首页 计算机学科 java spring springboot 高级

# Java 监听机制

SpringBoot 的监听机制，其实是对 Java 提供的事件监听机制的封装。

<span alt='solid'>Java 中的事件监听机制定义了以下几个角色 </span>：

  1. 事件：Event，继承 java.util.EventObject 类的对象
  2. 事件源：Source，任意对象 Object
  3. 监听器：Listener，实现 java.util.EventListener 接口的对象

<span alt='solid'>SpringBoot 监听机制 </span>：

SpringBoot 在项目启动时，会对几个监听器进行回调，我们可以实现这些监听器接口，在项目启动时完成一些操作

SpringBoot 中监听器分为：ApplicationContextInitializer，SpringApplicationRunListener，CommandLineRunner，ApplicationRunner

项目结构：

image-20230913201836361

ApplicationContextInitializer

import org.springframework.context.ApplicationContextInitializer;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.stereotype.Component;
@Component
public class MyApplicationContextInitializer implements ApplicationContextInitializer {
    @Override
    public void initialize(ConfigurableApplicationContext applicationContext) {
        System.out.println("ApplicationContextInitializer...initialize");
    }
}
SpringApplicationRunListener
import org.springframework.boot.ConfigurableBootstrapContext;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.SpringApplicationRunListener;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.env.ConfigurableEnvironment;
import java.time.Duration;
public class MySpringApplicationRunListener implements SpringApplicationRunListener {
    // 需要重写构造函数否则报错初始化异常
    public MySpringApplicationRunListener(SpringApplication application, String[] args) {}
    @Override
    public void starting(ConfigurableBootstrapContext bootstrapContext) {
        System.out.println("starting...项目启动中");
    }
    @Override
    public void environmentPrepared(ConfigurableBootstrapContext bootstrapContext,
                                     ConfigurableEnvironment environment) {
        System.out.println("environmentPrepared...环境对象开始准备");
    }
    @Override
    public void contextPrepared(ConfigurableApplicationContext context) {
        System.out.println("contextPrepared...上下文对象开始准备");
    }
    @Override
    public void contextLoaded(ConfigurableApplicationContext context) {
        System.out.println("contextLoaded...上下文对象开始加载");
    }
    @Deprecated
    public void started(ConfigurableApplicationContext context) {
        System.out.println("started...上下文对象加载完成");
    }
    @Deprecated
    public void running(ConfigurableApplicationContext context) {
        System.out.println("running...项目启动完成, 开始运行");
    }
    public void failed(ConfigurableApplicationContext context, Throwable exception) {
        System.out.println("failed...项目启动失败");
    }
    @Override
    public void started(ConfigurableApplicationContext context, Duration timeTaken) {
        started(context);
        System.out.println("started...");
    }
    public void ready(ConfigurableApplicationContext context, Duration timeTaken) {
        running(context);
        System.out.println("ready...");
    }
}
CommandLineRunner
import org.springframework.boot.CommandLineRunner;
import org.springframework.stereotype.Component;
import java.util.Arrays;
@Component
public class MyCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("CommandLineRunner...run");
        System.out.println(Arrays.toString(args));
    }
}
ApplicationRunner
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.stereotype.Component;
import java.util.Arrays;
@Component
public class MyApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("ApplicationRunner ... run");
        System.out.println(Arrays.toString(args.getSourceArgs()));
    }
}
spring.factories
org.springframework.context.ApplicationContextInitializer=\
  com.dkx.listener.MyApplicationContextInitializer
org.springframework.boot.SpringApplicationRunListener=\
  com.dkx.listener.MySpringApplicationRunListener
启动类
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
@SpringBootApplication
public class SpringbootListenerApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringbootListenerApplication.class, args);
    }
}
运行结果：
image-20230913202225065