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。

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

@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

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。

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。

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

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

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

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

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。

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

builder.initializers(new ServletContextApplicationContextInitializer(servletContext));

内部实现就是

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

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

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);
	}