### 歡迎訪問我的GitHub
> 這裡分類和彙總了欣宸的全部原創(含配套原始碼)：[https://github.com/zq2599/blog_demos](https://github.com/zq2599/blog_demos)

### 關於擴充套件
- 在之前學習spring環境初始化原始碼的過程中，見到有些地方能透過子類來實現自定義擴充套件，從本章開始，我們來逐個實踐這些擴充套件，除了加深對spring的理解，有的擴充套件也能解決一些通用的問題；
- 文中涉及的spring版本號為4.1.8.RELEASE；
### 相關文章連結
- 為了方便開發和測試，我們的擴充套件實戰是在SpringBoot框架下進行的，在SpringBoot自定義spring擴充套件的方式請參考[《SpringBoot應用使用自定義的ApplicationContext實現類》](http://blog.itpub.net/70017844/viewspace-2899777/)
### 擴充套件功能介紹
- 今天實戰的內容，是透過spring容器來確保環境變數**MYSQL_HOST**一定存在，如果不存在應用就會啟動失敗；
### 分析spring原始碼
- 透過分析spring原始碼來確定如何擴充套件；
- 在spring環境初始化的時候，AbstractApplicationContext的prepareRefresh方法會被呼叫，原始碼如下
```java
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是個空方法，留給子類擴充套件；
- 來看看getEnvironment().validateRequiredProperties()的原始碼，對應的是AbstractPropertyResolver類的validateRequiredProperties方法：
```java
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;
		}
	}
```
- 可見spring容器初始化的時候，會從集合requiredProperties中取出所有key，然後獲取這些key的環境變數（包括系統環境變數和程式環境變數），如果有一個key對應的環境變數為空，就會丟擲異常，導致spring容器初始化失敗；
### 擴充套件功能分析
- 看了AbstractPropertyResolver類的validateRequiredProperties方法原始碼後，可以確定該方法能強制要求一些環境變數必須存在，否則停止spring啟動，我們只要把我們認為必要的環境變數的key存入集合requiredProperties中即可，達到此目標需要解決下面兩個問題：

- 如何將環境變數的key存入集合requiredProperties？
- **呼叫AbstractPropertyResolver類的setRequiredProperties方法，注意該方法是向集合requiredProperties中新增資料，並不會將已有資料清除；**

- 在什麼時候執行AbstractPropertyResolver類的setRequiredProperties方法設定key？

- **建立一個AbstractApplicationContext的子類，重寫initPropertySources方法，在此方法中執行AbstractPropertyResolver類的setRequiredProperties；**

### 開始實戰
- 用maven建立一個SpringBoot工程，工程名為customizepropertyverify，pom.xml內容如下：
```xml
<?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></relativepath> 
    </parent>

<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>
```
- 建立一個類CustomApplicationContext，繼承自AnnotationConfigServletWebServerApplicationContext，重寫initPropertySources方法，將"MYSQL_HOST"作為啟動時必須要存在的環境變數：

```java
package com.bolingcavalry.customizepropertyverify.context;

import org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext;

/**
 * @Description : AnnotationConfigServletWebServerApplicationContext，重寫了initPropertySources方法，
 * 要求spring啟動的時候環境變數MYSQL_HOST必須存在
 * @Author : zq2599@gmail.com
 * @Date : 2018-08-10 21:40
 */
public class CustomApplicationContext extends AnnotationConfigServletWebServerApplicationContext {

@Override
    protected void initPropertySources() {
        super.initPropertySources();
        //把"MYSQL_HOST"作為啟動的時候必須驗證的環境變數
        getEnvironment().setRequiredProperties("MYSQL_HOST");
    }
}
```
- 建立應用啟動類CustomizepropertyverifyApplication，指定ApplicationContext的class為CustomApplicationContext：

```java
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檔案所在目錄下執行命令**mvn clean package -U -DskipTests**，命令執行完成後，即可在target目錄找到檔案customizepropertyverify-0.0.1-SNAPSHOT.jar；

### 實戰原始碼下載
- 本章實戰的原始碼可以在github下載，地址和連結資訊如下表所示：

| 名稱      |   連結 |   備註|
| :-------- | :----| :----|
| 專案主頁| https://github.com/zq2599/blog_demos | 該專案在GitHub上的主頁 |
| git倉庫地址(https)| https://github.com/zq2599/blog_demos.git | 該專案原始碼的倉庫地址，https協議 |
| git倉庫地址(ssh)| git@github.com:zq2599/blog_demos.git | 該專案原始碼的倉庫地址，ssh協議 |

- 這個git專案中有多個資料夾，本章原始碼在資料夾customizepropertyverify下，如下圖紅框所示：

![image.png](https://bbs-img.huaweicloud.com/blogs/img/20220612/1655019838036798863.png)

### 驗證
- 接下來我們來驗證自定義的ApplicationContext是否實現了環境變數檢查的功能；

- "MYSQL_HOST"這個環境變數是不存在的，所以我們先驗證環境變數校驗不透過導致spring容器啟動失敗的情況，在target目錄執行命令**java -jar customizepropertyverify-0.0.1-SNAPSHOT.jar**，會發現應用啟動失敗，日誌中顯示由於找不到環境變數"MYSQL_HOST"，在AbstractPropertyResolver.validateRequiredProperties位置丟擲異常，如下所示：

```shell
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: default
2018-08-12 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 hierarchy
2018-08-12 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]
...
```

- 接下來驗證環境變數"MYSQL_HOST"存在的時候應用是否能初始化成功，執行命令**java -DMYSQL_HOST="192.168.0.1" -jar customizepropertyverify-0.0.1-SNAPSHOT.jar**，這個命令可以將"MYSQL_HOST"設定到程式環境變數中，這次順利透過校驗，應用啟動成功：

```shell
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: default
2018-08-12 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 hierarchy
2018-08-12 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/8.5.32
2018-08-12 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 WebApplicationContext
2018-08-12 14:15:32.868  INFO 15328 --- [ost-startStop-1] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1021 ms
2018-08-12 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 hierarchy
2018-08-12 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.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 startup
2018-08-12 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 started
2018-08-12 14:15:39.645  INFO 15328 --- [nio-8080-exec-1] o.s.web.servlet.DispatcherServlet        : FrameworkServlet 'dispatcherServlet': initialization completed in 16 ms
```

- 驗證完成，我們可以透過自定義子類來強制要求指定的環境變數必須存在；

- 至此，我們spring擴充套件實戰的第一章就結束了，接下來的章節我們會進行更多的實戰，來了解spring強大的擴充套件機制；

### 歡迎關注ITpub：程式設計師欣宸
> [學習路上，你不孤單，欣宸原創一路相伴...](http://blog.itpub.net/70017844/)</java.util.map>

spring4.1.8擴充套件實戰之一：自定義環境變數驗證

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