越来越方便了

 java技术生态发展近25年，框架也越来越方便使用了，简直so easy！！！我就以Spring衍生出的Spring boot做演示，Spring boot会让你开发应用更快速。

快速启动spring boot 请参照官网 Spring | Quickstart

代码如下：

@SpringBootApplication
@RestController
public class SpringBootTestApplication {public static void main(String[] args) {SpringApplication.run(SpringBootTestApplication.class, args);}@GetMapping("/hello")public String hello(@RequestParam(value = "name", defaultValue = "World") String name) {return String.format("Hello %s!", name);}
}

注maven的pom文件 （部分）：

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

运行

通过浏览器方法http://localhost:8080/hello

至此一个简单的spring boot应用开发完毕，要是公司企业的展示性网站用这种方式极快。

我们重点关注一下这个@SpringBootApplication注解，就是因为它，程序运行主类，相当于跑了一个tomcat中间件，既然能通过web访问，说明是一个web应用程序。

@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 {
/*** Exclude specific auto-configuration classes such that they will never be applied.* @return the classes to exclude*/@AliasFor(annotation = EnableAutoConfiguration.class)Class<?>[] exclude() default {};/*** Exclude specific auto-configuration class names such that they will never be* applied.* @return the class names to exclude* @since 1.3.0*/@AliasFor(annotation = EnableAutoConfiguration.class)String[] excludeName() default {};/*** Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}* for a type-safe alternative to String-based package names.* <p>* <strong>Note:</strong> this setting is an alias for* {@link ComponentScan @ComponentScan} only. It has no effect on {@code @Entity}* scanning or Spring Data {@link Repository} scanning. For those you should add* {@link org.springframework.boot.autoconfigure.domain.EntityScan @EntityScan} and* {@code @Enable...Repositories} annotations.* @return base packages to scan* @since 1.3.0*/@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")String[] scanBasePackages() default {};/*** Type-safe alternative to {@link #scanBasePackages} for specifying the packages to* scan for annotated components. The package of each class specified will be scanned.* <p>* Consider creating a special no-op marker class or interface in each package that* serves no purpose other than being referenced by this attribute.* <p>* <strong>Note:</strong> this setting is an alias for* {@link ComponentScan @ComponentScan} only. It has no effect on {@code @Entity}* scanning or Spring Data {@link Repository} scanning. For those you should add* {@link org.springframework.boot.autoconfigure.domain.EntityScan @EntityScan} and* {@code @Enable...Repositories} annotations.* @return base packages to scan* @since 1.3.0*/@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")Class<?>[] scanBasePackageClasses() default {};......略了
}

发现@SpringBootApplication注解是一个组合型注解，有3个Spring annotations：@SpringBootConfiguration, @ComponentScan, and @EnableAutoConfiguration ，记住@EnableAutoConfiguration注解很重要，其次@ComponentScan注解。

@EnableAutoConfiguration注解，spring boot 会基于classpath路径的jar、注解和配置信息，会自动配置我们的应用程序，所有的这些注解会帮助spring boot 自动配置我们的应用程序，我们不必担心配置它们。我演示的代码，spring boot会检查classpath路径，由于有依赖spring-boot-starter-web，Spring boot会把项目配置成web应用项目，另外Spring Boot将把它的HelloController当作一个web控制器，而且由于我的应用程序依赖于Tomcat服务器（spring-boot-starter-tomcat），Spring Boot也将使用这个Tomcat服务器来运行我的应用程序。

特别注意，框架里是否装载Bean会配合条件表达式一起使用

 真正使用时是混合使用的

@EnableAutoConfiguration代码：

package org.springframework.boot.autoconfigure;@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";/*** Exclude specific auto-configuration classes such that they will never be applied.* @return the classes to exclude*/Class<?>[] exclude() default {};/*** Exclude specific auto-configuration class names such that they will never be* applied.* @return the class names to exclude* @since 1.3.0*/String[] excludeName() default {};}

又有两个需要理解的元注解meta-annotation : @AutoConfigurationPackage 和@Import(AutoConfigurationImportSelector.class)

@AutoConfigurationPackage代码：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {String[] basePackages() default {};Class<?>[] basePackageClasses() default {};
}

这个注解又用了一个@Import， 它的value值是：  AutoConfigurationPackages.Registrar.class.，而

AutoConfigurationPackages.Registrar 实现ImportBeanDefinitionRegistrar。还记得我以前写的博文Spring Bean注册的几种方式Spring Bean注册的几种方式_filterregistrationbean beandefinitionregistrypostp_董广明的博客-CSDN博客吗

@Import(AutoConfigurationImportSelector.class)

这个注解是自动配置机制，AutoConfigurationImportSelector实现了 DeferredImportSelector.

这个选择器的实现使用spring core功能方法：SpringFactoriesLoader.loadFactoryNames() ，该方法从META-INF/spring.factories加载配置类

而这个引导配置类从spring-boot-autoconfigure-2.3.0.RELEASE-sources.jar!/META-INF/spring.factories文件加载，该文件有键org.springframework.boot.autoconfigure.EnableAutoConfiguration

注意spring引导自动配置模块隐式包含在所有引导应用程序中。

参考：

1. SpringBoot中@EnableAutoConfiguration注解的作用  SpringBoot中@EnableAutoConfiguration注解的作用_51CTO博客_@enableautoconfiguration注解报错

2. Talking about how Spring Boot works  Talking about how Spring Boot works - Huong Dan Java

3. How Spring Boot Works？ Spring Boot Rock’n’Roll -王福强的个人博客：一个架构士的思考与沉淀
4.  How Spring Boot auto-configuration works  How Spring Boot auto-configuration works | Java Development Journal
5. Spring Boot - How auto configuration works? https://www.logicbig.com/tutorials/spring-framework/spring-boot/auto-config-mechanism.html

6. SpringBoot 启动原理  SpringBoot 启动原理 | Server 运维论坛

7. How SpringBoot AutoConfiguration magic works? https://www.sivalabs.in/how-springboot-autoconfiguration-magic/

8. @EnableAutoConfiguration Annotation in Spring Boot @EnableAutoConfiguration Annotation in Spring Boot