SpringBoot从零单排 ------初级入门篇
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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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