SpringBoot内置tomcat启动原理

前言

         不得不说springboot的开发者是在为大众程序猿谋福利，把大家都惯成了懒汉，xml不配置了，连tomcat也懒的配置了，典型的一键启动系统，那么tomcat在springboot是怎么启动的呢？ 

内置tomcat

         开发阶段对我们来说使用内置的tomcat是非常够用了，当然也可以使用jetty。

<dependency>
   <groupid>org.springframework.boot</groupid>
   <artifactid>spring-boot-starter-web</artifactid>
   <version>2.1.6.release</version>
</dependency>

@springbootapplication
public class myspringboottomcatstarter{
    public static void main(string[] args) {
        long time=system.currenttimemillis();
        springapplication.run(myspringboottomcatstarter.class);
        system.out.println("===应用启动耗时："+(system.currenttimemillis()-time)+"===");
    }
}

        这里是main函数入口，两句代码最耀眼，分别是springbootapplication注解和springapplication.run()方法。

发布生产

     发布的时候，目前大多数的做法还是排除内置的tomcat，打瓦包（war）然后部署在生产的tomcat中，好吧，那打包的时候应该怎么处理？

<dependency>
    <groupid>org.springframework.boot</groupid>
    <artifactid>spring-boot-starter-web</artifactid>
    <!-- 移除嵌入式tomcat插件 -->
    <exclusions>
        <exclusion>
            <groupid>org.springframework.boot</groupid>
            <artifactid>spring-boot-starter-tomcat</artifactid>
        </exclusion>
    </exclusions>
</dependency>
<!--添加servlet-api依赖--->
<dependency>
    <groupid>javax.servlet</groupid>
    <artifactid>javax.servlet-api</artifactid>
    <version>3.1.0</version>
    <scope>provided</scope>
</dependency>

       更新main函数，主要是继承springbootservletinitializer，并重写configure()方法。

@springbootapplication
public class myspringboottomcatstarter extends springbootservletinitializer {
    public static void main(string[] args) {
        long time=system.currenttimemillis();
        springapplication.run(myspringboottomcatstarter.class);
        system.out.println("===应用启动耗时："+(system.currenttimemillis()-time)+"===");
    }

    @override
    protected springapplicationbuilder configure(springapplicationbuilder builder) {
        return builder.sources(this.getclass());
    }
}

从main函数说起

public static configurableapplicationcontext run(class<?> primarysource, string... args) {
    return run(new class[]{primarysource}, args);
}

--这里run方法返回的是configurableapplicationcontext
public static configurableapplicationcontext run(class<?>[] primarysources, string[] args) {
    return (new springapplication(primarysources)).run(args);
}

public configurableapplicationcontext run(string... args) {
    configurableapplicationcontext context = null;
    collection<springbootexceptionreporter> exceptionreporters = new arraylist();
    this.configureheadlessproperty();
    springapplicationrunlisteners listeners = this.getrunlisteners(args);
    listeners.starting();

    collection exceptionreporters;
    try {
        applicationarguments applicationarguments = new defaultapplicationarguments(args);
        configurableenvironment environment = this.prepareenvironment(listeners, applicationarguments);
        this.configureignorebeaninfo(environment);
        
        //打印banner，这里你可以自己涂鸦一下，换成自己项目的logo
        banner printedbanner = this.printbanner(environment);
        
        //创建应用上下文
        context = this.createapplicationcontext();
        exceptionreporters = this.getspringfactoriesinstances(springbootexceptionreporter.class, new class[]{configurableapplicationcontext.class}, context);

        //预处理上下文
        this.preparecontext(context, environment, listeners, applicationarguments, printedbanner);
        
        //刷新上下文
        this.refreshcontext(context);
        
        //再刷新上下文
        this.afterrefresh(context, applicationarguments);
        
        listeners.started(context);
        this.callrunners(context, applicationarguments);
    } catch (throwable var10) {
        
    }

    try {
        listeners.running(context);
        return context;
    } catch (throwable var9) {
        
    }
}

        既然我们想知道tomcat在springboot中是怎么启动的，那么run方法中，重点关注创建应用上下文（createapplicationcontext）和刷新上下文（refreshcontext）。  

创建上下文

//创建上下文
protected configurableapplicationcontext createapplicationcontext() {
    class<?> contextclass = this.applicationcontextclass;
    if (contextclass == null) {
        try {
            switch(this.webapplicationtype) {
                case servlet:
                    //创建annotationconfigservletwebserverapplicationcontext
                    contextclass = class.forname("org.springframework.boot.web.servlet.context.annotationconfigservletwebserverapplicationcontext");
                    break;
                case reactive:
                    contextclass = class.forname("org.springframework.boot.web.reactive.context.annotationconfigreactivewebserverapplicationcontext");
                    break;
                default:
                    contextclass = class.forname("org.springframework.context.annotation.annotationconfigapplicationcontext");
            }
        } catch (classnotfoundexception var3) {
            throw new illegalstateexception("unable create a default applicationcontext, please specify an applicationcontextclass", var3);
        }
    }

    return (configurableapplicationcontext)beanutils.instantiateclass(contextclass);
}

这里会创建annotationconfigservletwebserverapplicationcontext类。
而annotationconfigservletwebserverapplicationcontext类继承了servletwebserverapplicationcontext，而这个类是最终集成了abstractapplicationcontext。

刷新上下文

//springapplication.java
//刷新上下文
private void refreshcontext(configurableapplicationcontext context) {
    this.refresh(context);
    if (this.registershutdownhook) {
        try {
            context.registershutdownhook();
        } catch (accesscontrolexception var3) {
        }
    }
}

//这里直接调用最终父类abstractapplicationcontext.refresh()方法
protected void refresh(applicationcontext applicationcontext) {
    ((abstractapplicationcontext)applicationcontext).refresh();
}

//abstractapplicationcontext.java
public void refresh() throws beansexception, illegalstateexception {
    synchronized(this.startupshutdownmonitor) {
        this.preparerefresh();
        configurablelistablebeanfactory beanfactory = this.obtainfreshbeanfactory();
        this.preparebeanfactory(beanfactory);

        try {
            this.postprocessbeanfactory(beanfactory);
            this.invokebeanfactorypostprocessors(beanfactory);
            this.registerbeanpostprocessors(beanfactory);
            this.initmessagesource();
            this.initapplicationeventmulticaster();
            //调用各个子类的onrefresh()方法，也就说这里要回到子类：servletwebserverapplicationcontext，调用该类的onrefresh()方法
            this.onrefresh();
            this.registerlisteners();
            this.finishbeanfactoryinitialization(beanfactory);
            this.finishrefresh();
        } catch (beansexception var9) {
            this.destroybeans();
            this.cancelrefresh(var9);
            throw var9;
        } finally {
            this.resetcommoncaches();
        }

    }
}

//servletwebserverapplicationcontext.java
//在这个方法里看到了熟悉的面孔，this.createwebserver，神秘的面纱就要揭开了。
protected void onrefresh() {
    super.onrefresh();
    try {
        this.createwebserver();
    } catch (throwable var2) {
        
    }
}

//servletwebserverapplicationcontext.java
//这里是创建webserver，但是还没有启动tomcat，这里是通过servletwebserverfactory创建，那么接着看下servletwebserverfactory
private void createwebserver() {
    webserver webserver = this.webserver;
    servletcontext servletcontext = this.getservletcontext();
    if (webserver == null && servletcontext == null) {
        servletwebserverfactory factory = this.getwebserverfactory();
        this.webserver = factory.getwebserver(new servletcontextinitializer[]{this.getselfinitializer()});
    } else if (servletcontext != null) {
        try {
            this.getselfinitializer().onstartup(servletcontext);
        } catch (servletexception var4) {
        
        }
    }

    this.initpropertysources();
}

//接口
public interface servletwebserverfactory {
    webserver getwebserver(servletcontextinitializer... initializers);
}

//实现
abstractservletwebserverfactory
jettyservletwebserverfactory
tomcatservletwebserverfactory
undertowservletwebserverfactory

 这里servletwebserverfactory接口有4个实现类

而其中我们常用的有两个：tomcatservletwebserverfactory和jettyservletwebserverfactory。

//tomcatservletwebserverfactory.java
//这里我们使用的tomcat，所以我们查看tomcatservletwebserverfactory。到这里总算是看到了tomcat的踪迹。
@override
public webserver getwebserver(servletcontextinitializer... initializers) {
    tomcat tomcat = new tomcat();
    file basedir = (this.basedirectory != null) ? this.basedirectory : createtempdir("tomcat");
    tomcat.setbasedir(basedir.getabsolutepath());
    //创建connector对象
    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);
}

protected tomcatwebserver gettomcatwebserver(tomcat tomcat) {
    return new tomcatwebserver(tomcat, getport() >= 0);
}

//tomcat.java
//返回engine容器，看到这里，如果熟悉tomcat源码的话，对engine不会感到陌生。
public engine getengine() {
    service service = getserver().findservices()[0];
    if (service.getcontainer() != null) {
        return service.getcontainer();
    }
    engine engine = new standardengine();
    engine.setname( "tomcat" );
    engine.setdefaulthost(hostname);
    engine.setrealm(createdefaultrealm());
    service.setcontainer(engine);
    return engine;
}
//engine是*别容器，host是engine的子容器，context是host的子容器，wrapper是context的子容器

           getwebserver这个方法创建了tomcat对象，并且做了两件重要的事情：把connector对象添加到tomcat中，configureengine(tomcat.getengine());
           getwebserver方法返回的是tomcatwebserver。

//tomcatwebserver.java
//这里调用构造函数实例化tomcatwebserver
public tomcatwebserver(tomcat tomcat, boolean autostart) {
    assert.notnull(tomcat, "tomcat server must not be null");
    this.tomcat = tomcat;
    this.autostart = autostart;
    initialize();
}

private void initialize() throws webserverexception {
    //在控制台会看到这句日志
    logger.info("tomcat initialized with port(s): " + getportsdescription(false));
    synchronized (this.monitor) {
        try {
            addinstanceidtoenginename();

            context context = findcontext();
            context.addlifecyclelistener((event) -> {
                if (context.equals(event.getsource()) && lifecycle.start_event.equals(event.gettype())) {
                    removeserviceconnectors();
                }
            });

            //===启动tomcat服务===
            this.tomcat.start();

            rethrowdeferredstartupexceptions();

            try {
                contextbindings.bindclassloader(context, context.getnamingtoken(), getclass().getclassloader());
            }
            catch (namingexception ex) {
                
            }
            
            //开启阻塞非守护进程
            startdaemonawaitthread();
        }
        catch (exception ex) {
            stopsilently();
            destroysilently();
            throw new webserverexception("unable to start embedded tomcat", ex);
        }
    }
}

//tomcat.java
public void start() throws lifecycleexception {
    getserver();
    server.start();
}
//这里server.start又会回到tomcatwebserver的
public void stop() throws lifecycleexception {
    getserver();
    server.stop();
}

//tomcatwebserver.java
//启动tomcat服务
@override
public void start() throws webserverexception {
    synchronized (this.monitor) {
        if (this.started) {
            return;
        }
        try {
            addpreviouslyremovedconnectors();
            connector connector = this.tomcat.getconnector();
            if (connector != null && this.autostart) {
                performdeferredloadonstartup();
            }
            checkthatconnectorshavestarted();
            this.started = true;
            //在控制台打印这句日志，如果在yml设置了上下文，这里会打印
            logger.info("tomcat started on port(s): " + getportsdescription(true) + " with context path '"
                    + getcontextpath() + "'");
        }
        catch (connectorstartfailedexception ex) {
            stopsilently();
            throw ex;
        }
        catch (exception ex) {
            throw new webserverexception("unable to start embedded tomcat server", ex);
        }
        finally {
            context context = findcontext();
            contextbindings.unbindclassloader(context, context.getnamingtoken(), getclass().getclassloader());
        }
    }
}

//关闭tomcat服务
@override
public void stop() throws webserverexception {
    synchronized (this.monitor) {
        boolean wasstarted = this.started;
        try {
            this.started = false;
            try {
                stoptomcat();
                this.tomcat.destroy();
            }
            catch (lifecycleexception ex) {
                
            }
        }
        catch (exception ex) {
            throw new webserverexception("unable to stop embedded tomcat", ex);
        }
        finally {
            if (wasstarted) {
                containercounter.decrementandget();
            }
        }
    }
}

 

附：tomcat顶层结构图

      tomcat最顶层容器是server，代表着整个服务器，一个server包含多个service。从上图可以看除service主要包括多个connector和一个container。connector用来处理连接相关的事情，并提供socket到request和response相关转化。container用于封装和管理servlet，以及处理具体的request请求。那么上文提到的engine>host>context>wrapper容器又是怎么回事呢？ 我们来看下图：

      综上所述，一个tomcat只包含一个server，一个server可以包含多个service，一个service只有一个container，但有多个connector，这样一个服务可以处理多个连接。
      多个connector和一个container就形成了一个service，有了service就可以对外提供服务了，但是service要提供服务又必须提供一个宿主环境，那就非server莫属了，所以整个tomcat的声明周期都由server控制。

总结

   springboot的启动主要是通过实例化springapplication来启动的，启动过程主要做了以下几件事情：配置属性、获取监听器，发布应用开始启动事件初、始化输入参数、配置环境，输出banner、创建上下文、预处理上下文、刷新上下文、再刷新上下文、发布应用已经启动事件、发布应用启动完成事件。在springboot中启动tomcat的工作在刷新上下这一步。而tomcat的启动主要是实例化两个组件：connector、container，一个tomcat实例就是一个server，一个server包含多个service，也就是多个应用程序，每个service包含多个connector和一个container，而一个container下又包含多个子容器。