Spring Boot启动流程扩展

Spring Boot 2019-05-02

Spring Boot启动可扩展流程

通过上一篇Spring Boot启动流程详解，我们可知Spring Boot在以下几点可以进行扩展

初始化器(Initializer)
监听器(Listener)
Runners

下面依次来看下如何添加扩展

Initializer

Spring Boot在启动时会从META-INF/spring.factories文件中，加载ApplicationContextInitializer，下面是其定义

    public interface ApplicationContextInitializer<C extends ConfigurableApplicationContext> {
        void initialize(ConfigurableApplicationContext configurableApplicationContext);
    }

下面，我们定义一个类来实现该接口

@Slf4j
public class CustomApplicationInitializer implements ApplicationContextInitializer {
    @Override
    public void initialize(ConfigurableApplicationContext configurableApplicationContext) {
        log.info("自定义Initializer启动");
    }
}

加载自定义的initializer的方式有两种，一是在main方法中add initializer另一种则更符合Spring Boot style，在META-INF/spring.factories中增加自定义initializer的配置

org.springframework.context.ApplicationContextInitializer=com.wbl.spingbootdemo.initializer.CustomApplicationInitializer

启动Spring Boot则可以看到如下输出

Listener

Spring Boot启动的时候会从META-INF/spring.factories文件中加载ApplicationListener,
下面是其定义

@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    void onApplicationEvent(ApplicationEvent applicationEvent;
}

ApplicationListener实现了EventListener接口，这是JDK中定义的一个类，用来实现观察者模式。只不过ApplicationListener只监听ApplicationEvent事件

下面实现一个自定的ApplicationListener，监听的几个事件都是继承SpringApplicationEvent的

@Slf4j
public class CustomApplicationListener implements ApplicationListener {
    @Override
    public void onApplicationEvent(ApplicationEvent applicationEvent) {
        if (applicationEvent instanceof ApplicationStartingEvent){
            log.info("receive application starting event");
        }

        if (applicationEvent instanceof ApplicationStartedEvent){
            log.info("receive application started event");
        }

        if (applicationEvent instanceof ApplicationEnvironmentPreparedEvent) {
            log.info("receive application environment prepared event");
        }

        if (applicationEvent instanceof ApplicationPreparedEvent){
            log.info("receive application application prepared event");
        }

        if (applicationEvent instanceof ApplicationContextInitializedEvent){
            log.info("receive application context initialized event");
        }

        if (applicationEvent instanceof ApplicationReadyEvent){
            log.info("receive application ready event");
        }
    }
}

启动Spring Boot可以看到如下输出

总共监听到了以下事件

ApplicationEnvironmentPreparedEvent
ApplicationContextInitializedEvent
ApplicationPreparedEvent
ApplicationStartedEvent
ApplicationReadyEvent

而没有监听到的事件是ApplicationStartingEvent，这是因为在发送此事件时，日志对象还没有初始化成功，所有无法打印日志，如果使用System.out.println则可以看到输出

Runner

Runner的实现分为两种

ApplicationRunner
CommandLineRunner

这两个实现本质上没有什么不同，除了接收参数不同

ApplicationRunner

下面实现了一个自定义的ApplicationRunner

@Service
@Slf4j
@Order(1)
public class CustomApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        log.info("启动自定义ApplicationRunner");
    }
}

@Order用来决定Runner执行的顺序，数字越小，表示优先级越高，越先执行

CommandLineRunner

下面实现一个自定义的CommandLineRunner

@Service
@Slf4j
@Order(2)
public class CustomCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        log.info("启动自定义CommandLineRunner");
    }
}

启动两个Runner可以看到如下输出

总结

Initializer需要实现ApplicationContextInitializer接口
Listener需要实现ApplicationListener
Initializer和Listener可以在META-INF/spring.factories进行配置，以便Spring Boot加载
Runner有ApplicationRunner和CommandLineRunner两种实现，两者除了接收参数不同外(一个接收ApplicationArguments，一个接收String[])，没有其他的区别
@Order注解用来决定Runner的执行顺序