spring4.1.8 扩展实战之一：自定义环境变量验证

作者：程序员欣宸

2022 年 6 月 12 日
本文字数：9244 字
阅读完需：约 30 分钟

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关于扩展

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；

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();  }

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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;    }  }

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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>

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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");    }}

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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;
@SpringBootApplicationpublic class CustomizepropertyverifyApplication {
    public static void main(String[] args) {        SpringApplication springApplication = new SpringApplication(CustomizepropertyverifyApplication.class);        springApplication.setApplicationContextClass(CustomApplicationContext.class);        springApplication.run(args);    }}

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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：

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：

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 14:10:16.165  INFO 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 14:10:16.168  INFO 19624 --- [           main] c.b.c.CustomizepropertyverifyApplication : No active profile set, falling back to default profiles: default2018-08-12 14:10:16.218  INFO 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 hierarchy2018-08-12 14:10:16.221  WARN 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 failed
org.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]...

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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 14:15:31.811  INFO 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 14:15:31.813  INFO 15328 --- [           main] c.b.c.CustomizepropertyverifyApplication : No active profile set, falling back to default profiles: default2018-08-12 14:15:31.850  INFO 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 hierarchy2018-08-12 14:15:32.776  INFO 15328 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)2018-08-12 14:15:32.796  INFO 15328 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]2018-08-12 14:15:32.796  INFO 15328 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet Engine: Apache Tomcat/8.5.322018-08-12 14:15:32.805  INFO 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 14:15:32.867  INFO 15328 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext2018-08-12 14:15:32.868  INFO 15328 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1021 ms2018-08-12 14:15:32.919  INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean  : Servlet dispatcherServlet mapped to [/]2018-08-12 14:15:32.923  INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'characterEncodingFilter' to: [/*]2018-08-12 14:15:32.923  INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]2018-08-12 14:15:32.926  INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'httpPutFormContentFilter' to: [/*]2018-08-12 14:15:32.927  INFO 15328 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean   : Mapping filter: 'requestContextFilter' to: [/*]2018-08-12 14:15:33.006  INFO 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 14:15:33.136  INFO 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 hierarchy2018-08-12 14:15:33.190  INFO 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 14:15:33.194  INFO 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 14:15:33.195  INFO 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 14:15:33.226  INFO 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 14:15:33.227  INFO 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 14:15:33.314  INFO 15328 --- [           main] o.s.j.e.a.AnnotationMBeanExporter        : Registering beans for JMX exposure on startup2018-08-12 14:15:33.343  INFO 15328 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''2018-08-12 14:15:33.346  INFO 15328 --- [           main] c.b.c.CustomizepropertyverifyApplication : Started CustomizepropertyverifyApplication in 1.761 seconds (JVM running for 2.113)2018-08-12 14:15:39.626  INFO 15328 --- [nio-8080-exec-1] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring FrameworkServlet 'dispatcherServlet'2018-08-12 14:15:39.626  INFO 15328 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization started2018-08-12 14:15:39.645  INFO 15328 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization completed in 16 ms

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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；

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spring4.1.8 扩展实战之一：自定义环境变量验证

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