SpringBoot之启动加载器

一、什么是启动加载器？

在项目启动的时候做一些初始化工作。

二、启动类加载器实践

2.1 实现 CommandLineRunner 接口

@Component
public class FirstCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("run FirstCommandLineRunner");
    }
}

2.2 实现 ApplicationRunner 接口

@Component
public class FirstApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("run FirstApplicationRunner");
    }
}

启动项目，观察控制台输出：

run FirstApplicationRunner
run FirstCommandLineRunner

可以看到默认实现 ApplicationRunner 接口比 CommandLineRunner 优先级高。

2.3 使用 @Order 注解

我们试试看增加 @Order 注解之后会怎么样

@Component
@Order(1)
public class FirstCommandLineRunner implements CommandLineRunner {
	......
}    
@Component
@Order(2)
public class FirstApplicationRunner implements ApplicationRunner {
    ......
}    

启动项目，观察控制台输出：

run FirstCommandLineRunner
run FirstApplicationRunner

发现 @Order 注解可以改变执行顺序。

三、启动类加载器原理

进入 SpringApplication.run 方法中，callRunners 方法就是入口：

public ConfigurableApplicationContext run(String... args) {
	......
	callRunners(context, applicationArguments);
	......
}

查看 callRunners 方法实现：

private void callRunners(ApplicationContext context, ApplicationArguments args) {
	List<Object> runners = new ArrayList<>();
    // 添加接口实现类
	runners.addAll(context.getBeansOfType(ApplicationRunner.class).values());
	runners.addAll(context.getBeansOfType(CommandLineRunner.class).values());
    // 进行排序
	AnnotationAwareOrderComparator.sort(runners);
    // 循环调用 callRunner 方法
	for (Object runner : new LinkedHashSet<>(runners)) {
		if (runner instanceof ApplicationRunner) {
			callRunner((ApplicationRunner) runner, args);
		}
		if (runner instanceof CommandLineRunner) {
			callRunner((CommandLineRunner) runner, args);
		}
	}
}

可以看到先添加的 ApplicationRunner 接口实现，然后添加 CommandLineRunner 接口实现，所以默认优先级以 ApplicationRunner 为先，后面有个排序实现，以 @Order 进行优先级排序

我们再来分别看下两个接口各自的 callRunner 的实现：

private void callRunner(ApplicationRunner runner, ApplicationArguments args) {
	try {
		(runner).run(args);
	}
	catch (Exception ex) {
		throw new IllegalStateException("Failed to execute ApplicationRunner", ex);
	}
}

private void callRunner(CommandLineRunner runner, ApplicationArguments args) {
	try {
		(runner).run(args.getSourceArgs());
	}
	catch (Exception ex) {
		throw new IllegalStateException("Failed to execute CommandLineRunner", ex);
	}
}