分析SpringBoot底层机制

Tomcat启动分析，Spring容器初始化，Tomcat如何关联Spring容器？

1.创建SpringBoot环境

（1）创建Maven程序，创建SpringBoot环境

（2）pom.xml导入SpringBoot的父工程和依赖

<!--导入SpringBoot父工程-规定写法-->
<parent>
    <artifactId>spring-boot-starter-parent</artifactId>
    <groupId>org.springframework.boot</groupId>
    <version>2.5.3</version>
</parent>
<dependencies>
    <!--导入web项目场景启动器：会自动导入和web开发相关的所有依赖[jar包]-->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
</dependencies>

（3）创建主程序MainApp.java

package com.li.springboot;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

/**
 * @author 李
 * @version 1.0
 */
@SpringBootApplication//表示SpringBoot项目
public class MainApp {
    public static void main(String[] args) {
        //启动SpringBoot项目
        ConfigurableApplicationContext ioc =
                SpringApplication.run(MainApp.class, args);
    }
}

（4）启动项目，我们可以注意到Tomcat也随之启动了。

问题一：当我们执行run方法时，为什么会启动我们内置的tomcat？它的底层是如何实现的？

2.Spring容器初始化（@Configuration+@Bean）

我们知道，如果在一个类上添加了注解@Configuration，那么这个类就会变成配置类；配置类中通过@Bean注解，可以将方法中 new 出来的Bean对象注入到容器中，该bean对象的id默认为方法名。

配置类本身也会作为bean注入到容器中

容器初始化的底层机制仍然是我们之前分析的Spring容器的机制（IO/文件扫描+注解+反射+集合+映射）

对比：

1. Spring通过@ComponentScan，指定要扫描的包；而SpringBoot默认从主程序所在的包开始扫描，同时也可以指定要扫描的包（scanBasePackages = {"xxx.xx"}）。
2. Spring通过xml或者注解，指定要注入的bean；SpringBoot通过扫描配置类（对应spring的xml）的@Bean或者注解，指定注入bean

3.SpringBoot怎样启动Tomcat，并能支持访问@Controller？

由前面的例子1中可以看到，当启动SpringBoot时，tomcat也会随之启动。那么问题来了：

1. SpringBoot是怎么内嵌Tomcat，并启动Tomcat的？
2. 而且底层是怎样让@Controller修饰的控制器也可以被访问的？

3.1源码分析SpringApplication.run()

SpringApplication.run()方法会完成两个重要任务：

1. 创建容器
2. 容器的刷新：包括参数的刷新+启动Tomcat

（1）创建一个控制器

package com.li.springboot.controller;

import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

/**
 * @author 李
 * @version 1.0
 * HiController被标注后，作为一个控制器注入容器中
 */
@RestController//相当于@Controller+@ResponseBody
public class HiController {

    @RequestMapping("/hi")
    public String hi() {
        return "hi,HiController";
    }
}

（2）启动主程序MainApp.java，进行debug

（3）首先进入SpringApplication.java的run方法

（4）点击step into，进入如下方法

public ConfigurableApplicationContext run(String... args) {
    ...
    try {
        ...
        context = this.createApplicationContext();//严重分析，创建容器
        context.setApplicationStartup(this.applicationStartup);
        this.prepareContext(bootstrapContext, context, environment, listeners, applicationArguments, printedBanner);
        this.refreshContext(context);//刷新应用上下文，比如初始化默认设置/注入相关bean/启动Tomcat
        this.afterRefresh(context, applicationArguments);
        stopWatch.stop();
        ...

    } catch (Throwable var10) {...}
    ...
}

（5）分别对 **createApplicationContext() **和 refreshContext(context) 方法进行分析：

（5.1）step into 进入 **createApplicationContext() ** 方法中：

//springApplication.java
//容器类型很多，会根据你的this.webApplicationType创建对应的容器，默认this.webApplicationType
//的类型为SERVLET，也就是web容器(可以处理servlet)
protected ConfigurableApplicationContext createApplicationContext() {
    return this.applicationContextFactory.create(this.webApplicationType);
}

（5.2）点击进入下一层

//接口 ApplicationContextFactory.java

//该方法根据webApplicationType创建不同的容器
ApplicationContextFactory DEFAULT = (webApplicationType) -> {
    try {
        switch(webApplicationType) {
        case SERVLET://默认进入这一分支，返回
                //AnnotationConfigServletWebServerApplicationContext容器
            return new AnnotationConfigServletWebServerApplicationContext();
        case REACTIVE:
            return new AnnotationConfigReactiveWebServerApplicationContext();
        default:
            return new AnnotationConfigApplicationContext();
        }
    } catch (Exception var2) {
        throw new IllegalStateException("Unable create a default ApplicationContext instance, you may need a custom ApplicationContextFactory", var2);
    }
};

总结：createApplicationContext()方法中创建了容器，但是还没有将bean注入到容器中。

（5.3）step into 进入 refreshContext(context) 方法中：

//springApplication.java
private void refreshContext(ConfigurableApplicationContext context) {
    if (this.registerShutdownHook) {
        shutdownHook.registerApplicationContext(context);
    }

    this.refresh(context);//核心，真正执行相关任务
}

（5.4）在this.refresh(context);这一步进入下一层：

//springApplication.java
protected void refresh(ConfigurableApplicationContext applicationContext) {
    applicationContext.refresh();
}

（5.5）继续进入下一层：

protected void refresh(ConfigurableApplicationContext applicationContext) {
    applicationContext.refresh();
}

（5.6）继续进入下一层：

//ServletWebServerApplicationContext.java
public final void refresh() throws BeansException, IllegalStateException {
    try {
        super.refresh();
    } catch (RuntimeException var3) {
        WebServer webServer = this.webServer;
        if (webServer != null) {
            webServer.stop();
        }

        throw var3;
    }
}

（5.7）在super.refresh();这一步进入下一层：

//AbstractApplicationContext.java

@Override
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
      StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");
     ...
      try {
        ...
         // Initialize other special beans in specific context subclasses.
         //在上下文的子类初始化指定的bean
         onRefresh(); //当父类完成通用的工作后，再重新用动态绑定机制回到子类
        ...
      }

      catch (BeansException ex) {...}

      finally {...}
   }
}

（5.8）在onRefresh();这一步step into，会重新返回上一层：

//ServletWebServerApplicationContext.java
protected void onRefresh() {
    super.onRefresh();

    try {
        this.createWebServer();//创建一个webserver，可以理解成创建我们指定的web服务-Tomcat
    } catch (Throwable var2) {
        throw new ApplicationContextException("Unable to start web server", var2);
    }
}

（5.9）在this.createWebServer();这一步step into：

//ServletWebServerApplicationContext.java

private void createWebServer() {
    WebServer webServer = this.webServer;
    ServletContext servletContext = this.getServletContext();
    if (webServer == null && servletContext == null) {
        StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
        ServletWebServerFactory factory = this.getWebServerFactory();
        createWebServer.tag("factory", factory.getClass().toString());
        this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});//使用TomcatServletWebServerFactory创建一个TomcatWebServer
        createWebServer.end();
        this.getBeanFactory().registerSingleton("webServerGracefulShutdown", new WebServerGracefulShutdownLifecycle(this.webServer));
        this.getBeanFactory().registerSingleton("webServerStartStop", new WebServerStartStopLifecycle(this, this.webServer));
    } else if (servletContext != null) {
        try {
            this.getSelfInitializer().onStartup(servletContext);
        } catch (ServletException var5) {
            throw new ApplicationContextException("Cannot initialize servlet context", var5);
        }
    }

    this.initPropertySources();
}

（5.10）在this.webServer = factory.getWebServer(new ServletContextInitializer[]{this.getSelfInitializer()});这一步step into：

//TomcatServletWebServerFactory.java会创建Tomcat，并启动Tomcat

public WebServer getWebServer(ServletContextInitializer... initializers) {
    if (this.disableMBeanRegistry) {
        Registry.disableRegistry();
    }

    Tomcat tomcat = new Tomcat();//创建了Tomcat对象，下面是一系列的初始化任务
    File baseDir = this.baseDirectory != null ? this.baseDirectory : this.createTempDir("tomcat");
    tomcat.setBaseDir(baseDir.getAbsolutePath());
    Connector connector = new Connector(this.protocol);
    connector.setThrowOnFailure(true);
    tomcat.getService().addConnector(connector);
    this.customizeConnector(connector);
    tomcat.setConnector(connector);
    tomcat.getHost().setAutoDeploy(false);
    this.configureEngine(tomcat.getEngine());
    Iterator var5 = this.additionalTomcatConnectors.iterator();

    while(var5.hasNext()) {
        Connector additionalConnector = (Connector)var5.next();
        tomcat.getService().addConnector(additionalConnector);
    }

    this.prepareContext(tomcat.getHost(), initializers);
    return this.getTomcatWebServer(tomcat);
}

（5.11）在return this.getTomcatWebServer(tomcat);这一步step into：

//TomcatServletWebServerFactory.java

//这里做了端口校验，创建了TomcatWebServer
protected TomcatWebServer getTomcatWebServer(Tomcat tomcat) {
    return new TomcatWebServer(tomcat, this.getPort() >= 0, this.getShutdown());
}

（5.12）继续step into进入下一层

//TomcatServletWebServerFactory.java

public TomcatWebServer(Tomcat tomcat, boolean autoStart, Shutdown shutdown) {
    this.monitor = new Object();
    this.serviceConnectors = new HashMap();
    Assert.notNull(tomcat, "Tomcat Server must not be null");
    this.tomcat = tomcat;
    this.autoStart = autoStart;
    this.gracefulShutdown = shutdown == Shutdown.GRACEFUL ? new GracefulShutdown(tomcat) : null;
    this.initialize();//进行初始化，并启动tomcat
}

（5.13）this.initialize();继续step into：

//TomcatServletWebServerFactory.java

private void initialize() throws WebServerException {
    logger.info("Tomcat initialized with port(s): " + this.getPortsDescription(false));
    synchronized(this.monitor) {
        try {
            this.addInstanceIdToEngineName();
            Context context = this.findContext();
            context.addLifecycleListener((event) -> {
                if (context.equals(event.getSource()) && "start".equals(event.getType())) {
                    this.removeServiceConnectors();
                }

            });
            this.tomcat.start();//启动Tomcat！
            this.rethrowDeferredStartupExceptions();

            try {
                ContextBindings.bindClassLoader(context, context.getNamingToken(), this.getClass().getClassLoader());
            } catch (NamingException var5) {
            }

            this.startDaemonAwaitThread();
        } catch (Exception var6) {
            this.stopSilently();
            this.destroySilently();
            throw new WebServerException("Unable to start embedded Tomcat", var6);
        }

    }
}

（6）一路返回上层，然后终于执行完refreshContext(context)方法，此时context为已经注入了bean