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前情提要

最近遇到一个需求，在单个服务进程上开启多网络端口，将API的访问通过端口强行隔离开。

发散探索

1. SpringBoot自带的多端口配置，server.port和management.port，将常规的API与管理相关API通过端口拆分开；
2. 1中的端口是基于HTTP/1.1通信的，如果需要基于HTTP/2.0（如gRPC)又会有些差别；
3. 一般一个进程内SpringApplication对象只有1个，理论上搞多个自然就能开启多个端口，略显粗暴；
   本着探索的态度，我会逐个探讨上述3个方向，本文先关注在方向1上。如果你有更NB的问题或者建议，欢迎评论区留言。

从management.port开始

配置生效依赖SpringBoot的AutoConfiguration机制，management相关配置体现在ManagementContextAutoConfiguration中，完成ManagementContext的配置；其中涉及的问题如下。

确定否需要开启额外端口

@ConditionalOnManagementPort决定了是否开启新端口，该注解声明如下

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnManagementPortCondition.class)
public @interface ConditionalOnManagementPort {/*** The {@link ManagementPortType} to match.* @return the port type*/ManagementPortType value();}

其依赖的OnManagementPortCondition会在配置过程中被处理，处理概要过程如下：

1. 基于当前的Environment决定实际的ManagementPortType；
2. 获得当前注解中value对应的设定ManagementPortType;
3. 基于1和2决定被其注解的bean是否初始化，ManagementPortType的定义如下

public enum ManagementPortType {/*** The management port has been disabled.*/DISABLED,/*** The management port is the same as the server port.*/SAME,/*** The management port and server port are different.*/DIFFERENT;/*** Look at the given environment to determine if the {@link ManagementPortType} is* {@link #DISABLED}, {@link #SAME} or {@link #DIFFERENT}.* @param environment the Spring environment* @return {@link #DISABLED} if {@code management.server.port} is set to a negative* value, {@link #SAME} if {@code management.server.port} is not specified or equal to* {@code server.port} and {@link #DIFFERENT} otherwise.* @since 2.1.4*/public static ManagementPortType get(Environment environment) {Integer managementPort = getPortProperty(environment, "management.server.");if (managementPort != null && managementPort < 0) {return DISABLED;}Integer serverPort = getPortProperty(environment, "server.");return ((managementPort == null || (serverPort == null && managementPort.equals(8080))|| (managementPort != 0 && managementPort.equals(serverPort))) ? SAME : DIFFERENT);}private static Integer getPortProperty(Environment environment, String prefix) {return environment.getProperty(prefix + "port", Integer.class);}}

额外端口是如何开启的

到这里，我们可以回到ManagementContextAutoConfiguration，针对性地关注@ConditionalOnManagementPort(ManagementPortType.DIFFERENT)相关类, 也就是下面这段

    @Configuration(proxyBeanMethods = false)@ConditionalOnManagementPort(ManagementPortType.DIFFERENT)static class DifferentManagementContextConfiguration implements ApplicationListener<WebServerInitializedEvent> {private final ApplicationContext applicationContext;private final ManagementContextFactory managementContextFactory;DifferentManagementContextConfiguration(ApplicationContext applicationContext,ManagementContextFactory managementContextFactory) {this.applicationContext = applicationContext;this.managementContextFactory = managementContextFactory;}@Overridepublic void onApplicationEvent(WebServerInitializedEvent event) {if (event.getApplicationContext().equals(this.applicationContext)) {ConfigurableWebServerApplicationContext managementContext = this.managementContextFactory.createManagementContext(this.applicationContext,EnableChildManagementContextConfiguration.class,PropertyPlaceholderAutoConfiguration.class);if (isLazyInitialization()) {managementContext.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());}managementContext.setServerNamespace("management");managementContext.setId(this.applicationContext.getId() + ":management");setClassLoaderIfPossible(managementContext);CloseManagementContextListener.addIfPossible(this.applicationContext, managementContext);managementContext.refresh();}}protected boolean isLazyInitialization() {AbstractApplicationContext context = (AbstractApplicationContext) this.applicationContext;List<BeanFactoryPostProcessor> postProcessors = context.getBeanFactoryPostProcessors();return postProcessors.stream().anyMatch(LazyInitializationBeanFactoryPostProcessor.class::isInstance);}private void setClassLoaderIfPossible(ConfigurableApplicationContext child) {if (child instanceof DefaultResourceLoader) {((DefaultResourceLoader) child).setClassLoader(this.applicationContext.getClassLoader());}}}

从中我们可以发现几个点：

1. ManagementContext是在主WebApplicationContext发布事件WebServerInitializedEvent后开始初始化;
2. ManagementContext全新的ConfigurableWebServerApplicationContext，这意味着最终启动后，内存中存在至少两个ApplicationContext；如果说开启两个SpringApplication对象是表象的话，那么创建两个WebApplicationContext应该说就是底层的本质了；
3. 整个处理过程就是创建ConfigurableWebServerApplicationContext，后续做必要的配置，最后refresh。如果你分析过SpringApplication.run()，那想必你看到了熟悉的味道。

ManagementContextFactory的故事

从哪儿来

对应的子类有两个，ReactiveManagementContextFactory和ServletManagementContextFactory，具体初始化则由xxxManagementContextFactoryAutoConfiguration影响，更进一步的细节藏在
@ConditionalOnWebApplication(type=xxx)中。

创建过程

以ReactiveManagementContextFactory为例

@Overridepublic ConfigurableWebServerApplicationContext createManagementContext(ApplicationContext parent,Class<?>... configClasses) {// 复用EnvironmentEnvironment parentEnvironment = parent.getEnvironment();ConfigurableEnvironment childEnvironment = ApplicationContextFactory.DEFAULT.createEnvironment(WebApplicationType.REACTIVE);if (parentEnvironment instanceof ConfigurableEnvironment) {// 复用ConversionServicechildEnvironment.setConversionService(((ConfigurableEnvironment) parentEnvironment).getConversionService());}AnnotationConfigReactiveWebServerApplicationContext child = new AnnotationConfigReactiveWebServerApplicationContext();child.setEnvironment(childEnvironment);// 这里可以看到最终会是parent和child关系child.setParent(parent);/*
这里以编程的方式完成BeanDefinition注册, 这里的configClass =     {EnableChildManagementContextConfiguration.class,PropertyPlaceholderAutoConfiguration.class}		*/Class<?>[] combinedClasses = ObjectUtils.addObjectToArray(configClasses,ReactiveWebServerFactoryAutoConfiguration.class);child.register(combinedClasses);// 最后保持parent和child的WebFactory一致, 直接复用parent的BeanDefinitionregisterReactiveWebServerFactory(parent, child);return child;}

management 相关API如何被注册

经过前面一波分析，我们已经看到了创建ApplicationContext，注册必要的Class，而后refresh整个容器就要启动，但没有看到management相关API，其实就藏在EnableChildManagementContextConfiguration.class这里, 从这里出发最终找到配置文件ManagementContextConfiguration.imports，链路如下

EnableManagementContext -> ManagementContextConfigurationImportSelector -> META-INFO/spring/org.springframework.boot.actuate.autoconfigure.web.ManagementContextConfiguration.imports

ManagementContextConfiguration.imports的内容如下：

org.springframework.boot.actuate.autoconfigure.endpoint.web.ServletEndpointManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.endpoint.web.reactive.WebFluxEndpointManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.endpoint.web.servlet.WebMvcEndpointManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.endpoint.web.jersey.JerseyWebEndpointManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.security.servlet.SecurityRequestMatchersManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.web.jersey.JerseySameManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.web.jersey.JerseyChildManagementContextConfiguration
org.springframework.boot.actuate.autoconfigure.web.reactive.ReactiveManagementChildContextConfiguration
org.springframework.boot.actuate.autoconfigure.web.servlet.ServletManagementChildContextConfiguration
org.springframework.boot.actuate.autoconfigure.web.servlet.WebMvcEndpointChildContextConfiguration

这些Configuration最终受如下2个注解的解决决定是否初始化
@ManagementContextConfiguration(value = ManagementContextType.xxx, proxyBeanMethods = false)
@ConditionalOnWebApplication(type = Type.xxx)
最终不同的EndPoint由不同的EndPointDiscover对整个ApplicationContext中的类进行扫描并提取出来，组装为EndPointHandlerMapping。

到这里可以解释一个现象，就是在manage端口上，无法访问到应用端口上的API，因为两者各自关联了一个独立的HandlerMapping。虽然通常child context可以访问到parent context中的所有bean实例，但是各自可见的API被各自的RequestHandlerMapping限制而隔离。

小结

以上是针对mangement API开放独立端口的探究，从中有几个点值得借鉴：

1. 独立的ApplicationContext可以作为资源隔离的一种方式，同时又不完全失去与parent的联系；
2. 条件初始化在SpringBoot自动配置中的广泛应用；
3. WebApplicationContext与ApplicationContext的区别在于WebServerFactory是否存在，以及RequestHandlerMapping+必要的注解识别，有了这点发现，做API隔离可用的方案也会更加丰富；
   后续会对另外2种实现方式做探讨，感谢你的阅读。