SpringBoot的重要类及接口

ApplicationContextInitializer 

首先看spring官网的介绍：

翻译下来就是说：

• 用于在spring容器刷新之前初始化Spring ConfigurableApplicationContext的回调接口。（就是在容器刷新之前调用该类的 initialize 方法。并将 ConfigurableApplicationContext 类的实例传递给该方法）
• 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三种实现方式

首先新建一个类 MyApplicationContextInitializer并实现 ApplicationContextInitializer 接口。

public class MyApplicationContextInitializer implements ApplicationContextInitializer {
   @Override
   public void initialize(ConfigurableApplicationContext applicationContext) {
       System.out.println("-----MyApplicationContextInitializer initialize-----");
   }
}

1、main函数中添加

@SpringBootApplication
public class MySpringBootApplication {
    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(MySpringBootApplication.class);
        application.addInitializers(new MyApplicationContextInitializer());
        application.run(args);
    }
}

2、配置文件中配置

context.initializer.classes=org.springframework.boot.demo.common.MyApplicationContextInitializer 

3、SpringBoot的SPI扩展---META-INF/spring.factories中配置

org.springframework.context.ApplicationContextInitializer=org.springframework.boot.demo.common.MyApplicationContextInitializer

排序问题

给 MyApplicationContextInitializer 加上Order注解：我们指定其拥有最高的排序级别。（值越小越早执行）

@Order(Ordered.HIGHEST_PRECEDENCE)
public class MyApplicationContextInitializer implements ApplicationContextInitializer{
    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {
        System.out.println("-----MyApplicationContextInitializer initialize-----");
    }
}

除了使用Order注解，还可以采用实现Ordered接口的方式，排序验证结果都是一样的。

通过源码分析ApplicationContextInitializer何时被调用

public static ConfigurableApplicationContext run(Class<?> primarySource, String... args) {
    return run(new Class[]{primarySource}, args);
}

public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {
    return (new SpringApplication(primarySources)).run(args);
}

public ConfigurableApplicationContext run(String... args) {
    StopWatch stopWatch = new StopWatch();
    stopWatch.start();
    ConfigurableApplicationContext context = null;
    Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList();
    this.configureHeadlessProperty();
    SpringApplicationRunListeners listeners = this.getRunListeners(args);
    listeners.starting();

    Collection exceptionReporters;
    try {
        ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
        ConfigurableEnvironment environment = this.prepareEnvironment(listeners, applicationArguments);
        this.configureIgnoreBeanInfo(environment);
        Banner printedBanner = this.printBanner(environment);
        context = this.createApplicationContext();
        exceptionReporters = this.getSpringFactoriesInstances(SpringBootExceptionReporter.class, new Class[]{ConfigurableApplicationContext.class}, context);
        // 
        this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);
        this.refreshContext(context);
        this.afterRefresh(context, applicationArguments);
        stopWatch.stop();
        if (this.logStartupInfo) {
            (new StartupInfoLogger(this.mainApplicationClass)).logStarted(this.getApplicationLog(), stopWatch);
        }

        listeners.started(context);
        this.callRunners(context, applicationArguments);
    } catch (Throwable var10) {
        this.handleRunFailure(context, var10, exceptionReporters, listeners);
        throw new IllegalStateException(var10);
    }

    try {
        listeners.running(context);
        return context;
    } catch (Throwable var9) {
        this.handleRunFailure(context, var9, exceptionReporters, (SpringApplicationRunListeners)null);
        throw new IllegalStateException(var9);
    }
}

在SpringBoot 2.3.6.RELEASE版本中，在this.prepareContext(context, environment, listeners, applicationArguments, printedBanner);方法中调用。

ApplicationRunner or CommandLineRunner

应用服务启动时，加载一些数据和执行一些应用的初始化动作。如：删除临时文件，清除缓存信息，读取配置文件信息，数据库连接等。 

SpringBoot提供了CommandLineRunner和ApplicationRunner接口。当接口有多个实现类时，提供了@order注解实现自定义执行顺序，也可以实现Ordered接口来自定义顺序。

注意：数字越小，优先级越高，也就是@Order(1)注解的类会在@Order(2)注解的类之前执行。

两者的区别在于：

ApplicationRunner中run方法的参数为ApplicationArguments，而CommandLineRunner接口中run方法的参数为String数组。想要更详细地获取命令行参数，那就使用ApplicationRunner接口。

ApplicationRunner

@Component
@Order(value = 10)
public class AgentApplicationRun2 implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments applicationArguments) throws Exception {
 
    }
}

CommandLineRunner

@Component
@Order(value = 11)
public class AgentApplicationRun implements CommandLineRunner {
 
    @Override
    public void run(String... strings) throws Exception {
 
    }
}

接口不同于：

ApplicationRunner中run方法的参数为ApplicationArguments，而CommandLineRunner接口中run方法的参数为String数组。