一、 ApplicationContextInitializer 介绍
首先看spring官网的介绍：
翻译一下：
- 用于在spring容器刷新之前初始化Spring ConfigurableApplicationContext的回调接口。（剪短说就是在容器刷新之前调用该类的 initialize 方法。并将 ConfigurableApplicationContext
- 通常用于需要对应用程序上下文进行编程初始化的web应用程序中。例如，根据上下文环境注册属性源或激活配置文件等。
- 可排序的（实现Ordered接口，或者添加@Order注解）
ApplicationContextInitializer
二、三种实现方式
MyApplicationContextInitializer 并实现 ApplicationContextInitializer
1 public class MyApplicationContextInitializer implements ApplicationContextInitializer {
2 @Override
3 public void initialize(ConfigurableApplicationContext applicationContext) {
4 System.out.println("-----MyApplicationContextInitializer initialize-----");
5 }
6 }
2.1、mian函数中添加
优雅的写一个SpringBoot的main方法
1 @SpringBootApplication
2 public class MySpringBootApplication {
3 public static void main(String[] args) {
4 SpringApplication application = new SpringApplication(MySpringBootApplication.class);
5 application.addInitializers(new MyApplicationContextInitializer());
6 application.run(args);
7 }
8 }
运行，查看控制台：生效了
2.2、配置文件中配置
context.initializer.classes=org.springframework.boot.demo.common.MyApplicationContextInitializer
2.3、SpringBoot的SPI扩展---META-INF/spring.factories中配置
org.springframework.context.ApplicationContextInitializer=org.springframework.boot.demo.common.MyApplicationContextInitializer
三、排序问题
如图所示改造一下mian方法。打一个断点，debug查看排序情况。
MyApplicationContextInitializer
1 @Order(Ordered.HIGHEST_PRECEDENCE)
2 public class MyApplicationContextInitializer implements ApplicationContextInitializer{
3 @Override
4 public void initialize(ConfigurableApplicationContext applicationContext) {
5 System.out.println("-----MyApplicationContextInitializer initialize-----");
6 }
7 }
下面我们通过debug分别验证二章节中提到的三种方法排序是否都是可以的。
application.getInitializers()
然后再分别验证2.2和2.3章节中的方法。排序都是可以实现的。
@Order(Ordered.LOWEST_PRECEDENCE)
四、通过源码分析ApplicationContextInitializer何时被调用
MyApplicationContextInitializer
ApplicationContextInitializer 在容器刷新前的准备阶段被调用。 refreshContext(context);
ApplicationContextInitializer
1 public ConfigurableApplicationContext run(String... args) {
2 //记录程序运行时间
3 StopWatch stopWatch = new StopWatch();
4 stopWatch.start();
5 // ConfigurableApplicationContext Spring 的上下文
6 ConfigurableApplicationContext context = null;
7 Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
8 configureHeadlessProperty();
9 //从META-INF/spring.factories中获取监听器
10 //1、获取并启动监听器
11 SpringApplicationRunListeners listeners = getRunListeners(args);
12 listeners.starting();
13 try {
14 ApplicationArguments applicationArguments = new DefaultApplicationArguments(
15 args);
16 //2、构造容器环境
17 ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
18 //处理需要忽略的Bean
19 configureIgnoreBeanInfo(environment);
20 //打印banner
21 Banner printedBanner = printBanner(environment);
22 ///3、初始化容器
23 context = createApplicationContext();
24 //实例化SpringBootExceptionReporter.class，用来支持报告关于启动的错误
25 exceptionReporters = getSpringFactoriesInstances(
26 SpringBootExceptionReporter.class,
27 new Class[]{ConfigurableApplicationContext.class}, context);
28 //4、刷新容器前的准备阶段
29 prepareContext(context, environment, listeners, applicationArguments, printedBanner);
30 //5、刷新容器
31 refreshContext(context);
32 //刷新容器后的扩展接口
33 afterRefresh(context, applicationArguments);
34 stopWatch.stop();
35 if (this.logStartupInfo) {
36 new StartupInfoLogger(this.mainApplicationClass)
37 .logStarted(getApplicationLog(), stopWatch);
38 }
39 listeners.started(context);
40 callRunners(context, applicationArguments);
41 } catch (Throwable ex) {
42 handleRunFailure(context, ex, exceptionReporters, listeners);
43 throw new IllegalStateException(ex);
44 }
45
46 try {
47 listeners.running(context);
48 } catch (Throwable ex) {
49 handleRunFailure(context, ex, exceptionReporters, null);
50 throw new IllegalStateException(ex);
51 }
52 return context;
53 }
refreshContext(context);
1 private void prepareContext(ConfigurableApplicationContext context,
2 ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
3 ApplicationArguments applicationArguments, Banner printedBanner) {
4 context.setEnvironment(environment);
5 postProcessApplicationContext(context);
6 applyInitializers(context);
7 ...
8 }
applyInitializers 中遍历调用每一个被加载的 ApplicationContextInitializer 的 initialize(context); 方法，并将 ConfigurableApplicationContext 的实例传递给 initialize
1 protected void applyInitializers(ConfigurableApplicationContext context) {
2 for (ApplicationContextInitializer initializer : getInitializers()) {
3 Class<?> requiredType = GenericTypeResolver.resolveTypeArgument(
4 initializer.getClass(), ApplicationContextInitializer.class);
5 Assert.isInstanceOf(requiredType, context, "Unable to call initializer.");
6 initializer.initialize(context);
7 }
8 }
ApplicationContextInitializer
