SpringBoot（一）原理剖析：SpringApplication启动原理

　　通常搭建一个基于spring的web应用，我们需要做以下工作：

pom文件中引入相关jar包，包括spring、springmvc、redis、mybaits、log4j、mysql-connector-java 等等相关jar ...
配置web.xml，Listener配置、Filter配置、Servlet配置、log4j配置、error配置 ...
配置数据库连接、配置spring事务
配置视图解析器
开启注解、自动扫描功能
配置完成后部署tomcat、启动调试
......

　　而用springboot后，一切都变得很简便快速。

一、springboot的启动类入口

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

二、@SpringBootApplication注解分析

　　SpringBootApplication注解如下：

 1 @Target(ElementType.TYPE)  //注解的适用范围，其中Type用于描述类、接口或Enum声明 
 2 @Retention(RetentionPolicy.RUNTIME)  //注解的生命周期，保留到Class文件中
 3 @Documented  //表明这个注解应该被javadoc记录
 4 @Inherited   //子类可以继承该注解
 5 @SpringBootConfiguration  //继承了Configuration，表示当前是注解类
 6 @EnableAutoConfiguration  //开启SpringBoot的注解功能，借助@import的支持，收集和注册依赖包中的bean定义
 7 @ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
 8         @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) }) //自动扫描并加载符合条件的组件
 9 public @interface SpringBootApplication {
10 
11     /**
12      * Exclude specific auto-configuration classes such that they will never be applied.
13      * @return the classes to exclude
14      */
15     @AliasFor(annotation = EnableAutoConfiguration.class)
16     Class<?>[] exclude() default {};
17 
18     /**
19      * Exclude specific auto-configuration class names such that they will never be
20      * applied.
21      */
22     @AliasFor(annotation = EnableAutoConfiguration.class)
23     String[] excludeName() default {};
24 
25     /**
26      * Base packages to scan for annotated components. Use {@link #scanBasePackageClasses}
27      * for a type-safe alternative to String-based package names.
28      */
29     @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
30     String[] scanBasePackages() default {};
31 
32     /**
33      * Type-safe alternative to {@link #scanBasePackages} for specifying the packages to
34      * scan for annotated components. The package of each class specified will be scanned.
35      */
36     @AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
37     Class<?>[] scanBasePackageClasses() default {};
38 
39     /**
40      * The {@link BeanNameGenerator} class to be used for naming detected components
41      * within the Spring container.
42      */
43     @AliasFor(annotation = ComponentScan.class, attribute = "nameGenerator")
44     Class<? extends BeanNameGenerator> nameGenerator() default BeanNameGenerator.class;
45 
46     /**
47      * Specify whether {@link Bean @Bean} methods should get proxied in order to enforce
48      * bean lifecycle behavior
49      */
50     @AliasFor(annotation = Configuration.class)
51     boolean proxyBeanMethods() default true;
52 
53 }

SpringBootApplication