SpringBoot從零單排 ------初級入門篇

　　有人說SSM已死，將來是SpringBoot和SpringCloud的天下，這個觀點可能有點極端，但不能否認的是已經愈來愈多的公司開始使用SpringBoot。因此我將平時學習SpringBoot的內容作個記錄，查漏補缺吧前端

一、建立SpringBoot項目java

能夠經過官方提供的Spring initializer工具來初始化springboot，同時IntelliJ IDEA 也集成了這個工具。所以能夠根據我的需求選擇不一樣的建立方式node

一、官方工具Spring initializer
網址：https://start.spring.io
mysql

下載下的壓縮包進行解壓導入到編輯器中便可。程序員

二、Idea建立項目

New -> Project - > spring initializer -> 選擇SDK->填寫Group& Artifact->next->選擇所需jar的依賴(也可暫時勾選)->next->修改項目名->finishweb

建立成功以後的目錄spring

三、啓動項目

啓動springboot咱們只須要執行上圖中的ManApplication中的main方法就能夠了。sql

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); } }

這個啓動類能夠分爲兩部分
一、@SpringBootApplication
二、SpringApplication.run數據庫

SpringBootApplicationapache

查看源代碼咱們發現@SpringBootApplication是一個複合註解，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

其實源代碼中的註釋已經描述了這個註解的做用了

/**
 * 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 主要意思是能夠替代Spring的@Configuration註解。做用是將當前類中用@Bean註解標註的方法實力注入到Spring容器中，實例名就是方法名。
寫個代碼，舉個栗子

定義一個配置類，

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 容器啓動初始化");
    }

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

在main方法中能夠直接獲取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("用戶姓名爲：" + user.getUserName() + "，今年" + user.getAge() + "歲了");
	}
}

啓動項目控制後臺輸出以下

.   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: 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 容器啓動初始化
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)
用戶姓名爲：Object Man，今年18歲了

SpringBoot的核心理念約定優於配置，所以經過註解的形式取代了xml配置文件，減小了工做量，也使代碼變得簡潔。

@EnableAutoConfiguration

仍是老規矩先看源碼

@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 {};
}

發現有個Import導入了AutoConfigurationImportSelector類。那麼這個類是幹嗎的呢？查看源碼咱們發現有個selectImports方法

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

loadMetadata的實現代碼和相關代碼以下：

//定義一個路徑
	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 {
			//找到自動配置的屬性文件
			Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
					: ClassLoader.getSystemResources(path);
			//建立一個properties對象，將全部配置文件加載到properties對象中
			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路徑下被自動配置的類有

結論：EnableAutoConfiguration會將SpringBoot鎖須要的將配置加載到容器中。

@ComponentScan
能夠把它理解爲一個掃描器，一個項目中可能會有好多個控制器，咱們就是經過ComponentScan去發現指定路徑下的@Controller（@RestController）、@Service、@Repository 、@Component並將他們裝入bean容器中。
他有以下幾個屬性

public enum FilterType {


   ANNOTATION, //按照註解過濾

   ASSIGNABLE_TYPE, //按照給定的類型過濾

   ASPECTJ, //使用ASPECTJ表達式

   REGEX, //經過正則

   CUSTOM //自定義規則

}

SpringApplication.run

該過程首先建立了一個SpringApplication對象實例，而後完某些實例的初始化。以後調用run方法。具體詳情能夠參考源碼和下圖

HelloWorld

添加依賴

pom文件中有個parent標籤

　　　　　<parent>
		<!-- 一個很是牛x的依賴，使用以後後面經常使用的依賴包能夠不用寫version了 -->
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>2.1.1.RELEASE</version>
	</parent>

這是SpringBoot的一個父級依賴，使用以後相關依賴的時候能夠不用填寫版本、默認和父級依賴的版本同樣。而後咱們須要在dependencies標籤中添加web依賴模塊

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

編寫Controller

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

　　而後啓動項目瀏覽器訪問：http://localhost:8080/ 就能夠看到瀏覽器輸出"Hello World"

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