spring boot之hello

自己使用springboot也已经写过一段时间的代码，但是对springboot真正运行的流程还是有点模糊，今天写出自己对springboot的认识，如有不对，还请各位大佬不吝赐教，话不多说，直接上代码

工程的目录结构如下

pom文件如下:

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">

    <groupId>com.rookie.bigdata</groupId>
    <version>1.0-SNAPSHOT</version>

    <modelVersion>4.0.0</modelVersion>

    <artifactId>springboot-helloworld</artifactId>

    <name>springboot-helloworld</name>
    <!-- FIXME change it to the project's website -->
    <url>http://www.example.com</url>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.5.RELEASE</version>
    </parent>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <maven.compiler.source>1.8</maven.compiler.source>
        <maven.compiler.target>1.8</maven.compiler.target>
    </properties>

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

    </dependencies>

    <build>

    </build>
</project>

代码如下：

Application.java

package com.rookie.bigdata;

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

/**
 * Created by  on 2018/9/20.
 */
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);

    }
}

HelloController.java

package com.rookie.bigdata.web;

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

/**
 * Created by on 2018/9/20.
 */
@RestController
public class HelloController {

    @RequestMapping("/hello")
    public String home() {

        return "hello world";
    }
}

运行Application.java中的主类，访问localhost:8080/hello，出现如下界面表示成功

@SpringBootApplication注解是一个spring boot的一个重要注解，用来进行快速的启动，源码如下

@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 {

 @SpringBootConfiguration标注在某个类上，表示这是一个Spring Boot的配置类。源码如下：

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

@EnableAutoConfiguratio  通知SpringBoot开启自动配置功能，这样自动配置才能生效。源码如下：

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@AutoConfigurationPackage//根据条件来注册我们需要的bean实例，@Import注解支持导入普通的java类,并将其声明成一个bean.给容器中导入组件@Import(AutoConfigurationImportSelector.class)public @interface EnableAutoConfiguration {

@AutoConfigurationPackage  自动配置包注解，源码如下：

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
//@Import(AutoConfigurationPackages.Registrar.class)：默认将主配置类(@SpringBootApplication)所在的包及其子包里面的所有组件扫描到Spring容器中。
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

@Import(AutoConfigurationPackages.Registrar.class)注解源码如下：

    /**
     * {@link ImportBeanDefinitionRegistrar} to store the base package from the importing
     * configuration.
     */
    static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

        @Override
        public void registerBeanDefinitions(AnnotationMetadata metadata,
                BeanDefinitionRegistry registry) {
        //默认将会扫描@SpringBootApplication标注的主配置类所在的包及其子包下所有组件
            register(registry, new PackageImport(metadata).getPackageName());
        }

        @Override
        public Set<Object> determineImports(AnnotationMetadata metadata) {
            return Collections.singleton(new PackageImport(metadata));
        }

    }

这样就明白了为什么Application这个类要在最外面的一层，是为了扫描@SpringBootApplication标注的主配置类所在的包及其子包下所有组件，并根据每个类上面的注解实例化为一个bean对象