationMetadata var1, BeanDefinitionRegistry var2);
}

很好理解，手动将需要注入的bean的definition放入BeanDefinitionRegistry 中

2. 以 DataSource 自动配置为例子

所有的开始都是源于 DataSourceAutoConfiguration 这个类，就是说如果你想自动化生成 DataSrouce 你只要在你的配置类引入该类：

@Configuration
@Import({DataSourceAutoConfiguration.class})
public class Application {
    // ...
}

DataSourceAutoConfiguration 中引入了其他配置类，并且使用 @Bean 来生成需要的组件：

@Configuration
@ConditionalOnClass({DataSource.class, EmbeddedDatabaseType.class})
@EnableConfigurationProperties({DataSourceProperties.class})
@Import({Registrar.class, DataSourcePoolMetadataProvidersConfiguration.class})
public class DataSourceAutoConfiguration {
}

引入了 Registrar 和 DataSourcePoolMetadataProvidersCofniguration ，这里以 Registrar 为例介绍：

static class Registrar implements ImportBeanDefinitionRegistrar {
    private static final String BEAN_NAME = "dataSourceInitializerPostProcessor";

    Registrar() {
    }

    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        if(!registry.containsBeanDefinition("dataSourceInitializerPostProcessor")) {
            GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
            beanDefinition.setBeanClass(DataSourceInitializerPostProcessor.class);
            beanDefinition.setRole(2);
            beanDefinition.setSynthetic(true);
            registry.registerBeanDefinition("dataSourceInitializerPostProcessor", beanDefinition);
        }

    }
}

这是符合上面的第3种方式，目的就是注册一个PostProcessor 来处理注册进来的 DataSource，凡是有DataSource实例，就实例化 DataSourceInitializer （用于预跑一些初始化的SQL脚步）。

重点是下面

@Configuration
@Conditional({DataSourceAutoConfiguration.PooledDataSourceCondition.class})
@ConditionalOnMissingBean({DataSource.class, XADataSource.class})
@Import({Tomcat.class, Hikari.class, Dbcp.class, Dbcp2.class})
protected static class PooledDataSourceConfiguration {
    protected PooledDataSourceConfiguration() {
    }
}

引入三个配置类，Tomcat、Hikari、Dbcp、Dbcp2，处理逻辑基本一致，都是判断是否有对于的类和配置，以Dbcp为例：

@ConditionalOnClass({org.apache.commons.dbcp.BasicDataSource.class})
@ConditionalOnProperty(
    name = {"spring.datasource.type"},
    havingValue = "org.apache.commons.dbcp.BasicDataSource",
    matchIfMissing = true
)
static class Dbcp extends DataSourceConfiguration {
    Dbcp() {
    }

    @Bean
    @ConfigurationProperties("spring.datasource.dbcp")
    public org.apache.commons.dbcp.BasicDataSource dataSource(DataSourceProperties properties) {
        org.apache.commons.dbcp.BasicDataSource dataSource = (org.apache.commons.dbcp.BasicDataSource)this.createDataSource(properties, org.apache.commons.dbcp.BasicDataSource.class);
        DatabaseDriver databaseDriver = DatabaseDriver.fromJdbcUrl(properties.determineUrl());
        String validationQuery = databaseDriver.getValidationQuery();
        if(validationQuery != null) {
            dataSource.setTestOnBorrow(true);
            dataSource.setValidationQuery(validationQuery);
        }

        return dataSource;
    }
}

最终 @Bean 生成 dataSource 实例。

3. 自动化引入配置类

这个秘密就隐藏在 @SpringBootApplication 中

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

从源码中可见，@SpringBootApplication = @EnableAutoConfiguration + @SpringBootConfiguration + @ComponentScan

很明显，EnableAutoConfiguration 是自动化配置的关键

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

EnableAutoConfigurationImportSelector implements DeferredImportSelector

符合上面第3中引入方式，EnableAutoConfigurationImportSelector 的主要功能就是将spring.factories 配置的config获取，返回出来

spring.factories 中都是些什么：

# 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,\
org.springframework.boot.autoconfigure.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\
org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.hornetq.HornetQAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,\
org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,\
org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,\
org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,\
org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.velocity.VelocityAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

前面分析的 DataSourceAutoConfiguration 就在上面，由此可见，这些AutoConfig都被自动引入。

这么多配置可能由于的就一半都不到，所以如果优化启动速度，那么就手动@Import 即可，不过有点麻烦。

SpringBoot 的启动

    public static void main(String[] args) throws LifecycleException, InterruptedException {
        SpringApplication.run(Application.class, args);
    }

一切的开始都是从这段代码，所以 SpringApplication.run 是分析入口，最终追踪到源码：

public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
  return (new SpringApplication(sources)).run(args);
}

实例化，然后run。

实例化中，还调用判断了是否Web环境，原理是判断是否存在两个class：

private static final String[] WEB_ENVIRONMENT_CLASSES = new String[]{"javax.servlet.Servlet", "org.springframework.web.context.ConfigurableWebApplicationContext"};    
    
    private boolean deduceWebEnvironment() {
        String[] var1 = WEB_ENVIRONMENT_CLASSES;
        int var2 = var1.length;

        for(int var3 = 0; var3 < var2; ++var3) {
            String className = var1[var3];
            if(!ClassUtils.isPresent(className, (ClassLoader)null)) {
                return false;
            }
        }

        return true;
    }

run的源码：

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    this.configureHeadlessProperty();
    SpringApplicationRunListeners listeners = this.getRunListeners(args);
    listeners.started();

    try {
        DefaultApplicationArguments ex = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = this.prepareEnvironment(listeners, ex); // env配置处理, application.properties等
        Banner printedBanner = this.printBanner(environment); // 打印 Banner
        context = this.createApplicationContext(); // 关键（1）
        this.prepareContext(context, environment, listeners, ex, printedBanner);// 一些赋值，及调用initer
        this.refreshContext(context); // refresh ctx, 视为启动ctx 关键（2）
        this.afterRefresh(context, ex);
        listeners.finished(context, (Throwable)null);
        stopWatch.stop();
        if(this.logStartupInfo) {
            (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
        }

        return context;
    } catch (Throwable var8) {
        this.handleRunFailure(context, listeners, var8);
        throw new IllegalStateException(var8);
    }
}

分析关键（1）的源码

    protected ConfigurableApplicationContext createApplicationContext() {
        Class contextClass = this.applicationContextClass;
        if(contextClass == null) {
            try {
                contextClass = Class.forName(this.webEnvironment?"org.springframework.boot.context.embedded.AnnotationConfigEmbeddedWebApplicationContext":"org.springframework.context.annotation.AnnotationConfigApplicationContext");
            } catch (ClassNotFoundException var3) {
                throw new IllegalStateException("Unable create a default ApplicationContext, please specify an ApplicationContextClass", var3);
            }
        }

        return (ConfigurableApplicationContext)BeanUtils.instantiate(contextClass);
    }

很简单，判断是否 web环境，如果是就是要使用 AnnotationConfigEmbeddedWebApplicationContext 这个Ctx类实例化Context。

关键（2）的源码

    protected void refresh(ApplicationContext applicationContext) {
        Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
        ((AbstractApplicationContext)applicationContext).refresh();
    }

那么 AnnotationConfigEmbeddedWebApplicationContext 的 onRefresh 和普通的 context的区别在于:

    protected void onRefresh() {
        super.onRefresh();

        try {
            this.createEmbeddedServletContainer();  // 创建自带的web容器
        } catch (Throwable var2) {
            throw new ApplicationContextException("Unable to start embedded container", var2);
        }
    }

    private void createEmbeddedServletContainer() {
        EmbeddedServletContainer localContainer = this.embeddedServletContainer;
        ServletContext localServletContext = this.getServletContext();
        if(localContainer == null && localServletContext == null) {
            EmbeddedServletContainerFactory ex = this.getEmbeddedServletContainerFactory(); // 获取ContainerFactory
            this.embeddedServletContainer = ex.getEmbeddedServletContainer(new ServletContextInitializer[]{this.getSelfInitializer()});
        } else if(localServletContext != null) {
            //...
        }

        this.initPropertySources();
    }

那么 ContainerFactory 肯定是自动引入的配置咯！

EmbeddedServletContainerAutoConfiguration 中：

@Configuration
@ConditionalOnClass({Servlet.class, Tomcat.class})
@ConditionalOnMissingBean(
    value = {EmbeddedServletContainerFactory.class},
    search = SearchStrategy.CURRENT
)
public static class EmbeddedTomcat {
    public EmbeddedTomcat() {
    }

    @Bean
    public TomcatEmbeddedServletContainerFactory tomcatEmbeddedServletContainerFactory() {
        return new TomcatEmbeddedServletContainerFactory();
    }
}

当然，该类中还有其他web容器的引入配置，形同上面 Tomcat的，逻辑也类似就是判断是否存在一些关键类：

@ConditionalOnClass({Servlet.class, Tomcat.class})

其他的，refresh 过程跟一般的spring context一致，不作分析。

总结

SpringBoot 远不止如此，且学且记录吧！

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