使用spring boot很方便，一个jar包就可以启动了，因为它里面内嵌了tomcat等服务器。

但是spring boot也提供了部署到独立服务器的方法。

如果你看文档的话，从jar转换为war包很简单，pom.xml的配置修改略去不讲。

只看source的修改，很简单，只要一个配置类，继承自SpringBootServletInitializer， 并覆盖configure方法。

@SpringBootApplication
public class TestApplication extends SpringBootServletInitializer{


    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {
        return builder.sources(TestApplication .class);
    }

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

@SpringBootApplicationpublic class TestApplication extends SpringBootServletInitializer{        @Override    protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {        return builder.sources(TestApplication .class);    }    public static void main(String[] args) {        SpringApplication.run(TestApplication.class, args);    }}

 对，你没看错，就这么简单。

但是，我觉得但凡有点儿好奇心的人都不甘于就这么用它，总会想知道为啥这样就行了？

那么我们根据调用关系来弄个究竟。

SpringBootServletInitializer.configure

<-createRootApplicationContext

<-onStartup

<-SpringServletContainerInitializer.onStartup

 

SpringServletContainerInitializer这个类比较特殊，实现的是interface ServletContainerInitializer，这个类的onStartup方法，是由tomcat调用了。

那么tomcat是怎么找到它的呢？是搜寻的这个资源文件META-INF/services/javax.servlet.ServletContainerInitializer

而在spring的包spring-web-xxxx.jar包里正好有这个文件，它注册的恰恰就是这个类

 

写道

org.springframework.web.SpringServletContainerInitializer

 

这个类有个注解@HandlesTypes(WebApplicationInitializer.class)。

调用SpringServletContainerInitializer.onStartup方法时，会把所有的WebApplicationInitializer类以及子类都传过来。

然后再通过条件过滤一下。

if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&
                        WebApplicationInitializer.class.isAssignableFrom(waiClass)) {
                    try {
                        initializers.add((WebApplicationInitializer) waiClass.newInstance());
                    }
                    catch (Throwable ex) {
                        throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);
                    }
                }

if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&                        WebApplicationInitializer.class.isAssignableFrom(waiClass)) {                    try {                        initializers.add((WebApplicationInitializer) waiClass.newInstance());                    }                    catch (Throwable ex) {                        throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);                    }                }

 也就是只要是非interface，且非抽象类，并都是WebApplicationInitializer的字类的话，就会被实例化，并最终调用。

然后，在SpringBootServletInitializer的createRootApplicationContext方法里，最终会初始化SpringApplication，调用其run方法，跟直接运行入口的main方法是一样的了。

 

 既然从，SpringApplication.run方法以后走的逻辑是一样的，那么是不是需要启动内嵌web服务器的分支是在哪儿呢？

顺着这条线往下走。

SpringApplication.run(String...)
    SpringApplication.createAndRefreshContext(SpringApplicationRunListeners, ApplicationArguments)
        SpringApplication.refresh(ApplicationContext)
            AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).refresh()
                AnnotationConfigEmbeddedWebApplicationContext(AbstractApplicationContext).refresh()
                    AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).onRefresh()
                        AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).createEmbeddedServletContainer()

SpringApplication.run(String...)    SpringApplication.createAndRefreshContext(SpringApplicationRunListeners, ApplicationArguments)        SpringApplication.refresh(ApplicationContext)            AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).refresh()                AnnotationConfigEmbeddedWebApplicationContext(AbstractApplicationContext).refresh()                    AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).onRefresh()                        AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).createEmbeddedServletContainer()

 有下面一个分支代码

if (localContainer == null && localServletContext == null) {
    EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();
    this.embeddedServletContainer = containerFactory
            .getEmbeddedServletContainer(getSelfInitializer());
}

if (localContainer == null && localServletContext == null) {            EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();            this.embeddedServletContainer = containerFactory                    .getEmbeddedServletContainer(getSelfInitializer());        }

 localContainer在初始化的时候没有赋值过程，一直会是null，主要是localServletContext，看看什么时候为null，什么时候有值。

它的赋值有三个地方，两个构造函数，一个set方法

public GenericWebApplicationContext(ServletContext servletContext) {
    this.servletContext = servletContext;
}
public GenericWebApplicationContext(DefaultListableBeanFactory beanFactory, ServletContext servletContext) {
    super(beanFactory);
    this.servletContext = servletContext;
}
public void setServletContext(ServletContext servletContext) {
    this.servletContext = servletContext;
}

public GenericWebApplicationContext(ServletContext servletContext) {        this.servletContext = servletContext;    }    public GenericWebApplicationContext(DefaultListableBeanFactory beanFactory, ServletContext servletContext) {        super(beanFactory);        this.servletContext = servletContext;    }    public void setServletContext(ServletContext servletContext) {        this.servletContext = servletContext;    }

 查找一下，发现构造函数并没有地方调用，调用的是这个set方法，过程如下

SpringApplication.run(String...)
    SpringApplication.createAndRefreshContext(SpringApplicationRunListeners, ApplicationArguments)
        SpringApplication.applyInitializers(ConfigurableApplicationContext)
            ServletContextApplicationContextInitializer.initialize(ConfigurableApplicationContext)
                ServletContextApplicationContextInitializer.initialize(ConfigurableWebApplicationContext)
                    AnnotationConfigEmbeddedWebApplicationContext(GenericWebApplicationContext).setServletContext(ServletContext)

SpringApplication.run(String...)    SpringApplication.createAndRefreshContext(SpringApplicationRunListeners, ApplicationArguments)        SpringApplication.applyInitializers(ConfigurableApplicationContext)            ServletContextApplicationContextInitializer.initialize(ConfigurableApplicationContext)                ServletContextApplicationContextInitializer.initialize(ConfigurableWebApplicationContext)                    AnnotationConfigEmbeddedWebApplicationContext(GenericWebApplicationContext).setServletContext(ServletContext)

 你会发现，至少到SpringApplication.applyInitializers(ConfigurableApplicationContext)这一步，部署不部署到tomcat，都会执行这个方法的，那么区别在哪儿呢？

先看看这个方法的内容

protected void applyInitializers(ConfigurableApplicationContext context) {
    for (ApplicationContextInitializer initializer : getInitializers()) {
        Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
                initializer.getClass(), ApplicationContextInitializer.class);
        Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
        initializer.initialize(context);
    }
}

protected void applyInitializers(ConfigurableApplicationContext context) {        for (ApplicationContextInitializer initializer : getInitializers()) {            Class
       requiredType = GenericTypeResolver.resolveTypeArgument(                    initializer.getClass(), ApplicationContextInitializer.class);            Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");            initializer.initialize(context);        }    }

 也就是说，如果注入的initializers里是否包含了ServletContextApplicationContextInitializer，就能决定是否会调用以后的逻辑。

那么返回到文章的开头，看看抽象类SpringBootServletInitializer，就会发现在方法createRootApplicationContext里，类ServletContextApplicationContextInitializer的注入过程。

builder.initializers(new ServletContextApplicationContextInitializer(servletContext));

builder.initializers(new ServletContextApplicationContextInitializer(servletContext));

 内部实现就是

public SpringApplicationBuilder initializers(
        ApplicationContextInitializer<?>... initializers) {
    this.application.addInitializers(initializers);
    return this;
}

public SpringApplicationBuilder initializers(            ApplicationContextInitializer
      ... initializers) {        this.application.addInitializers(initializers);        return this;    }

 

至于spring的御用servlet——DispatcherServlet，则是通过动态添加方式添加到ServletContext里的。类EmbeddedWebApplicationContext

private void selfInitialize(ServletContext servletContext) throws ServletException {
--------省略------------
    for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
        beans.onStartup(servletContext);
    }
}

private void selfInitialize(ServletContext servletContext) throws ServletException {    --------省略------------        for (ServletContextInitializer beans : getServletContextInitializerBeans()) {            beans.onStartup(servletContext);        }    }

 类ServletRegistrationBean

@Override
public void onStartup(ServletContext servletContext) throws ServletException {
    Assert.notNull(this.servlet, "Servlet must not be null");
    String name = getServletName();
    if (!isEnabled()) {
        logger.info("Servlet " + name + " was not registered (disabled)");
        return;
    }
    logger.info("Mapping servlet: '" + name + "' to " + this.urlMappings);
    Dynamic added = servletContext.addServlet(name, this.servlet);
    if (added == null) {
        logger.info("Servlet " + name + " was not registered "
                + "(possibly already registered?)");
        return;
    }
    configure(added);
}

@Override    public void onStartup(ServletContext servletContext) throws ServletException {        Assert.notNull(this.servlet, "Servlet must not be null");        String name = getServletName();        if (!isEnabled()) {            logger.info("Servlet " + name + " was not registered (disabled)");            return;        }        logger.info("Mapping servlet: '" + name + "' to " + this.urlMappings);        Dynamic added = servletContext.addServlet(name, this.servlet);        if (added == null) {            logger.info("Servlet " + name + " was not registered "                    + "(possibly already registered?)");            return;        }        configure(added);    }