Spring boot传统部署

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。

Java代码  

1. @SpringBootApplication  
2. public class TestApplication extends SpringBootServletInitializer{  
3.   
4.       
5.     @Override  
6.     protected SpringApplicationBuilder configure(SpringApplicationBuilder builder) {  
7.         return builder.sources(TestApplication .class);  
8.     }  
9.   
10.     public static void main(String[] args) {  
11.         SpringApplication.run(TestApplication.class, args);  
12.     }  
13. }  

@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);
    }
}

 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

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。

Java代码  

1. if (!waiClass.isInterface() && !Modifier.isAbstract(waiClass.getModifiers()) &&  
2.                         WebApplicationInitializer.class.isAssignableFrom(waiClass)) {  
3.                     try {  
4.                         initializers.add((WebApplicationInitializer) waiClass.newInstance());  
5.                     }  
6.                     catch (Throwable ex) {  
7.                         throw new ServletException("Failed to instantiate WebApplicationInitializer class", ex);  
8.                     }  
9.                 }  

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。

Java代码  

1. SpringApplication.run(String...)  
2.     SpringApplication.createAndRefreshContext(SpringApplicationRunListeners, ApplicationArguments)  
3.         SpringApplication.refresh(ApplicationContext)  
4.             AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).refresh()  
5.                 AnnotationConfigEmbeddedWebApplicationContext(AbstractApplicationContext).refresh()  
6.                     AnnotationConfigEmbeddedWebApplicationContext(EmbeddedWebApplicationContext).onRefresh()  
7.                         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

Java代码  

1. if (localContainer == null && localServletContext == null) {  
2.     EmbeddedServletContainerFactory containerFactory = getEmbeddedServletContainerFactory();  
3.     this.embeddedServletContainer = containerFactory  
4.             .getEmbeddedServletContainer(getSelfInitializer());  
5. }  

       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

Java代码  

1. public GenericWebApplicationContext(ServletContext servletContext) {  
2.     this.servletContext = servletContext;  
3. }  
4. public GenericWebApplicationContext(DefaultListableBeanFactory beanFactory, ServletContext servletContext) {  
5.     super(beanFactory);  
6.     this.servletContext = servletContext;  
7. }  
8. public void setServletContext(ServletContext servletContext) {  
9.     this.servletContext = servletContext;  
10. }  

    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

Java代码  

1. SpringApplication.run(String...)  
2.     SpringApplication.createAndRefreshContext(SpringApplicationRunListeners, ApplicationArguments)  
3.         SpringApplication.applyInitializers(ConfigurableApplicationContext)  
4.             ServletContextApplicationContextInitializer.initialize(ConfigurableApplicationContext)  
5.                 ServletContextApplicationContextInitializer.initialize(ConfigurableWebApplicationContext)  
6.                     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

Java代码  

1. protected void applyInitializers(ConfigurableApplicationContext context) {  
2.     for (ApplicationContextInitializer initializer : getInitializers()) {  
3.         Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(  
4.                 initializer.getClass(), ApplicationContextInitializer.class);  
5.         Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");  
6.         initializer.initialize(context);  
7.     }  
8. }  

 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。

Java代码  

1. builder.initializers(new ServletContextApplicationContextInitializer(servletContext));  

builder.initializers(new ServletContextApplicationContextInitializer(servletContext));

 内部实现就是

Java代码  

1. public SpringApplicationBuilder initializers(  
2.         ApplicationContextInitializer<?>... initializers) {  
3.     this.application.addInitializers(initializers);  
4.     return this;  
5. }  

 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

Java代码  

1. private void selfInitialize(ServletContext servletContext) throws ServletException {  
2. --------省略------------  
3.     for (ServletContextInitializer beans : getServletContextInitializerBeans()) {  
4.         beans.onStartup(servletContext);  
5.     }  
6. }  

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

 类ServletRegistrationBean

Java代码  

1. @Override  
2. public void onStartup(ServletContext servletContext) throws ServletException {  
3.     Assert.notNull(this.servlet, "Servlet must not be null");  
4.     String name = getServletName();  
5.     if (!isEnabled()) {  
6.         logger.info("Servlet " + name + " was not registered (disabled)");  
7.         return;  
8.     }  
9.     logger.info("Mapping servlet: ‘" + name + "‘ to " + this.urlMappings);  
10.     Dynamic added = servletContext.addServlet(name, this.servlet);  
11.     if (added == null) {  
12.         logger.info("Servlet " + name + " was not registered "  
13.                 + "(possibly already registered?)");  
14.         return;  
15.     }  
16.     configure(added);  
17. }  

    @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);
    }