【Spring】原來SpringBoot是這樣玩的

渠成發表於2020-06-29
Spring Boot

菜瓜：我自己去調Mvc的原始碼差點沒給Spring的邏輯秀死。。。難受

水稻：那今天我們們看一個簡單易用的SpringBoot吧

菜瓜：可以，這個我熟悉

水稻：熟悉？

菜瓜：當我沒說，請開始你的表演

水稻：我沒有別的意思，就是單純的反問（手動狗頭）。平時工作中用多了SpringBoot。我們們今天帶著幾個問題來看看它的操作吧

  1. 如何啟動Spring容器
  2. 如何內嵌Tomcat容器
  3. 如何完成自動裝配，就是0配置

菜瓜：你確定這是我熟悉的SpringBoot？？？

水稻：。。。看過來

  • 啟動類點進去
  • public ConfigurableApplicationContext run(String... args) {
   StopWatch stopWatch = new StopWatch();
   stopWatch.start();
   ...
   listeners.starting();
   try {
      ...
      // ①建立Spring上下文容器物件 - 預設Servlet容器
      context = createApplicationContext();
      ...
      // ②呼叫refresh方法 - 回到熟悉的容器啟動流程
      refreshContext(context);
      afterRefresh(context, applicationArguments);
      ...   
   ...
   return context;
}
  • ① 建立上下文物件 
    • protected ConfigurableApplicationContext createApplicationContext() {
   Class<?> contextClass = this.applicationContextClass;
   if (contextClass == null) {
      try {
         switch (this.webApplicationType) {
         case SERVLET:
            contextClass = Class.forName(DEFAULT_SERVLET_WEB_CONTEXT_CLASS);
            break;
         case REACTIVE:
            contextClass = Class.forName(DEFAULT_REACTIVE_WEB_CONTEXT_CLASS);
            break;
         default:
            contextClass = Class.forName(DEFAULT_CONTEXT_CLASS);
         }
      }
      catch (ClassNotFoundException ex) {
         ...
      }
   }
   return (ConfigurableApplicationContext) BeanUtils.instantiateClass(contextClass);
}
  • ②啟動容器 
    •     @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            prepareRefresh();
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

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

            try {
                ...
                // ①springboot 內嵌tomcat容器
                onRefresh();
                ...

    }

    @Override
    protected void onRefresh() {
        super.onRefresh();
        try {
      // ②建立Servlet容器 預設tomcat
            createWebServer();
        }
        ...
    }      
     
      
    private void createWebServer() {
        WebServer webServer = this.webServer;
        ServletContext servletContext = getServletContext();
        if (webServer == null && servletContext == null) {
            ServletWebServerFactory factory = getWebServerFactory();
      // ③看進去 回到mvc整合tomcat的場景
            this.webServer = factory.getWebServer(getSelfInitializer());
        }
        ...
        initPropertySources();
    }      
         
    @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);
    }

水稻：好了，第一步和第二步完成了

菜瓜：就這？？？

水稻：是不是極其簡單，令人髮指。重頭戲是後面的自動裝配

  • 回到我們們啟動類的註解上
  • ...
// 標記自身被掃描  
@SpringBootConfiguration
// 下一步 - 自動裝配入口 
@EnableAutoConfiguration
// 掃描bean路徑 - 約定是啟動類所在的包：所以沒事別把啟動類挪走(都是淚)
@ComponentScan(excludeFilters = {
        @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
        @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication 
  
->   
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration 
  
-> 重要
public class AutoConfigurationImportSelector ... {
  @Override
  public void process(AnnotationMetadata annotationMetadata,
        DeferredImportSelector deferredImportSelector) {
     ...
     // 獲取以EnableAutoConfiguration命名的/META-INF/Spring.factories檔案中的value去重 
     AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector).getAutoConfigurationEntry(getAutoConfigurationMetadata(),            annotationMetadata);
     // 啟動的時候斷點可以看到
         this.autoConfigurationEntries.add(autoConfigurationEntry);
     for (String importClassName : autoConfigurationEntry.getConfigurations()) {
        this.entries.putIfAbsent(importClassName, annotationMetadata);
     }
  }
}
  • AutoConfigurationImportSelector 中的process是被ConfigurationClassPostProcessor通過processConfigBeanDefinitions方法呼叫（呼叫鏈如下）
1. this.processConfigBeanDefinitions(registry);
2. parser.parse(candidates);
3. this.parse(((AnnotatedBeanDefinition)bd).getMetadata(), holder.getBeanName());
4. sourceClass = this.doProcessConfigurationClass(configClass, sourceClass);
5. this.processImports(configClass, sourceClass, this.getImports(sourceClass), true);
6. this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector)selector);  
7. handler.processGroupImports();
8. grouping.getImports().forEach...
9. this.group.process(...);
-- 
蒐集到需要自動裝配的類，封裝成BeanDefinition後續例項化，實現自動裝配功能
譬如引入WebMvcAutoConfiguration類 - webmvc功能自動整合
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration 

菜瓜：原來如此。你把呼叫鏈拎出來就簡單了很多。自動裝配就是通過SPI載入org.springframework.boot.autoconfigure包下的class，封裝成BeanDefinition後交給容器載入

 

總結：SpringBoot只需要一行程式碼便能啟動一個Java應用。完全解放開發者複雜的配置

  • 內嵌Servlet容器，預設tomcat
  • 啟動SpringWeb容器
  • 自動裝配了簡單web應用需要的工具和組建

 

相關文章