为什么要自定义application context
在学习spring容器初始化的过程中，发现spring容器预留了一些扩展点，我们可以写子类来做功能扩展，例如AbstractApplicationContext类的initPropertySources、postProcessBeanFactory、onRefresh等方法都是空方法，留给子类来扩展用；
在传统spring框架下的扩展方式
传统的spring框架下使用哪个ApplicationContext实现类，是自己写代码来指定的，下面是基于spring框架的应用的启动代码：
public static void main(String[] args) {
ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("classpath:applicationContext.xml");
Simple bean = context.getBean(Simple.class);
bean.execute();
context.close();
}如果我们要用自定的ApplicationContext实现类，只要将上面的ClassPathXmlApplicationContext改成我们做的类即可；
探寻SpringBoot框架下的扩展方式
先看一段代码，以下是一个SpringBoot应用启动的代码：
@SpringBootApplication
public class CustomizeapplicationcontextApplication {
public static void main(String[] args) {
SpringApplication springApplication = new SpringApplication(CustomizeapplicationcontextApplication.class);
springApplication.run(args);
}
}从上述代码可知，我们需要去查看SpingApplication的run方法源码，那里应该有ApplicationContext初始化相关的信息，在SpingApplication的run方法中，果然找到了有用信息，如下图红框所示，是createApplicationContext方法创建了ApplicationContext实现类：
去看看createApplicationContext方法，如下图：
上图中有两点需要注意：
1. 红框中显示，ApplicationContext实现类来自成员变量applicationContextClass，只要我们能设置成员变量applicationContextClass，就达到了目的：在应用中使用自定义的ApplicationContext实现类；
2. 绿框中显示，如果没有设置成员变量applicationContextClass，就把AnnotationConfigServletWebServerApplicationContext作为ApplicationContext实现类，用在spring环境中，所以，我们在自定义ApplicationContext实现类时，用AnnotationConfigServletWebServerApplicationContext作为父类最合适（除了我们自己扩展的东西，其他部分都和默认的保持一致）；
如何设置自定义的ApplicationContext实现类
从createApplicationContext方法可以看出，设置了成员变量applicationContextClass，就达到了使用自定义ApplicationContext实现类的目的，那么如何设置成员变量applicationContextClass呢？就在下面这个setApplicationContextClass方法：
public void setApplicationContextClass(
Class<? extends ConfigurableApplicationContext> applicationContextClass) {
this.applicationContextClass = applicationContextClass;
if (!isWebApplicationContext(applicationContextClass)) {
this.webApplicationType = WebApplicationType.NONE;
}
}我们只要在启动应用的时候，在调用springApplication的run方法之前先调用setApplicationContextClass方法就能指定ApplicationContext实现类了；
分析到此，您是否和我一样有疑虑：setApplicationContextClass方法会不会被在其他地方被调用，导致我们的设置无效呢？通过以下两种方法来检查：
1. 在IntelliJ IDEA中，通过”Find Usages”的方式查找该方法被调用的地方，结果只发现了一处SpringBootServletInitializer类的onStartup方法，这是war包形式的应用在配置Initializer的时候才会用到，main方法启动的方式中不会用到；
2. 在setApplicationContextClass方法中打断点，debug启动应用，确认不会走进来；
实战使用自定义的ApplicationContext实现类
理论分析完毕，可以实战验证了：
1. 基于maven创建一个SpringBoot的web应用，pom.xml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>customizeapplicationcontext</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
<name>customizeapplicationcontext</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>
2. 创建类CustomizeApplicationContext，继承自AnnotationConfigServletWebServerApplicationContext，重写了父类的几个方法，如下：
package com.bolingcavalry.customizeapplicationcontext;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.boot.web.servlet.context.AnnotationConfigServletWebServerApplicationContext;
/**
* @Description : 自定义的ApplicationContext实现类
* @Author : [email protected]
* @Date : 2018-08-11 17:12
*/
public class CustomizeApplicationContext extends AnnotationConfigServletWebServerApplicationContext {
Log logger = LogFactory.getLog(CustomizeApplicationContext.class);
@Override
protected void initPropertySources() {
super.initPropertySources();
logger.info("execute override initPropertySources");
}
@Override
protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
super.postProcessBeanFactory(beanFactory);
logger.info("execute override postProcessBeanFactory");
}
@Override
protected void onRefresh() {
super.onRefresh();
logger.info("execute override onRefresh");
}
}
3. 启动类CustomizeapplicationcontextApplication的main方法中，调用setApplicationContextClass方法，将ApplicationContext实现类设置为CustomizeApplicationContext：
package com.bolingcavalry.customizeapplicationcontext;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class CustomizeapplicationcontextApplication {
public static void main(String[] args) {
SpringApplication springApplication = new SpringApplication(CustomizeapplicationcontextApplication.class);
springApplication.setApplicationContextClass(CustomizeApplicationContext.class);
springApplication.run(args);
}
}
4. 启动应用，查看日志如下，CustomizeApplicationContext中重写的方法都被执行了，并且initPropertySources被执行了两次，那是因为除了AbstractApplicationContext类中有调用，在ServletWebServerApplicationContext类的onRefresh中会执行createWebServer方法，而createWebServer方法中也会调用一次initPropertySources方法：
2018-08-11 17:21:18.833 INFO 5132 --- [ main] c.CustomizeapplicationcontextApplication : Starting CustomizeapplicationcontextApplication on zhaoqinwin10 with PID 5132 (D:\temp\201808\09\customizeapplicationcontext\target\classes started by 12167 in D:\temp\201808\09\customizeapplicationcontext)
2018-08-11 17:21:18.840 INFO 5132 --- [ main] c.CustomizeapplicationcontextApplication : No active profile set, falling back to default profiles: default
2018-08-11 17:21:18.957 INFO 5132 --- [ main] c.b.c.CustomizeApplicationContext : Refreshing com.bolingcavalry.customizeapplicationcontext.CustomizeApplicationContext@5906ebcb: startup date [Sat Aug 11 17:21:18 CST 2018]; root of context hierarchy
2018-08-11 17:21:18.958 INFO 5132 --- [ main] c.b.c.CustomizeApplicationContext : execute override initPropertySources
2018-08-11 17:21:18.967 INFO 5132 --- [ main] c.b.c.CustomizeApplicationContext : execute override postProcessBeanFactory
2018-08-11 17:21:20.085 INFO 5132 --- [ main] o.s.b.w.embedded.tomcat.TomcatWebServer : Tomcat initialized with port(s): 8080 (http)
2018-08-11 17:21:20.102 INFO 5132 --- [ main] o.apache.catalina.core.StandardService : Starting service [Tomcat]
2018-08-11 17:21:20.103 INFO 5132 --- [ main] org.apache.catalina.core.StandardEngine : Starting Servlet Engine: Apache Tomcat/8.5.32
2018-08-11 17:21:20.106 INFO 5132 --- [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:\jdk\bin;C:\WINDOWS\Sun\Java\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\ProgramData\Oracle\Java\javapath;C:\Program Files (x86)\Intel\iCLS Client\;C:\Program Files\Intel\iCLS Client\;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:\Program Files\Intel\WiFi\bin\;C:\Program Files\Common Files\Intel\WirelessCommon\;C:\software\apache-maven-3.3.3\bin\;C:\software\Git\bin;E:\work\dubbo\zookeeper-3.3.6/bin;E:\work\dubbo\zookeeper-3.3.6/conf;D:\soft\gradle-1.1\bin;C:\jdk\bin;C:\software\svn\bin;D:\software\AndroidSdk\platform-tools;D:\software\calibre\;D:\software\gradle-2.14.1\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\System32\Wbem;C:\WINDOWS\System32\WindowsPowerShell\v1.0\;C:\Program Files\Git\cmd;C:\Users\12167\AppData\Local\Microsoft\WindowsApps;C:\Users\12167\AppData\Local\Programs\Fiddler;%USERPROFILE%\AppData\Local\Microsoft\WindowsApps;.]
2018-08-11 17:21:20.226 INFO 5132 --- [ost-startStop-1] o.a.c.c.C.[Tomcat].[localhost].[/] : Initializing Spring embedded WebApplicationContext
2018-08-11 17:21:20.227 INFO 5132 --- [ost-startStop-1] o.s.web.context.ContextLoader : Root WebApplicationContext: initialization completed in 1273 ms
2018-08-11 17:21:20.305 INFO 5132 --- [ost-startStop-1] o.s.b.w.servlet.ServletRegistrationBean : Servlet dispatcherServlet mapped to [/]
2018-08-11 17:21:20.310 INFO 5132 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'characterEncodingFilter' to: [/*]
2018-08-11 17:21:20.310 INFO 5132 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'hiddenHttpMethodFilter' to: [/*]
2018-08-11 17:21:20.310 INFO 5132 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'httpPutFormContentFilter' to: [/*]
2018-08-11 17:21:20.310 INFO 5132 --- [ost-startStop-1] o.s.b.w.servlet.FilterRegistrationBean : Mapping filter: 'requestContextFilter' to: [/*]
2018-08-11 17:21:20.330 INFO 5132 --- [ main] c.b.c.CustomizeApplicationContext : execute override initPropertySources
2018-08-11 17:21:20.330 INFO 5132 --- [ main] c.b.c.CustomizeApplicationContext : execute override onRefresh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) | [email protected]: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customizeapplicationcontext下，如下图红框所示：
至此，我们通过查看SpringApplication的源码，搞清楚了spring环境的ApplicationContext实现类在SpringBoot框架下如何指定，也做了一次简单的自定义实战，今后在研究和学习spring过程中，如果需要扩展spring容器就能在SpringBoot环境下进行了，相比传统的war包部署、以及指定多个jar包的classpath等操作，springboot应用的单个jar包更方便省事。