SpringBoot從零單排 ------初級入門篇
摘要：
有人說SSM已死，未來是SpringBoot和SpringCloud的天下，這個觀點可能有點極端，但不可否認的是已經越來越多的公司開始使用SpringBoot。所以我將平時學習SpringBoot的內容做個記錄，查漏補缺吧
1、建立SpringBoot專案
可以通過官方提供的Sprin...
有人說SSM已死，未來是SpringBoot和SpringCloud的天下，這個觀點可能有點極端，但不可否認的是已經越來越多的公司開始使用SpringBoot。所以我將平時學習SpringBoot的內容做個記錄，查漏補缺吧
1、建立SpringBoot專案
可以通過官方提供的Spring initializer工具來初始化springboot，同時IntelliJ IDEA 也集成了這個工具。因此可以根據個人需求選擇不同的建立方式
1、官方工具Spring initializer
網址 ：https://start.spring.io
下載下的壓縮包進行解壓匯入到編輯器中即可。
2、Idea建立專案
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->修改專案名->finish
建立成功之後的目錄
3、啟動專案
啟動springboot我們只需要執行上圖中的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);
}
}
這個啟動類可以分為兩部分
1、@SpringBootApplication
2、SpringApplication.run
SpringBootApplication
檢視原始碼我們發現@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
其實原始碼中的註釋已經描述了這個註解的作用了
/**
* 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註解。作用是將當前類中用@Bean註解標註的方法實力注入到Spring容器中，例項名就是方法名。
寫個程式碼，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 容器啟動初始化");
}
@Bean
public User createUser() {
User user = new User();
user.setUserName("Object Man");
user.setAge(18);
return user;
}
}
在main方法中可以直接獲取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("使用者姓名為：" + user.getUserName() + "，今年" + user.getAge() + "歲了");
}
}
啟動專案控制後臺輸出如下
._______ _ _ /\\ / ___'_ __ _ _(_)_ ____ _ \ \ \ \ ( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \ \\/___)| |_)| | | | | || (_| |) ) ) ) '|____| .__|_| |_|_| |_\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::(v2.1.1.RELEASE) 2019-01-05 20:25:18.761INFO 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 20:25:18.789INFO 71172 --- [main] c.o.s.SpringbootStudyApplication: No active profile set, falling back to default profiles: default 2019-01-05 20:25:19.787INFO 71172 --- [main] o.s.b.w.embedded.tomcat.TomcatWebServer: Tomcat initialized with port(s): 8080 (http) 2019-01-05 20:25:19.809INFO 71172 --- [main] o.apache.catalina.core.StandardService: Starting service [Tomcat] 2019-01-05 20:25:19.809INFO 71172 --- [main] org.apache.catalina.core.StandardEngine: Starting Servlet Engine: Apache Tomcat/9.0.13 2019-01-05 20:25:19.816INFO 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 20:25:19.900INFO 71172 --- [main] o.a.c.c.C.[Tomcat].[localhost].[/]: Initializing Spring embedded WebApplicationContext 2019-01-05 20:25:19.900INFO 71172 --- [main] o.s.web.context.ContextLoader: Root WebApplicationContext: initialization completed in 1001 ms =====>>>>> Configuration_Test 容器啟動初始化 2019-01-05 20:25:20.124INFO 71172 --- [main] o.s.s.concurrent.ThreadPoolTaskExecutor: Initializing ExecutorService 'applicationTaskExecutor' 2019-01-05 20:25:20.300INFO 71172 --- [main] o.s.b.w.embedded.tomcat.TomcatWebServer: Tomcat started on port(s): 8080 (http) with context path '' 2019-01-05 20:25:20.304INFO 71172 --- [main] c.o.s.SpringbootStudyApplication: Started SpringbootStudyApplication in 1.945 seconds (JVM running for 2.714) 使用者姓名為：Object Man，今年18歲了
SpringBoot的核心理念約定優於配置，因此通過註解的形式取代了xml配置檔案，減少了工作量，也使程式碼變得簡潔。
@EnableAutoConfiguration
還是老規矩先看原始碼
@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匯入了AutoConfigurationImportSelector類。那麼這個類是幹嘛的呢？檢視原始碼我們發現有個selectImports方法
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
//AutoConfigurationMetadataLoader是springboot autoconfigure載入AutoConfigurationMetadata的內部工具類
AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
.loadMetadata(this.beanClassLoader);
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
autoConfigurationMetadata, annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
loadMetadata的實現程式碼和相關程式碼如下：
//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檔案
Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
: ClassLoader.getSystemResources(path);
//建立一個properties物件，將所有配置檔案載入到properties物件中
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鎖需要的將配置載入到容器中。
@ComponentScan
可以把它理解為一個掃描器，一個專案中可能會有好多個控制器，我們就是通過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。
他有如下幾個屬性
public enum FilterType {
ANNOTATION, //按照註解過濾
ASSIGNABLE_TYPE, //按照給定的型別過濾
ASPECTJ, //使用ASPECTJ表示式
REGEX, //通過正則
CUSTOM //自定義規則
}
SpringApplication.run
該過程首先建立了一個SpringApplication物件例項，然後完某些例項的初始化。之後呼叫run方法。具體詳情可以參考原始碼和下圖
HelloWorld
新增依賴
pom檔案中有個parent標籤
<parent> <!-- 一個非常牛x的依賴，使用之後後面常用的依賴包可以不用寫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，使用之後相關依賴的時候可以不用填寫版本、預設和父級依賴的版本一樣。然後我們需要在dependencies標籤中新增web依賴模組
<!-- 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請求對映到方法上
*/
@RequestMapping("/")
public String HelloWorld() {
return "Hello World";
}
}
然後啟動專案瀏覽器訪問： http://localhost:8080/ 就可以看到瀏覽器輸出"Hello World"
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