归志宁无五亩园，读书本意在元元。这篇文章主要讲述spring4.1.8扩展实战之一：自定义环境变量验证相关的知识，希望能为你提供帮助。
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关于扩展

• 【spring4.1.8扩展实战之一（自定义环境变量验证）】在之前学习spring环境初始化源码的过程中，见到有些地方能通过子类来实现自定义扩展，从本章开始，我们来逐个实践这些扩展，除了加深对spring的理解，有的扩展也能解决一些通用的问题；
  
• 文中涉及的spring版本号为4.1.8.RELEASE；

相关文章链接

• 为了方便开发和测试，我们的扩展实战是在SpringBoot框架下进行的，在SpringBoot自定义spring扩展的方式请参考《SpringBoot应用使用自定义的ApplicationContext实现类》

扩展功能介绍

• 今天实战的内容，是通过spring容器来确保环境变量MYSQL_HOST一定存在，如果不存在应用就会启动失败；

分析spring源码

• 通过分析spring源码来确定如何扩展；
  
• 在spring环境初始化的时候，AbstractApplicationContext的prepareRefresh方法会被调用，源码如下

protected void prepareRefresh() this.startupDate = System.currentTimeMillis(); this.closed.set(false); this.active.set(true); if (logger.isInfoEnabled()) logger.info("Refreshing " + this); // Initialize any placeholder property sources in the context environment initPropertySources(); // Validate that all properties marked as required are resolvable // see ConfigurablePropertyResolver#setRequiredProperties getEnvironment().validateRequiredProperties();

• initPropertySources是个空方法，留给子类扩展；
  
• 来看看getEnvironment().validateRequiredProperties()的源码，对应的是AbstractPropertyResolver类的validateRequiredProperties方法：

public void validateRequiredProperties() MissingRequiredPropertiesException ex = new MissingRequiredPropertiesException(); for (String key : this.requiredProperties) if (this.getProperty(key) == null) ex.addMissingRequiredProperty(key); if (!ex.getMissingRequiredProperties().isEmpty()) throw ex;

• 可见spring容器初始化的时候，会从集合requiredProperties中取出所有key，然后获取这些key的环境变量（包括系统环境变量和进程环境变量），如果有一个key对应的环境变量为空，就会抛出异常，导致spring容器初始化失败；

扩展功能分析

• 看了AbstractPropertyResolver类的validateRequiredProperties方法源码后，可以确定该方法能强制要求一些环境变量必须存在，否则停止spring启动，我们只要把我们认为必要的环境变量的key存入集合requiredProperties中即可，达到此目标需要解决下面两个问题：
  
• 如何将环境变量的key存入集合requiredProperties？
  调用AbstractPropertyResolver类的setRequiredProperties方法，注意该方法是向集合requiredProperties中添加数据，并不会将已有数据清除；
  
• 在什么时候执行AbstractPropertyResolver类的setRequiredProperties方法设置key？
  创建一个AbstractApplicationContext的子类，重写initPropertySources方法，在此方法中执行AbstractPropertyResolver类的setRequiredProperties；

开始实战

• 用maven创建一个SpringBoot工程，工程名为customizepropertyverify，pom.xml内容如下：

< ?xml version="1.0" encoding="UTF-8"?> < project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> < modelVersion> 4.0.0< /modelVersion> < groupId> com.bolingcavalry< /groupId> < artifactId> customizepropertyverify< /artifactId> < version> 0.0.1-SNAPSHOT< /version> < packaging> jar< /packaging> < name> customizepropertyverify< /name> < description> Demo project for Spring Boot< /description> < parent> < groupId> org.springframework.boot< /groupId> < artifactId> spring-boot-starter-parent< /artifactId> < version> 2.0.4.RELEASE< /version> < relativePath/> < !-- lookup parent from repository --> < /parent> < properties> < project.build.sourceEncoding> UTF-8< /project.build.sourceEncoding> < project.reporting.outputEncoding> UTF-8< /project.reporting.outputEncoding> < java.version> 1.8< /java.version> < /properties> < dependencies> < dependency> < groupId> org.springframework.boot< /groupId> < artifactId> spring-boot-starter-web< /artifactId> < /dependency> < dependency> < groupId> org.springframework.boot< /groupId> < artifactId> spring-boot-starter-test< /artifactId> < scope> test< /scope> < /dependency> < /dependencies> < build> < plugins> < plugin> < groupId> org.springframework.boot< /groupId> < artifactId> spring-boot-maven-plugin< /artifactId> < /plugin> < /plugins> < /build> < /project>

• 创建一个类CustomApplicationContext，继承自AnnotationConfigServletWebServerApplicationContext，重写initPropertySources方法，将" mysql_HOST" 作为启动时必须要存在的环境变量：

package com.bolingcavalry.customizepropertyverify.context; import org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext; /** * @Description : AnnotationConfigServletWebServerApplicationContext，重写了initPropertySources方法， * 要求spring启动的时候环境变量MYSQL_HOST必须存在 * @Author : zq2599@gmail.com * @Date : 2018-08-10 21:40 */ public class CustomApplicationContext extends AnnotationConfigServletWebServerApplicationContext @Override protected void initPropertySources() super.initPropertySources(); //把"MYSQL_HOST"作为启动的时候必须验证的环境变量 getEnvironment().setRequiredProperties("MYSQL_HOST");

• 创建应用启动类CustomizepropertyverifyApplication，指定ApplicationContext的class为CustomApplicationContext：

package com.bolingcavalry.customizepropertyverify; import com.bolingcavalry.customizepropertyverify.context.CustomApplicationContext; import org.springframework.boot.SpringApplication; import org.springframework.boot.autoconfigure.SpringBootApplication; @SpringBootApplication public class CustomizepropertyverifyApplication public static void main(String[] args) SpringApplication springApplication = new SpringApplication(CustomizepropertyverifyApplication.class); springApplication.setApplicationContextClass(CustomApplicationContext.class); springApplication.run(args);

• 打包，在pom.xml文件所在目录下执行命令mvn clean package -U -DskipTests，命令执行完成后，即可在target目录找到文件customizepropertyverify-0.0.1-SNAPSHOT.jar；

实战源码下载

• 本章实战的源码可以在github下载，地址和链接信息如下表所示：

名称	链接	备注
项目主页	https://github.com/zq2599/blog_demos	该项目在GitHub上的主页
git仓库地址(https)	https://github.com/zq2599/blog_demos.git	该项目源码的仓库地址，https协议
git仓库地址(ssh)	git@github.com:zq2599/blog_demos.git	该项目源码的仓库地址，ssh协议

• 这个git项目中有多个文件夹，本章源码在文件夹customizepropertyverify下，如下图红框所示：

文章图片

验证

• 接下来我们来验证自定义的ApplicationContext是否实现了环境变量检查的功能；
  
• " MYSQL_HOST" 这个环境变量是不存在的，所以我们先验证环境变量校验不通过导致spring容器启动失败的情况，在target目录执行命令java -jar customizepropertyverify-0.0.1-SNAPSHOT.jar，会发现应用启动失败，日志中显示由于找不到环境变量" MYSQL_HOST" ，在AbstractPropertyResolver.validateRequiredProperties位置抛出异常，如下所示：

D:\\github\\blog_demos\\customizepropertyverify\\target> java -jar customizepropertyverify-0.0.1-SNAPSHOT.jar._______ _ _ /\\\\ / ____ __ _ _(_)_ ____ _ \\ \\ \\ \\ ( ( )\\___ | _ | _| | _ \\/ _` | \\ \\ \\ \\ \\\\/___)| |_)| | | | | || (_| |) ) ) ) |____| .__|_| |_|_| |_\\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::(v2.0.4.RELEASE)2018-08-12 14:10:16.165INFO 19624 --- [main] c.b.c.CustomizepropertyverifyApplication : Starting CustomizepropertyverifyApplication v0.0.1-SNAPSHOT on DESKTOP-82CCEBN with PID 19624 (D:\\github\\blog_demos\\customizepropertyverify\\target\\customizepropertyverify-0.0.1-SNAPSHOT.jar started by 12167 in D:\\github\\blog_demos\\customizepropertyverify\\target) 2018-08-12 14:10:16.168INFO 19624 --- [main] c.b.c.CustomizepropertyverifyApplication : No active profile set, falling back to default profiles: default 2018-08-12 14:10:16.218INFO 19624 --- [main] c.b.c.context.CustomApplicationContext: Refreshing com.bolingcavalry.customizepropertyverify.context.CustomApplicationContext@6d1e7682: startup date [Sun Aug 12 14:10:16 GMT+08:00 2018]; root of context hierarchy 2018-08-12 14:10:16.221WARN 19624 --- [main] o.s.boot.SpringApplication: Error handling failed (ApplicationEventMulticaster not initialized - call refresh before multicasting events via the context: com.bolingcavalry.customizepropertyverify.context.CustomApplicationContext@6d1e7682: startup date [Sun Aug 12 14:10:16 GMT+08:00 2018]; root of context hierarchy) 2018-08-12 14:10:16.226 ERROR 19624 --- [main] o.s.boot.SpringApplication: Application run failedorg.springframework.core.env.MissingRequiredPropertiesException: The following properties were declared as required but could not be resolved: [MYSQL_HOST] at org.springframework.core.env.AbstractPropertyResolver.validateRequiredProperties(AbstractPropertyResolver.java:146) ~[spring-core-5.0.8.RELEASE.jar!/:5.0.8.RELEASE] at org.springframework.core.env.AbstractEnvironment.validateRequiredProperties(AbstractEnvironment.java:519) ~[spring-core-5.0.8.RELEASE.jar!/:5.0.8.RELEASE] at org.springframework.context.support.AbstractApplicationContext.prepareRefresh(AbstractApplicationContext.java:598) ~[spring-context-5.0.8.RELEASE.jar!/:5.0.8.RELEASE] at org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext.prepareRefresh(AnnotationConfigServletWebServerApplicationContext.java:200) ~[spring-boot-2.0.4.RELEASE.jar!/:2.0.4.RELEASE] at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:519) ~[spring-context-5.0.8.RELEASE.jar!/:5.0.8.RELEASE] at org.springframework.boot.web.servlet.context.ServletWebServerApplicationContext.refresh(ServletWebServerApplicationContext.java:140) ~[spring-boot-2.0.4.RELEASE.jar!/:2.0.4.RELEASE] at org.springframework.boot.SpringApplication.refresh(SpringApplication.java:762) ~[spring-boot-2.0.4.RELEASE.jar!/:2.0.4.RELEASE] at org.springframework.boot.SpringApplication.refreshContext(SpringApplication.java:398) ~[spring-boot-2.0.4.RELEASE.jar!/:2.0.4.RELEASE] at org.springframework.boot.SpringApplication.run(SpringApplication.java:330) ~[spring-boot-2.0.4.RELEASE.jar!/:2.0.4.RELEASE] at com.bolingcavalry.customizepropertyverify.CustomizepropertyverifyApplication.main(CustomizepropertyverifyApplication.java:13) [classes!/:0.0.1-SNAPSHOT] at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_181] at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_181] at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source) ~[na:1.8.0_181] at java.lang.reflect.Method.invoke(Unknown Source) ~[na:1.8.0_181] at org.springframework.boot.loader.MainMethodRunner.run(MainMethodRunner.java:48) [customizepropertyverify-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT] at org.springframework.boot.loader.Launcher.launch(Launcher.java:87) [customizepropertyverify-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT] at org.springframework.boot.loader.Launcher.launch(Launcher.java:50) [customizepropertyverify-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT] at org.springframework.boot.loader.JarLauncher.main(JarLauncher.java:51) [customizepropertyverify-0.0.1-SNAPSHOT.jar:0.0.1-SNAPSHOT] ...

• 接下来验证环境变量" MYSQL_HOST" 存在的时候应用是否能初始化成功，执行命令java -DMYSQL_HOST=" 192.168.0.1" -jar customizepropertyverify-0.0.1-SNAPSHOT.jar，这个命令可以将" MYSQL_HOST" 设置到进程环境变量中，这次顺利通过校验，应用启动成功：
  

  D:\\github\\blog_demos\\customizepropertyverify\\target> java -DMYSQL_HOST="192.168.0.1" -jar customizepropertyverify-0.0.1-SNAPSHOT.jar._______ _ _ /\\\\ / ____ __ _ _(_)_ ____ _ \\ \\ \\ \\ ( ( )\\___ | _ | _| | _ \\/ _` | \\ \\ \\ \\ \\\\/___)| |_)| | | | | || (_| |) ) ) ) |____| .__|_| |_|_| |_\\__, | / / / / =========|_|==============|___/=/_/_/_/ :: Spring Boot ::(v2.0.4.RELEASE)

  
  

2018-08-12 14:15:31.811INFO 15328 --- [main] c.b.c.CustomizepropertyverifyApplication : Starting CustomizepropertyverifyApplication v0.0.1-SNAPSHOT on DESKTOP-82CCEBN with PID 15328 (D:\\github\\blog_demos\\customizepropertyverify\\target\\customizepropertyverify-0.0.1-SNAPSHOT.jar started by 12167 in D:\\github\\blog_demos\\customizepropertyverify\\target)
2018-08-12 14:15:31.813INFO 15328 --- [main] c.b.c.CustomizepropertyverifyApplication : No active profile set, falling back to default profiles: default
2018-08-12 14:15:31.850INFO 15328 --- [main] c.b.c.context.CustomApplicationContext: Refreshing com.bolingcavalry.customizepropertyverify.context.CustomApplicationContext@3339ad8e: startup date [Sun Aug 12 14:15:31 GMT+08:00 2018]; root of context hierarchy
2018-08-12 14:15:32.776INFO 15328 --- [main] o.s.b.w.embedded.tomcat.TomcatWebServer: Tomcat initialized with port(s): 8080 (http)
2018-08-12 14:15:32.796INFO 15328 --- [main] o.apache.catalina.core.StandardService: Starting service [Tomcat]
2018-08-12 14:15:32.796INFO 15328 --- [main] org.apache.catalina.core.StandardEngine: Starting Servlet Engine: Apache Tomcat/8.5.32
2018-08-12 14:15:32.805INFO 15328 --- [ost-startStop-1] o.a.catalina.core.AprLifecycleListener: The APR based Apache Tomcat Native library which allows optimal performance in production environments was not found on the java.library.path: [C:\\Program Files (x86)\\Common Files\\Oracle\\Java\\javapath; C:\\WINDOWS\\Sun\\Java\\bin; C:\\WINDOWS\\system32; C:\\WINDOWS; C:\\Program Files (x86)\\Common Files\\Oracle\\Java\\javapath; C:\\Program Files (x86)\\Intel\\Intel(R) Management Engine Components\\iCLS\\; C:\\Program Files\\Intel\\Intel(R) Management Engine Components\\iCLS\\; C:\\windows\\system32; C:\\windows; C:\\windows\\System32\\Wbem; C:\\windows\\System32\\WindowsPowerShell\\v1.0\\; C:\\Program Files (x86)\\Intel\\Intel(R) Management Engine Components\\DAL; C:\\Program Files\\Intel\\Intel(R) Management Engine Components\\DAL; C:\\Program Files (x86)\\Intel\\Intel(R) Management Engine Components\\IPT; C:\\Program Files\\Intel\\Intel(R) Management Engine Components\\IPT; C:\\Program Files (x86)\\NVIDIA Corporation\\PhysX\\Common; C:\\jdk\\bin; C:\\software\\Git\\cmd; C:\\software\\apache-maven-3.5.0\\bin; ; %SystemRoot%\\system32; %SystemRoot%; %SystemRoot%\\System32\\Wbem; %SYSTEMROOT%\\System32\\WindowsPowerShell\\v1.0\\; %SYSTEMROOT%\\System32\\OpenSSH\\; C:\\Users\\12167\\AppData\\Local\\Microsoft\\WindowsApps; ; .]
2018-08-12 14:15:32.867INFO 15328 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]: Initializing Spring embedded WebApplicationContext
2018-08-12 14:15:32.868INFO 15328 --- [ost-startStop-1] o.s.web.context.ContextLoader: Root WebApplicationContext: initialization completed in 1021 ms
2018-08-12 14:15:32.919INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean: Servlet dispatcherServlet mapped to [/]
2018-08-12 14:15:32.923INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean: Mapping filter: characterEncodingFilter to: [/]
2018-08-12 14:15:32.923INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean: Mapping filter: hiddenHttpMethodFilter to: [/]
2018-08-12 14:15:32.926INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean: Mapping filter: httpPutFormContentFilter to: [/]
2018-08-12 14:15:32.927INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean: Mapping filter: requestContextFilter to: [/]
2018-08-12 14:15:33.006INFO 15328 --- [main] o.s.w.s.handler.SimpleUrlHandlerMapping: Mapped URL path [//favicon.ico] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-08-12 14:15:33.136INFO 15328 --- [main] s.w.s.m.m.a.RequestMappingHandlerAdapter : Looking for @ControllerAdvice: com.bolingcavalry.customizepropertyverify.context.CustomApplicationContext@3339ad8e: startup date [Sun Aug 12 14:15:31 GMT+08:00 2018]; root of context hierarchy
2018-08-12 14:15:33.190INFO 15328 --- [main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped " [/hello]" onto public java.lang.String com.bolingcavalry.customizepropertyverify.controller.HelloWorldController.hello()
2018-08-12 14:15:33.194INFO 15328 --- [main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped " [/error]" onto public org.springframework.http.ResponseEntity< java.util.Map< java.lang.String, java.lang.Object> > org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.error(javax.servlet.http.HttpServletRequest)
2018-08-12 14:15:33.195INFO 15328 --- [main] s.w.s.m.m.a.RequestMappingHandlerMapping : Mapped " [/error],produces=[text/html]" onto public org.springframework.web.servlet.ModelAndView org.springframework.boot.autoconfigure.web.servlet.error.BasicErrorController.errorHtml(javax.servlet.http.HttpServletRequest,javax.servlet.http.HttpServletResponse)
2018-08-12 14:15:33.226INFO 15328 --- [main] o.s.w.s.handler.SimpleUrlHandlerMapping: Mapped URL path [/webjars/] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-08-12 14:15:33.227INFO 15328 --- [main] o.s.w.s.handler.SimpleUrlHandlerMapping: Mapped URL path [/**] onto handler of type [class org.springframework.web.servlet.resource.ResourceHttpRequestHandler]
2018-08-12 14:15:33.314INFO 15328 --- [main] o.s.j.e.a.AnnotationMBeanExporter: Registering beans for JMX exposure on startup
2018-08-12 14:15:33.343INFO 15328 --- [main] o.s.b.w.embedded.tomcat.TomcatWebServer: Tomcat started on port(s): 8080 (http) with context path
2018-08-12 14:15:33.346INFO 15328 --- [main] c.b.c.CustomizepropertyverifyApplication : Started CustomizepropertyverifyApplication in 1.761 seconds (JVM running for 2.113)
2018-08-12 14:15:39.626INFO 15328 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]: Initializing Spring FrameworkServlet dispatcherServlet
2018-08-12 14:15:39.626INFO 15328 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet: FrameworkServlet dispatcherServlet: initialization started
2018-08-12 14:15:39.645INFO 15328 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet: FrameworkServlet dispatcherServlet: initialization completed in 16 ms

- 验证完成，我们可以通过自定义子类来强制要求指定的环境变量必须存在；- 至此，我们spring扩展实战的第一章就结束了，接下来的章节我们会进行更多的实战，来了解spring强大的扩展机制；### 欢迎关注51CTO博客：程序员欣宸 > [学习路上，你不孤单，欣宸原创一路相伴...](https://blog.51cto.com/u_13674465)

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