springBoot 是什么

springboot 是一个框架，它依赖于spring，在过去我们使用spring项目的时候，需要大量各种繁琐的配置文件才能让spring跑起来，但自从有了springboot之后，它简化了使用方式，做到了无xml文件的配置即可运行，最大的特点是让配置文件变得简单，得益于自身的自动装配功能，部署也变得更简单（springboot内置web容器服务器tomcat），加上热部署后，可以快速开发和运行一个项目；

spring大家都知道，boot是启动的意思。所以，spring boot其实就是一个启动spring项目的一个工具而已，总而言之，springboot 是一个服务于框架的框架；

Spring Boot的核心功能

1、可独立运行的Spring项目：Spring Boot可以以jar包的形式独立运行。

2、内嵌的Servlet容器：Spring Boot可以选择内嵌Tomcat、Jetty或者Undertow，无须以war包形式部署项目。

3、简化的Maven配置：Spring提供推荐的基础 POM 文件来简化Maven 配置。

4、自动配置Spring：Spring Boot会根据项目依赖来自动配置Spring 框架，极大地减少项目要使用的配置。

5、提供生产就绪型功能：提供可以直接在生产环境中使用的功能，如性能指标、应用信息和应用健康检查。

6、无代码生成和xml配置：Spring Boot不生成代码。完全不需要任何xml配置即可实现Spring的所有配置。

第一种启动方式 @EnableAutoConfiguration

@EnableAutoConfiguration 作用

开启自动装配，帮助SpringBoot应用将所有符合条件的@Configuration配置都加载到当前SpringBoot，并创建对应配置类的Bean，并把该Bean实体交给IoC容器进行管理。

在使用springboot之前，我们需要先导入相关的jar包


<?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">
    <modelVersion>4.0.0</modelVersion>
 
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>2.0.8.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.spring</groupId>
    <artifactId>springBoot-demo1</artifactId>
    <version>1.0-SNAPSHOT</version>
 
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
    </dependencies>
</project>

新建文件 IndexController.java


package com.spring;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
 
@EnableAutoConfiguration
@RestController
public class IndexController {
 
 
     // 访问路径   http://localhost:8080/index
    @RequestMapping("/index")
    public String index(){
        System.out.println("我进来了");
        return "index controller";
    }
 
    public static void main(String[] args) {
        // 启动springboot
        SpringApplication.run(IndexController.class,args);
    }
}

运行main方法，后即可启动项目了


 
 
  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.0.8.RELEASE)
 
2021-06-03 20:37:26.010  INFO 942 --- [           main] com.spring.IndexController               : Starting IndexController on xiexindongdeMacBook-Pro.local with PID 942 (/Users/yexindong/Documents/java/java_project/springBootdemo1/target/classes started by yexindong in /Users/yexindong/Documents/java/java_project)
2021-06-03 20:37:26.014  INFO 942 --- [           main] com.spring.IndexController               : No active profile set, falling back to default profiles: default
2021-06-03 20:37:26.064  INFO 942 --- [           main] ConfigServletWebServerApplicationContext : Refreshing org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@105fece7: startup date [Thu Jun 03 20:37:26 CST 2021]; root of context hierarchy
2021-06-03 20:37:27.329  INFO 942 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2021-06-03 20:37:27.365  INFO 942 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2021-06-03 20:37:27.365  INFO 942 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/8.5.37
2021-06-03 20:37:27.375  INFO 942 --- [ost-startStop-1] o.a.catalina.core.AprLifecycleListener   : The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [/Users/yexindong/Library/Java/Extensions:/Library/Java/Extensions:/Network/Library/Java/Extensions:/System/Library/Java/Extensions:/usr/lib/java:.]
2021-06-03 20:37:27.474  INFO 942 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2021-06-03 20:37:27.474  INFO 942 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1413 ms
2021-06-03 20:37:27.520  INFO 942 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean  : Servlet dispatcherServlet mapped to [/]
2021-06-03 20:37:27.523  INFO 942 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'characterEncodingFilter' to: [/*]
2021-06-03 20:37:27.523  INFO 942 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
2021-06-03 20:37:27.523  INFO 942 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'httpPutFormContentFilter' to: [/*]
2021-06-03 20:37:27.523  INFO 942 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'requestContextFilter' to: [/*]
2021-06-03 20:37:27.619  INFO 942 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**/favicon.ico] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2021-06-03 20:37:27.750  INFO 942 --- [           main] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext@105fece7: startup date [Thu Jun 03 20:37:26 CST 2021]; root of context hierarchy
2021-06-03 20:37:27.805  INFO 942 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/index]}" onto public java.lang.String com.spring.IndexController.index()
2021-06-03 20:37:27.808  INFO 942 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error]}" onto public org.springframework.http.ResponseEntity<java.util.Map<java.lang.String, java.lang.Object>> org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
2021-06-03 20:37:27.808  INFO 942 --- [           main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped "{[/error],produces=[text/html]}" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
2021-06-03 20:37:27.825  INFO 942 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/webjars/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2021-06-03 20:37:27.825  INFO 942 --- [           main] o.s.w.s.handler.SimpleUrlHandlerMapping  : Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2021-06-03 20:37:27.938  INFO 942 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup
2021-06-03 20:37:27.986  INFO 942 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2021-06-03 20:37:27.990  INFO 942 --- [           main] com.spring.IndexController               : Started IndexController in 2.649 seconds (JVM running for 3.063)
2021-06-03 20:37:40.475  INFO 942 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring FrameworkServlet 'dispatcherServlet'
2021-06-03 20:37:40.475  INFO 942 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization started
2021-06-03 20:37:40.491  INFO 942 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization completed in 16 ms

在浏览器输入网址访问 controller ：http://localhost:8080/index 页面结果如下：

控制台也输出了打印的内容，是不是很简单呢？

第二种启动方式 @ComponentScan

@ComponentScan() 注解作用

根据定义的扫描路径，将符合规则的类加载到spring容器中，比如在类中加入了以下注解 @Controller、@Service、@Mapper 、@Component、@Configuration 等等；

接下来我们测试一下，创建启动类 App.java


package com.spring.controller;
 
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.context.annotation.ComponentScan;
 
@EnableAutoConfiguration  // 开启自动装配
@ComponentScan("com.spring.controller")
public class App {
    
    public static void main(String[] args) {
        // 启动springboot
        SpringApplication.run(App.class,args);
    }
}

控制层 OrderController.java


package com.spring.controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class OrderController {
    
    @RequestMapping("/order")
    public String order(){
        System.out.println("我进来了 order controller 的 index 方法");
        return "order controller";
    }
}

启动后访问接口 http://localhost:8080/order

控制台打印结果

第三种方式 @SpringBootApplication

@SpringBootApplication 注解作用

标注这是一个springboot的应用，被标注的类是一个主程序， SpringApplication.run(App.class, args);传入的类App.class必须是被@SpringBootApplication标注的类。

@SpringBootApplication是一个组合注解，组合了其他相关的注解，点进去注解后我们可以看到，这个注解集成了以上2种启动方式的注解；在这里的 @ComponentScan() 注解有一堆东西，它的作用是 将主配置类所在包及其下面所有后代包的所有注解扫描；

SpringBootApplication 使用起来更加简单，只需要一个注解即可完成，


package com.spring.controller;
 
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
 
 
@SpringBootApplication
public class App {
 
    public static void main(String[] args) {
        // 启动springboot
        SpringApplication.run(App.class,args);
    }
}

启动后控制台打印的信息

SpringBoot的三种启动方式