SpringBoot从零单排 ------初级入门篇

　　有人说SSM已死，未来是SpringBoot和SpringCloud的天下，这个观点可能有点极端，但不可否认的是已经越来越多的公司开始使用SpringBoot。所以我将平时学习SpringBoot的内容做个记录，查漏补缺吧

1、创建SpringBoot项目

可以通过官方提供的Spring initializer工具来初始化springboot，同时IntelliJ IDEA 也集成了这个工具。因此可以根据个人需求选择不同的创建方式

1、官方工具Spring initializer
网址 ：https://start.spring.io

下载下的压缩包进行解压导入到编辑器中即可。

2、Idea创建项目

New -> Project - > spring initializer -> 选择SDK->填写Group& Artifact->next->选择所需jar的依赖(也可暂时勾选)->next->修改项目名->finish

创建成功之后的目录

3、启动项目

启动springboot我们只需要执行上图中的ManApplication中的main方法就可以了。

package com.objectman.springboot_study;

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

@SpringBootApplication
public class SpringbootStudyApplication {

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

}

这个启动类可以分为两部分
1、@SpringBootApplication
2、SpringApplication.run

SpringBootApplication

查看源代码我们发现@SpringBootApplication是一个复合注解，主要包括了
@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan

@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

其实源代码中的注释已经描述了这个注解的作用了

/**
 * Indicates that a class provides Spring Boot application
 * {@link Configuration @Configuration}. Can be used as an alternative to the Spring's
 * standard {@code @Configuration} annotation so that configuration can be found
 * automatically (for example in tests).
 * <p>
 * Application should only ever include <em>one</em> {@code @SpringBootConfiguration} and
 * most idiomatic Spring Boot applications will inherit it from
 * {@code @SpringBootApplication}.
 *
 * @author Phillip Webb
 * @since 1.4.0
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {
}

　　

Can be used as an alternative to the Spring’s standard {@code @Configuration} annotation so that configuration can be found 主要意思是可以替代Spring的@Configuration注解。作用是将当前类中用@Bean注解标注的方法实力注入到Spring容器中，实例名就是方法名。
写个代码，举个栗子

定义一个配置类，

import com.objectman.springboot_study.User;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.context.annotation.Bean;

@SpringBootConfiguration
public class Configuration_Test {

    public Configuration_Test() {
        System.out.println("=====>>>>> Configuration_Test 容器启动初始化");
    }

    @Bean
    public User createUser() {
        User user = new User();
        user.setUserName("Object Man");
        user.setAge(18);
        return user;
    }
}

在main方法中可以直接获取bean。

@SpringBootApplication
public class SpringbootStudyApplication {

	public static void main(String[] args) {
		ConfigurableApplicationContext context = SpringApplication.run(SpringbootStudyApplication.class, args);
		User user = (User) context.getBean("createUser");
		System.out.println("用户姓名为：" + user.getUserName() + "，今年" + user.getAge() + "岁了");
	}
}

启动项目控制后台输出如下

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.1.1.RELEASE)

2019-01-05 20:25:18.761  INFO 71172 --- [           main] c.o.s.SpringbootStudyApplication         : Starting SpringbootStudyApplication on MicroWin10-1123 with PID 71172 (C:\Users\Administrator\IdeaProjects\springboot_study\target\classes started by Administrator in C:\Users\Administrator\IdeaProjects\springboot_study)
2019-01-05 20:25:18.789  INFO 71172 --- [           main] c.o.s.SpringbootStudyApplication         : No active profile set, falling back to default profiles: default
2019-01-05 20:25:19.787  INFO 71172 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2019-01-05 20:25:19.809  INFO 71172 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2019-01-05 20:25:19.809  INFO 71172 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/9.0.13
2019-01-05 20:25:19.816  INFO 71172 --- [           main] 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: [E:\JDK\jdk1.8.0_131\bin;C:\WINDOWS\Sun\Java\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\ProgramData\Oracle\Java\javapath;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;E:\JDK\jdk1.8.0_131\bin;E:\JDK\jdk1.8.0_131\jre\bin;C:\WINDOWS\System32\OpenSSH\;E:\apache-maven-3.5.4\bin;C:\Program Files\NVIDIA Corporation\NVIDIA NvDLISR;E:\program\MySql\mysql-8.0.13-winx64\bin;E:\program\node\;C:\Program Files\Git\cmd;E:\Python\Python_Controller\Scripts\;E:\Python\Python_Controller\;C:\Users\Administrator\AppData\Local\Microsoft\WindowsApps;C:\Users\Administrator\AppData\Roaming\npm;.]
2019-01-05 20:25:19.900  INFO 71172 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2019-01-05 20:25:19.900  INFO 71172 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1001 ms
=====>>>>> Configuration_Test 容器启动初始化
2019-01-05 20:25:20.124  INFO 71172 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2019-01-05 20:25:20.300  INFO 71172 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2019-01-05 20:25:20.304  INFO 71172 --- [           main] c.o.s.SpringbootStudyApplication         : Started SpringbootStudyApplication in 1.945 seconds (JVM running for 2.714)
用户姓名为：Object Man，今年18岁了

　　

SpringBoot的核心理念约定优于配置，因此通过注解的形式取代了xml配置文件，减少了工作量，也使代码变得简洁。

@EnableAutoConfiguration

还是老规矩先看源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
	Class<?>[] exclude() default {};
	String[] excludeName() default {};
}

发现有个Import导入了AutoConfigurationImportSelector类。那么这个类是干嘛的呢？查看源码我们发现有个selectImports方法

	public String[] selectImports(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return NO_IMPORTS;
		}
		//AutoConfigurationMetadataLoader是springboot autoconfigure加载AutoConfigurationMetadata的内部工具类
		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
				.loadMetadata(this.beanClassLoader);
		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
				autoConfigurationMetadata, annotationMetadata);
		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
	}

loadMetadata的实现代码和相关代码如下：

//定义一个路径
	protected static final String PATH = "META-INF/"
			+ "spring-autoconfigure-metadata.properties";

	public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
		return loadMetadata(classLoader, PATH);
	}
	
	static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
		try {
			//找到自动配置的属性文件
			Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
					: ClassLoader.getSystemResources(path);
			//创建一个properties对象，将所有配置文件加载到properties对象中
			Properties properties = new Properties();
			while (urls.hasMoreElements()) {
				properties.putAll(PropertiesLoaderUtils
						.loadProperties(new UrlResource(urls.nextElement())));
			}
			return loadMetadata(properties);
		}
		catch (IOException ex) {
			throw new IllegalArgumentException(
					"Unable to load @ConditionalOnClass location [" + path + "]", ex);
		}
	}

PATH路径下被自动配置的类有

结论：EnableAutoConfiguration会将SpringBoot锁需要的将配置加载到容器中。

@ComponentScan
可以把它理解为一个扫描器，一个项目中可能会有好多个控制器，我们就是通过ComponentScan去发现指定路径下的@Controller（@RestController）、@Service、@Repository 、@Component并将他们装入bean容器中。
他有如下几个属性

public enum FilterType {


   ANNOTATION, //按照注解过滤

   ASSIGNABLE_TYPE, //按照给定的类型过滤

   ASPECTJ, //使用ASPECTJ表达式

   REGEX, //通过正则

   CUSTOM //自定义规则

}

　　

SpringApplication.run

该过程首先创建了一个SpringApplication对象实例，然后完某些实例的初始化。之后调用run方法。具体详情可以参考源码和下图

HelloWorld

添加依赖

pom文件中有个parent标签

　　　　　<parent>
		<!-- 一个非常牛x的依赖，使用之后后面常用的依赖包可以不用写version了 -->
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.1.1.RELEASE</version>
	</parent>

　　

这是SpringBoot的一个父级依赖，使用之后相关依赖的时候可以不用填写版本、默认和父级依赖的版本一样。然后我们需要在dependencies标签中添加web依赖模块

<!-- WEB依赖包 -->
	<dependency>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-web</artifactId>
</dependency>

编写Controller

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
/**
 * @RestController = @Controller + @ResponseBody
 *
 */
@RestController
public class HelloWorldController {
    /**
     * RequestMapping 将Http请求映射到方法上
     */
    @RequestMapping("/")
    public String HelloWorld() {
        return "Hello World";
    }
}

　　然后启动项目浏览器访问：http://localhost:8080/ 就可以看到浏览器输出"Hello World"

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