浅谈SpringBoot核心注解原理

时间 2019-12-09

原文原文链接

SpringBoot核心注解原理java

今天跟你们来探讨下SpringBoot的核心注解@SpringBootApplication以及run方法，理解下springBoot为何不须要XML，达到零配置git

首先咱们先来看段代码github

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

咱们点进@SpringBootApplication来看web

@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 {

}

上面的元注解咱们在这里不在作解释，相信你们在开发当中确定知道，咱们要来讲@SpringBootConfiguration @EnableAutoConfiguration 这两个注解,到这里咱们知道 SpringBootApplication注解里除了元注解，咱们能够看到又是@SpringBootConfiguration，@EnableAutoConfiguration，@ComponentScan的组合注解，官网上也有详细说明，那咱们如今把目光投向这三个注解。redis

首先咱们先来看 @SpringBootConfiguration，那咱们点进来看spring

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Configuration
public @interface SpringBootConfiguration {

}

咱们能够看到这个注解除了元注解之外，就只有一个@Configuration，那也就是说这个注解至关于@Configuration，因此这两个注解做用是同样的，那他是干吗的呢，相信不少人都知道，它是让咱们可以去注册一些额外的Bean，而且导入一些额外的配置。那@Configuration还有一个做用就是把该类变成一个配置类，不须要额外的XML进行配置。因此@SpringBootConfiguration就至关于@Configuration。tomcat

那咱们继续来看下一个@EnableAutoConfiguration，这个注解官网说是让Spring自动去进行一些配置，那咱们点进来看springboot

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(EnableAutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}

能够看到它是由 @AutoConfigurationPackage，@Import(EnableAutoConfigurationImportSelector.class)这两个而组成的，咱们先说@AutoConfigurationPackage，他是说：让包中的类以及子包中的类可以被自动扫描到spring容器中。咱们来看@Import(EnableAutoConfigurationImportSelector.class)这个是核心，以前咱们说自动配置，那他到底帮咱们配置了什么，怎么配置的？就和@Import(EnableAutoConfigurationImportSelector.class)息息相关，程序中默认使用的类就自动帮咱们找到。咱们来看EnableAutoConfigurationImportSelector.classapp

public class EnableAutoConfigurationImportSelector
      extends AutoConfigurationImportSelector {

   @Override
   protected boolean isEnabled(AnnotationMetadata metadata) {
      if (getClass().equals(EnableAutoConfigurationImportSelector.class)) {
         return getEnvironment().getProperty(
               EnableAutoConfiguration.ENABLED_OVERRIDE_PROPERTY, Boolean.class,
               true);
      }
      return true;
   }

}

能够看到他继承了AutoConfigurationImportSelector咱们继续来看AutoConfigurationImportSelector，这个类有一个方法less

public String[] selectImports(AnnotationMetadata annotationMetadata) {
   if (!isEnabled(annotationMetadata)) {
      return NO_IMPORTS;
   }
   try {
      AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
            .loadMetadata(this.beanClassLoader);
      AnnotationAttributes attributes = getAttributes(annotationMetadata);
      List<String> configurations = getCandidateConfigurations(annotationMetadata,
            attributes);
      configurations = removeDuplicates(configurations);
      configurations = sort(configurations, autoConfigurationMetadata);
      Set<String> exclusions = getExclusions(annotationMetadata, attributes);
      checkExcludedClasses(configurations, exclusions);
      configurations.removeAll(exclusions);
      configurations = filter(configurations, autoConfigurationMetadata);
      fireAutoConfigurationImportEvents(configurations, exclusions);
      return configurations.toArray(new String[configurations.size()]);
   }
   catch (IOException ex) {
      throw new IllegalStateException(ex);
   }
}

这个类会帮你扫描那些类自动去添加到程序当中。咱们能够看到getCandidateConfigurations()这个方法，他的做用就是引入系统已经加载好的一些类，究竟是那些类呢，咱们点进去看一下

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
      AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
         getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
   Assert.notEmpty(configurations,
         "No auto configuration classes found in META-INF/spring.factories. If you "
               + "are using a custom packaging, make sure that file is correct.");
   return configurations;
}

这个类回去寻找的一个目录为META-INF/spring.factories，也就是说他帮你加载让你去使用也就是在这个META-INF/spring.factories目录装配的，他在哪里？

咱们点进spring.factories来看

咱们能够发现帮咱们配置了不少类的全路径，好比你想整合activemq，或者说Servlet

能够看到他都已经帮咱们引入了进来，我看随便拿几个来看

org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,\

好比咱们常常用的security，能够看到已经帮你配置好，因此咱们的EnableAutoConfiguration主要做用就是让你自动去配置，但并非全部都是建立好的，是根据你程序去进行决定。那咱们继续来看

@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

这个注解你们应该都不陌生，扫描包，放入spring容器，那他在springboot当中作了什么策略呢？咱们能够点跟烟去思考，帮咱们作了一个排除策略，他在这里结合SpringBootConfiguration去使用，为何是排除，由于不可能一上来所有加载，由于内存有限。

那么咱们来总结下@SpringbootApplication：就是说，他已经把不少东西准备好，具体是否使用取决于咱们的程序或者说配置，那咱们到底用不用？那咱们继续来看一行代码

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

那们来看下在执行run方法到底有没有用到哪些自动配置的东西，好比说内置的Tomcat，那咱们来找找内置Tomcat，咱们点进run

public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
        return new SpringApplication(sources).run(args);
    }

而后他调用又一个run方法，咱们点进来看

public ConfigurableApplicationContext run(String... args) {
   //计时器
   StopWatch stopWatch = new StopWatch();
   stopWatch.start();
   ConfigurableApplicationContext context = null;
   FailureAnalyzers analyzers = null;
   configureHeadlessProperty();
   //监听器
   SpringApplicationRunListeners listeners = getRunListeners(args);
   listeners.starting();
   try {
      ApplicationArguments applicationArguments = new DefaultApplicationArguments(
            args);
      ConfigurableEnvironment environment = prepareEnvironment(listeners,
            applicationArguments);
      Banner printedBanner = printBanner(environment);
      //准备上下文
      context = createApplicationContext();
      analyzers = new FailureAnalyzers(context);
         //预刷新context
      prepareContext(context, environment, listeners, applicationArguments,
            printedBanner);
     //刷新context
      refreshContext(context);
     //刷新以后的context
      afterRefresh(context, applicationArguments);
      listeners.finished(context, null);
      stopWatch.stop();
      if (this.logStartupInfo) {
         new StartupInfoLogger(this.mainApplicationClass)
               .logStarted(getApplicationLog(), stopWatch);
      }
      return context;
   }
   catch (Throwable ex) {
      handleRunFailure(context, listeners, analyzers, ex);
      throw new IllegalStateException(ex);
   }
}

那咱们关注的就是 refreshContext(context); 刷新context，咱们点进来看

private void refreshContext(ConfigurableApplicationContext context) {
   refresh(context);
   if (this.registerShutdownHook) {
      try {
         context.registerShutdownHook();
      }
      catch (AccessControlException ex) {
         // Not allowed in some environments.
      }
   }
}

咱们继续点进refresh(context);

protected void refresh(ApplicationContext applicationContext) {
   Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
   ((AbstractApplicationContext) applicationContext).refresh();
}

他会调用 ((AbstractApplicationContext) applicationContext).refresh();方法，咱们点进来看

public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      // Prepare this context for refreshing.
      prepareRefresh();

      // Tell the subclass to refresh the internal bean factory.
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      // Prepare the bean factory for use in this context.
      prepareBeanFactory(beanFactory);

      try {
         // Allows post-processing of the bean factory in context subclasses.
         postProcessBeanFactory(beanFactory);

         // Invoke factory processors registered as beans in the context.
         invokeBeanFactoryPostProcessors(beanFactory);

         // Register bean processors that intercept bean creation.
         registerBeanPostProcessors(beanFactory);

         // Initialize message source for this context.
         initMessageSource();

         // Initialize event multicaster for this context.
         initApplicationEventMulticaster();

         // Initialize other special beans in specific context subclasses.
         onRefresh();

         // Check for listener beans and register them.
         registerListeners();

         // Instantiate all remaining (non-lazy-init) singletons.
         finishBeanFactoryInitialization(beanFactory);

         // Last step: publish corresponding event.
         finishRefresh();
      }

      catch (BeansException ex) {
         if (logger.isWarnEnabled()) {
            logger.warn("Exception encountered during context initialization - " +
                  "cancelling refresh attempt: " + ex);
         }

         // Destroy already created singletons to avoid dangling resources.
         destroyBeans();

         // Reset 'active' flag.
         cancelRefresh(ex);

         // Propagate exception to caller.
         throw ex;
      }

      finally {
         // Reset common introspection caches in Spring's core, since we
         // might not ever need metadata for singleton beans anymore...
         resetCommonCaches();
      }
   }
}

这点代码似曾相识啊没错，就是一个spring的bean的加载过程我在，解析springIOC加载过程的时候介绍过这里面的方法，若是你看过Spring源码的话，应该知道这些方法都是作什么的。如今咱们不关心其余的，咱们来看一个方法叫作 onRefresh();方法

protected void onRefresh() throws BeansException {
   // For subclasses: do nothing by default.
}

他在这里并无实现，可是咱们找他的其余实现，咱们来找

咱们既然要找Tomcat那就确定跟web有关，咱们能够看到有个ServletWebServerApplicationContext

@Override
protected void onRefresh() {
   super.onRefresh();
   try {
      createWebServer();
   }
   catch (Throwable ex) {
      throw new ApplicationContextException("Unable to start web server", ex);
   }
}

咱们能够看到有一个createWebServer();方法他是建立web容器的，而Tomcat不就是web容器，那他是怎么建立的呢，咱们继续看

private void createWebServer() {
   WebServer webServer = this.webServer;
   ServletContext servletContext = getServletContext();
   if (webServer == null && servletContext == null) {
      ServletWebServerFactory factory = getWebServerFactory();
      this.webServer = factory.getWebServer(getSelfInitializer());
   }
   else if (servletContext != null) {
      try {
         getSelfInitializer().onStartup(servletContext);
      }
      catch (ServletException ex) {
         throw new ApplicationContextException("Cannot initialize servlet context",
               ex);
      }
   }
   initPropertySources();
}

factory.getWebServer(getSelfInitializer());他是经过工厂的方式建立的

public interface ServletWebServerFactory {

   WebServer getWebServer(ServletContextInitializer... initializers);

}

能够看到它是一个接口，为何会是接口。由于咱们不止是Tomcat一种web容器。

咱们看到还有Jetty，那咱们来看TomcatServletWebServerFactory

@Override
public WebServer getWebServer(ServletContextInitializer... initializers) {
   Tomcat tomcat = new Tomcat();
   File baseDir = (this.baseDirectory != null) ? this.baseDirectory
         : createTempDir("tomcat");
   tomcat.setBaseDir(baseDir.getAbsolutePath());
   Connector connector = new Connector(this.protocol);
   tomcat.getService().addConnector(connector);
   customizeConnector(connector);
   tomcat.setConnector(connector);
   tomcat.getHost().setAutoDeploy(false);
   configureEngine(tomcat.getEngine());
   for (Connector additionalConnector : this.additionalTomcatConnectors) {
      tomcat.getService().addConnector(additionalConnector);
   }
   prepareContext(tomcat.getHost(), initializers);
   return getTomcatWebServer(tomcat);
}

那这块代码，就是咱们要寻找的内置Tomcat，在这个过程中，咱们能够看到建立Tomcat的一个流程。由于run方法里面加载的东西不少，因此今天就浅谈到这里。若是不明白的话，咱们在用另外一种方式来理解下，你们要应该都知道stater举点例子

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-data-redis</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-freemarker</artifactId>
</dependency>

因此咱们不防不定义一个stater来理解下，咱们作一个需求，就是定制化不一样的人跟你们说大家好，咱们来看

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.4.RELEASE</version>
    <relativePath/>
</parent>
<groupId>com.zgw</groupId>
<artifactId>gw-spring-boot-srater</artifactId>
<version>1.0-SNAPSHOT</version>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
    </dependency>
</dependencies>

咱们先来看maven配置写入版本号，若是自定义一个stater的话必须依赖spring-boot-autoconfigure这个包,咱们先看下项目目录

public class GwServiceImpl  implements GwService{
    @Autowired
    GwProperties properties;

    @Override
    public void Hello()
    {
        String name=properties.getName();
        System.out.println(name+"说:大家好啊");
    }
}

咱们作的就是经过配置文件来定制name这个是具体实现,

@Component
@ConfigurationProperties(prefix = "spring.gwname")
public class GwProperties {

    String name="zgw";

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

这个类能够经过@ConfigurationProperties读取配置文件

@Configuration
@ConditionalOnClass(GwService.class)  //扫描类
@EnableConfigurationProperties(GwProperties.class) //让配置类生效
public class GwAutoConfiguration {

    /**
    * 功能描述 托管给spring
    * @author zgw
    * @return
    */
    @Bean
    @ConditionalOnMissingBean
    public GwService gwService()
    {
        return new GwServiceImpl();
    }
}

这个为配置类，为何这么写由于，spring-boot的stater都是这么写的，咱们能够参照他仿写stater，以达到自动配置的目的，而后咱们在经过spring.factories也来进行配置

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.gw.GwAutoConfiguration

而后这样一个简单的stater就完成了，而后能够进行maven的打包，在其余项目引入就可使用，在这里列出代码地址

https://github.com/zgw1469039806/gwspringbootsrater

到此本文结束,欢迎转载，请标注出处和做者，谢谢！