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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