A, ApplicationContextInitializer Introduction
First, look at the introduction spring official website:
translate:
- Before spring refresh for container initialization Spring ConfigurableApplicationContext callback interface. (Cut, that is to call the class before the container refresh initialize method. And ConfigurableApplicationContext pass an instance of the class to the method)
- Typically used for the application context for web application programming initialization. For example, according to the properties of the source register context activation profile or the like.
- Sortable (Ordered interface implemented or add annotations @Order)
After reading this explanation, in order to facilitate the explanation, we look at the custom ApplicationContextInitializer three ways. And through SpringBoot source code analysis to take effect and functions to achieve.
Second, the three ways
First create a new class MyApplicationContextInitializer and implement ApplicationContextInitializer interface.
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 function added
The elegant main method to write a SpringBoot
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 }
Run, view the console: the entry into force
2.2 configuration file is
context.initializer.classes=org.springframework.boot.demo.common.MyApplicationContextInitializer
2.3, SpringBoot the SPI extended --- META-INF / spring.factories configuration
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。
给 MyApplicationContextInitializer 加上Order注解：我们指定其拥有最高的排序级别。（越高越早执行）
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。
首先验证2.1章节中采用的main函数中添加：debug，断点处查看 application.getInitializers() 这行代码的结果可见，排序生效了。
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。
然而当采用2.3中的SPI扩展的方式，排序指定 @Order(Ordered.LOWEST_PRECEDENCE) 排序并没有生效。当然采用实现Ordered接口的方式，排序验证结果都是一样的。
四、通过源码分析ApplicationContextInitializer何时被调用
debug差看上文中自定的 MyApplicationContextInitializer 的调用栈。
可见 ApplicationContextInitializer 在容器刷新前的准备阶段被调用。 refreshContext(context);
在SpringBoot的启动函数中， 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 }
然后看在 refreshContext(context); 具体是怎么被调用的。
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 }
然后在 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 方法。
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 }
OK，到这里通过源码说明了 ApplicationContextInitializer 是何时及如何被调用的。