SpringBoot初體驗及原理解析

1、前言

​  上篇文章,咱們聊到了SpringBoot得以實現的幕後推手,此次咱們來用SpringBoot開始HelloWorld之旅。SpringBoot是Spring框架對「約定大於配置(Convention over Configuration)」理念的最佳實踐。SpringBoot應用本質上就是一個基於Spring框架的應用。咱們大多數程序猿已經對Spring特別熟悉了,那隨着咱們的深刻挖掘,會發現SpringBoot中並無什麼新鮮事,若是你不信,那就一塊兒走着瞧唄!html

2、SpringBoot初體驗

首先,咱們按照下圖中的步驟生成一個SpringBoot項目:vue

  

解壓後的項目文件在idea中打開之後,咱們會看到以下的項目結構:java

  

這時候,咱們在com.hafiz.springbootdemo包下新建controller包,而後再在該包下面新建DemoController.java文件,內容以下:react

package com.hafiz.springbootdemo.controller;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

/** * @author hafiz.zhang * @description: first spring boot controller * @date Created in 2018/6/3 16:49. */
@RestController
public class DemoController {

    @GetMapping("/hello")
    public String sayHello(String name) {
        return "Hello, " + name;
    }
}

而後咱們運行SpringbootDemoApplication.java這個main方法,啓動成功後,在控制檯中會出現如下日誌,證實項目啓動成功:web

  

咱們從日誌中能夠看出,已經註冊了get方式的/hello的路徑以及名爲/error的路徑,而且項目在tomat中運行在8080端口,而後咱們在瀏覽器中訪問localhost:8080/hello?name=hafiz.zhang會看到以下信息:redis

這樣咱們就完成了demo項目的開發,先不用這裏面到底發生了啥,我就想問問你:是否是很爽?徹底沒有傳統項目的繁瑣的xml配置,爽到爆有木有?搭建一個項目骨架只要幾秒鐘!這就是SpringBoot帶給咱們的便利~ 就是這麼神奇!就是這麼牛逼!spring

3、項目簡單解析

首先咱們來看,生成好的項目中的SpringbootDemoApplication.java文件,內容以下:json

package com.hafiz.springbootdemo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class SpringbootDemoApplication {

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

代碼很簡單,但最耀眼的就是@SpringBootApplication註解以及在main方法中運行的SpringAppliation.run()了,那咱們要揭開SpringBoot應用的奧祕,很明顯就要拿這二位開刀了!瀏覽器

@SpringBootApplication註解解析

先看看@SpringBootApplication註解的源碼:springboot

@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 {
    ...
}

咱們看到@SpringBootApplication實際上是一個複合的註解,起主要做用的就是@SpringBootConfiguration@EnableAutoConfiguration以及@ComponentScan 三個註解組成,因此若是咱們把SpringBoot啓動類改寫成以下方式,整個SpringBoot應用依然能夠與以前的啓動類功能同樣:

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public class SpringbootDemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootDemoApplication.class, args);
    }
}

由於咱們每次新建項目時都要寫上三個註解來完成配置,這顯然太繁瑣了,SpringBoot就爲咱們提供了@SpringBootApplication這樣一個複合註解來簡化咱們的操做。簡直不能再貼心一點了!

@SpringBootConfiguration註解解析

接着咱們來看@SpringBootConfiguration註解的源碼:

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

咱們能夠看到,這裏面依舊沒有什麼新東西,它SpringBoot爲了區別@Configuration而新提供的專屬於SpringBoot的註解,功能和@Configuration如出一轍。而這裏的@Configuration註解對於咱們來講並不陌生,咱們在上篇文章中進行了詳細的介紹。SpringBoot的啓動類標註了這個註解,毫無疑問,它自己也是個IoC容器的配置類。瞭解到了這裏,咱們其實能夠把SpringBoot的啓動類來拆成兩個類,拆完之後就很是清楚明瞭了:

@Configuration
@EnableAutoConfiguration
@ComponentScan
public class DemoConfiguration {

}
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoConfiguration.class, args);
    }
}

因此呢,啓動類DemoApplication其實就是一個標準的Standalone類型的Java程序的main函數啓動類,也並無什麼特殊的東西。

@EnableAutoConfiguration 的神奇之處

看到這貨,咱們不只聯想出Spring 中不少以「@Enable」開頭的註解,好比:@EnableScheduling@EnableCaching以及@EnableMBeanExport等,@EnableAutoConfiguration註解的理念和工做原理和它們其實一脈相承。簡單的來講,就是該註解藉助@Import註解的支持,Spring的IoC容器收集和註冊特定場景相關的Bean定義:

  • @EnableScheduling是經過@Import將Spring調度框架相關的bean都加載到IoC容器。
  • @EnableMBeanExport是經過@Import將JMX相關的bean定義加載到IoC容器。

@EnableAutoConfiguration註解也是藉助@Import將全部複合配置條件的bean定義加載到IoC容器,僅此而已!而@EnableAutoConfiguration註解的源碼以下:

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

這其中最關鍵的就是@Import(EnableAutoConfigurationImportSelector.class)了,它藉助EnableAutoConfigurationImportSelector.class能夠幫助SpringBoot應用將全部符合條件的@Configuration配置類都加載到當前SpringBoot建立並使用的IoC容器,就像一個「八爪魚」同樣。

  

下面咱們給出EnableAutoConfigurationImportSelector.java的父類AutoConfigurationImportSelector.java的部分源碼,來解釋和驗證上圖:

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    protected List<AutoConfigurationImportFilter> getAutoConfigurationImportFilters() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportFilter.class,
                this.beanClassLoader);
    }
    protected List<AutoConfigurationImportListener> getAutoConfigurationImportListeners() {
        return SpringFactoriesLoader.loadFactories(AutoConfigurationImportListener.class,
                this.beanClassLoader);
    }
}

以上源碼能夠看出,@EnableAutoConfiguration正是藉助SpringFactoriesLoader的支持,才能完成如此偉大的壯舉!

幕後英雄SpringFactoriesLoader詳解

SpringFactoriesLoader屬於Spring框架專屬的一種擴展方案(其功能和使用方式相似於Java的SPI方案:java.util.ServiceLoader),它的主要功能就是從指定的配置文件META-INF/spring.factories中加載配置,spring.factories是一個很是經典的java properties文件,內容格式是Key=Value形式,只不過這Key以及Value都很是特殊,爲Java類的完整類名(Fully qualified name),好比:

com.hafiz.service.DemoService=com.hafiz.service.impl.DemoServiceImpl,com.hafiz.service.impl.DemoServiceImpl2

而後Spring框架就能夠根據某個類型做爲Key來查找對應的類型名稱列表了,SpringFactories源碼以下:

public abstract class SpringFactoriesLoader {

    private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);

    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

    public static <T> List<T> loadFactories(Class<T> factoryClass, ClassLoader classLoader){
        ...
    }

    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        ...
    }
    // ...
}

對於@EnableAutoConfiguraion來講,SpringFactoriesLoader的用途和其本意稍微不一樣,它本意是爲了提供SPI擴展,而在@EnableAutoConfiguration這個場景下,它更多的是提供了一種配置查找的功能的支持,也就是根據@EnableAutoConfiguration的完整類名org.springframework.boot.autoconfigure.EnableAutoConfiguration做爲Key來獲取一組對應的@Configuration類:

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.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.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
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.ldap.LdapDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
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.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.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
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.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
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

在SpringBoot的autoconfigure依賴包中的META-INF文件下的spring.factories文件中,咱們能夠找到以上內容,這就很好的解釋了爲何。

總結來講,@EnableAutoConfiguration能實現自動配置的原理就是:SpringFactoriesLoader從classpath中搜尋全部META-INF/spring.fatories文件,並將其中Key[org.springframework.boot.autoconfigure.EnableAutoConfiguration]對應的Value配置項經過反射的方式實例化爲對應的標註了@Configuration的JavaConfig形式的IoC容器配置類,而後彙總到當前使用的IoC容器中。

非必須的@ComponentScan解析

爲何說這個註解是非必需的呢?由於咱們知道做爲Spring框架裏的老成員,@ComponentScan的功能就是自動掃描並加載複合條件的組件或Bean定義,最終將這些bean定義加載到當前使用的容器中。這個過程,咱們能夠手工單個進行註冊,不是必定要經過這個註解批量掃描和註冊,因此說@ComponentScan是非必需的。

因此,若是咱們當前應用沒有任何bean定義須要經過@ComponentScan加載到當前SpringBoot應用對應的IoC容器,那麼,去掉@ComponentScan註解,當前的SpringBoot應用依舊能夠完美運行!那是否是到目前爲止,依舊沒有什麼新鮮的事物出現?

關於咱們用到的@RestController以及@GetMapping

@RestController註解也是一個複合註解,沒啥新東西,就是自動返回json類型的數據,源碼以下:

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Controller
@ResponseBody
public @interface RestController {
    String value() default "";
}

@GetMapping註解也是一個複合註解,依舊沒啥新東西,就是用來定義只支持GET請求的URL,源碼以下:

@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@RequestMapping(method = RequestMethod.GET)
public @interface GetMapping {
    ...
}

4、總結

  剛開始沒有深刻接觸SpringBoot的時候,感受這貨真的好神奇,居然能夠省去那麼多搭建步驟。這特麼簡直就是開發人員的福音啊,再回頭想一想這貨爲啥會火呢?咱們都知道近幾年微服務的概念很火,不少企業也開始把本身的項目作成微服務了,那隨着項目愈來愈多,按照咱們之前那種方式配置項目,很繁瑣,光建立項目就要耗費很大的精力和時間,這個時候拯救世界的SpringBoot出現了,它簡化了配置方式,實乃應運而生,順勢而爲啊!再不火,沒天理不是。下篇文章咱們來聊一聊SpringBoot的啓動流程。期待的小夥伴,評論扣1.

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